futex: Address the robust futex unlock race for real
Subsystems (16)
Thomas Gleixner
maintainerv5 cover futex: Address the robust futex unlock race for real
This is a follow up to v4 which can be found here:
https://lore.kernel.org/[email protected]
The v1 cover letter contains a detailed analysis of the underlying problem:
https://lore.kernel.org/[email protected]
TLDR:
The robust futex unlock mechanism is racy in respect to the clearing of the robust_list_head::list_op_pending pointer because unlock and clearing the pointer are not atomic. The race window is between the unlock and clearing the pending op pointer. If the task is forced to exit in this window, exit will access a potentially invalid pending op pointer when cleaning up the robust list. That happens if another task manages to unmap the object containing the lock before the cleanup, which results in an UAF. In the worst case this UAF can lead to memory corruption when unrelated content has been mapped to the same address by the time the access happens.
User space can't solve this problem without help from the kernel. This series provides the kernel side infrastructure to help it along:
1) Combined unlock, pointer clearing, wake-up for the contended case
2) VDSO based unlock and pointer clearing helpers with a fix-up function in the kernel when user space was interrupted within the critical section.
Both ensure that the pointer clearing happens _before_ a task exits and the kernel cleans up the robust list during the exit procedure.
Changes since v4:
- Fixed the build fails caused by using __percpu in the types header, which unearthed a nasty header inclusion order and works by chance problem. Reported by 0-day and Nam
- Changed the range initializer to take the full address - Andre
- Picked up tags where appropriate
- Added the debug vdso image to sysfs. This allows CI and general debugging to find the debug.so in a uniform place. The patch is marked RFC and can be discussed and applied/rejected separately of the rest. The images are small enough and only one instance of each variant is required, so the memory overhead is minimal.
The delta patch (w/o the RFC VDSO sysfs change) against the previous version can be found below.
The series applies on v7.1-rc2 and is also available via git:
git://git.kernel.org/pub/scm/linux/kernel/git/tglx/devel.git locking-futex-v5
Thanks,
tglx
---
arch/um/Makefile +2 -1
@@ -71,7 +71,8 @@ USER_CFLAGS = $(patsubst $(KERNEL_DEFINES),,$(patsubst -I%,,$(KBUILD_CFLAGS))) \ -D_FILE_OFFSET_BITS=64 -idirafter $(srctree)/include \ -idirafter $(objtree)/include -D__KERNEL__ -D__UM_HOST__ \ -include $(srctree)/include/linux/compiler-version.h \ - -include $(srctree)/include/linux/kconfig.h + -include $(srctree)/include/linux/kconfig.h \ + -idirafter $(ARCH_DIR)/include/generated #This will adjust *FLAGS accordingly to the platform. include $(srctree)/$(ARCH_DIR)/Makefile-os-Linux
arch/um/include/asm/Kbuild +1 -0
@@ -16,6 +16,7 @@ generic-y += module.h generic-y += module.lds.h generic-y += parport.h generic-y += percpu.h +generic-y += percpu_types.h generic-y += preempt.h generic-y += runtime-const.h generic-y += softirq_stack.h
arch/x86/entry/vdso/common/vfutex.c +1 -1
@@ -32,7 +32,7 @@#define CLEAR_POPQ "movq %[zero], %a[pop]\n"#define CLEAR_POPL "movl %k[zero], %a[pop]\n" -#define futex_robust_try_unlock (prefix, clear_pop, __lock, __tid, __pop) \ +#define futex_robust_try_unlock (prefix, clear_pop, __lock, __tid, __pop)\ ({ \ asm volatile ( \ " \n" \
arch/x86/entry/vdso/vma.c +4 -4
@@ -85,14 +85,14 @@ static void vdso_futex_robust_unlock_update_ips(void)futex_reset_cs_ranges (fd);#ifdef CONFIG_X86_64 -futex_set_vdso_cs_range (fd, idx, vdso, image->sym___futex_list64_try_unlock_cs_start, - image->sym___futex_list64_try_unlock_cs_end, false); +futex_set_vdso_cs_range (fd, idx, vdso + image->sym___futex_list64_try_unlock_cs_start, + vdso + image->sym___futex_list64_try_unlock_cs_end, false); idx++;#endif /* CONFIG_X86_64 */ #if defined(CONFIG_X86_32 ) || defined(CONFIG_COMPAT ) -futex_set_vdso_cs_range (fd, idx, vdso, image->sym___futex_list32_try_unlock_cs_start, - image->sym___futex_list32_try_unlock_cs_end, true); +futex_set_vdso_cs_range (fd, idx, vdso + image->sym___futex_list32_try_unlock_cs_start, + vdso + image->sym___futex_list32_try_unlock_cs_end, true); #endif /* CONFIG_X86_32 || CONFIG_COMPAT */ }#else
arch/x86/include/asm/percpu.h +0 -5
@@ -40,12 +40,10 @@#endif #define __percpu_prefix -#define __percpu_seg_override CONCATENATE(__seg_, __percpu_seg)#else /* !CONFIG_CC_HAS_NAMED_AS: */ #define __percpu_prefix __force_percpu_prefix -#define __percpu_seg_override#endif /* CONFIG_CC_HAS_NAMED_AS */
@@ -82,7 +80,6 @@#define __force_percpu_prefix#define __percpu_prefix -#define __percpu_seg_override#define PER_CPU_VAR (var) (var)__percpu_rel
@@ -92,8 +89,6 @@# define __my_cpu_type (var) typeof(var)# define __my_cpu_ptr (ptr) (ptr)# define __my_cpu_var (var) (var) - -# define __percpu_qual __percpu_seg_override#else # define __my_cpu_type (var) typeof(var) __percpu_seg_override# define __my_cpu_ptr (ptr) (__my_cpu_type(*(ptr))*)(__force uintptr_t)(ptr)
arch/x86/include/asm/percpu_types.h new file +17 -0
@@ -0,0 +1,17 @@ +/* SPDX-License-Identifier: GPL-2.0 */ +#ifndef _ASM_X86_PERCPU_TYPES_H +#define _ASM_X86_PERCPU_TYPES_H + +#if defined(CONFIG_SMP ) && defined(CONFIG_CC_HAS_NAMED_AS ) +#define __percpu_seg_override CONCATENATE(__seg_, __percpu_seg) +#else /* !CONFIG_CC_HAS_NAMED_AS: */ +#define __percpu_seg_override +#endif + +#if defined(CONFIG_USE_X86_SEG_SUPPORT ) && defined(USE_TYPEOF_UNQUAL ) +#define __percpu_qual __percpu_seg_override +#endif + +#include <asm-generic/percpu_types.h> + +#endif
include/asm-generic/Kbuild +1 -0
@@ -44,6 +44,7 @@ mandatory-y += module.lds.h mandatory-y += msi.h mandatory-y += pci.h mandatory-y += percpu.h +mandatory-y += percpu_types.h mandatory-y += pgalloc.h mandatory-y += preempt.h mandatory-y += rqspinlock.h
include/asm-generic/percpu_types.h new file +19 -0
@@ -0,0 +1,19 @@ +/* SPDX-License-Identifier: GPL-2.0 */ +#ifndef _ASM_GENERIC_PERCPU_TYPES_H_ +#define _ASM_GENERIC_PERCPU_TYPES_H_ + +#ifndef __ASSEMBLER__ +/* +* __percpu_qual is the qualifier for the percpu named address space. +* +* Most architectures use generic named address space for percpu variables but +* some architectures define percpu variables in different named address space. +* E.g. on x86, percpu variable may be declared as being relative to the %fs or +* %gs segments using __seg_fs or __seg_gs named address space qualifier. +*/ +#ifndef __percpu_qual +# define __percpu_qual +#endif + +#endif /* __ASSEMBLER__ */ +#endif /* _ASM_GENERIC_PERCPU_TYPES_H_ */
include/linux/compiler_types.h +3 -0
@@ -634,6 +634,9 @@ struct ftrace_likely_data {#else #define __unqual_scalar_typeof (x) __typeof_unqual__(x)#endif + +#include <asm/percpu_types.h> +#endif /* !__ASSEMBLY__ */ /*
include/linux/futex.h +2 -3
@@ -142,10 +142,9 @@ static inline void futex_fixup_robust_unlock(struct pt_regs *regs) }static inline void futex_set_vdso_cs_range (struct futex_mm_data *fd,unsigned int idx, -unsigned long vdso,unsigned long start, -unsigned long end,bool sz32) +unsigned long start,unsigned long end,bool sz32) { - fd->unlock.cs_ranges[idx].start_ip = vdso + start; + fd->unlock.cs_ranges[idx].start_ip = start; fd->unlock.cs_ranges[idx].len = end - start; fd->unlock.cs_ranges[idx].pop_size32 = sz32; }
include/linux/futex_types.h +1 -0
@@ -3,6 +3,7 @@#define _LINUX_FUTEX_TYPES_H#ifdef CONFIG_FUTEX +#include <linux/compiler_types.h> #include <linux/mutex_types.h> #include <linux/types.h>
include/linux/percpu.h +5 -4
@@ -3,13 +3,14 @@#define __LINUX_PERCPU_H#include <linux/alloc_tag.h> +#include <linux/cleanup.h> +#include <linux/compiler_types.h> +#include <linux/init.h> #include <linux/mmdebug.h> -#include <linux/preempt.h> -#include <linux/smp.h> #include <linux/pfn.h> -#include <linux/init.h> -#include <linux/cleanup.h> +#include <linux/preempt.h> #include <linux/sched.h> +#include <linux/smp.h> #include <asm/percpu.h>
kernel/futex/core.c +2 -2
@@ -1510,7 +1510,7 @@ void futex_exit_recursive(struct task_struct *tsk) }static void futex_cleanup_begin (struct task_struct *tsk) - __acquires(&tsk->futex_exit_mutex) + __acquires(&tsk->futex.exit_mutex) {/* * Prevent various race issues against a concurrent incoming waiter
@@ -1537,7 +1537,7 @@ static void futex_cleanup_begin(struct task_struct *tsk) }static void futex_cleanup_end (struct task_struct *tsk,int state) - __releases(&tsk->futex_exit_mutex) + __releases(&tsk->futex.exit_mutex) {/* * Lockless store. The only side effect is that an observer might
tools/testing/selftests/futex/functional/robust_list.c +1 -1
@@ -45,7 +45,7 @@#define SLEEP_US 100 -#if UINTPTR_MAX ==0xffffffffffffffff +#if __SIZEOF_LONG__ ==8 # define BUILD_64 #endif
Thomas Gleixner
maintainerPATCH v5 1/16 percpu: Sanitize __percpu_qual include hell
Slapping __percpu_qual into the next available header is sloppy at best.
It's required by __percpu which is defined in compiler_types.h and that is meant to be included without requiring a boatload of other headers so that a struct or function declaration can contain a __percpu qualifier w/o further prerequisites.
This implicit dependency on linux/percpu.h makes that impossible and causes a major problem when trying to separate headers.
Create asm/percpu_types.h and move it there. Include that from compiler_types.h and the whole recursion problem goes away.
Fix up UM so it uses the generic header and includes it in the UM_HOST build, which pulls in compiler_types.h. The USER_CFLAGS fix was suggested by Richard.
---
V5: New. Address 0-day __percpu fallout
--- arch/um/Makefile | 3 ++- arch/um/include/asm/Kbuild | 1 + arch/x86/include/asm/percpu.h | 5 ----- arch/x86/include/asm/percpu_types.h | 17 +++++++++++++++++ include/asm-generic/Kbuild | 1 + include/asm-generic/percpu_types.h | 19 +++++++++++++++++++ include/linux/compiler_types.h | 3 +++ include/linux/percpu.h | 9 +++++---- 8 files changed, 48 insertions(+), 10 deletions(-)
arch/um/Makefile +2 -1
@@ -71,7 +71,8 @@ USER_CFLAGS = $(patsubst $(KERNEL_DEFINE -D_FILE_OFFSET_BITS=64 -idirafter $(srctree)/include \ -idirafter $(objtree)/include -D__KERNEL__ -D__UM_HOST__ \ -include $(srctree)/include/linux/compiler-version.h \ - -include $(srctree)/include/linux/kconfig.h + -include $(srctree)/include/linux/kconfig.h \ + -idirafter $(ARCH_DIR)/include/generated #This will adjust *FLAGS accordingly to the platform. include $(srctree)/$(ARCH_DIR)/Makefile-os-Linux
arch/um/include/asm/Kbuild +1 -0
@@ -16,6 +16,7 @@ generic-y += module.h generic-y += module.lds.h generic-y += parport.h generic-y += percpu.h +generic-y += percpu_types.h generic-y += preempt.h generic-y += runtime-const.h generic-y += softirq_stack.h
arch/x86/include/asm/percpu.h +0 -5
@@ -40,12 +40,10 @@#endif #define __percpu_prefix -#define __percpu_seg_override CONCATENATE(__seg_, __percpu_seg)#else /* !CONFIG_CC_HAS_NAMED_AS: */ #define __percpu_prefix __force_percpu_prefix -#define __percpu_seg_override#endif /* CONFIG_CC_HAS_NAMED_AS */
@@ -82,7 +80,6 @@#define __force_percpu_prefix#define __percpu_prefix -#define __percpu_seg_override#define PER_CPU_VAR (var) (var)__percpu_rel
@@ -92,8 +89,6 @@# define __my_cpu_type (var) typeof(var)# define __my_cpu_ptr (ptr) (ptr)# define __my_cpu_var (var) (var) - -# define __percpu_qual __percpu_seg_override#else # define __my_cpu_type (var) typeof(var) __percpu_seg_override# define __my_cpu_ptr (ptr) (__my_cpu_type(*(ptr))*)(__force uintptr_t)(ptr)
arch/x86/include/asm/percpu_types.h new file +17 -0
@@ -0,0 +1,17 @@ +/* SPDX-License-Identifier: GPL-2.0 */ +#ifndef _ASM_X86_PERCPU_TYPES_H +#define _ASM_X86_PERCPU_TYPES_H + +#if defined(CONFIG_SMP ) && defined(CONFIG_CC_HAS_NAMED_AS ) +#define __percpu_seg_override CONCATENATE(__seg_, __percpu_seg) +#else /* !CONFIG_CC_HAS_NAMED_AS: */ +#define __percpu_seg_override +#endif + +#if defined(CONFIG_USE_X86_SEG_SUPPORT ) && defined(USE_TYPEOF_UNQUAL ) +#define __percpu_qual __percpu_seg_override +#endif + +#include <asm-generic/percpu_types.h> + +#endif
include/asm-generic/Kbuild +1 -0
@@ -44,6 +44,7 @@ mandatory-y += module.lds.h mandatory-y += msi.h mandatory-y += pci.h mandatory-y += percpu.h +mandatory-y += percpu_types.h mandatory-y += pgalloc.h mandatory-y += preempt.h mandatory-y += rqspinlock.h
include/asm-generic/percpu_types.h new file +19 -0
@@ -0,0 +1,19 @@ +/* SPDX-License-Identifier: GPL-2.0 */ +#ifndef _ASM_GENERIC_PERCPU_TYPES_H_ +#define _ASM_GENERIC_PERCPU_TYPES_H_ + +#ifndef __ASSEMBLER__ +/* +* __percpu_qual is the qualifier for the percpu named address space. +* +* Most architectures use generic named address space for percpu variables but +* some architectures define percpu variables in different named address space. +* E.g. on x86, percpu variable may be declared as being relative to the %fs or +* %gs segments using __seg_fs or __seg_gs named address space qualifier. +*/ +#ifndef __percpu_qual +# define __percpu_qual +#endif + +#endif /* __ASSEMBLER__ */ +#endif /* _ASM_GENERIC_PERCPU_TYPES_H_ */
include/linux/compiler_types.h +3 -0
@@ -634,6 +634,9 @@ struct ftrace_likely_data {#else #define __unqual_scalar_typeof (x) __typeof_unqual__(x)#endif + +#include <asm/percpu_types.h> +#endif /* !__ASSEMBLY__ */ /*
include/linux/percpu.h +5 -4
@@ -3,13 +3,14 @@#define __LINUX_PERCPU_H#include <linux/alloc_tag.h> +#include <linux/cleanup.h> +#include <linux/compiler_types.h> +#include <linux/init.h> #include <linux/mmdebug.h> -#include <linux/preempt.h> -#include <linux/smp.h> #include <linux/pfn.h> -#include <linux/init.h> -#include <linux/cleanup.h> +#include <linux/preempt.h> #include <linux/sched.h> +#include <linux/smp.h> #include <asm/percpu.h>
[tip: locking/core] percpu: Sanitize __percpu_qual include hell
The following commit has been merged into the locking/core branch of tip:
The following commit has been merged into the locking/core branch of tip:
Commit-ID: c06cd66387da92e6cdac44e16c7b5ef9219c53ac Gitweb: https://git.kernel.org/tip/c06cd66387da92e6cdac44e16c7b5ef9219c53ac Author: Thomas Gleixner <[email protected]> AuthorDate: Tue, 02 Jun 2026 11:09:21 +02:00 Committer: Peter Zijlstra <[email protected]> CommitterDate: Wed, 03 Jun 2026 11:38:48 +02:00
percpu: Sanitize __percpu_qual include hell
Slapping __percpu_qual into the next available header is sloppy at best.
It's required by __percpu which is defined in compiler_types.h and that is meant to be included without requiring a boatload of other headers so that a struct or function declaration can contain a __percpu qualifier w/o further prerequisites.
This implicit dependency on linux/percpu.h makes that impossible and causes a major problem when trying to separate headers.
Create asm/percpu_types.h and move it there. Include that from compiler_types.h and the whole recursion problem goes away.
Fix up UM so it uses the generic header and includes it in the UM_HOST build, which pulls in compiler_types.h. The USER_CFLAGS fix was suggested by Richard.
--- arch/um/Makefile | 3 ++- arch/um/include/asm/Kbuild | 1 + arch/x86/include/asm/percpu.h | 5 ----- arch/x86/include/asm/percpu_types.h | 17 +++++++++++++++++ include/asm-generic/Kbuild | 1 + include/asm-generic/percpu_types.h | 19 +++++++++++++++++++ include/linux/compiler_types.h | 3 +++ include/linux/percpu.h | 9 +++++---- 8 files changed, 48 insertions(+), 10 deletions(-) create mode 100644 arch/x86/include/asm/percpu_types.h create mode 100644 include/asm-generic/percpu_types.h
arch/um/Makefile +2 -1
@@ -71,7 +71,8 @@ USER_CFLAGS = $(patsubst $(KERNEL_DEFINES),,$(patsubst -I%,,$(KBUILD_CFLAGS))) \ -D_FILE_OFFSET_BITS=64 -idirafter $(srctree)/include \ -idirafter $(objtree)/include -D__KERNEL__ -D__UM_HOST__ \ -include $(srctree)/include/linux/compiler-version.h \ - -include $(srctree)/include/linux/kconfig.h + -include $(srctree)/include/linux/kconfig.h \ + -idirafter $(ARCH_DIR)/include/generated #This will adjust *FLAGS accordingly to the platform. include $(srctree)/$(ARCH_DIR)/Makefile-os-Linux
arch/um/include/asm/Kbuild +1 -0
@@ -16,6 +16,7 @@ generic-y += module.h generic-y += module.lds.h generic-y += parport.h generic-y += percpu.h +generic-y += percpu_types.h generic-y += preempt.h generic-y += runtime-const.h generic-y += softirq_stack.h
arch/x86/include/asm/percpu.h +0 -5
@@ -40,12 +40,10 @@#endif #define __percpu_prefix -#define __percpu_seg_override CONCATENATE(__seg_, __percpu_seg)#else /* !CONFIG_CC_HAS_NAMED_AS: */ #define __percpu_prefix __force_percpu_prefix -#define __percpu_seg_override#endif /* CONFIG_CC_HAS_NAMED_AS */
@@ -82,7 +80,6 @@#define __force_percpu_prefix#define __percpu_prefix -#define __percpu_seg_override#define PER_CPU_VAR (var) (var)__percpu_rel
@@ -92,8 +89,6 @@# define __my_cpu_type (var) typeof(var)# define __my_cpu_ptr (ptr) (ptr)# define __my_cpu_var (var) (var) - -# define __percpu_qual __percpu_seg_override#else # define __my_cpu_type (var) typeof(var) __percpu_seg_override# define __my_cpu_ptr (ptr) (__my_cpu_type(*(ptr))*)(__force uintptr_t)(ptr)
arch/x86/include/asm/percpu_types.h new file +17 -0
@@ -0,0 +1,17 @@ +/* SPDX-License-Identifier: GPL-2.0 */ +#ifndef _ASM_X86_PERCPU_TYPES_H +#define _ASM_X86_PERCPU_TYPES_H + +#if defined(CONFIG_SMP ) && defined(CONFIG_CC_HAS_NAMED_AS ) +#define __percpu_seg_override CONCATENATE(__seg_, __percpu_seg) +#else /* !CONFIG_CC_HAS_NAMED_AS: */ +#define __percpu_seg_override +#endif + +#if defined(CONFIG_USE_X86_SEG_SUPPORT ) && defined(USE_TYPEOF_UNQUAL ) +#define __percpu_qual __percpu_seg_override +#endif + +#include <asm-generic/percpu_types.h> + +#endif
include/asm-generic/Kbuild +1 -0
@@ -44,6 +44,7 @@ mandatory-y += module.lds.h mandatory-y += msi.h mandatory-y += pci.h mandatory-y += percpu.h +mandatory-y += percpu_types.h mandatory-y += pgalloc.h mandatory-y += preempt.h mandatory-y += rqspinlock.h
include/asm-generic/percpu_types.h new file +19 -0
@@ -0,0 +1,19 @@ +/* SPDX-License-Identifier: GPL-2.0 */ +#ifndef _ASM_GENERIC_PERCPU_TYPES_H_ +#define _ASM_GENERIC_PERCPU_TYPES_H_ + +#ifndef __ASSEMBLER__ +/* +* __percpu_qual is the qualifier for the percpu named address space. +* +* Most architectures use generic named address space for percpu variables but +* some architectures define percpu variables in different named address space. +* E.g. on x86, percpu variable may be declared as being relative to the %fs or +* %gs segments using __seg_fs or __seg_gs named address space qualifier. +*/ +#ifndef __percpu_qual +# define __percpu_qual +#endif + +#endif /* __ASSEMBLER__ */ +#endif /* _ASM_GENERIC_PERCPU_TYPES_H_ */
include/linux/compiler_types.h +3 -0
@@ -634,6 +634,9 @@ struct ftrace_likely_data {#else #define __unqual_scalar_typeof (x) __typeof_unqual__(x)#endif + +#include <asm/percpu_types.h> +#endif /* !__ASSEMBLY__ */ /*
include/linux/percpu.h +5 -4
@@ -3,13 +3,14 @@#define __LINUX_PERCPU_H#include <linux/alloc_tag.h> +#include <linux/cleanup.h> +#include <linux/compiler_types.h> +#include <linux/init.h> #include <linux/mmdebug.h> -#include <linux/preempt.h> -#include <linux/smp.h> #include <linux/pfn.h> -#include <linux/init.h> -#include <linux/cleanup.h> +#include <linux/preempt.h> #include <linux/sched.h> +#include <linux/smp.h> #include <asm/percpu.h>
Thomas Gleixner
maintainerPATCH v5 2/16 futex: Move futex task related data into a struct
In reply to Thomas GleixnerHaving all these members in task_struct along with the required #ifdeffery is annoying, does not allow efficient initializing of the data with memset() and makes extending it tedious.
Move it into a data structure and fix up all usage sites.
---
V2: Rename the struct and add the missing kernel doc - Andre
--- Documentation/locking/robust-futexes.rst | 8 ++-- include/linux/futex.h | 12 ++---- include/linux/futex_types.h | 36 ++++++++++++++++++ include/linux/sched.h | 16 ++------ kernel/exit.c | 4 +- kernel/futex/core.c | 59 +++++++++++++++---------------- kernel/futex/pi.c | 26 ++++++------- kernel/futex/syscalls.c | 23 ++++-------- 8 files changed, 101 insertions(+), 83 deletions(-)
Documentation/locking/robust-futexes.rst +4 -4
@@ -94,7 +94,7 @@ time, the kernel checks this user-space locks to be cleaned up? In the common case, at do_exit() time, there is no list registered, so -the cost of robust futexes is just a simple current->robust_list != NULL +the cost of robust futexes is just a current->futex.robust_list != NULL comparison. If the thread has registered a list, then normally the list is empty. If the thread/process crashed or terminated in some incorrect way then the list might be non-empty: in this case the kernel carefully
@@ -178,9 +178,9 @@ The patch adds two new syscalls: one to size_t __user *len_ptr); List registration is very fast: the pointer is simply stored in -current->robust_list. [Note that in the future, if robust futexes become -widespread, we could extend sys_clone() to register a robust-list head -for new threads, without the need of another syscall.] +current->futex.robust_list. [Note that in the future, if robust futexes +become widespread, we could extend sys_clone() to register a robust-list +head for new threads, without the need of another syscall.] So there is virtually zero overhead for tasks not using robust futexes, and even for robust futex users, there is only one extra syscall per
include/linux/futex.h +4 -8
@@ -64,14 +64,10 @@ enum {static inline void futex_init_task (struct task_struct *tsk) { - tsk->robust_list =NULL ; -#ifdef CONFIG_COMPAT - tsk->compat_robust_list =NULL ; -#endif -INIT_LIST_HEAD (&tsk->pi_state_list); - tsk->pi_state_cache =NULL ; - tsk->futex_state =FUTEX_STATE_OK ; -mutex_init (&tsk->futex_exit_mutex); +memset (&tsk->futex,0 ,sizeof (tsk->futex)); +INIT_LIST_HEAD (&tsk->futex.pi_state_list); + tsk->futex.state =FUTEX_STATE_OK ; +mutex_init (&tsk->futex.exit_mutex); }void futex_exit_recursive (struct task_struct *tsk);
include/linux/futex_types.h new file +36 -0
@@ -0,0 +1,36 @@ +/* SPDX-License-Identifier: GPL-2.0 */ +#ifndef _LINUX_FUTEX_TYPES_H +#define _LINUX_FUTEX_TYPES_H + +#ifdef CONFIG_FUTEX +#include <linux/mutex_types.h> +#include <linux/types.h> + +struct compat_robust_list_head ; +struct futex_pi_state ; +struct robust_list_head ; + +/** +* struct futex_sched_data - Futex related per task data +* @robust_list: User space registered robust list pointer +* @compat_robust_list: User space registered robust list pointer for compat tasks +* @pi_state_list: List head for Priority Inheritance (PI) state management +* @pi_state_cache: Pointer to cache one PI state object per task +* @exit_mutex: Mutex for serializing exit +* @state: Futex handling state to handle exit races correctly +*/ +struct futex_sched_data { +struct robust_list_head __user *robust_list; +#ifdef CONFIG_COMPAT +struct compat_robust_list_head __user *compat_robust_list; +#endif +struct list_head pi_state_list; +struct futex_pi_state *pi_state_cache; +struct mutex exit_mutex; +unsigned int state; +}; +#else +struct futex_sched_data { }; +#endif /* !CONFIG_FUTEX */ + +#endif /* _LINUX_FUTEX_TYPES_H */
include/linux/sched.h +4 -12
@@ -16,6 +16,7 @@#include <linux/cpumask_types.h> #include <linux/cache.h> +#include <linux/futex_types.h> #include <linux/irqflags_types.h> #include <linux/smp_types.h> #include <linux/pid_types.h>
@@ -64,7 +65,6 @@ struct bpf_net_context;struct capture_control ;struct cfs_rq ;struct fs_struct ; -struct futex_pi_state ;struct io_context ;struct io_uring_task ;struct mempolicy ;
@@ -76,7 +76,6 @@ struct pid_namespace;struct pipe_inode_info ;struct rcu_node ;struct reclaim_state ; -struct robust_list_head ;struct root_domain ;struct rq ;struct sched_attr ;
@@ -1331,16 +1330,9 @@ struct task_struct {u32 closid;u32 rmid;#endif -#ifdef CONFIG_FUTEX -struct robust_list_head __user *robust_list; -#ifdef CONFIG_COMPAT -struct compat_robust_list_head __user *compat_robust_list; -#endif -struct list_head pi_state_list; -struct futex_pi_state *pi_state_cache; -struct mutex futex_exit_mutex; -unsigned int futex_state; -#endif + +struct futex_sched_data futex; +#ifdef CONFIG_PERF_EVENTS u8 perf_recursion[PERF_NR_CONTEXTS ];struct perf_event_context *perf_event_ctxp;
kernel/exit.c +2 -2
@@ -988,8 +988,8 @@ void __noreturn do_exit(long code)proc_exit_connector (tsk);mpol_put_task_policy (tsk);#ifdef CONFIG_FUTEX -if (unlikely (current->pi_state_cache)) -kfree (current->pi_state_cache); +if (unlikely (current->futex.pi_state_cache)) +kfree (current->futex.pi_state_cache);#endif /* * Make sure we are holding no locks:
kernel/futex/core.c +30 -29
155 lines not on this page · raw message
kernel/futex/pi.c +13 -13
@@ -14,7 +14,7 @@ int refill_pi_state_cache(void) {struct futex_pi_state *pi_state; -if (likely (current->pi_state_cache)) +if (likely (current->futex.pi_state_cache))return 0 ; pi_state =kzalloc_obj (*pi_state);
@@ -28,17 +28,17 @@ int refill_pi_state_cache(void)refcount_set (&pi_state->refcount,1 ); pi_state->key =FUTEX_KEY_INIT ; - current->pi_state_cache = pi_state; + current->futex.pi_state_cache = pi_state;return 0 ; }static struct futex_pi_state *alloc_pi_state (void ) { -struct futex_pi_state *pi_state = current->pi_state_cache; +struct futex_pi_state *pi_state = current->futex.pi_state_cache;WARN_ON (!pi_state); - current->pi_state_cache =NULL ; + current->futex.pi_state_cache =NULL ;return pi_state; }
@@ -60,7 +60,7 @@ static void pi_state_update_owner(structif (new_owner) {raw_spin_lock (&new_owner->pi_lock);WARN_ON (!list_empty (&pi_state->list)); -list_add (&pi_state->list, &new_owner->pi_state_list); +list_add (&pi_state->list, &new_owner->futex.pi_state_list); pi_state->owner = new_owner;raw_spin_unlock (&new_owner->pi_lock); }
@@ -96,7 +96,7 @@ void put_pi_state(struct futex_pi_stateraw_spin_unlock_irqrestore (&pi_state->pi_mutex.wait_lock, flags); } -if (current->pi_state_cache) { +if (current->futex.pi_state_cache) {kfree (pi_state); }else {/*
@@ -106,7 +106,7 @@ void put_pi_state(struct futex_pi_state*/ pi_state->owner =NULL ;refcount_set (&pi_state->refcount,1 ); - current->pi_state_cache = pi_state; + current->futex.pi_state_cache = pi_state; } }
@@ -179,7 +179,7 @@ void put_pi_state(struct futex_pi_state* * p->pi_lock: * -* p->pi_state_list -> pi_state->list, relation +* p->futex.pi_state_list -> pi_state->list, relation * pi_mutex->owner -> pi_state->owner, relation * * pi_state->refcount:
@@ -327,7 +327,7 @@ static int handle_exit_race(u32 __user ** If the futex exit state is not yet FUTEX_STATE_DEAD, tell the * caller that the alleged owner is busy. */ -if (tsk && tsk->futex_state !=FUTEX_STATE_DEAD ) +if (tsk && tsk->futex.state !=FUTEX_STATE_DEAD )return -EBUSY ;/*
@@ -346,8 +346,8 @@ static int handle_exit_race(u32 __user ** *uaddr = 0xC0000000; tsk = get_task(PID); * } if (!tsk->flags & PF_EXITING) { * ... attach(); -* tsk->futex_state = } else { -* FUTEX_STATE_DEAD; if (tsk->futex_state != +* tsk->futex.state = } else { +* FUTEX_STATE_DEAD; if (tsk->futex.state != * FUTEX_STATE_DEAD) * return -EAGAIN; * return -ESRCH; <--- FAIL
@@ -396,7 +396,7 @@ static void __attach_to_pi_owner(struct pi_state->key = *key;WARN_ON (!list_empty (&pi_state->list)); -list_add (&pi_state->list, &p->pi_state_list); +list_add (&pi_state->list, &p->futex.pi_state_list);/* * Assignment without holding pi_state->pi_mutex.wait_lock is safe * because there is no concurrency as the object is not published yet.
@@ -440,7 +440,7 @@ static int attach_to_pi_owner(u32 __user* in futex_exit_release(), we do this protected by p->pi_lock: */ raw_spin_lock_irq (&p->pi_lock); -if (unlikely (p->futex_state !=FUTEX_STATE_OK )) { +if (unlikely (p->futex.state !=FUTEX_STATE_OK )) {/* * The task is on the way out. When the futex state is * FUTEX_STATE_DEAD, we know that the task has finished
kernel/futex/syscalls.c +8 -15
@@ -25,17 +25,13 @@* @head: pointer to the list-head * @len: length of the list-head, as userspace expects */ -SYSCALL_DEFINE2(set_robust_list, struct robust_list_head __user *, head, - size_t, len) +SYSCALL_DEFINE2(set_robust_list, struct robust_list_head __user *, head, size_t, len) { -/* -* The kernel knows only one size for now: -*/ +/* The kernel knows only one size for now. */ if (unlikely (len !=sizeof (*head)))return -EINVAL ; - current->robust_list = head; - + current->futex.robust_list = head;return 0 ; }
@@ -43,9 +39,9 @@ static inline void __user *futex_task_ro {#ifdef CONFIG_COMPAT if (compat) -return p->compat_robust_list; +return p->futex.compat_robust_list;#endif -return p->robust_list; +return p->futex.robust_list; }static void __user *futex_get_robust_list_common (int pid,bool compat)
@@ -475,15 +471,13 @@ SYSCALL_DEFINE4(futex_requeue, }#ifdef CONFIG_COMPAT -COMPAT_SYSCALL_DEFINE2 (set_robust_list, - struct compat_robust_list_head __user *, head, - compat_size_t, len) +COMPAT_SYSCALL_DEFINE2(set_robust_list, struct compat_robust_list_head __user *, head, + compat_size_t, len) {if (unlikely (len !=sizeof (*head)))return -EINVAL ; - current->compat_robust_list = head; - + current->futex.compat_robust_list = head;return 0 ; }
@@ -523,4 +517,3 @@ SYSCALL_DEFINE6(futex_time32, u32 __user return do_futex(uaddr, op, val, tp, uaddr2, (unsigned long)utime, val3); }#endif /* CONFIG_COMPAT_32BIT_TIME */ -
[tip: locking/core] futex: Move futex task related data into a struct
The following commit has been merged into the locking/core branch of tip:
The following commit has been merged into the locking/core branch of tip:
Commit-ID: c1ffc9c6e4f8a13dd68e97920c9a24d095c6e41a Gitweb: https://git.kernel.org/tip/c1ffc9c6e4f8a13dd68e97920c9a24d095c6e41a Author: Thomas Gleixner <[email protected]> AuthorDate: Tue, 02 Jun 2026 11:09:25 +02:00 Committer: Peter Zijlstra <[email protected]> CommitterDate: Wed, 03 Jun 2026 11:38:49 +02:00
futex: Move futex task related data into a struct
Having all these members in task_struct along with the required #ifdeffery is annoying, does not allow efficient initializing of the data with memset() and makes extending it tedious.
Move it into a data structure and fix up all usage sites.
--- Documentation/locking/robust-futexes.rst | 8 +-- include/linux/futex.h | 12 +---- include/linux/futex_types.h | 36 ++++++++++++++- include/linux/sched.h | 16 +----- kernel/exit.c | 4 +- kernel/futex/core.c | 59 +++++++++++------------ kernel/futex/pi.c | 26 +++++----- kernel/futex/syscalls.c | 23 +++------ 8 files changed, 101 insertions(+), 83 deletions(-) create mode 100644 include/linux/futex_types.h
Documentation/locking/robust-futexes.rst +4 -4
@@ -94,7 +94,7 @@ time, the kernel checks this user-space list: are there any robust futex locks to be cleaned up? In the common case, at do_exit() time, there is no list registered, so -the cost of robust futexes is just a simple current->robust_list != NULL +the cost of robust futexes is just a current->futex.robust_list != NULL comparison. If the thread has registered a list, then normally the list is empty. If the thread/process crashed or terminated in some incorrect way then the list might be non-empty: in this case the kernel carefully
@@ -178,9 +178,9 @@ one to query the registered list pointer:: size_t __user *len_ptr); List registration is very fast: the pointer is simply stored in -current->robust_list. [Note that in the future, if robust futexes become -widespread, we could extend sys_clone() to register a robust-list head -for new threads, without the need of another syscall.] +current->futex.robust_list. [Note that in the future, if robust futexes +become widespread, we could extend sys_clone() to register a robust-list +head for new threads, without the need of another syscall.] So there is virtually zero overhead for tasks not using robust futexes, and even for robust futex users, there is only one extra syscall per
include/linux/futex.h +4 -8
@@ -64,14 +64,10 @@ enum {static inline void futex_init_task (struct task_struct *tsk) { - tsk->robust_list =NULL ; -#ifdef CONFIG_COMPAT - tsk->compat_robust_list =NULL ; -#endif -INIT_LIST_HEAD (&tsk->pi_state_list); - tsk->pi_state_cache =NULL ; - tsk->futex_state =FUTEX_STATE_OK ; -mutex_init (&tsk->futex_exit_mutex); +memset (&tsk->futex,0 ,sizeof (tsk->futex)); +INIT_LIST_HEAD (&tsk->futex.pi_state_list); + tsk->futex.state =FUTEX_STATE_OK ; +mutex_init (&tsk->futex.exit_mutex); }void futex_exit_recursive (struct task_struct *tsk);
include/linux/futex_types.h new file +36 -0
@@ -0,0 +1,36 @@ +/* SPDX-License-Identifier: GPL-2.0 */ +#ifndef _LINUX_FUTEX_TYPES_H +#define _LINUX_FUTEX_TYPES_H + +#ifdef CONFIG_FUTEX +#include <linux/mutex_types.h> +#include <linux/types.h> + +struct compat_robust_list_head ; +struct futex_pi_state ; +struct robust_list_head ; + +/** +* struct futex_sched_data - Futex related per task data +* @robust_list: User space registered robust list pointer +* @compat_robust_list: User space registered robust list pointer for compat tasks +* @pi_state_list: List head for Priority Inheritance (PI) state management +* @pi_state_cache: Pointer to cache one PI state object per task +* @exit_mutex: Mutex for serializing exit +* @state: Futex handling state to handle exit races correctly +*/ +struct futex_sched_data { +struct robust_list_head __user *robust_list; +#ifdef CONFIG_COMPAT +struct compat_robust_list_head __user *compat_robust_list; +#endif +struct list_head pi_state_list; +struct futex_pi_state *pi_state_cache; +struct mutex exit_mutex; +unsigned int state; +}; +#else +struct futex_sched_data { }; +#endif /* !CONFIG_FUTEX */ + +#endif /* _LINUX_FUTEX_TYPES_H */
include/linux/sched.h +4 -12
@@ -16,6 +16,7 @@#include <linux/cpumask_types.h> #include <linux/cache.h> +#include <linux/futex_types.h> #include <linux/irqflags_types.h> #include <linux/smp_types.h> #include <linux/pid_types.h>
@@ -64,7 +65,6 @@ struct bpf_net_context;struct capture_control ;struct cfs_rq ;struct fs_struct ; -struct futex_pi_state ;struct io_context ;struct io_uring_task ;struct mempolicy ;
@@ -76,7 +76,6 @@ struct pid_namespace;struct pipe_inode_info ;struct rcu_node ;struct reclaim_state ; -struct robust_list_head ;struct root_domain ;struct rq ;struct sched_attr ;
@@ -1331,16 +1330,9 @@ struct task_struct {u32 closid;u32 rmid;#endif -#ifdef CONFIG_FUTEX -struct robust_list_head __user *robust_list; -#ifdef CONFIG_COMPAT -struct compat_robust_list_head __user *compat_robust_list; -#endif -struct list_head pi_state_list; -struct futex_pi_state *pi_state_cache; -struct mutex futex_exit_mutex; -unsigned int futex_state; -#endif + +struct futex_sched_data futex; +#ifdef CONFIG_PERF_EVENTS u8 perf_recursion[PERF_NR_CONTEXTS ];struct perf_event_context *perf_event_ctxp;
kernel/exit.c +2 -2
@@ -988,8 +988,8 @@ void __noreturn do_exit(long code)proc_exit_connector (tsk);mpol_put_task_policy (tsk);#ifdef CONFIG_FUTEX -if (unlikely (current->pi_state_cache)) -kfree (current->pi_state_cache); +if (unlikely (current->futex.pi_state_cache)) +kfree (current->futex.pi_state_cache);#endif /* * Make sure we are holding no locks:
kernel/futex/core.c +30 -29
155 lines not on this page · raw message
kernel/futex/pi.c +13 -13
@@ -14,7 +14,7 @@ int refill_pi_state_cache(void) {struct futex_pi_state *pi_state; -if (likely (current->pi_state_cache)) +if (likely (current->futex.pi_state_cache))return 0 ; pi_state =kzalloc_obj (*pi_state);
@@ -28,17 +28,17 @@ int refill_pi_state_cache(void)refcount_set (&pi_state->refcount,1 ); pi_state->key =FUTEX_KEY_INIT ; - current->pi_state_cache = pi_state; + current->futex.pi_state_cache = pi_state;return 0 ; }static struct futex_pi_state *alloc_pi_state (void ) { -struct futex_pi_state *pi_state = current->pi_state_cache; +struct futex_pi_state *pi_state = current->futex.pi_state_cache;WARN_ON (!pi_state); - current->pi_state_cache =NULL ; + current->futex.pi_state_cache =NULL ;return pi_state; }
@@ -60,7 +60,7 @@ static void pi_state_update_owner(struct futex_pi_state *pi_state,if (new_owner) {raw_spin_lock (&new_owner->pi_lock);WARN_ON (!list_empty (&pi_state->list)); -list_add (&pi_state->list, &new_owner->pi_state_list); +list_add (&pi_state->list, &new_owner->futex.pi_state_list); pi_state->owner = new_owner;raw_spin_unlock (&new_owner->pi_lock); }
@@ -96,7 +96,7 @@ void put_pi_state(struct futex_pi_state *pi_state)raw_spin_unlock_irqrestore (&pi_state->pi_mutex.wait_lock, flags); } -if (current->pi_state_cache) { +if (current->futex.pi_state_cache) {kfree (pi_state); }else {/*
@@ -106,7 +106,7 @@ void put_pi_state(struct futex_pi_state *pi_state)*/ pi_state->owner =NULL ;refcount_set (&pi_state->refcount,1 ); - current->pi_state_cache = pi_state; + current->futex.pi_state_cache = pi_state; } }
@@ -179,7 +179,7 @@ void put_pi_state(struct futex_pi_state *pi_state)* * p->pi_lock: * -* p->pi_state_list -> pi_state->list, relation +* p->futex.pi_state_list -> pi_state->list, relation * pi_mutex->owner -> pi_state->owner, relation * * pi_state->refcount:
@@ -327,7 +327,7 @@ static int handle_exit_race(u32 __user *uaddr, u32 uval,* If the futex exit state is not yet FUTEX_STATE_DEAD, tell the * caller that the alleged owner is busy. */ -if (tsk && tsk->futex_state !=FUTEX_STATE_DEAD ) +if (tsk && tsk->futex.state !=FUTEX_STATE_DEAD )return -EBUSY ;/*
@@ -346,8 +346,8 @@ static int handle_exit_race(u32 __user *uaddr, u32 uval,* *uaddr = 0xC0000000; tsk = get_task(PID); * } if (!tsk->flags & PF_EXITING) { * ... attach(); -* tsk->futex_state = } else { -* FUTEX_STATE_DEAD; if (tsk->futex_state != +* tsk->futex.state = } else { +* FUTEX_STATE_DEAD; if (tsk->futex.state != * FUTEX_STATE_DEAD) * return -EAGAIN; * return -ESRCH; <--- FAIL
@@ -396,7 +396,7 @@ static void __attach_to_pi_owner(struct task_struct *p, union futex_key *key, pi_state->key = *key;WARN_ON (!list_empty (&pi_state->list)); -list_add (&pi_state->list, &p->pi_state_list); +list_add (&pi_state->list, &p->futex.pi_state_list);/* * Assignment without holding pi_state->pi_mutex.wait_lock is safe * because there is no concurrency as the object is not published yet.
@@ -440,7 +440,7 @@ static int attach_to_pi_owner(u32 __user *uaddr, u32 uval, union futex_key *key,* in futex_exit_release(), we do this protected by p->pi_lock: */ raw_spin_lock_irq (&p->pi_lock); -if (unlikely (p->futex_state !=FUTEX_STATE_OK )) { +if (unlikely (p->futex.state !=FUTEX_STATE_OK )) {/* * The task is on the way out. When the futex state is * FUTEX_STATE_DEAD, we know that the task has finished
kernel/futex/syscalls.c +8 -15
@@ -25,17 +25,13 @@* @head: pointer to the list-head * @len: length of the list-head, as userspace expects */ -SYSCALL_DEFINE2(set_robust_list, struct robust_list_head __user *, head, - size_t, len) +SYSCALL_DEFINE2(set_robust_list, struct robust_list_head __user *, head, size_t, len) { -/* -* The kernel knows only one size for now: -*/ +/* The kernel knows only one size for now. */ if (unlikely (len !=sizeof (*head)))return -EINVAL ; - current->robust_list = head; - + current->futex.robust_list = head;return 0 ; }
@@ -43,9 +39,9 @@ static inline void __user *futex_task_robust_list(struct task_struct *p, bool co {#ifdef CONFIG_COMPAT if (compat) -return p->compat_robust_list; +return p->futex.compat_robust_list;#endif -return p->robust_list; +return p->futex.robust_list; }static void __user *futex_get_robust_list_common (int pid,bool compat)
@@ -475,15 +471,13 @@ SYSCALL_DEFINE4(futex_requeue, }#ifdef CONFIG_COMPAT -COMPAT_SYSCALL_DEFINE2 (set_robust_list, - struct compat_robust_list_head __user *, head, - compat_size_t, len) +COMPAT_SYSCALL_DEFINE2(set_robust_list, struct compat_robust_list_head __user *, head, + compat_size_t, len) {if (unlikely (len !=sizeof (*head)))return -EINVAL ; - current->compat_robust_list = head; - + current->futex.compat_robust_list = head;return 0 ; }
@@ -523,4 +517,3 @@ SYSCALL_DEFINE6(futex_time32, u32 __user *, uaddr, int, op, u32, val, return do_futex(uaddr, op, val, tp, uaddr2, (unsigned long)utime, val3); }#endif /* CONFIG_COMPAT_32BIT_TIME */ -
Thomas Gleixner
maintainerPATCH v5 3/16 futex: Make futex_mm_init() void
In reply to Thomas GleixnerNothing fails there. Mop up the leftovers of the early version of this, which did an allocation.
While at it clean up the stubs and the #ifdef comments to make the header file readable.
--- include/linux/futex.h | 28 +++++++++++----------------- kernel/fork.c | 8 ++------ kernel/futex/core.c | 3 +-- 3 files changed, 14 insertions(+), 25 deletions(-)
include/linux/futex.h +11 -17
@@ -81,22 +81,20 @@ int futex_hash_prctl(unsigned long arg2,#ifdef CONFIG_FUTEX_PRIVATE_HASH int futex_hash_allocate_default (void );void futex_hash_free (struct mm_struct *mm); -int futex_mm_init (struct mm_struct *mm); - -#else /* !CONFIG_FUTEX_PRIVATE_HASH */ +void futex_mm_init (struct mm_struct *mm); +#else /* CONFIG_FUTEX_PRIVATE_HASH */ static inline int futex_hash_allocate_default (void ) {return 0 ; }static inline int futex_hash_free (struct mm_struct *mm) {return 0 ; } -static inline int futex_mm_init (struct mm_struct *mm) {return 0 ; } -#endif /* CONFIG_FUTEX_PRIVATE_HASH */ +static inline void futex_mm_init (struct mm_struct *mm) { } +#endif /* !CONFIG_FUTEX_PRIVATE_HASH */ -#else /* !CONFIG_FUTEX */ +#else /* CONFIG_FUTEX */ static inline void futex_init_task (struct task_struct *tsk) { }static inline void futex_exit_recursive (struct task_struct *tsk) { }static inline void futex_exit_release (struct task_struct *tsk) { }static inline void futex_exec_release (struct task_struct *tsk) { } -static inline long do_futex (u32 __user *uaddr,int op,u32 val, -ktime_t *timeout,u32 __user *uaddr2, -u32 val2,u32 val3) +static inline long do_futex (u32 __user *uaddr,int op,u32 val,ktime_t *timeout, +u32 __user *uaddr2,u32 val2,u32 val3) {return -EINVAL ; }
@@ -104,13 +102,9 @@ static inline int futex_hash_prctl(unsig {return -EINVAL ; } -static inline int futex_hash_allocate_default (void ) -{ -return 0 ; -} +static inline int futex_hash_allocate_default (void ) {return 0 ; }static inline int futex_hash_free (struct mm_struct *mm) {return 0 ; } -static inline int futex_mm_init (struct mm_struct *mm) {return 0 ; } - -#endif +static inline void futex_mm_init (struct mm_struct *mm) { } +#endif /* !CONFIG_FUTEX */ -#endif +#endif /* _LINUX_FUTEX_H */
kernel/fork.c +2 -6
@@ -1101,6 +1101,7 @@ static struct mm_struct *mm_init(struct#endif mm_init_uprobes_state (mm);hugetlb_count_init (mm); +futex_mm_init (mm);mm_flags_clear_all (mm);if (current->mm) {
@@ -1113,11 +1114,8 @@ static struct mm_struct *mm_init(struct mm->def_flags =0 ; } -if (futex_mm_init (mm)) - goto fail_mm_init; -if (mm_alloc_pgd (mm)) - goto fail_nopgd; + goto fail_mm_init;if (mm_alloc_id (mm)) goto fail_noid;
@@ -1144,8 +1142,6 @@ static struct mm_struct *mm_init(structmm_free_id (mm); fail_noid:mm_free_pgd (mm); -fail_nopgd: -futex_hash_free (mm); fail_mm_init:free_mm (mm);return NULL ;
kernel/futex/core.c +1 -2
@@ -1720,7 +1720,7 @@ static bool futex_ref_is_dead(struct futreturn atomic_long_read (&mm->futex_atomic) ==0 ; } -int futex_mm_init (struct mm_struct *mm) +void futex_mm_init (struct mm_struct *mm) {mutex_init (&mm->futex_hash_lock);RCU_INIT_POINTER (mm->futex_phash,NULL );
@@ -1729,7 +1729,6 @@ int futex_mm_init(struct mm_struct *mm) mm->futex_ref =NULL ;atomic_long_set (&mm->futex_atomic,0 ); mm->futex_batches =get_state_synchronize_rcu (); -return 0 ; }void futex_hash_free (struct mm_struct *mm)
[tip: locking/core] futex: Make futex_mm_init() void
The following commit has been merged into the locking/core branch of tip:
The following commit has been merged into the locking/core branch of tip:
Commit-ID: d7b3f52c861f54ba2fff15696d3798277fb4c19f Gitweb: https://git.kernel.org/tip/d7b3f52c861f54ba2fff15696d3798277fb4c19f Author: Thomas Gleixner <[email protected]> AuthorDate: Tue, 02 Jun 2026 11:09:29 +02:00 Committer: Peter Zijlstra <[email protected]> CommitterDate: Wed, 03 Jun 2026 11:38:49 +02:00
futex: Make futex_mm_init() void
Nothing fails there. Mop up the leftovers of the early version of this, which did an allocation.
While at it clean up the stubs and the #ifdef comments to make the header file readable.
--- include/linux/futex.h | 28 +++++++++++----------------- kernel/fork.c | 8 ++------ kernel/futex/core.c | 3 +-- 3 files changed, 14 insertions(+), 25 deletions(-)
include/linux/futex.h +11 -17
@@ -81,22 +81,20 @@ int futex_hash_prctl(unsigned long arg2, unsigned long arg3, unsigned long arg4)#ifdef CONFIG_FUTEX_PRIVATE_HASH int futex_hash_allocate_default (void );void futex_hash_free (struct mm_struct *mm); -int futex_mm_init (struct mm_struct *mm); - -#else /* !CONFIG_FUTEX_PRIVATE_HASH */ +void futex_mm_init (struct mm_struct *mm); +#else /* CONFIG_FUTEX_PRIVATE_HASH */ static inline int futex_hash_allocate_default (void ) {return 0 ; }static inline int futex_hash_free (struct mm_struct *mm) {return 0 ; } -static inline int futex_mm_init (struct mm_struct *mm) {return 0 ; } -#endif /* CONFIG_FUTEX_PRIVATE_HASH */ +static inline void futex_mm_init (struct mm_struct *mm) { } +#endif /* !CONFIG_FUTEX_PRIVATE_HASH */ -#else /* !CONFIG_FUTEX */ +#else /* CONFIG_FUTEX */ static inline void futex_init_task (struct task_struct *tsk) { }static inline void futex_exit_recursive (struct task_struct *tsk) { }static inline void futex_exit_release (struct task_struct *tsk) { }static inline void futex_exec_release (struct task_struct *tsk) { } -static inline long do_futex (u32 __user *uaddr,int op,u32 val, -ktime_t *timeout,u32 __user *uaddr2, -u32 val2,u32 val3) +static inline long do_futex (u32 __user *uaddr,int op,u32 val,ktime_t *timeout, +u32 __user *uaddr2,u32 val2,u32 val3) {return -EINVAL ; }
@@ -104,13 +102,9 @@ static inline int futex_hash_prctl(unsigned long arg2, unsigned long arg3, unsig {return -EINVAL ; } -static inline int futex_hash_allocate_default (void ) -{ -return 0 ; -} +static inline int futex_hash_allocate_default (void ) {return 0 ; }static inline int futex_hash_free (struct mm_struct *mm) {return 0 ; } -static inline int futex_mm_init (struct mm_struct *mm) {return 0 ; } - -#endif +static inline void futex_mm_init (struct mm_struct *mm) { } +#endif /* !CONFIG_FUTEX */ -#endif +#endif /* _LINUX_FUTEX_H */
kernel/fork.c +2 -6
@@ -1101,6 +1101,7 @@ static struct mm_struct *mm_init(struct mm_struct *mm, struct task_struct *p,#endif mm_init_uprobes_state (mm);hugetlb_count_init (mm); +futex_mm_init (mm);mm_flags_clear_all (mm);if (current->mm) {
@@ -1113,11 +1114,8 @@ static struct mm_struct *mm_init(struct mm_struct *mm, struct task_struct *p, mm->def_flags =0 ; } -if (futex_mm_init (mm)) - goto fail_mm_init; -if (mm_alloc_pgd (mm)) - goto fail_nopgd; + goto fail_mm_init;if (mm_alloc_id (mm)) goto fail_noid;
@@ -1144,8 +1142,6 @@ fail_nocontext:mm_free_id (mm); fail_noid:mm_free_pgd (mm); -fail_nopgd: -futex_hash_free (mm); fail_mm_init:free_mm (mm);return NULL ;
kernel/futex/core.c +1 -2
@@ -1720,7 +1720,7 @@ static bool futex_ref_is_dead(struct futex_private_hash *fph)return atomic_long_read (&mm->futex_atomic) ==0 ; } -int futex_mm_init (struct mm_struct *mm) +void futex_mm_init (struct mm_struct *mm) {mutex_init (&mm->futex_hash_lock);RCU_INIT_POINTER (mm->futex_phash,NULL );
@@ -1729,7 +1729,6 @@ int futex_mm_init(struct mm_struct *mm) mm->futex_ref =NULL ;atomic_long_set (&mm->futex_atomic,0 ); mm->futex_batches =get_state_synchronize_rcu (); -return 0 ; }void futex_hash_free (struct mm_struct *mm)
Thomas Gleixner
maintainerPATCH v5 4/16 futex: Move futex related mm_struct data into a struct
In reply to Thomas GleixnerHaving all these members in mm_struct along with the required #ifdeffery is annoying, does not allow efficient initializing of the data with memset() and makes extending it tedious.
Move it into a data structure and fix up all usage sites.
The extra struct for the private hash is intentional to make integration of other conditional mechanisms easier in terms of initialization and separation.
---
V5: Include compiler_types.h so that __percpu is defined - Nam
V3: Split out the private hash data
V2: Use an empty stub struct as for the others - Mathieu
--- include/linux/futex_types.h | 36 +++++++++++ include/linux/mm_types.h | 12 --- kernel/futex/core.c | 133 ++++++++++++++++++++------------------------ 3 files changed, 98 insertions(+), 83 deletions(-)
include/linux/futex_types.h +35 -1
@@ -3,6 +3,7 @@#define _LINUX_FUTEX_TYPES_H#ifdef CONFIG_FUTEX +#include <linux/compiler_types.h> #include <linux/mutex_types.h> #include <linux/types.h>
@@ -29,8 +30,41 @@ struct futex_sched_data {struct mutex exit_mutex;unsigned int state; }; -#else + +#ifdef CONFIG_FUTEX_PRIVATE_HASH +/** +* struct futex_mm_phash - Futex private hash related per MM data +* @lock: Mutex to protect the private hash operations +* @hash: RCU managed pointer to the private hash +* @hash_new: Pointer to a newly allocated private hash +* @batches: Batch state for RCU synchronization +* @rcu: RCU head for call_rcu() +* @atomic: Aggregate value for @hash_ref +* @ref: Per CPU reference counter for a private hash +*/ +struct futex_mm_phash { +struct mutex lock; +struct futex_private_hash __rcu *hash; +struct futex_private_hash *hash_new; +unsigned long batches; +struct rcu_head rcu; +atomic_long_t atomic; +unsigned int __percpu *ref; +}; +#else /* CONFIG_FUTEX_ROBUST_UNLOCK */ +struct futex_mm_phash { }; +#endif /* !CONFIG_FUTEX_ROBUST_UNLOCK */ + +/** +* struct futex_mm_data - Futex related per MM data +* @phash: Futex private hash related data +*/ +struct futex_mm_data { +struct futex_mm_phash phash; +}; +#else /* CONFIG_FUTEX */ struct futex_sched_data { }; +struct futex_mm_data { };#endif /* !CONFIG_FUTEX */ #endif /* _LINUX_FUTEX_TYPES_H */
include/linux/mm_types.h +2 -10
@@ -20,6 +20,7 @@#include <linux/seqlock.h> #include <linux/percpu_counter.h> #include <linux/types.h> +#include <linux/futex_types.h> #include <linux/rseq_types.h> #include <linux/bitmap.h>
@@ -1270,16 +1271,7 @@ struct mm_struct {*/ seqcount_t mm_lock_seq;#endif -#ifdef CONFIG_FUTEX_PRIVATE_HASH -struct mutex futex_hash_lock; -struct futex_private_hash __rcu *futex_phash; -struct futex_private_hash *futex_phash_new; -/* futex-ref */ -unsigned long futex_batches; -struct rcu_head futex_rcu; -atomic_long_t futex_atomic; -unsigned int __percpu *futex_ref; -#endif +struct futex_mm_data futex;unsigned long hiwater_rss;/* High-watermark of RSS usage */ unsigned long hiwater_vm;/* High-water virtual memory usage */
kernel/futex/core.c +61 -72
355 lines not on this page · raw message
[tip: locking/core] futex: Move futex related mm_struct data into a struct
The following commit has been merged into the locking/core branch of tip:
The following commit has been merged into the locking/core branch of tip:
Commit-ID: 1f7f4816b9b05e5110bc1c8a05c3c478e2dae11b Gitweb: https://git.kernel.org/tip/1f7f4816b9b05e5110bc1c8a05c3c478e2dae11b Author: Thomas Gleixner <[email protected]> AuthorDate: Tue, 02 Jun 2026 11:09:34 +02:00 Committer: Peter Zijlstra <[email protected]> CommitterDate: Wed, 03 Jun 2026 11:38:49 +02:00
futex: Move futex related mm_struct data into a struct
Having all these members in mm_struct along with the required #ifdeffery is annoying, does not allow efficient initializing of the data with memset() and makes extending it tedious.
Move it into a data structure and fix up all usage sites.
The extra struct for the private hash is intentional to make integration of other conditional mechanisms easier in terms of initialization and separation.
--- include/linux/futex_types.h | 36 ++++++++- include/linux/mm_types.h | 12 +--- kernel/futex/core.c | 133 ++++++++++++++++------------------- 3 files changed, 98 insertions(+), 83 deletions(-)
include/linux/futex_types.h +35 -1
@@ -3,6 +3,7 @@#define _LINUX_FUTEX_TYPES_H#ifdef CONFIG_FUTEX +#include <linux/compiler_types.h> #include <linux/mutex_types.h> #include <linux/types.h>
@@ -29,8 +30,41 @@ struct futex_sched_data {struct mutex exit_mutex;unsigned int state; }; -#else + +#ifdef CONFIG_FUTEX_PRIVATE_HASH +/** +* struct futex_mm_phash - Futex private hash related per MM data +* @lock: Mutex to protect the private hash operations +* @hash: RCU managed pointer to the private hash +* @hash_new: Pointer to a newly allocated private hash +* @batches: Batch state for RCU synchronization +* @rcu: RCU head for call_rcu() +* @atomic: Aggregate value for @hash_ref +* @ref: Per CPU reference counter for a private hash +*/ +struct futex_mm_phash { +struct mutex lock; +struct futex_private_hash __rcu *hash; +struct futex_private_hash *hash_new; +unsigned long batches; +struct rcu_head rcu; +atomic_long_t atomic; +unsigned int __percpu *ref; +}; +#else /* CONFIG_FUTEX_ROBUST_UNLOCK */ +struct futex_mm_phash { }; +#endif /* !CONFIG_FUTEX_ROBUST_UNLOCK */ + +/** +* struct futex_mm_data - Futex related per MM data +* @phash: Futex private hash related data +*/ +struct futex_mm_data { +struct futex_mm_phash phash; +}; +#else /* CONFIG_FUTEX */ struct futex_sched_data { }; +struct futex_mm_data { };#endif /* !CONFIG_FUTEX */ #endif /* _LINUX_FUTEX_TYPES_H */
include/linux/mm_types.h +2 -10
@@ -20,6 +20,7 @@#include <linux/seqlock.h> #include <linux/percpu_counter.h> #include <linux/types.h> +#include <linux/futex_types.h> #include <linux/rseq_types.h> #include <linux/bitmap.h>
@@ -1270,16 +1271,7 @@ struct mm_struct {*/ seqcount_t mm_lock_seq;#endif -#ifdef CONFIG_FUTEX_PRIVATE_HASH -struct mutex futex_hash_lock; -struct futex_private_hash __rcu *futex_phash; -struct futex_private_hash *futex_phash_new; -/* futex-ref */ -unsigned long futex_batches; -struct rcu_head futex_rcu; -atomic_long_t futex_atomic; -unsigned int __percpu *futex_ref; -#endif +struct futex_mm_data futex;unsigned long hiwater_rss;/* High-watermark of RSS usage */ unsigned long hiwater_vm;/* High-water virtual memory usage */
kernel/futex/core.c +61 -72
355 lines not on this page · raw message
Thomas Gleixner
maintainerPATCH v5 5/16 futex: Provide UABI defines for robust list entry modifiers
In reply to Thomas GleixnerThe marker for PI futexes in the robust list is a hardcoded 0x1 which lacks any sensible form of documentation.
Provide proper defines for the bit and the mask and fix up the usage sites. Thereby convert the boolean pi argument into a modifier argument, which allows new modifier bits to be trivially added and conveyed.
---
V2: Explain the code shuffling - Andre
--- include/uapi/linux/futex.h | 4 +++ kernel/futex/core.c | 53 +++++++++++++++++++++------------------------ 2 files changed, 29 insertions(+), 28 deletions(-)
include/uapi/linux/futex.h +4 -0
@@ -177,6 +177,10 @@ struct robust_list_head {*/ #define ROBUST_LIST_LIMIT 2048 +/* Modifiers for robust_list_head::list_op_pending */ +#define FUTEX_ROBUST_MOD_PI (0x1UL) +#define FUTEX_ROBUST_MOD_MASK (FUTEX_ROBUST_MOD_PI) +/* * bitset with all bits set for the FUTEX_xxx_BITSET OPs to request a * match of any bit.
kernel/futex/core.c +25 -28
177 lines not on this page · raw message
[tip: locking/core] futex: Provide UABI defines for robust list entry modifiers
The following commit has been merged into the locking/core branch of tip:
The following commit has been merged into the locking/core branch of tip:
Commit-ID: 2cb5251d3d64d57c172185b9b608f704b3015f26 Gitweb: https://git.kernel.org/tip/2cb5251d3d64d57c172185b9b608f704b3015f26 Author: Thomas Gleixner <[email protected]> AuthorDate: Tue, 02 Jun 2026 11:09:38 +02:00 Committer: Peter Zijlstra <[email protected]> CommitterDate: Wed, 03 Jun 2026 11:38:50 +02:00
futex: Provide UABI defines for robust list entry modifiers
The marker for PI futexes in the robust list is a hardcoded 0x1 which lacks any sensible form of documentation.
Provide proper defines for the bit and the mask and fix up the usage sites. Thereby convert the boolean pi argument into a modifier argument, which allows new modifier bits to be trivially added and conveyed.
--- include/uapi/linux/futex.h | 4 +++- kernel/futex/core.c | 53 +++++++++++++++++-------------------- 2 files changed, 29 insertions(+), 28 deletions(-)
include/uapi/linux/futex.h +4 -0
@@ -177,6 +177,10 @@ struct robust_list_head {*/ #define ROBUST_LIST_LIMIT 2048 +/* Modifiers for robust_list_head::list_op_pending */ +#define FUTEX_ROBUST_MOD_PI (0x1UL) +#define FUTEX_ROBUST_MOD_MASK (FUTEX_ROBUST_MOD_PI) +/* * bitset with all bits set for the FUTEX_xxx_BITSET OPs to request a * match of any bit.
kernel/futex/core.c +25 -28
177 lines not on this page · raw message
Thomas Gleixner
maintainerPATCH v5 6/16 uaccess: Provide unsafe_atomic_store_release_user()
In reply to Thomas GleixnerThe upcoming support for unlocking robust futexes in the kernel requires store release semantics. Syscalls do not imply memory ordering on all architectures so the unlock operation requires a barrier.
This barrier can be avoided when stores imply release like on x86.
Provide a generic version with a smp_mb() before the unsafe_put_user(), which can be overridden by architectures.
Provide also a ARCH_MEMORY_ORDER_TSO Kconfig option, which can be selected by architectures with Total Store Order (TSO), where store implies release, so that the smp_mb() in the generic implementation can be avoided.
If that is set a barrier() is used instead of smp_mb(), which is not required for the use case at hand, but makes it future proof for other usage to prevent the compiler from reordering.
---
V4: Rename it really ....
Add a barrier when TSO=y
V3: Rename to CONFIG_ARCH_MEMORY_ORDER_TSO - Peter
V2: New patch
--- arch/Kconfig | 4 ++++ include/linux/uaccess.h | 11 +++++++++++ 2 files changed, 15 insertions(+)
arch/Kconfig +4 -0
@@ -403,6 +403,10 @@ config ARCH_32BIT_OFF_T config ARCH_32BIT_USTAT_F_TINODE bool +# Selected by architectures with Total Store Order (TSO) +config ARCH_MEMORY_ORDER_TSO + bool + config HAVE_ASM_MODVERSIONS bool help
include/linux/uaccess.h +11 -0
@@ -649,6 +649,17 @@ static inline void user_access_restore(u#define user_read_access_end user_access_end#endif +#ifndef unsafe_atomic_store_release_user +# define unsafe_atomic_store_release_user (val, uptr, elbl) \ + do { \ + if (!IS_ENABLED(CONFIG_ARCH_MEMORY_ORDER_TSO)) \ + smp_mb(); \ + else \ + barrier(); \ + unsafe_put_user(val, uptr, elbl); \ + } while (0) +#endif +/* Define RW variant so the below _mode macro expansion works */ #define masked_user_rw_access_begin (u) masked_user_access_begin(u)#define user_rw_access_begin (u, s) user_access_begin(u, s)
[tip: locking/core] uaccess: Provide unsafe_atomic_store_release_user()
The following commit has been merged into the locking/core branch of tip:
The following commit has been merged into the locking/core branch of tip:
Commit-ID: 6149fc36c09b91050b62e8e68a91027df8df7345 Gitweb: https://git.kernel.org/tip/6149fc36c09b91050b62e8e68a91027df8df7345 Author: Thomas Gleixner <[email protected]> AuthorDate: Tue, 02 Jun 2026 11:09:42 +02:00 Committer: Peter Zijlstra <[email protected]> CommitterDate: Wed, 03 Jun 2026 11:38:50 +02:00
uaccess: Provide unsafe_atomic_store_release_user()
The upcoming support for unlocking robust futexes in the kernel requires store release semantics. Syscalls do not imply memory ordering on all architectures so the unlock operation requires a barrier.
This barrier can be avoided when stores imply release like on x86.
Provide a generic version with a smp_mb() before the unsafe_put_user(), which can be overridden by architectures.
Provide also a ARCH_MEMORY_ORDER_TSO Kconfig option, which can be selected by architectures with Total Store Order (TSO), where store implies release, so that the smp_mb() in the generic implementation can be avoided.
If that is set a barrier() is used instead of smp_mb(), which is not required for the use case at hand, but makes it future proof for other usage to prevent the compiler from reordering.
--- arch/Kconfig | 4 ++++ include/linux/uaccess.h | 11 +++++++++++ 2 files changed, 15 insertions(+)
arch/Kconfig +4 -0
@@ -403,6 +403,10 @@ config ARCH_32BIT_OFF_T config ARCH_32BIT_USTAT_F_TINODE bool +# Selected by architectures with Total Store Order (TSO) +config ARCH_MEMORY_ORDER_TSO + bool + config HAVE_ASM_MODVERSIONS bool help
include/linux/uaccess.h +11 -0
@@ -649,6 +649,17 @@ static inline void user_access_restore(unsigned long flags) { }#define user_read_access_end user_access_end#endif +#ifndef unsafe_atomic_store_release_user +# define unsafe_atomic_store_release_user (val, uptr, elbl) \ + do { \ + if (!IS_ENABLED(CONFIG_ARCH_MEMORY_ORDER_TSO)) \ + smp_mb(); \ + else \ + barrier(); \ + unsafe_put_user(val, uptr, elbl); \ + } while (0) +#endif +/* Define RW variant so the below _mode macro expansion works */ #define masked_user_rw_access_begin (u) masked_user_access_begin(u)#define user_rw_access_begin (u, s) user_access_begin(u, s)
Thomas Gleixner
maintainerPATCH v5 7/16 x86: Select ARCH_MEMORY_ORDER_TSO
In reply to Thomas GleixnerThe generic unsafe_atomic_store_release_user() implementation does:
if (!IS_ENABLED(CONFIG_ARCH_MEMORY_ORDER_TSO))
smp_mb();
unsafe_put_user();As x86 implements Total Store Order (TSO) which means stores imply release, select ARCH_MEMORY_ORDER_TSO to avoid the unnecessary smp_mb().
---
V4: Rename it correctly
V3: Rename to TOS - Peter
V2: New patch
--- arch/x86/Kconfig | 1 + 1 file changed, 1 insertion(+)
arch/x86/Kconfig +1 -0
@@ -113,6 +113,7 @@ config X86 select ARCH_HAS_ZONE_DMA_SET if EXPERT select ARCH_HAVE_NMI_SAFE_CMPXCHG select ARCH_HAVE_EXTRA_ELF_NOTES + select ARCH_MEMORY_ORDER_TSO select ARCH_MHP_MEMMAP_ON_MEMORY_ENABLE select ARCH_MIGHT_HAVE_ACPI_PDC if ACPI select ARCH_MIGHT_HAVE_PC_PARPORT
[tip: locking/core] x86: Select ARCH_MEMORY_ORDER_TSO
The following commit has been merged into the locking/core branch of tip:
The following commit has been merged into the locking/core branch of tip:
Commit-ID: 7b125c44d0b7f617ee81dffd14ce116149d03cb6 Gitweb: https://git.kernel.org/tip/7b125c44d0b7f617ee81dffd14ce116149d03cb6 Author: Thomas Gleixner <[email protected]> AuthorDate: Tue, 02 Jun 2026 11:09:47 +02:00 Committer: Peter Zijlstra <[email protected]> CommitterDate: Wed, 03 Jun 2026 11:38:50 +02:00
x86: Select ARCH_MEMORY_ORDER_TSO
The generic unsafe_atomic_store_release_user() implementation does:
if (!IS_ENABLED(CONFIG_ARCH_MEMORY_ORDER_TSO))
smp_mb();
unsafe_put_user();As x86 implements Total Store Order (TSO) which means stores imply release, select ARCH_MEMORY_ORDER_TSO to avoid the unnecessary smp_mb().
--- arch/x86/Kconfig | 1 + 1 file changed, 1 insertion(+)
arch/x86/Kconfig +1 -0
@@ -113,6 +113,7 @@ config X86 select ARCH_HAS_ZONE_DMA_SET if EXPERT select ARCH_HAVE_NMI_SAFE_CMPXCHG select ARCH_HAVE_EXTRA_ELF_NOTES + select ARCH_MEMORY_ORDER_TSO select ARCH_MHP_MEMMAP_ON_MEMORY_ENABLE select ARCH_MIGHT_HAVE_ACPI_PDC if ACPI select ARCH_MIGHT_HAVE_PC_PARPORT
Thomas Gleixner
maintainerPATCH v5 8/16 futex: Cleanup UAPI defines
In reply to Thomas GleixnerMake the operand defines tabular for readability sake.
---
V2: New patch
--- include/uapi/linux/futex.h | 27 +++++++++++++-------------- 1 file changed, 13 insertions(+), 14 deletions(-)
include/uapi/linux/futex.h +13 -14
@@ -25,23 +25,22 @@#define FUTEX_PRIVATE_FLAG 128#define FUTEX_CLOCK_REALTIME 256 -#define FUTEX_CMD_MASK ~(FUTEX_PRIVATE_FLAG | FUTEX_CLOCK_REALTIME) -#define FUTEX_WAIT_PRIVATE (FUTEX_WAIT | FUTEX_PRIVATE_FLAG) -#define FUTEX_WAKE_PRIVATE (FUTEX_WAKE | FUTEX_PRIVATE_FLAG) -#define FUTEX_REQUEUE_PRIVATE (FUTEX_REQUEUE | FUTEX_PRIVATE_FLAG) -#define FUTEX_CMP_REQUEUE_PRIVATE (FUTEX_CMP_REQUEUE | FUTEX_PRIVATE_FLAG) -#define FUTEX_WAKE_OP_PRIVATE (FUTEX_WAKE_OP | FUTEX_PRIVATE_FLAG) -#define FUTEX_LOCK_PI_PRIVATE (FUTEX_LOCK_PI | FUTEX_PRIVATE_FLAG) -#define FUTEX_LOCK_PI2_PRIVATE (FUTEX_LOCK_PI2 | FUTEX_PRIVATE_FLAG) -#define FUTEX_UNLOCK_PI_PRIVATE (FUTEX_UNLOCK_PI | FUTEX_PRIVATE_FLAG) -#define FUTEX_TRYLOCK_PI_PRIVATE (FUTEX_TRYLOCK_PI | FUTEX_PRIVATE_FLAG) +#define FUTEX_CMD_MASK ~(FUTEX_PRIVATE_FLAG | FUTEX_CLOCK_REALTIME) + +#define FUTEX_WAIT_PRIVATE (FUTEX_WAIT | FUTEX_PRIVATE_FLAG) +#define FUTEX_WAKE_PRIVATE (FUTEX_WAKE | FUTEX_PRIVATE_FLAG) +#define FUTEX_REQUEUE_PRIVATE (FUTEX_REQUEUE | FUTEX_PRIVATE_FLAG) +#define FUTEX_CMP_REQUEUE_PRIVATE (FUTEX_CMP_REQUEUE | FUTEX_PRIVATE_FLAG) +#define FUTEX_WAKE_OP_PRIVATE (FUTEX_WAKE_OP | FUTEX_PRIVATE_FLAG) +#define FUTEX_LOCK_PI_PRIVATE (FUTEX_LOCK_PI | FUTEX_PRIVATE_FLAG) +#define FUTEX_LOCK_PI2_PRIVATE (FUTEX_LOCK_PI2 | FUTEX_PRIVATE_FLAG) +#define FUTEX_UNLOCK_PI_PRIVATE (FUTEX_UNLOCK_PI | FUTEX_PRIVATE_FLAG) +#define FUTEX_TRYLOCK_PI_PRIVATE (FUTEX_TRYLOCK_PI | FUTEX_PRIVATE_FLAG)#define FUTEX_WAIT_BITSET_PRIVATE (FUTEX_WAIT_BITSET | FUTEX_PRIVATE_FLAG)#define FUTEX_WAKE_BITSET_PRIVATE (FUTEX_WAKE_BITSET | FUTEX_PRIVATE_FLAG) -#define FUTEX_WAIT_REQUEUE_PI_PRIVATE (FUTEX_WAIT_REQUEUE_PI | \ - FUTEX_PRIVATE_FLAG) -#define FUTEX_CMP_REQUEUE_PI_PRIVATE (FUTEX_CMP_REQUEUE_PI | \ - FUTEX_PRIVATE_FLAG) +#define FUTEX_WAIT_REQUEUE_PI_PRIVATE (FUTEX_WAIT_REQUEUE_PI | FUTEX_PRIVATE_FLAG) +#define FUTEX_CMP_REQUEUE_PI_PRIVATE (FUTEX_CMP_REQUEUE_PI | FUTEX_PRIVATE_FLAG)/* * Flags for futex2 syscalls.
[tip: locking/core] futex: Cleanup UAPI defines
The following commit has been merged into the locking/core branch of tip:
The following commit has been merged into the locking/core branch of tip:
Commit-ID: 1fd053d26f0333485cdbaa9d6e7b8cb53f54de95 Gitweb: https://git.kernel.org/tip/1fd053d26f0333485cdbaa9d6e7b8cb53f54de95 Author: Thomas Gleixner <[email protected]> AuthorDate: Tue, 02 Jun 2026 11:09:51 +02:00 Committer: Peter Zijlstra <[email protected]> CommitterDate: Wed, 03 Jun 2026 11:38:51 +02:00
futex: Cleanup UAPI defines
Make the operand defines tabular for readability sake.
--- include/uapi/linux/futex.h | 29 ++++++++++++++--------------- 1 file changed, 14 insertions(+), 15 deletions(-)
include/uapi/linux/futex.h +14 -15
@@ -25,23 +25,22 @@#define FUTEX_PRIVATE_FLAG 128#define FUTEX_CLOCK_REALTIME 256 -#define FUTEX_CMD_MASK ~(FUTEX_PRIVATE_FLAG | FUTEX_CLOCK_REALTIME) - -#define FUTEX_WAIT_PRIVATE (FUTEX_WAIT | FUTEX_PRIVATE_FLAG) -#define FUTEX_WAKE_PRIVATE (FUTEX_WAKE | FUTEX_PRIVATE_FLAG) -#define FUTEX_REQUEUE_PRIVATE (FUTEX_REQUEUE | FUTEX_PRIVATE_FLAG) -#define FUTEX_CMP_REQUEUE_PRIVATE (FUTEX_CMP_REQUEUE | FUTEX_PRIVATE_FLAG) -#define FUTEX_WAKE_OP_PRIVATE (FUTEX_WAKE_OP | FUTEX_PRIVATE_FLAG) -#define FUTEX_LOCK_PI_PRIVATE (FUTEX_LOCK_PI | FUTEX_PRIVATE_FLAG) -#define FUTEX_LOCK_PI2_PRIVATE (FUTEX_LOCK_PI2 | FUTEX_PRIVATE_FLAG) -#define FUTEX_UNLOCK_PI_PRIVATE (FUTEX_UNLOCK_PI | FUTEX_PRIVATE_FLAG) -#define FUTEX_TRYLOCK_PI_PRIVATE (FUTEX_TRYLOCK_PI | FUTEX_PRIVATE_FLAG) + +#define FUTEX_CMD_MASK ~(FUTEX_PRIVATE_FLAG | FUTEX_CLOCK_REALTIME) + +#define FUTEX_WAIT_PRIVATE (FUTEX_WAIT | FUTEX_PRIVATE_FLAG) +#define FUTEX_WAKE_PRIVATE (FUTEX_WAKE | FUTEX_PRIVATE_FLAG) +#define FUTEX_REQUEUE_PRIVATE (FUTEX_REQUEUE | FUTEX_PRIVATE_FLAG) +#define FUTEX_CMP_REQUEUE_PRIVATE (FUTEX_CMP_REQUEUE | FUTEX_PRIVATE_FLAG) +#define FUTEX_WAKE_OP_PRIVATE (FUTEX_WAKE_OP | FUTEX_PRIVATE_FLAG) +#define FUTEX_LOCK_PI_PRIVATE (FUTEX_LOCK_PI | FUTEX_PRIVATE_FLAG) +#define FUTEX_LOCK_PI2_PRIVATE (FUTEX_LOCK_PI2 | FUTEX_PRIVATE_FLAG) +#define FUTEX_UNLOCK_PI_PRIVATE (FUTEX_UNLOCK_PI | FUTEX_PRIVATE_FLAG) +#define FUTEX_TRYLOCK_PI_PRIVATE (FUTEX_TRYLOCK_PI | FUTEX_PRIVATE_FLAG)#define FUTEX_WAIT_BITSET_PRIVATE (FUTEX_WAIT_BITSET | FUTEX_PRIVATE_FLAG)#define FUTEX_WAKE_BITSET_PRIVATE (FUTEX_WAKE_BITSET | FUTEX_PRIVATE_FLAG) -#define FUTEX_WAIT_REQUEUE_PI_PRIVATE (FUTEX_WAIT_REQUEUE_PI | \ - FUTEX_PRIVATE_FLAG) -#define FUTEX_CMP_REQUEUE_PI_PRIVATE (FUTEX_CMP_REQUEUE_PI | \ - FUTEX_PRIVATE_FLAG) +#define FUTEX_WAIT_REQUEUE_PI_PRIVATE (FUTEX_WAIT_REQUEUE_PI | FUTEX_PRIVATE_FLAG) +#define FUTEX_CMP_REQUEUE_PI_PRIVATE (FUTEX_CMP_REQUEUE_PI | FUTEX_PRIVATE_FLAG)/* * Flags for futex2 syscalls.
Re: [patch V5 08/16] futex: Cleanup UAPI defines
In reply to Thomas GleixnerQuoted 1 line · On Tue, 02 Jun 2026 11:09:51 +0200 Thomas Gleixner <[email protected]> wrote:
Make the operand defines tabular for readability sake.
Too late for now, but putting FUTEX_PRIVATE_FLAG first would help even more.
-- David
Quoted 50 lines
Signed-off-by Thomas Gleixner <[email protected]>Reviewed-by André Almeida <[email protected]>---
V2: New patch--- include/uapi/linux/futex.h | 27 +++++++++++++-------------- 1 file changed, 13 insertions(+), 14 deletions(-)include/uapi/linux/futex.h +13
-14@@ -25,23 +25,22 @@#define FUTEX_PRIVATE_FLAG 128#define FUTEX_CLOCK_REALTIME 256 -#define FUTEX_CMD_MASK ~(FUTEX_PRIVATE_FLAG | FUTEX_CLOCK_REALTIME) -#define FUTEX_WAIT_PRIVATE (FUTEX_WAIT | FUTEX_PRIVATE_FLAG) -#define FUTEX_WAKE_PRIVATE (FUTEX_WAKE | FUTEX_PRIVATE_FLAG) -#define FUTEX_REQUEUE_PRIVATE (FUTEX_REQUEUE | FUTEX_PRIVATE_FLAG) -#define FUTEX_CMP_REQUEUE_PRIVATE (FUTEX_CMP_REQUEUE | FUTEX_PRIVATE_FLAG) -#define FUTEX_WAKE_OP_PRIVATE (FUTEX_WAKE_OP | FUTEX_PRIVATE_FLAG) -#define FUTEX_LOCK_PI_PRIVATE (FUTEX_LOCK_PI | FUTEX_PRIVATE_FLAG) -#define FUTEX_LOCK_PI2_PRIVATE (FUTEX_LOCK_PI2 | FUTEX_PRIVATE_FLAG) -#define FUTEX_UNLOCK_PI_PRIVATE (FUTEX_UNLOCK_PI | FUTEX_PRIVATE_FLAG) -#define FUTEX_TRYLOCK_PI_PRIVATE (FUTEX_TRYLOCK_PI | FUTEX_PRIVATE_FLAG) +#define FUTEX_CMD_MASK ~(FUTEX_PRIVATE_FLAG | FUTEX_CLOCK_REALTIME) + +#define FUTEX_WAIT_PRIVATE (FUTEX_WAIT | FUTEX_PRIVATE_FLAG) +#define FUTEX_WAKE_PRIVATE (FUTEX_WAKE | FUTEX_PRIVATE_FLAG) +#define FUTEX_REQUEUE_PRIVATE (FUTEX_REQUEUE | FUTEX_PRIVATE_FLAG) +#define FUTEX_CMP_REQUEUE_PRIVATE (FUTEX_CMP_REQUEUE | FUTEX_PRIVATE_FLAG) +#define FUTEX_WAKE_OP_PRIVATE (FUTEX_WAKE_OP | FUTEX_PRIVATE_FLAG) +#define FUTEX_LOCK_PI_PRIVATE (FUTEX_LOCK_PI | FUTEX_PRIVATE_FLAG) +#define FUTEX_LOCK_PI2_PRIVATE (FUTEX_LOCK_PI2 | FUTEX_PRIVATE_FLAG) +#define FUTEX_UNLOCK_PI_PRIVATE (FUTEX_UNLOCK_PI | FUTEX_PRIVATE_FLAG) +#define FUTEX_TRYLOCK_PI_PRIVATE (FUTEX_TRYLOCK_PI | FUTEX_PRIVATE_FLAG)#define FUTEX_WAIT_BITSET_PRIVATE (FUTEX_WAIT_BITSET | FUTEX_PRIVATE_FLAG)#define FUTEX_WAKE_BITSET_PRIVATE (FUTEX_WAKE_BITSET | FUTEX_PRIVATE_FLAG) -#define FUTEX_WAIT_REQUEUE_PI_PRIVATE (FUTEX_WAIT_REQUEUE_PI | \ - FUTEX_PRIVATE_FLAG) -#define FUTEX_CMP_REQUEUE_PI_PRIVATE (FUTEX_CMP_REQUEUE_PI | \ - FUTEX_PRIVATE_FLAG) +#define FUTEX_WAIT_REQUEUE_PI_PRIVATE (FUTEX_WAIT_REQUEUE_PI | FUTEX_PRIVATE_FLAG) +#define FUTEX_CMP_REQUEUE_PI_PRIVATE (FUTEX_CMP_REQUEUE_PI | FUTEX_PRIVATE_FLAG)/* * Flags for futex2 syscalls.
Thomas Gleixner
maintainerPATCH v5 9/16 futex: Add support for unlocking robust futexes
In reply to Thomas GleixnerUnlocking robust non-PI futexes happens in user space with the following sequence:
1) robust_list_set_op_pending(mutex); 2) robust_list_remove(mutex); lval = 0; 3) lval = atomic_xchg(lock, lval); 4) if (lval & WAITERS) 5) sys_futex(WAKE,....); 6) robust_list_clear_op_pending();
That opens a window between #3 and #6 where the mutex could be acquired by some other task which observes that it is the last user and:
A) unmaps the mutex memory
B) maps a different file, which ends up covering the same address
When the original task exits before reaching #6 then the kernel robust list handling observes the pending op entry and tries to fix up user space.
In case that the newly mapped data contains the TID of the exiting thread at the address of the mutex/futex the kernel will set the owner died bit in that memory and therefore corrupting unrelated data.
PI futexes have a similar problem both for the non-contented user space unlock and the in kernel unlock:
1) robust_list_set_op_pending(mutex); 2) robust_list_remove(mutex); lval = gettid(); 3) if (!atomic_try_cmpxchg(lock, lval, 0)) 4) sys_futex(UNLOCK_PI,....); 5) robust_list_clear_op_pending();
Address the first part of the problem where the futexes have waiters and need to enter the kernel anyway. Add a new FUTEX_ROBUST_UNLOCK flag, which is valid for the sys_futex() FUTEX_UNLOCK_PI, FUTEX_WAKE, FUTEX_WAKE_BITSET operations.
This deliberately omits FUTEX_WAKE_OP from this treatment as it's unclear whether this is needed and there is no usage of it in glibc either to investigate.
For the futex2 syscall family this needs to be implemented with a new syscall.
The sys_futex() case [ab]uses the @uaddr2 argument to hand the pointer to robust_list_head::list_pending_op into the kernel. This argument is only evaluated when the FUTEX_ROBUST_UNLOCK bit is set and is therefore backward compatible.
This is an explicit argument to avoid the lookup of the robust list pointer and retrieving the pending op pointer from there. User space has the pointer already available so it can just put it into the @uaddr2 argument. Aside of that this allows the usage of multiple robust lists in the future without any changes to the internal functions as they just operate on the provided pointer.
This requires a second flag FUTEX_ROBUST_LIST32 which indicates that the robust list pointer points to an u32 and not to an u64. This is required for two reasons:
1) sys_futex() has no compat variant
2) The gaming emulators use both both 64-bit and compat 32-bit robust lists in the same 64-bit application
As a consequence 32-bit applications have to set this flag unconditionally so they can run on a 64-bit kernel in compat mode unmodified. 32-bit kernels return an error code when the flag is not set. 64-bit kernels will happily clear the full 64 bits if user space fails to set it.
In case of FUTEX_UNLOCK_PI this clears the robust list pending op when the unlock succeeded. In case of errors, the user space value is still locked by the caller and therefore the above cannot happen.
In case of FUTEX_WAKE* this does the unlock of the futex in the kernel and clears the robust list pending op when the unlock was successful. If not, the user space value is still locked and user space has to deal with the returned error. That means that the unlocking of non-PI robust futexes has to use the same try_cmpxchg() unlock scheme as PI futexes.
If the clearing of the pending list op fails (fault) then the kernel clears the registered robust list pointer if it matches to prevent that exit() will try to handle invalid data. That's a valid paranoid decision because the robust list head sits usually in the TLS and if the TLS is not longer accessible then the chance for fixing up the resulting mess is very close to zero.
The problem of non-contended unlocks still exists and will be addressed separately.
---
V4: Fix the uapi defines
V3: Expand changelog to explain @uaddr2 - Andre
V2: Use store release for unlock - Andre, Peter
Use a separate FLAG for 32bit lists - Florian
Add command defines
--- include/uapi/linux/futex.h | 29 +++++++++++++++++++++++- io_uring/futex.c | 2 - kernel/futex/core.c | 53 +++++++++++++++++++++++++++++++++++++++++++-- kernel/futex/futex.h | 15 +++++++++++- kernel/futex/pi.c | 15 +++++++++++- kernel/futex/syscalls.c | 13 ++++++++--- kernel/futex/waitwake.c | 30 +++++++++++++++++++++++-- 7 files changed, 144 insertions(+), 13 deletions(-)
include/uapi/linux/futex.h +28 -1
@@ -25,8 +25,11 @@#define FUTEX_PRIVATE_FLAG 128#define FUTEX_CLOCK_REALTIME 256 +#define FUTEX_UNLOCK_ROBUST 512 +#define FUTEX_ROBUST_LIST32 1024 -#define FUTEX_CMD_MASK ~(FUTEX_PRIVATE_FLAG | FUTEX_CLOCK_REALTIME) +#define FUTEX_CMD_MASK ~(FUTEX_PRIVATE_FLAG | FUTEX_CLOCK_REALTIME | \ + FUTEX_UNLOCK_ROBUST | FUTEX_ROBUST_LIST32)#define FUTEX_WAIT_PRIVATE (FUTEX_WAIT | FUTEX_PRIVATE_FLAG)#define FUTEX_WAKE_PRIVATE (FUTEX_WAKE | FUTEX_PRIVATE_FLAG)
@@ -43,6 +46,30 @@#define FUTEX_CMP_REQUEUE_PI_PRIVATE (FUTEX_CMP_REQUEUE_PI | FUTEX_PRIVATE_FLAG)/* +* Operations to unlock a futex, clear the robust list pending op pointer and +* wake waiters. +*/ +#define FUTEX_UNLOCK_PI_LIST64 (FUTEX_UNLOCK_PI | FUTEX_UNLOCK_ROBUST) +#define FUTEX_UNLOCK_PI_LIST64_PRIVATE (FUTEX_UNLOCK_PI_LIST64 | FUTEX_PRIVATE_FLAG) +#define FUTEX_UNLOCK_PI_LIST32 (FUTEX_UNLOCK_PI | FUTEX_UNLOCK_ROBUST | \ + FUTEX_ROBUST_LIST32) +#define FUTEX_UNLOCK_PI_LIST32_PRIVATE (FUTEX_UNLOCK_PI_LIST32 | FUTEX_PRIVATE_FLAG) + +#define FUTEX_UNLOCK_WAKE_LIST64 (FUTEX_WAKE | FUTEX_UNLOCK_ROBUST) +#define FUTEX_UNLOCK_WAKE_LIST64_PRIVATE (FUTEX_UNLOCK_WAKE_LIST64 | FUTEX_PRIVATE_FLAG) + +#define FUTEX_UNLOCK_WAKE_LIST32 (FUTEX_WAKE | FUTEX_UNLOCK_ROBUST | \ + FUTEX_ROBUST_LIST32) +#define FUTEX_UNLOCK_WAKE_LIST32_PRIVATE (FUTEX_UNLOCK_WAKE_LIST32 | FUTEX_PRIVATE_FLAG) + +#define FUTEX_UNLOCK_BITSET_LIST64 (FUTEX_WAKE_BITSET | FUTEX_UNLOCK_ROBUST) +#define FUTEX_UNLOCK_BITSET_LIST64_PRIVATE (FUTEX_UNLOCK_BITSET_LIST64 | FUTEX_PRIVATE_FLAG) + +#define FUTEX_UNLOCK_BITSET_LIST32 (FUTEX_WAKE_BITSET | FUTEX_UNLOCK_ROBUST | \ + FUTEX_ROBUST_LIST32) +#define FUTEX_UNLOCK_BITSET_LIST32_PRIVATE (FUTEX_UNLOCK_BITSET_LIST32 | FUTEX_PRIVATE_FLAG) + +/* * Flags for futex2 syscalls. * * NOTE: these are not pure flags, they can also be seen as:
io_uring/futex.c +1 -1
@@ -327,7 +327,7 @@ int io_futex_wake(struct io_kiocb *req,* Strict flags - ensure that waking 0 futexes yields a 0 result. * See commit 43adf8449510 ("futex: FLAGS_STRICT") for details. */ - ret =futex_wake (iof->uaddr,FLAGS_STRICT | iof->futex_flags, + ret =futex_wake (iof->uaddr,FLAGS_STRICT | iof->futex_flags,NULL , iof->futex_val, iof->futex_mask);if (ret <0 )req_set_fail (req);
kernel/futex/core.c +51 -2
83 lines not on this page · raw message
kernel/futex/futex.h +13 -2
@@ -40,6 +40,8 @@#define FLAGS_NUMA 0x0080#define FLAGS_STRICT 0x0100#define FLAGS_MPOL 0x0200 +#define FLAGS_UNLOCK_ROBUST 0x0400 +#define FLAGS_ROBUST_LIST32 0x0800/* FUTEX_ to FLAGS_ */ static inline unsigned int futex_to_flags (unsigned int op)
@@ -52,6 +54,12 @@ static inline unsigned int futex_to_flagif (op &FUTEX_CLOCK_REALTIME ) flags |=FLAGS_CLOCKRT ; +if (op &FUTEX_UNLOCK_ROBUST ) + flags |=FLAGS_UNLOCK_ROBUST ; + +if (op &FUTEX_ROBUST_LIST32 ) + flags |=FLAGS_ROBUST_LIST32 ; +return flags; }
@@ -449,13 +457,16 @@ extern int futex_unqueue_multiple(structextern int futex_wait_multiple (struct futex_vector *vs,unsigned int count,struct hrtimer_sleeper *to); -extern int futex_wake (u32 __user *uaddr,unsigned int flags,int nr_wake,u32 bitset); +extern int futex_wake (u32 __user *uaddr,unsigned int flags,void __user *pop, +int nr_wake,u32 bitset);extern int futex_wake_op (u32 __user *uaddr1,unsigned int flags,u32 __user *uaddr2,int nr_wake,int nr_wake2,int op); -extern int futex_unlock_pi (u32 __user *uaddr,unsigned int flags); +extern int futex_unlock_pi (u32 __user *uaddr,unsigned int flags,void __user *pop);extern int futex_lock_pi (u32 __user *uaddr,unsigned int flags,ktime_t *time,int trylock); +bool futex_robust_list_clear_pending (void __user *pop,unsigned int flags); +#endif /* _FUTEX_H */
kernel/futex/pi.c +13 -2
@@ -1139,7 +1139,7 @@ int futex_lock_pi(u32 __user *uaddr, uns* This is the in-kernel slowpath: we look up the PI state (if any), * and do the rt-mutex unlock. */ -int futex_unlock_pi (u32 __user *uaddr,unsigned int flags) +static int __futex_unlock_pi (u32 __user *uaddr,unsigned int flags) {u32 curval, uval, vpid =task_pid_vnr (current);union futex_key key =FUTEX_KEY_INIT ;
@@ -1148,7 +1148,6 @@ int futex_unlock_pi(u32 __user *uaddr, uif (!IS_ENABLED (CONFIG_FUTEX_PI ))return -ENOSYS ; - retry:if (get_user (uval, uaddr))return -EFAULT ;
@@ -1302,3 +1301,15 @@ int futex_unlock_pi(u32 __user *uaddr, ureturn ret; } +int futex_unlock_pi (u32 __user *uaddr,unsigned int flags,void __user *pop) +{ +int ret =__futex_unlock_pi (uaddr, flags); + +if (ret || !(flags &FLAGS_UNLOCK_ROBUST )) +return ret; + +if (!futex_robust_list_clear_pending (pop, flags)) +return -EFAULT ; + +return 0 ; +}
kernel/futex/syscalls.c +10 -3
@@ -118,6 +118,13 @@ long do_futex(u32 __user *uaddr, int op,return -ENOSYS ; } +if (flags &FLAGS_UNLOCK_ROBUST ) { +if (cmd !=FUTEX_WAKE && + cmd !=FUTEX_WAKE_BITSET && + cmd !=FUTEX_UNLOCK_PI ) +return -ENOSYS ; + } +switch (cmd) {case FUTEX_WAIT : val3 =FUTEX_BITSET_MATCH_ANY ;
@@ -128,7 +135,7 @@ long do_futex(u32 __user *uaddr, int op, val3 =FUTEX_BITSET_MATCH_ANY ; fallthrough;case FUTEX_WAKE_BITSET : -return futex_wake (uaddr, flags, val, val3); +return futex_wake (uaddr, flags, uaddr2, val, val3);case FUTEX_REQUEUE :return futex_requeue (uaddr, flags, uaddr2, flags, val, val2,NULL ,0 );case FUTEX_CMP_REQUEUE :
@@ -141,7 +148,7 @@ long do_futex(u32 __user *uaddr, int op,case FUTEX_LOCK_PI2 :return futex_lock_pi (uaddr, flags, timeout,0 );case FUTEX_UNLOCK_PI : -return futex_unlock_pi (uaddr, flags); +return futex_unlock_pi (uaddr, flags, uaddr2);case FUTEX_TRYLOCK_PI :return futex_lock_pi (uaddr, flags,NULL ,1 );case FUTEX_WAIT_REQUEUE_PI :
@@ -375,7 +382,7 @@ SYSCALL_DEFINE4(futex_wake,if (!futex_validate_input (flags, mask))return -EINVAL ; -return futex_wake (uaddr,FLAGS_STRICT | flags, nr, mask); +return futex_wake (uaddr,FLAGS_STRICT | flags,NULL , nr, mask); }/*
kernel/futex/waitwake.c +28 -2
@@ -150,12 +150,35 @@ void futex_wake_mark(struct wake_q_head }/* +* If requested, clear the robust list pending op and unlock the futex +*/ +static bool futex_robust_unlock (u32 __user *uaddr,unsigned int flags,void __user *pop) +{ +if (!(flags &FLAGS_UNLOCK_ROBUST )) +return true; + +/* First unlock the futex, which requires release semantics. */ + scoped_user_write_access(uaddr, efault) +unsafe_atomic_store_release_user (0 , uaddr, efault); + +/* +* Clear the pending list op now. If that fails, then the task is in +* deeper trouble as the robust list head is usually part of the TLS. +* The chance of survival is close to zero. +*/ +return futex_robust_list_clear_pending (pop, flags); + +efault: +return false; +} + +/* * Wake up waiters matching bitset queued on this futex (uaddr). */ -int futex_wake (u32 __user *uaddr,unsigned int flags,int nr_wake,u32 bitset) +int futex_wake (u32 __user *uaddr,unsigned int flags,void __user *pop,int nr_wake,u32 bitset) { -struct futex_q *this, *next;union futex_key key =FUTEX_KEY_INIT ; +struct futex_q *this, *next;DEFINE_WAKE_Q (wake_q);int ret;
@@ -166,6 +189,9 @@ int futex_wake(u32 __user *uaddr, unsignif (unlikely (ret !=0 ))return ret; +if (!futex_robust_unlock (uaddr, flags, pop)) +return -EFAULT ; +if ((flags &FLAGS_STRICT ) && !nr_wake)return 0 ;
Peter Zijlstra
maintainerRe: [patch V5 09/16] futex: Add support for unlocking robust futexes
Quoted 14 lines · Thomas Gleixner
kernel/futex/futex.h +2
-0@@ -40,6 +40,8 @@#define FLAGS_NUMA 0x0080#define FLAGS_STRICT 0x0100#define FLAGS_MPOL 0x0200 +#define FLAGS_UNLOCK_ROBUST 0x0400 +#define FLAGS_ROBUST_LIST32 0x0800/* FUTEX_ to FLAGS_ */ static inline unsigned int futex_to_flags (unsigned int op)@@ -52,6 +54,12 @@ static inline unsigned int futex_to_flagif (op &FUTEX_CLOCK_REALTIME ) flags |=FLAGS_CLOCKRT ;
kernel/futex/futex.h static inline unsigned int futex_to_flag · line 65
Quoted 10 lines
kernel/futex/futex.h +6
-0+if (op &FUTEX_UNLOCK_ROBUST ) + flags |=FLAGS_UNLOCK_ROBUST ; + +if (op &FUTEX_ROBUST_LIST32 ) + flags |=FLAGS_ROBUST_LIST32 ; +return flags; }
Would you mind terribly if I did: 's/UNLOCK_ROBUST/ROBUST_UNLOCK/g' on the whole series?
Then we get:
FUTEX_ROBUST_UNLOCKFUTEX_ROBUST_LIST32
FLAGS_ROBUST_UNLOCKFLAGS_ROBUST_LIST32
which to me looks just a tad better.
Anyway, let me continue staring at things.
Peter Zijlstra
maintainerRe: [patch V5 09/16] futex: Add support for unlocking robust futexes
Quoted 2 lines · Peter Zijlstra
Would you mind terribly if I did: 's/UNLOCK_ROBUST/ROBUST_UNLOCK/g' on the whole series?
include/uapi/linux/futex.h +8 -8
@@ -25,11 +25,11 @@#define FUTEX_PRIVATE_FLAG 128#define FUTEX_CLOCK_REALTIME 256 -#define FUTEX_UNLOCK_ROBUST 512 +#define FUTEX_ROBUST_UNLOCK 512#define FUTEX_ROBUST_LIST32 1024#define FUTEX_CMD_MASK ~(FUTEX_PRIVATE_FLAG | FUTEX_CLOCK_REALTIME | \ - FUTEX_UNLOCK_ROBUST | FUTEX_ROBUST_LIST32) + FUTEX_ROBUST_UNLOCK | FUTEX_ROBUST_LIST32)#define FUTEX_WAIT_PRIVATE (FUTEX_WAIT | FUTEX_PRIVATE_FLAG)#define FUTEX_WAKE_PRIVATE (FUTEX_WAKE | FUTEX_PRIVATE_FLAG)
@@ -49,23 +49,23 @@* Operations to unlock a futex, clear the robust list pending op pointer and * wake waiters. */ -#define FUTEX_UNLOCK_PI_LIST64 (FUTEX_UNLOCK_PI | FUTEX_UNLOCK_ROBUST) +#define FUTEX_UNLOCK_PI_LIST64 (FUTEX_UNLOCK_PI | FUTEX_ROBUST_UNLOCK)#define FUTEX_UNLOCK_PI_LIST64_PRIVATE (FUTEX_UNLOCK_PI_LIST64 | FUTEX_PRIVATE_FLAG) -#define FUTEX_UNLOCK_PI_LIST32 (FUTEX_UNLOCK_PI | FUTEX_UNLOCK_ROBUST | \ +#define FUTEX_UNLOCK_PI_LIST32 (FUTEX_UNLOCK_PI | FUTEX_ROBUST_UNLOCK | \ FUTEX_ROBUST_LIST32)#define FUTEX_UNLOCK_PI_LIST32_PRIVATE (FUTEX_UNLOCK_PI_LIST32 | FUTEX_PRIVATE_FLAG) -#define FUTEX_UNLOCK_WAKE_LIST64 (FUTEX_WAKE | FUTEX_UNLOCK_ROBUST) +#define FUTEX_UNLOCK_WAKE_LIST64 (FUTEX_WAKE | FUTEX_ROBUST_UNLOCK)#define FUTEX_UNLOCK_WAKE_LIST64_PRIVATE (FUTEX_UNLOCK_WAKE_LIST64 | FUTEX_PRIVATE_FLAG) -#define FUTEX_UNLOCK_WAKE_LIST32 (FUTEX_WAKE | FUTEX_UNLOCK_ROBUST | \ +#define FUTEX_UNLOCK_WAKE_LIST32 (FUTEX_WAKE | FUTEX_ROBUST_UNLOCK | \ FUTEX_ROBUST_LIST32)#define FUTEX_UNLOCK_WAKE_LIST32_PRIVATE (FUTEX_UNLOCK_WAKE_LIST32 | FUTEX_PRIVATE_FLAG) -#define FUTEX_UNLOCK_BITSET_LIST64 (FUTEX_WAKE_BITSET | FUTEX_UNLOCK_ROBUST) +#define FUTEX_UNLOCK_BITSET_LIST64 (FUTEX_WAKE_BITSET | FUTEX_ROBUST_UNLOCK)#define FUTEX_UNLOCK_BITSET_LIST64_PRIVATE (FUTEX_UNLOCK_BITSET_LIST64 | FUTEX_PRIVATE_FLAG) -#define FUTEX_UNLOCK_BITSET_LIST32 (FUTEX_WAKE_BITSET | FUTEX_UNLOCK_ROBUST | \ +#define FUTEX_UNLOCK_BITSET_LIST32 (FUTEX_WAKE_BITSET | FUTEX_ROBUST_UNLOCK | \ FUTEX_ROBUST_LIST32)#define FUTEX_UNLOCK_BITSET_LIST32_PRIVATE (FUTEX_UNLOCK_BITSET_LIST32 | FUTEX_PRIVATE_FLAG)
kernel/futex/futex.h +3 -3
@@ -40,7 +40,7 @@#define FLAGS_NUMA 0x0080#define FLAGS_STRICT 0x0100#define FLAGS_MPOL 0x0200 -#define FLAGS_UNLOCK_ROBUST 0x0400 +#define FLAGS_ROBUST_UNLOCK 0x0400#define FLAGS_ROBUST_LIST32 0x0800/* FUTEX_ to FLAGS_ */
@@ -54,8 +54,8 @@ static inline unsigned int futex_to_flagif (op &FUTEX_CLOCK_REALTIME ) flags |=FLAGS_CLOCKRT ; -if (op &FUTEX_UNLOCK_ROBUST ) - flags |=FLAGS_UNLOCK_ROBUST ; +if (op &FUTEX_ROBUST_UNLOCK ) + flags |=FLAGS_ROBUST_UNLOCK ;if (op &FUTEX_ROBUST_LIST32 ) flags |=FLAGS_ROBUST_LIST32 ;
kernel/futex/pi.c +1 -1
@@ -1305,7 +1305,7 @@ int futex_unlock_pi(u32 __user *uaddr, u {int ret =__futex_unlock_pi (uaddr, flags); -if (ret || !(flags &FLAGS_UNLOCK_ROBUST )) +if (ret || !(flags &FLAGS_ROBUST_UNLOCK ))return ret;if (!futex_robust_list_clear_pending (pop, flags))
kernel/futex/syscalls.c +1 -1
@@ -118,7 +118,7 @@ long do_futex(u32 __user *uaddr, int op,return -ENOSYS ; } -if (flags &FLAGS_UNLOCK_ROBUST ) { +if (flags &FLAGS_ROBUST_UNLOCK ) {if (cmd !=FUTEX_WAKE && cmd !=FUTEX_WAKE_BITSET && cmd !=FUTEX_UNLOCK_PI )
kernel/futex/waitwake.c +1 -1
@@ -154,7 +154,7 @@ void futex_wake_mark(struct wake_q_head*/ static bool futex_robust_unlock (u32 __user *uaddr,unsigned int flags,void __user *pop) { -if (!(flags &FLAGS_UNLOCK_ROBUST )) +if (!(flags &FLAGS_ROBUST_UNLOCK ))return true;/* First unlock the futex, which requires release semantics. */
Thomas Gleixner
maintainerRe: [patch V5 09/16] futex: Add support for unlocking robust futexes
In reply to Peter ZijlstraQuoted 16 lines · Peter Zijlstra
Quoted 15 lines · Thomas Gleixner
kernel/futex/futex.h +2
-0@@ -40,6 +40,8 @@#define FLAGS_NUMA 0x0080#define FLAGS_STRICT 0x0100#define FLAGS_MPOL 0x0200 +#define FLAGS_UNLOCK_ROBUST 0x0400 +#define FLAGS_ROBUST_LIST32 0x0800/* FUTEX_ to FLAGS_ */ static inline unsigned int futex_to_flags (unsigned int op)@@ -52,6 +54,12 @@ static inline unsigned int futex_to_flagif (op &FUTEX_CLOCK_REALTIME ) flags |=FLAGS_CLOCKRT ;
Quoted 12 lines
kernel/futex/futex.h static inline unsigned int futex_to_flag · line 65
Quoted 9 lines
kernel/futex/futex.h +6
-0+if (op &FUTEX_UNLOCK_ROBUST ) + flags |=FLAGS_UNLOCK_ROBUST ; + +if (op &FUTEX_ROBUST_LIST32 ) + flags |=FLAGS_ROBUST_LIST32 ; +return flags; }Would you mind terribly if I did: 's/UNLOCK_ROBUST/ROBUST_UNLOCK/g' on the whole series?
No objections.
Peter Zijlstra
maintainerRe: [patch V5 09/16] futex: Add support for unlocking robust futexes
In reply to Thomas GleixnerQuoted 3 lines · Thomas Gleixner
This deliberately omits
FUTEX_WAKE_OPfrom this treatment as it's unclear whether this is needed and there is no usage of it in glibc either to investigate.
Well, that and because that already consumes uaddr2 its just not doable as-is.
Quoted 2 lines
For the futex2 syscall family this needs to be implemented with a new syscall.
Yeah, this would need to be part of futex_lock() / futex_unlock(), which we don't yet have.
[tip: locking/core] futex: Add support for unlocking robust futexes
In reply to Thomas GleixnerThe following commit has been merged into the locking/core branch of tip:
The following commit has been merged into the locking/core branch of tip:
Commit-ID: 3ca9595d9fb6cce6633a5b03d98c2aecb5499838 Gitweb: https://git.kernel.org/tip/3ca9595d9fb6cce6633a5b03d98c2aecb5499838 Author: Thomas Gleixner <[email protected]> AuthorDate: Tue, 02 Jun 2026 11:09:55 +02:00 Committer: Peter Zijlstra <[email protected]> CommitterDate: Wed, 03 Jun 2026 11:38:51 +02:00
futex: Add support for unlocking robust futexes
Unlocking robust non-PI futexes happens in user space with the following sequence:
1) robust_list_set_op_pending(mutex); 2) robust_list_remove(mutex); lval = 0; 3) lval = atomic_xchg(lock, lval); 4) if (lval & WAITERS) 5) sys_futex(WAKE,....); 6) robust_list_clear_op_pending();
That opens a window between #3 and #6 where the mutex could be acquired by some other task which observes that it is the last user and:
A) unmaps the mutex memory
B) maps a different file, which ends up covering the same address
When the original task exits before reaching #6 then the kernel robust list handling observes the pending op entry and tries to fix up user space.
In case that the newly mapped data contains the TID of the exiting thread at the address of the mutex/futex the kernel will set the owner died bit in that memory and therefore corrupting unrelated data.
PI futexes have a similar problem both for the non-contented user space unlock and the in kernel unlock:
1) robust_list_set_op_pending(mutex); 2) robust_list_remove(mutex); lval = gettid(); 3) if (!atomic_try_cmpxchg(lock, lval, 0)) 4) sys_futex(UNLOCK_PI,....); 5) robust_list_clear_op_pending();
Address the first part of the problem where the futexes have waiters and need to enter the kernel anyway. Add a new FUTEX_ROBUST_UNLOCK flag, which is valid for the sys_futex() FUTEX_UNLOCK_PI, FUTEX_WAKE, FUTEX_WAKE_BITSET operations.
This deliberately omits FUTEX_WAKE_OP from this treatment as it's unclear whether this is needed and there is no usage of it in glibc either to investigate.
For the futex2 syscall family this needs to be implemented with a new syscall.
The sys_futex() case [ab]uses the @uaddr2 argument to hand the pointer to robust_list_head::list_pending_op into the kernel. This argument is only evaluated when the FUTEX_ROBUST_UNLOCK bit is set and is therefore backward compatible.
This is an explicit argument to avoid the lookup of the robust list pointer and retrieving the pending op pointer from there. User space has the pointer already available so it can just put it into the @uaddr2 argument. Aside of that this allows the usage of multiple robust lists in the future without any changes to the internal functions as they just operate on the provided pointer.
This requires a second flag FUTEX_ROBUST_LIST32 which indicates that the robust list pointer points to an u32 and not to an u64. This is required for two reasons:
1) sys_futex() has no compat variant
2) The gaming emulators use both both 64-bit and compat 32-bit robust lists in the same 64-bit application
As a consequence 32-bit applications have to set this flag unconditionally so they can run on a 64-bit kernel in compat mode unmodified. 32-bit kernels return an error code when the flag is not set. 64-bit kernels will happily clear the full 64 bits if user space fails to set it.
In case of FUTEX_UNLOCK_PI this clears the robust list pending op when the unlock succeeded. In case of errors, the user space value is still locked by the caller and therefore the above cannot happen.
In case of FUTEX_WAKE* this does the unlock of the futex in the kernel and clears the robust list pending op when the unlock was successful. If not, the user space value is still locked and user space has to deal with the returned error. That means that the unlocking of non-PI robust futexes has to use the same try_cmpxchg() unlock scheme as PI futexes.
If the clearing of the pending list op fails (fault) then the kernel clears the registered robust list pointer if it matches to prevent that exit() will try to handle invalid data. That's a valid paranoid decision because the robust list head sits usually in the TLS and if the TLS is not longer accessible then the chance for fixing up the resulting mess is very close to zero.
The problem of non-contended unlocks still exists and will be addressed separately.
--- include/uapi/linux/futex.h | 29 +++++++++++++++++++- io_uring/futex.c | 2 +- kernel/futex/core.c | 53 +++++++++++++++++++++++++++++++++++-- kernel/futex/futex.h | 15 ++++++++-- kernel/futex/pi.c | 15 ++++++++-- kernel/futex/syscalls.c | 13 ++++++--- kernel/futex/waitwake.c | 30 +++++++++++++++++++-- 7 files changed, 144 insertions(+), 13 deletions(-)
include/uapi/linux/futex.h +28 -1
@@ -25,8 +25,11 @@#define FUTEX_PRIVATE_FLAG 128#define FUTEX_CLOCK_REALTIME 256 +#define FUTEX_ROBUST_UNLOCK 512 +#define FUTEX_ROBUST_LIST32 1024 -#define FUTEX_CMD_MASK ~(FUTEX_PRIVATE_FLAG | FUTEX_CLOCK_REALTIME) +#define FUTEX_CMD_MASK ~(FUTEX_PRIVATE_FLAG | FUTEX_CLOCK_REALTIME | \ + FUTEX_ROBUST_UNLOCK | FUTEX_ROBUST_LIST32)#define FUTEX_WAIT_PRIVATE (FUTEX_WAIT | FUTEX_PRIVATE_FLAG)#define FUTEX_WAKE_PRIVATE (FUTEX_WAKE | FUTEX_PRIVATE_FLAG)
@@ -43,6 +46,30 @@#define FUTEX_CMP_REQUEUE_PI_PRIVATE (FUTEX_CMP_REQUEUE_PI | FUTEX_PRIVATE_FLAG)/* +* Operations to unlock a futex, clear the robust list pending op pointer and +* wake waiters. +*/ +#define FUTEX_UNLOCK_PI_LIST64 (FUTEX_UNLOCK_PI | FUTEX_ROBUST_UNLOCK) +#define FUTEX_UNLOCK_PI_LIST64_PRIVATE (FUTEX_UNLOCK_PI_LIST64 | FUTEX_PRIVATE_FLAG) +#define FUTEX_UNLOCK_PI_LIST32 (FUTEX_UNLOCK_PI | FUTEX_ROBUST_UNLOCK | \ + FUTEX_ROBUST_LIST32) +#define FUTEX_UNLOCK_PI_LIST32_PRIVATE (FUTEX_UNLOCK_PI_LIST32 | FUTEX_PRIVATE_FLAG) + +#define FUTEX_UNLOCK_WAKE_LIST64 (FUTEX_WAKE | FUTEX_ROBUST_UNLOCK) +#define FUTEX_UNLOCK_WAKE_LIST64_PRIVATE (FUTEX_UNLOCK_WAKE_LIST64 | FUTEX_PRIVATE_FLAG) + +#define FUTEX_UNLOCK_WAKE_LIST32 (FUTEX_WAKE | FUTEX_ROBUST_UNLOCK | \ + FUTEX_ROBUST_LIST32) +#define FUTEX_UNLOCK_WAKE_LIST32_PRIVATE (FUTEX_UNLOCK_WAKE_LIST32 | FUTEX_PRIVATE_FLAG) + +#define FUTEX_UNLOCK_BITSET_LIST64 (FUTEX_WAKE_BITSET | FUTEX_ROBUST_UNLOCK) +#define FUTEX_UNLOCK_BITSET_LIST64_PRIVATE (FUTEX_UNLOCK_BITSET_LIST64 | FUTEX_PRIVATE_FLAG) + +#define FUTEX_UNLOCK_BITSET_LIST32 (FUTEX_WAKE_BITSET | FUTEX_ROBUST_UNLOCK | \ + FUTEX_ROBUST_LIST32) +#define FUTEX_UNLOCK_BITSET_LIST32_PRIVATE (FUTEX_UNLOCK_BITSET_LIST32 | FUTEX_PRIVATE_FLAG) + +/* * Flags for futex2 syscalls. * * NOTE: these are not pure flags, they can also be seen as:
io_uring/futex.c +1 -1
@@ -327,7 +327,7 @@ int io_futex_wake(struct io_kiocb *req, unsigned int issue_flags)* Strict flags - ensure that waking 0 futexes yields a 0 result. * See commit 43adf8449510 ("futex: FLAGS_STRICT") for details. */ - ret =futex_wake (iof->uaddr,FLAGS_STRICT | iof->futex_flags, + ret =futex_wake (iof->uaddr,FLAGS_STRICT | iof->futex_flags,NULL , iof->futex_val, iof->futex_mask);if (ret <0 )req_set_fail (req);
kernel/futex/core.c +51 -2
83 lines not on this page · raw message
kernel/futex/futex.h +13 -2
@@ -40,6 +40,8 @@#define FLAGS_NUMA 0x0080#define FLAGS_STRICT 0x0100#define FLAGS_MPOL 0x0200 +#define FLAGS_ROBUST_UNLOCK 0x0400 +#define FLAGS_ROBUST_LIST32 0x0800/* FUTEX_ to FLAGS_ */ static inline unsigned int futex_to_flags (unsigned int op)
@@ -52,6 +54,12 @@ static inline unsigned int futex_to_flags(unsigned int op)if (op &FUTEX_CLOCK_REALTIME ) flags |=FLAGS_CLOCKRT ; +if (op &FUTEX_ROBUST_UNLOCK ) + flags |=FLAGS_ROBUST_UNLOCK ; + +if (op &FUTEX_ROBUST_LIST32 ) + flags |=FLAGS_ROBUST_LIST32 ; +return flags; }
@@ -449,13 +457,16 @@ extern int futex_unqueue_multiple(struct futex_vector *v, int count);extern int futex_wait_multiple (struct futex_vector *vs,unsigned int count,struct hrtimer_sleeper *to); -extern int futex_wake (u32 __user *uaddr,unsigned int flags,int nr_wake,u32 bitset); +extern int futex_wake (u32 __user *uaddr,unsigned int flags,void __user *pop, +int nr_wake,u32 bitset);extern int futex_wake_op (u32 __user *uaddr1,unsigned int flags,u32 __user *uaddr2,int nr_wake,int nr_wake2,int op); -extern int futex_unlock_pi (u32 __user *uaddr,unsigned int flags); +extern int futex_unlock_pi (u32 __user *uaddr,unsigned int flags,void __user *pop);extern int futex_lock_pi (u32 __user *uaddr,unsigned int flags,ktime_t *time,int trylock); +bool futex_robust_list_clear_pending (void __user *pop,unsigned int flags); +#endif /* _FUTEX_H */
kernel/futex/pi.c +13 -2
@@ -1139,7 +1139,7 @@ out:* This is the in-kernel slowpath: we look up the PI state (if any), * and do the rt-mutex unlock. */ -int futex_unlock_pi (u32 __user *uaddr,unsigned int flags) +static int __futex_unlock_pi (u32 __user *uaddr,unsigned int flags) {u32 curval, uval, vpid =task_pid_vnr (current);union futex_key key =FUTEX_KEY_INIT ;
@@ -1148,7 +1148,6 @@ int futex_unlock_pi(u32 __user *uaddr, unsigned int flags)if (!IS_ENABLED (CONFIG_FUTEX_PI ))return -ENOSYS ; - retry:if (get_user (uval, uaddr))return -EFAULT ;
@@ -1302,3 +1301,15 @@ pi_faulted:return ret; } +int futex_unlock_pi (u32 __user *uaddr,unsigned int flags,void __user *pop) +{ +int ret =__futex_unlock_pi (uaddr, flags); + +if (ret || !(flags &FLAGS_ROBUST_UNLOCK )) +return ret; + +if (!futex_robust_list_clear_pending (pop, flags)) +return -EFAULT ; + +return 0 ; +}
kernel/futex/syscalls.c +10 -3
@@ -118,6 +118,13 @@ long do_futex(u32 __user *uaddr, int op, u32 val, ktime_t *timeout,return -ENOSYS ; } +if (flags &FLAGS_ROBUST_UNLOCK ) { +if (cmd !=FUTEX_WAKE && + cmd !=FUTEX_WAKE_BITSET && + cmd !=FUTEX_UNLOCK_PI ) +return -ENOSYS ; + } +switch (cmd) {case FUTEX_WAIT : val3 =FUTEX_BITSET_MATCH_ANY ;
@@ -128,7 +135,7 @@ long do_futex(u32 __user *uaddr, int op, u32 val, ktime_t *timeout, val3 =FUTEX_BITSET_MATCH_ANY ; fallthrough;case FUTEX_WAKE_BITSET : -return futex_wake (uaddr, flags, val, val3); +return futex_wake (uaddr, flags, uaddr2, val, val3);case FUTEX_REQUEUE :return futex_requeue (uaddr, flags, uaddr2, flags, val, val2,NULL ,0 );case FUTEX_CMP_REQUEUE :
@@ -141,7 +148,7 @@ long do_futex(u32 __user *uaddr, int op, u32 val, ktime_t *timeout,case FUTEX_LOCK_PI2 :return futex_lock_pi (uaddr, flags, timeout,0 );case FUTEX_UNLOCK_PI : -return futex_unlock_pi (uaddr, flags); +return futex_unlock_pi (uaddr, flags, uaddr2);case FUTEX_TRYLOCK_PI :return futex_lock_pi (uaddr, flags,NULL ,1 );case FUTEX_WAIT_REQUEUE_PI :
@@ -375,7 +382,7 @@ SYSCALL_DEFINE4(futex_wake,if (!futex_validate_input (flags, mask))return -EINVAL ; -return futex_wake (uaddr,FLAGS_STRICT | flags, nr, mask); +return futex_wake (uaddr,FLAGS_STRICT | flags,NULL , nr, mask); }/*
kernel/futex/waitwake.c +28 -2
@@ -150,12 +150,35 @@ void futex_wake_mark(struct wake_q_head *wake_q, struct futex_q *q) }/* +* If requested, clear the robust list pending op and unlock the futex +*/ +static bool futex_robust_unlock (u32 __user *uaddr,unsigned int flags,void __user *pop) +{ +if (!(flags &FLAGS_ROBUST_UNLOCK )) +return true; + +/* First unlock the futex, which requires release semantics. */ + scoped_user_write_access(uaddr, efault) +unsafe_atomic_store_release_user (0 , uaddr, efault); + +/* +* Clear the pending list op now. If that fails, then the task is in +* deeper trouble as the robust list head is usually part of the TLS. +* The chance of survival is close to zero. +*/ +return futex_robust_list_clear_pending (pop, flags); + +efault: +return false; +} + +/* * Wake up waiters matching bitset queued on this futex (uaddr). */ -int futex_wake (u32 __user *uaddr,unsigned int flags,int nr_wake,u32 bitset) +int futex_wake (u32 __user *uaddr,unsigned int flags,void __user *pop,int nr_wake,u32 bitset) { -struct futex_q *this, *next;union futex_key key =FUTEX_KEY_INIT ; +struct futex_q *this, *next;DEFINE_WAKE_Q (wake_q);int ret;
@@ -166,6 +189,9 @@ int futex_wake(u32 __user *uaddr, unsigned int flags, int nr_wake, u32 bitset)if (unlikely (ret !=0 ))return ret; +if (!futex_robust_unlock (uaddr, flags, pop)) +return -EFAULT ; +if ((flags &FLAGS_STRICT ) && !nr_wake)return 0 ;
Thomas Gleixner
maintainerPATCH v5 10/16 futex: Add robust futex unlock IP range
In reply to Thomas GleixnerThere will be a VDSO function to unlock robust futexes in user space. The unlock sequence is racy vs. clearing the list_pending_op pointer in the tasks robust list head. To plug this race the kernel needs to know the instruction window. As the VDSO is per MM the addresses are stored in mm_struct::futex.
Architectures which implement support for this have to update these addresses when the VDSO is (re)mapped and indicate the pending op pointer size which is matching the IP.
Arguably this could be resolved by chasing mm->context->vdso->image, but that's architecture specific and requires to touch quite some cache lines. Having it in mm::futex reduces the cache line impact and avoids having yet another set of architecture specific functionality.
To support multi size robust list applications (gaming) this provides two ranges when COMPAT is enabled.
---
V5: Let the caller provide the real start/end IP - Andre
V4: Guard futex_mm_init() for builds w/o HASH and UNLOCK
V3: Make the number of ranges depend on COMPAT - Peter
V2: Store ranges in a struct with size information and allow up to two ranges.
--- include/linux/futex.h | 21 +++++++++++++++++--- include/linux/futex_types.h | 28 ++++++++++++++++++++++++++ init/Kconfig | 6 +++++ kernel/futex/core.c | 46 +++++++++++++++++++++++++++++++++++--------- 4 files changed, 89 insertions(+), 12 deletions(-)
include/linux/futex.h +18 -3
@@ -81,11 +81,9 @@ int futex_hash_prctl(unsigned long arg2,#ifdef CONFIG_FUTEX_PRIVATE_HASH int futex_hash_allocate_default (void );void futex_hash_free (struct mm_struct *mm); -void futex_mm_init (struct mm_struct *mm);#else /* CONFIG_FUTEX_PRIVATE_HASH */ static inline int futex_hash_allocate_default (void ) {return 0 ; }static inline int futex_hash_free (struct mm_struct *mm) {return 0 ; } -static inline void futex_mm_init (struct mm_struct *mm) { }#endif /* !CONFIG_FUTEX_PRIVATE_HASH */ #else /* CONFIG_FUTEX */
@@ -104,7 +102,24 @@ static inline int futex_hash_prctl(unsig }static inline int futex_hash_allocate_default (void ) {return 0 ; }static inline int futex_hash_free (struct mm_struct *mm) {return 0 ; } -static inline void futex_mm_init (struct mm_struct *mm) { }#endif /* !CONFIG_FUTEX */ +#ifdef CONFIG_FUTEX_ROBUST_UNLOCK +void futex_reset_cs_ranges (struct futex_mm_data *fd); + +static inline void futex_set_vdso_cs_range (struct futex_mm_data *fd,unsigned int idx, +unsigned long start,unsigned long end,bool sz32) +{ + fd->unlock.cs_ranges[idx].start_ip = start; + fd->unlock.cs_ranges[idx].len = end - start; + fd->unlock.cs_ranges[idx].pop_size32 = sz32; +} +#endif /* CONFIG_FUTEX_ROBUST_UNLOCK */ + +#if defined(CONFIG_FUTEX_PRIVATE_HASH ) || defined(CONFIG_FUTEX_ROBUST_UNLOCK ) +void futex_mm_init (struct mm_struct *mm); +#else +static inline void futex_mm_init (struct mm_struct *mm) { } +#endif +#endif /* _LINUX_FUTEX_H */
include/linux/futex_types.h +28 -0
@@ -55,12 +55,40 @@ struct futex_mm_phash {struct futex_mm_phash { };#endif /* !CONFIG_FUTEX_ROBUST_UNLOCK */ +#ifdef CONFIG_FUTEX_ROBUST_UNLOCK +/** +* struct futex_unlock_cs_range - Range for the VDSO unlock critical section +* @start_ip: The start IP of the robust futex unlock critical section (inclusive) +* @len: The length of the robust futex unlock critical section +* @pop_size32: Pending OP pointer size indicator. 0 == 64-bit, 1 == 32-bit +*/ +struct futex_unlock_cs_range { +unsigned long start_ip; +unsigned int len; +unsigned int pop_size32; +}; + +#define FUTEX_ROBUST_MAX_CS_RANGES (1 + IS_ENABLED(CONFIG_COMPAT)) + +/** +* struct futex_unlock_cs_ranges - Futex unlock VSDO critical sections +* @cs_ranges: Array of critical section ranges +*/ +struct futex_unlock_cs_ranges { +struct futex_unlock_cs_range cs_ranges[FUTEX_ROBUST_MAX_CS_RANGES ]; +}; +#else /* CONFIG_FUTEX_ROBUST_UNLOCK */ +struct futex_unlock_cs_ranges { }; +#endif /* !CONFIG_FUTEX_ROBUST_UNLOCK */ +/** * struct futex_mm_data - Futex related per MM data * @phash: Futex private hash related data +* @unlock: Futex unlock VDSO critical sections */ struct futex_mm_data {struct futex_mm_phash phash; +struct futex_unlock_cs_ranges unlock; };#else /* CONFIG_FUTEX */ struct futex_sched_data { };
init/Kconfig +6 -0
@@ -1842,6 +1842,12 @@ config FUTEX_MPOL depends on FUTEX && NUMA default y +config HAVE_FUTEX_ROBUST_UNLOCK + bool + +config FUTEX_ROBUST_UNLOCK + def_bool FUTEX && HAVE_GENERIC_VDSO && GENERIC_IRQ_ENTRY && RSEQ && HAVE_FUTEX_ROBUST_UNLOCK + config EPOLL bool "Enable eventpoll support" if EXPERT default y
kernel/futex/core.c +37 -9
67 lines not on this page · raw message
[tip: locking/core] futex: Add robust futex unlock IP range
The following commit has been merged into the locking/core branch of tip:
The following commit has been merged into the locking/core branch of tip:
Commit-ID: 042df0c1d48609a85580dcbaff498c95ced20a5f Gitweb: https://git.kernel.org/tip/042df0c1d48609a85580dcbaff498c95ced20a5f Author: Thomas Gleixner <[email protected]> AuthorDate: Tue, 02 Jun 2026 11:09:59 +02:00 Committer: Peter Zijlstra <[email protected]> CommitterDate: Wed, 03 Jun 2026 11:38:51 +02:00
futex: Add robust futex unlock IP range
There will be a VDSO function to unlock robust futexes in user space. The unlock sequence is racy vs. clearing the list_pending_op pointer in the tasks robust list head. To plug this race the kernel needs to know the instruction window. As the VDSO is per MM the addresses are stored in mm_struct::futex.
Architectures which implement support for this have to update these addresses when the VDSO is (re)mapped and indicate the pending op pointer size which is matching the IP.
Arguably this could be resolved by chasing mm->context->vdso->image, but that's architecture specific and requires to touch quite some cache lines. Having it in mm::futex reduces the cache line impact and avoids having yet another set of architecture specific functionality.
To support multi size robust list applications (gaming) this provides two ranges when COMPAT is enabled.
--- include/linux/futex.h | 21 +++++++++++++--- include/linux/futex_types.h | 28 ++++++++++++++++++++++- init/Kconfig | 6 +++++- kernel/futex/core.c | 46 ++++++++++++++++++++++++++++-------- 4 files changed, 89 insertions(+), 12 deletions(-)
include/linux/futex.h +18 -3
@@ -81,11 +81,9 @@ int futex_hash_prctl(unsigned long arg2, unsigned long arg3, unsigned long arg4)#ifdef CONFIG_FUTEX_PRIVATE_HASH int futex_hash_allocate_default (void );void futex_hash_free (struct mm_struct *mm); -void futex_mm_init (struct mm_struct *mm);#else /* CONFIG_FUTEX_PRIVATE_HASH */ static inline int futex_hash_allocate_default (void ) {return 0 ; }static inline int futex_hash_free (struct mm_struct *mm) {return 0 ; } -static inline void futex_mm_init (struct mm_struct *mm) { }#endif /* !CONFIG_FUTEX_PRIVATE_HASH */ #else /* CONFIG_FUTEX */
@@ -104,7 +102,24 @@ static inline int futex_hash_prctl(unsigned long arg2, unsigned long arg3, unsig }static inline int futex_hash_allocate_default (void ) {return 0 ; }static inline int futex_hash_free (struct mm_struct *mm) {return 0 ; } -static inline void futex_mm_init (struct mm_struct *mm) { }#endif /* !CONFIG_FUTEX */ +#ifdef CONFIG_FUTEX_ROBUST_UNLOCK +void futex_reset_cs_ranges (struct futex_mm_data *fd); + +static inline void futex_set_vdso_cs_range (struct futex_mm_data *fd,unsigned int idx, +unsigned long start,unsigned long end,bool sz32) +{ + fd->unlock.cs_ranges[idx].start_ip = start; + fd->unlock.cs_ranges[idx].len = end - start; + fd->unlock.cs_ranges[idx].pop_size32 = sz32; +} +#endif /* CONFIG_FUTEX_ROBUST_UNLOCK */ + +#if defined(CONFIG_FUTEX_PRIVATE_HASH ) || defined(CONFIG_FUTEX_ROBUST_UNLOCK ) +void futex_mm_init (struct mm_struct *mm); +#else +static inline void futex_mm_init (struct mm_struct *mm) { } +#endif +#endif /* _LINUX_FUTEX_H */
include/linux/futex_types.h +28 -0
@@ -55,12 +55,40 @@ struct futex_mm_phash {struct futex_mm_phash { };#endif /* !CONFIG_FUTEX_ROBUST_UNLOCK */ +#ifdef CONFIG_FUTEX_ROBUST_UNLOCK +/** +* struct futex_unlock_cs_range - Range for the VDSO unlock critical section +* @start_ip: The start IP of the robust futex unlock critical section (inclusive) +* @len: The length of the robust futex unlock critical section +* @pop_size32: Pending OP pointer size indicator. 0 == 64-bit, 1 == 32-bit +*/ +struct futex_unlock_cs_range { +unsigned long start_ip; +unsigned int len; +unsigned int pop_size32; +}; + +#define FUTEX_ROBUST_MAX_CS_RANGES (1 + IS_ENABLED(CONFIG_COMPAT)) + +/** +* struct futex_unlock_cs_ranges - Futex unlock VSDO critical sections +* @cs_ranges: Array of critical section ranges +*/ +struct futex_unlock_cs_ranges { +struct futex_unlock_cs_range cs_ranges[FUTEX_ROBUST_MAX_CS_RANGES ]; +}; +#else /* CONFIG_FUTEX_ROBUST_UNLOCK */ +struct futex_unlock_cs_ranges { }; +#endif /* !CONFIG_FUTEX_ROBUST_UNLOCK */ +/** * struct futex_mm_data - Futex related per MM data * @phash: Futex private hash related data +* @unlock: Futex unlock VDSO critical sections */ struct futex_mm_data {struct futex_mm_phash phash; +struct futex_unlock_cs_ranges unlock; };#else /* CONFIG_FUTEX */ struct futex_sched_data { };
init/Kconfig +6 -0
@@ -1842,6 +1842,12 @@ config FUTEX_MPOL depends on FUTEX && NUMA default y +config HAVE_FUTEX_ROBUST_UNLOCK + bool + +config FUTEX_ROBUST_UNLOCK + def_bool FUTEX && HAVE_GENERIC_VDSO && GENERIC_IRQ_ENTRY && RSEQ && HAVE_FUTEX_ROBUST_UNLOCK + config EPOLL bool "Enable eventpoll support" if EXPERT default y
kernel/futex/core.c +37 -9
67 lines not on this page · raw message
Thomas Gleixner
maintainerPATCH v5 11/16 futex: Provide infrastructure to plug the non contended robust futex unlock race
In reply to Thomas GleixnerWhen the FUTEX_ROBUST_UNLOCK mechanism is used for unlocking (PI-)futexes, then the unlock sequence in user space looks like this:
1) robust_list_set_op_pending(mutex); 2) robust_list_remove(mutex); lval = gettid(); 3) if (atomic_try_cmpxchg(&mutex->lock, lval, 0)) 4) robust_list_clear_op_pending(); else 5) sys_futex(OP | FUTEX_ROBUST_UNLOCK, ....);
That still leaves a minimal race window between #3 and #4 where the mutex could be acquired by some other task, which observes that it is the last user and:
1) unmaps the mutex memory
2) maps a different file, which ends up covering the same address
When then the original task exits before reaching #5 then the kernel robust list handling observes the pending op entry and tries to fix up user space.
In case that the newly mapped data contains the TID of the exiting thread at the address of the mutex/futex the kernel will set the owner died bit in that memory and therefore corrupt unrelated data.
On X86 this boils down to this simplified assembly sequence:
mov %esi,%eax // Load TID into EAX
xor %ecx,%ecx // Set ECX to 0
#3 lock cmpxchg %ecx,(%rdi) // Try the TID -> 0 transition
.Lstart:
jnz .Lend
#4 movq %rcx,(%rdx) // Clear list_op_pending
.Lend:If the cmpxchg() succeeds and the task is interrupted before it can clear list_op_pending in the robust list head (#4) and the task crashes in a signal handler or gets killed then it ends up in do_exit() and subsequently in the robust list handling, which then might run into the unmap/map issue described above.
This is only relevant when user space was interrupted and a signal is pending. The fix-up has to be done before signal delivery is attempted because:
1) The signal might be fatal so get_signal() ends up in do_exit()
2) The signal handler might crash or the task is killed before returning from the handler. At that point the instruction pointer in pt_regs is not longer the instruction pointer of the initially interrupted unlock sequence.
The right place to handle this is in __exit_to_user_mode_loop() before invoking arch_do_signal_or_restart() as this covers obviously both scenarios.
As this is only relevant when the task was interrupted in user space, this is tied to RSEQ and the generic entry code as RSEQ keeps track of user space interrupts unconditionally even if the task does not have a RSEQ region installed. That makes the decision very lightweight:
if (current->rseq.user_irq && within(regs, csr->unlock_ip_range))
futex_fixup_robust_unlock(regs, csr);futex_fixup_robust_unlock() then invokes a architecture specific function to returen the pending op pointer or NULL. The function evaluates the register content to decide whether the pending ops pointer in the robust list head needs to be cleared.
Assuming the above unlock sequence, then on x86 this decision is the trivial evaluation of the zero flag:
return regs->eflags & X86_EFLAGS_ZF ? regs->dx : NULL;
Other architectures might need to do more complex evaluations due to LLSC, but the approach is valid in general. The size of the pointer is determined from the matching range struct, which covers both 32-bit and 64-bit builds including COMPAT.
The unlock sequence is going to be placed in the VDSO so that the kernel can keep everything synchronized, especially the register usage. The resulting code sequence for user space is:
if (__vdso_futex_robust_list$SZ_try_unlock(lock, tid, &pending_op) != tid)
err = sys_futex($OP | FUTEX_ROBUST_UNLOCK,....);
Both the VDSO unlock and the kernel side unlock ensure that the pending_op pointer is always cleared when the lock becomes unlocked.
---
V3: Fixup conversion leftover which was lost on the devel machine
V2: Convert to the struct range storage and simplify the fixup logic
--- include/linux/futex.h | 39 ++++++++++++++++++++++++++++++++++++- include/vdso/futex.h | 52 ++++++++++++++++++++++++++++++++++++++++++++++++++ kernel/entry/common.c | 9 +++++--- kernel/futex/core.c | 18 +++++++++++++++++ 4 files changed, 114 insertions(+), 4 deletions(-)
include/linux/futex.h +38 -1
@@ -105,7 +105,41 @@ static inline int futex_hash_free(struct#endif /* !CONFIG_FUTEX */ #ifdef CONFIG_FUTEX_ROBUST_UNLOCK +#include <asm/futex_robust.h> +void futex_reset_cs_ranges (struct futex_mm_data *fd); +void __futex_fixup_robust_unlock (struct pt_regs *regs,struct futex_unlock_cs_range *csr); + +static inline bool futex_within_robust_unlock (struct pt_regs *regs, +struct futex_unlock_cs_range *csr) +{ +unsigned long ip =instruction_pointer (regs); + +return ip >= csr->start_ip && ip < csr->start_ip + csr->len; +} + +static inline void futex_fixup_robust_unlock (struct pt_regs *regs) +{ +struct futex_unlock_cs_range *csr; + +/* +* Avoid dereferencing current->mm if not returning from interrupt. +* current->rseq.event is going to be used subsequently, so bringing the +* cache line in is not a big deal. +*/ +if (!current->rseq.event.user_irq) +return ; + + csr = current->mm->futex.unlock.cs_ranges; + +/* The loop is optimized out for !COMPAT */ +for (int r =0 ; r <FUTEX_ROBUST_MAX_CS_RANGES ; r++, csr++) { +if (unlikely (futex_within_robust_unlock (regs, csr))) { +__futex_fixup_robust_unlock (regs, csr); +return ; + } + } +}static inline void futex_set_vdso_cs_range (struct futex_mm_data *fd,unsigned int idx,unsigned long start,unsigned long end,bool sz32)
@@ -114,7 +148,10 @@ static inline void futex_set_vdso_cs_ran fd->unlock.cs_ranges[idx].len = end - start; fd->unlock.cs_ranges[idx].pop_size32 = sz32; } -#endif /* CONFIG_FUTEX_ROBUST_UNLOCK */ +#else /* CONFIG_FUTEX_ROBUST_UNLOCK */ +static inline void futex_fixup_robust_unlock (struct pt_regs *regs) { } +#endif /* !CONFIG_FUTEX_ROBUST_UNLOCK */ +#if defined(CONFIG_FUTEX_PRIVATE_HASH ) || defined(CONFIG_FUTEX_ROBUST_UNLOCK )void futex_mm_init (struct mm_struct *mm);
include/vdso/futex.h new file +52 -0
52 lines not on this page · raw message
kernel/entry/common.c +6 -3
@@ -1,11 +1,12 @@// SPDX-License-Identifier: GPL-2.0 -#include <linux/irq-entry-common.h> -#include <linux/resume_user_mode.h> +#include <linux/futex.h> #include <linux/highmem.h> +#include <linux/irq-entry-common.h> #include <linux/jump_label.h> #include <linux/kmsan.h> #include <linux/livepatch.h> +#include <linux/resume_user_mode.h> #include <linux/tick.h> /* Workaround to allow gradual conversion of architecture code */
@@ -60,8 +61,10 @@ static __always_inline unsigned long __eif (ti_work & _TIF_PATCH_PENDING)klp_update_patch_state (current); -if (ti_work & (_TIF_SIGPENDING | _TIF_NOTIFY_SIGNAL)) +if (ti_work & (_TIF_SIGPENDING | _TIF_NOTIFY_SIGNAL)) { +futex_fixup_robust_unlock (regs);arch_do_signal_or_restart (regs); + }if (ti_work & _TIF_NOTIFY_RESUME)resume_user_mode_work (regs);
kernel/futex/core.c +18 -0
@@ -46,6 +46,8 @@#include <linux/slab.h> #include <linux/vmalloc.h> +#include <vdso/futex.h> +#include "futex.h" #include "../locking/rtmutex_common.h"
@@ -1446,6 +1448,22 @@ bool futex_robust_list_clear_pending(voireturn robust_list_clear_pending (pop); } +#ifdef CONFIG_FUTEX_ROBUST_UNLOCK +void __futex_fixup_robust_unlock (struct pt_regs *regs,struct futex_unlock_cs_range *csr) +{ +/* +* arch_futex_robust_unlock_get_pop() returns the list pending op pointer from +* @regs if the try_cmpxchg() succeeded. +*/ +void __user *pop =arch_futex_robust_unlock_get_pop (regs); + +if (!pop) +return ; + +futex_robust_list_clear_pending (pop, csr->pop_size32 ?FLAGS_ROBUST_LIST32 :0 ); +} +#endif /* CONFIG_FUTEX_ROBUST_UNLOCK */ +static void futex_cleanup (struct task_struct *tsk) {if (unlikely (tsk->futex.robust_list)) {
Peter Zijlstra
maintainerRe: [patch V5 11/16] futex: Provide infrastructure to plug the non contended robust futex unlock race
Quoted 11 lines · Thomas Gleixner
When the
FUTEX_ROBUST_UNLOCKmechanism is used for unlocking (PI-)futexes, then the unlock sequence in user space looks like this:1) robust_list_set_op_pending(mutex); 2) robust_list_remove(mutex); lval = gettid(); 3) if (atomic_try_cmpxchg(&mutex->lock, lval, 0)) 4) robust_list_clear_op_pending(); else 5) sys_futex(OP | FUTEX_ROBUST_UNLOCK, ....);
Ah!, see, your uapi patch earlier called that FUTEX_UNLOCK_ROBUST.
I'll fix it all up if I don't fine real issues.
Peter Zijlstra
maintainerRe: [patch V5 11/16] futex: Provide infrastructure to plug the non contended robust futex unlock race
In reply to Thomas GleixnerQuoted 20 lines · Thomas Gleixner
On X86 this boils down to this simplified assembly sequence:
mov %esi,%eax // Load TID into EAX xor %ecx,%ecx // Set ECX to 0 #3 lock cmpxchg %ecx,(%rdi) // Try the TID -> 0 transition .Lstart: jnz .Lend #4 movq %rcx,(%rdx) // Clear list_op_pending .Lend:If the
cmpxchg()succeeds and the task is interrupted before it can clearlist_op_pendingin the robust list head (#4) and the task crashes in a signal handler or gets killed then it ends up indo_exit()and subsequently in the robust list handling, which then might run into the unmap/map issue described above.This is only relevant when user space was interrupted and a signal is pending. The fix-up has to be done before signal delivery is attempted because:
Quoted 6 lines
1) The signal might be fatal so
get_signal()ends up indo_exit()2) The signal handler might crash or the task is killed before returning from the handler. At that point the instruction pointer in
pt_regsis not longer the instruction pointer of the initially interrupted unlock sequence.
However, due to the pending field being strictly per thread (thread local storage and all that), the whole construct of futex robust unlock is not signal safe in the sense that signal handlers must not use it.
A signal handler trying to use this would result in nested use of the pending field, and that leads to corrupted state.
Quoted 20 lines
The right place to handle this is in
__exit_to_user_mode_loop()before invokingarch_do_signal_or_restart()as this covers obviously both scenarios.As this is only relevant when the task was interrupted in user space, this is tied to RSEQ and the generic entry code as RSEQ keeps track of user space interrupts unconditionally even if the task does not have a RSEQ region installed. That makes the decision very lightweight:
if (current->rseq.user_irq && within(regs, csr->unlock_ip_range)) futex_fixup_robust_unlock(regs, csr);
futex_fixup_robust_unlock()then invokes a architecture specific function to returen the pending op pointer or NULL. The function evaluates the register content to decide whether the pending ops pointer in the robust list head needs to be cleared.Assuming the above unlock sequence, then on x86 this decision is the trivial evaluation of the zero flag:
Quoted 6 lines
return regs->eflags & X86_EFLAGS_ZF ? regs->dx : NULL;Other architectures might need to do more complex evaluations due to LLSC, but the approach is valid in general. The size of the pointer is determined from the matching range struct, which covers both 32-bit and 64-bit builds including COMPAT.
So my initial thoughts today were that we should probably also move the IP to .Lend, to avoid userspace from writing to that location again.
However, due to the above mentioned restrictions vs signals, there cannot be a situation where this matters, and so the point is moot.
A double store is harmless and it makes the kernel just this little bit simpler.
The only reason I'm sending this email is to have this more explicitly documented for posterity I suppose ;-)
Quoted 9 lines
The unlock sequence is going to be placed in the VDSO so that the kernel can keep everything synchronized, especially the register usage. The resulting code sequence for user space is:
if (__vdso_futex_robust_list$SZ_try_unlock(lock, tid, &pending_op) != tid)err =
sys_futex($OP | FUTEX_ROBUST_UNLOCK,....);Both the VDSO unlock and the kernel side unlock ensure that the
pending_oppointer is always cleared when the lock becomes unlocked.
Thomas Gleixner
maintainerRe: [patch V5 11/16] futex: Provide infrastructure to plug the non contended robust futex unlock race
Quoted 12 lines · Peter Zijlstra
Quoted 10 lines · Thomas Gleixner
This is only relevant when user space was interrupted and a signal is pending. The fix-up has to be done before signal delivery is attempted because:
1) The signal might be fatal so
get_signal()ends up indo_exit()2) The signal handler might crash or the task is killed before returning from the handler. At that point the instruction pointer in
pt_regsis not longer the instruction pointer of the initially interrupted unlock sequence.
Quoted 6 lines
However, due to the pending field being strictly per thread (thread local storage and all that), the whole construct of futex robust unlock is not signal safe in the sense that signal handlers must not use it.
A signal handler trying to use this would result in nested use of the pending field, and that leads to corrupted state.
That's true already today and this unlock magic does not change it.
Quoted 8 lines
Quoted 4 lines
Other architectures might need to do more complex evaluations due to LLSC, but the approach is valid in general. The size of the pointer is determined from the matching range struct, which covers both 32-bit and 64-bit builds including COMPAT.
So my initial thoughts today were that we should probably also move the IP to .Lend, to avoid userspace from writing to that location again.
Quoted 8 lines
However, due to the above mentioned restrictions vs signals, there cannot be a situation where this matters, and so the point is moot.
A double store is harmless and it makes the kernel just this little bit simpler.
The only reason I'm sending this email is to have this more explicitly documented for posterity I suppose ;-)
Indeed :)
Peter Zijlstra
maintainerRe: [patch V5 11/16] futex: Provide infrastructure to plug the non contended robust futex unlock race
In reply to Thomas GleixnerQuoted 16 lines · Thomas Gleixner
When the
FUTEX_ROBUST_UNLOCKmechanism is used for unlocking (PI-)futexes, then the unlock sequence in user space looks like this:1) robust_list_set_op_pending(mutex); 2) robust_list_remove(mutex); lval = gettid(); 3) if (atomic_try_cmpxchg(&mutex->lock, lval, 0)) 4) robust_list_clear_op_pending(); else 5) sys_futex(OP | FUTEX_ROBUST_UNLOCK, ....);That still leaves a minimal race window between #3 and #4 where the mutex could be acquired by some other task, which observes that it is the last user and:
Quoted 5 lines
1) unmaps the mutex memory
2) maps a different file, which ends up covering the same addressWhen then the original task exits before reaching #5 then the kernel robust list handling observes the pending op entry and tries to fix up user space.
This #5 reference, should be #4, yeah? Same bit of Changelog is replicated in a later patch and has the same issue.
Thomas Gleixner
maintainerRe: [patch V5 11/16] futex: Provide infrastructure to plug the non contended robust futex unlock race
Quoted 17 lines · Peter Zijlstra
Quoted 16 lines · Thomas Gleixner
When the
FUTEX_ROBUST_UNLOCKmechanism is used for unlocking (PI-)futexes, then the unlock sequence in user space looks like this:1) robust_list_set_op_pending(mutex); 2) robust_list_remove(mutex); lval = gettid(); 3) if (atomic_try_cmpxchg(&mutex->lock, lval, 0)) 4) robust_list_clear_op_pending(); else 5) sys_futex(OP | FUTEX_ROBUST_UNLOCK, ....);That still leaves a minimal race window between #3 and #4 where the mutex could be acquired by some other task, which observes that it is the last user and:
Quoted 8 lines
Quoted 5 lines
1) unmaps the mutex memory
2) maps a different file, which ends up covering the same addressWhen then the original task exits before reaching #5 then the kernel robust list handling observes the pending op entry and tries to fix up user space.
This #5 reference, should be #4, yeah? Same bit of Changelog is replicated in a later patch and has the same issue.
Yes.
[tip: locking/core] futex: Provide infrastructure to plug the non contended robust futex unlock race
In reply to Thomas GleixnerThe following commit has been merged into the locking/core branch of tip:
The following commit has been merged into the locking/core branch of tip:
Commit-ID: 7010c39d8fc5063af69ee63f905e592e046f8e5d Gitweb: https://git.kernel.org/tip/7010c39d8fc5063af69ee63f905e592e046f8e5d Author: Thomas Gleixner <[email protected]> AuthorDate: Tue, 02 Jun 2026 11:10:04 +02:00 Committer: Peter Zijlstra <[email protected]> CommitterDate: Wed, 03 Jun 2026 11:38:52 +02:00
futex: Provide infrastructure to plug the non contended robust futex unlock race
When the FUTEX_ROBUST_UNLOCK mechanism is used for unlocking (PI-)futexes, then the unlock sequence in user space looks like this:
1) robust_list_set_op_pending(mutex); 2) robust_list_remove(mutex); lval = gettid(); 3) if (atomic_try_cmpxchg(&mutex->lock, lval, 0)) 4) robust_list_clear_op_pending(); else 5) sys_futex(OP | FUTEX_ROBUST_UNLOCK, ....);
That still leaves a minimal race window between #3 and #4 where the mutex could be acquired by some other task, which observes that it is the last user and:
1) unmaps the mutex memory
2) maps a different file, which ends up covering the same address
When then the original task exits before reaching #5 then the kernel robust list handling observes the pending op entry and tries to fix up user space.
In case that the newly mapped data contains the TID of the exiting thread at the address of the mutex/futex the kernel will set the owner died bit in that memory and therefore corrupt unrelated data.
On X86 this boils down to this simplified assembly sequence:
mov %esi,%eax // Load TID into EAX
xor %ecx,%ecx // Set ECX to 0
#3 lock cmpxchg %ecx,(%rdi) // Try the TID -> 0 transition
.Lstart:
jnz .Lend
#4 movq %rcx,(%rdx) // Clear list_op_pending
.Lend:If the cmpxchg() succeeds and the task is interrupted before it can clear list_op_pending in the robust list head (#4) and the task crashes in a signal handler or gets killed then it ends up in do_exit() and subsequently in the robust list handling, which then might run into the unmap/map issue described above.
This is only relevant when user space was interrupted and a signal is pending. The fix-up has to be done before signal delivery is attempted because:
1) The signal might be fatal so get_signal() ends up in do_exit()
2) The signal handler might crash or the task is killed before returning from the handler. At that point the instruction pointer in pt_regs is not longer the instruction pointer of the initially interrupted unlock sequence.
The right place to handle this is in __exit_to_user_mode_loop() before invoking arch_do_signal_or_restart() as this covers obviously both scenarios.
As this is only relevant when the task was interrupted in user space, this is tied to RSEQ and the generic entry code as RSEQ keeps track of user space interrupts unconditionally even if the task does not have a RSEQ region installed. That makes the decision very lightweight:
if (current->rseq.user_irq && within(regs, csr->unlock_ip_range))
futex_fixup_robust_unlock(regs, csr);futex_fixup_robust_unlock() then invokes a architecture specific function to return the pending op pointer or NULL. The function evaluates the register content to decide whether the pending ops pointer in the robust list head needs to be cleared.
Assuming the above unlock sequence, then on x86 this decision is the trivial evaluation of the zero flag:
return regs->eflags & X86_EFLAGS_ZF ? regs->dx : NULL;
Other architectures might need to do more complex evaluations due to LLSC, but the approach is valid in general. The size of the pointer is determined from the matching range struct, which covers both 32-bit and 64-bit builds including COMPAT.
The unlock sequence is going to be placed in the VDSO so that the kernel can keep everything synchronized, especially the register usage. The resulting code sequence for user space is:
if (__vdso_futex_robust_list$SZ_try_unlock(lock, tid, &pending_op) != tid)
err = sys_futex($OP | FUTEX_ROBUST_UNLOCK,....);
Both the VDSO unlock and the kernel side unlock ensure that the pending_op pointer is always cleared when the lock becomes unlocked.
--- include/linux/futex.h | 39 +++++++++++++++++++++++++++++++- include/vdso/futex.h | 52 ++++++++++++++++++++++++++++++++++++++++++- kernel/entry/common.c | 9 ++++--- kernel/futex/core.c | 18 +++++++++++++++- 4 files changed, 114 insertions(+), 4 deletions(-) create mode 100644 include/vdso/futex.h
include/linux/futex.h +38 -1
@@ -105,7 +105,41 @@ static inline int futex_hash_free(struct mm_struct *mm) { return 0; }#endif /* !CONFIG_FUTEX */ #ifdef CONFIG_FUTEX_ROBUST_UNLOCK +#include <asm/futex_robust.h> +void futex_reset_cs_ranges (struct futex_mm_data *fd); +void __futex_fixup_robust_unlock (struct pt_regs *regs,struct futex_unlock_cs_range *csr); + +static inline bool futex_within_robust_unlock (struct pt_regs *regs, +struct futex_unlock_cs_range *csr) +{ +unsigned long ip =instruction_pointer (regs); + +return ip >= csr->start_ip && ip < csr->start_ip + csr->len; +} + +static inline void futex_fixup_robust_unlock (struct pt_regs *regs) +{ +struct futex_unlock_cs_range *csr; + +/* +* Avoid dereferencing current->mm if not returning from interrupt. +* current->rseq.event is going to be used subsequently, so bringing the +* cache line in is not a big deal. +*/ +if (!current->rseq.event.user_irq) +return ; + + csr = current->mm->futex.unlock.cs_ranges; + +/* The loop is optimized out for !COMPAT */ +for (int r =0 ; r <FUTEX_ROBUST_MAX_CS_RANGES ; r++, csr++) { +if (unlikely (futex_within_robust_unlock (regs, csr))) { +__futex_fixup_robust_unlock (regs, csr); +return ; + } + } +}static inline void futex_set_vdso_cs_range (struct futex_mm_data *fd,unsigned int idx,unsigned long start,unsigned long end,bool sz32)
@@ -114,7 +148,10 @@ static inline void futex_set_vdso_cs_range(struct futex_mm_data *fd, unsigned in fd->unlock.cs_ranges[idx].len = end - start; fd->unlock.cs_ranges[idx].pop_size32 = sz32; } -#endif /* CONFIG_FUTEX_ROBUST_UNLOCK */ +#else /* CONFIG_FUTEX_ROBUST_UNLOCK */ +static inline void futex_fixup_robust_unlock (struct pt_regs *regs) { } +#endif /* !CONFIG_FUTEX_ROBUST_UNLOCK */ +#if defined(CONFIG_FUTEX_PRIVATE_HASH ) || defined(CONFIG_FUTEX_ROBUST_UNLOCK )void futex_mm_init (struct mm_struct *mm);
include/vdso/futex.h new file +52 -0
52 lines not on this page · raw message
kernel/entry/common.c +6 -3
@@ -1,11 +1,12 @@// SPDX-License-Identifier: GPL-2.0 -#include <linux/irq-entry-common.h> -#include <linux/resume_user_mode.h> +#include <linux/futex.h> #include <linux/highmem.h> +#include <linux/irq-entry-common.h> #include <linux/jump_label.h> #include <linux/kmsan.h> #include <linux/livepatch.h> +#include <linux/resume_user_mode.h> #include <linux/tick.h> /* Workaround to allow gradual conversion of architecture code */
@@ -60,8 +61,10 @@ static __always_inline unsigned long __exit_to_user_mode_loop(struct pt_regs *reif (ti_work & _TIF_PATCH_PENDING)klp_update_patch_state (current); -if (ti_work & (_TIF_SIGPENDING | _TIF_NOTIFY_SIGNAL)) +if (ti_work & (_TIF_SIGPENDING | _TIF_NOTIFY_SIGNAL)) { +futex_fixup_robust_unlock (regs);arch_do_signal_or_restart (regs); + }if (ti_work & _TIF_NOTIFY_RESUME)resume_user_mode_work (regs);
kernel/futex/core.c +18 -0
@@ -46,6 +46,8 @@#include <linux/slab.h> #include <linux/vmalloc.h> +#include <vdso/futex.h> +#include "futex.h" #include "../locking/rtmutex_common.h"
@@ -1446,6 +1448,22 @@ bool futex_robust_list_clear_pending(void __user *pop, unsigned int flags)return robust_list_clear_pending (pop); } +#ifdef CONFIG_FUTEX_ROBUST_UNLOCK +void __futex_fixup_robust_unlock (struct pt_regs *regs,struct futex_unlock_cs_range *csr) +{ +/* +* arch_futex_robust_unlock_get_pop() returns the list pending op pointer from +* @regs if the try_cmpxchg() succeeded. +*/ +void __user *pop =arch_futex_robust_unlock_get_pop (regs); + +if (!pop) +return ; + +futex_robust_list_clear_pending (pop, csr->pop_size32 ?FLAGS_ROBUST_LIST32 :0 ); +} +#endif /* CONFIG_FUTEX_ROBUST_UNLOCK */ +static void futex_cleanup (struct task_struct *tsk) {if (unlikely (tsk->futex.robust_list)) {
Thomas Gleixner
maintainerPATCH v5 12/16 x86/vdso: Prepare for robust futex unlock support
In reply to Thomas GleixnerThere will be a VDSO function to unlock non-contended robust futexes in user space. The unlock sequence is racy vs. clearing the list_pending_op pointer in the task's robust list head. To plug this race the kernel needs to know the critical section window so it can clear the pointer when the task is interrupted within that race window. The window is determined by labels in the inline assembly.
Add these symbols to the vdso2c generator and use them in the VDSO VMA code to update the critical section addresses in mm_struct::futex on (re)map().
The symbols are not exported to user space, but available in the debug version of the vDSO.
---
V3: Rename the symbols once more
V2: Rename the symbols
--- arch/x86/entry/vdso/vma.c | 29 +++++++++++++++++++++++++++++ arch/x86/include/asm/vdso.h | 4 ++++ arch/x86/tools/vdso2c.c | 16 ++++++++++------ 3 files changed, 43 insertions(+), 6 deletions(-)
arch/x86/entry/vdso/vma.c +29 -0
@@ -6,6 +6,7 @@*/ #include <linux/mm.h> #include <linux/err.h> +#include <linux/futex.h> #include <linux/sched.h> #include <linux/sched/task_stack.h> #include <linux/slab.h>
@@ -73,6 +74,31 @@ static void vdso_fix_landing(const struc regs->ip = new_vma->vm_start + ipoffset; } +#ifdef CONFIG_FUTEX_ROBUST_UNLOCK +static void vdso_futex_robust_unlock_update_ips (void ) +{ +const struct vdso_image *image = current->mm->context.vdso_image; +unsigned long vdso = (unsigned long ) current->mm->context.vdso; +struct futex_mm_data *fd = ¤t->mm->futex; +unsigned int idx =0 ; + +futex_reset_cs_ranges (fd); + +#ifdef CONFIG_X86_64 +futex_set_vdso_cs_range (fd, idx, vdso + image->sym___futex_list64_try_unlock_cs_start, + vdso + image->sym___futex_list64_try_unlock_cs_end, false); + idx++; +#endif /* CONFIG_X86_64 */ + +#if defined(CONFIG_X86_32 ) || defined(CONFIG_COMPAT ) +futex_set_vdso_cs_range (fd, idx, vdso + image->sym___futex_list32_try_unlock_cs_start, + vdso + image->sym___futex_list32_try_unlock_cs_end, true); +#endif /* CONFIG_X86_32 || CONFIG_COMPAT */ +} +#else +static inline void vdso_futex_robust_unlock_update_ips (void ) { } +#endif +static int vdso_mremap (const struct vm_special_mapping *sm,struct vm_area_struct *new_vma) {
@@ -80,6 +106,7 @@ static int vdso_mremap(const struct vm_svdso_fix_landing (image, new_vma); current->mm->context.vdso = (void __user *)new_vma->vm_start; +vdso_futex_robust_unlock_update_ips ();return 0 ; }
@@ -185,6 +212,8 @@ static int map_vdso(const struct vdso_im current->mm->context.vdso = (void __user *)text_start; current->mm->context.vdso_image = image; +vdso_futex_robust_unlock_update_ips (); + up_fail:mmap_write_unlock (mm);return ret;
arch/x86/include/asm/vdso.h +4 -0
@@ -24,6 +24,10 @@ struct vdso_image {long sym_int80_landing_pad;long sym_vdso32_sigreturn_landing_pad;long sym_vdso32_rt_sigreturn_landing_pad; +long sym___futex_list64_try_unlock_cs_start; +long sym___futex_list64_try_unlock_cs_end; +long sym___futex_list32_try_unlock_cs_start; +long sym___futex_list32_try_unlock_cs_end; };extern const struct vdso_image vdso64_image;
arch/x86/tools/vdso2c.c +10 -6
@@ -75,12 +75,16 @@ struct vdso_sym { };struct vdso_sym required_syms[] = { - {"__kernel_vsyscall" , true}, - {"__kernel_sigreturn" , true}, - {"__kernel_rt_sigreturn" , true}, - {"int80_landing_pad" , true}, - {"vdso32_rt_sigreturn_landing_pad" , true}, - {"vdso32_sigreturn_landing_pad" , true}, + {"__kernel_vsyscall" , true}, + {"__kernel_sigreturn" , true}, + {"__kernel_rt_sigreturn" , true}, + {"int80_landing_pad" , true}, + {"vdso32_rt_sigreturn_landing_pad" , true}, + {"vdso32_sigreturn_landing_pad" , true}, + {"__futex_list64_try_unlock_cs_start" , true}, + {"__futex_list64_try_unlock_cs_end" , true}, + {"__futex_list32_try_unlock_cs_start" , true}, + {"__futex_list32_try_unlock_cs_end" , true}, }; __attribute__((format(printf, 1, 2))) __attribute__((noreturn))
[tip: locking/core] x86/vdso: Prepare for robust futex unlock support
The following commit has been merged into the locking/core branch of tip:
The following commit has been merged into the locking/core branch of tip:
Commit-ID: 61cfc8e372d1971e0a96d3f1f8b5ee29916b3385 Gitweb: https://git.kernel.org/tip/61cfc8e372d1971e0a96d3f1f8b5ee29916b3385 Author: Thomas Gleixner <[email protected]> AuthorDate: Tue, 02 Jun 2026 11:10:08 +02:00 Committer: Peter Zijlstra <[email protected]> CommitterDate: Wed, 03 Jun 2026 11:38:52 +02:00
x86/vdso: Prepare for robust futex unlock support
There will be a VDSO function to unlock non-contended robust futexes in user space. The unlock sequence is racy vs. clearing the list_pending_op pointer in the task's robust list head. To plug this race the kernel needs to know the critical section window so it can clear the pointer when the task is interrupted within that race window. The window is determined by labels in the inline assembly.
Add these symbols to the vdso2c generator and use them in the VDSO VMA code to update the critical section addresses in mm_struct::futex on (re)map().
The symbols are not exported to user space, but available in the debug version of the vDSO.
--- arch/x86/entry/vdso/vma.c | 29 +++++++++++++++++++++++++++++ arch/x86/include/asm/vdso.h | 4 ++++ arch/x86/tools/vdso2c.c | 16 ++++++++++------ 3 files changed, 43 insertions(+), 6 deletions(-)
arch/x86/entry/vdso/vma.c +29 -0
@@ -6,6 +6,7 @@*/ #include <linux/mm.h> #include <linux/err.h> +#include <linux/futex.h> #include <linux/sched.h> #include <linux/sched/task_stack.h> #include <linux/slab.h>
@@ -73,6 +74,31 @@ static void vdso_fix_landing(const struct vdso_image *image, regs->ip = new_vma->vm_start + ipoffset; } +#ifdef CONFIG_FUTEX_ROBUST_UNLOCK +static void vdso_futex_robust_unlock_update_ips (void ) +{ +const struct vdso_image *image = current->mm->context.vdso_image; +unsigned long vdso = (unsigned long ) current->mm->context.vdso; +struct futex_mm_data *fd = ¤t->mm->futex; +unsigned int idx =0 ; + +futex_reset_cs_ranges (fd); + +#ifdef CONFIG_X86_64 +futex_set_vdso_cs_range (fd, idx, vdso + image->sym___futex_list64_try_unlock_cs_start, + vdso + image->sym___futex_list64_try_unlock_cs_end, false); + idx++; +#endif /* CONFIG_X86_64 */ + +#if defined(CONFIG_X86_32 ) || defined(CONFIG_COMPAT ) +futex_set_vdso_cs_range (fd, idx, vdso + image->sym___futex_list32_try_unlock_cs_start, + vdso + image->sym___futex_list32_try_unlock_cs_end, true); +#endif /* CONFIG_X86_32 || CONFIG_COMPAT */ +} +#else +static inline void vdso_futex_robust_unlock_update_ips (void ) { } +#endif +static int vdso_mremap (const struct vm_special_mapping *sm,struct vm_area_struct *new_vma) {
@@ -80,6 +106,7 @@ static int vdso_mremap(const struct vm_special_mapping *sm,vdso_fix_landing (image, new_vma); current->mm->context.vdso = (void __user *)new_vma->vm_start; +vdso_futex_robust_unlock_update_ips ();return 0 ; }
@@ -185,6 +212,8 @@ static int map_vdso(const struct vdso_image *image, unsigned long addr) current->mm->context.vdso = (void __user *)text_start; current->mm->context.vdso_image = image; +vdso_futex_robust_unlock_update_ips (); + up_fail:mmap_write_unlock (mm);return ret;
arch/x86/include/asm/vdso.h +4 -0
@@ -24,6 +24,10 @@ struct vdso_image {long sym_int80_landing_pad;long sym_vdso32_sigreturn_landing_pad;long sym_vdso32_rt_sigreturn_landing_pad; +long sym___futex_list64_try_unlock_cs_start; +long sym___futex_list64_try_unlock_cs_end; +long sym___futex_list32_try_unlock_cs_start; +long sym___futex_list32_try_unlock_cs_end; };extern const struct vdso_image vdso64_image;
arch/x86/tools/vdso2c.c +10 -6
@@ -75,12 +75,16 @@ struct vdso_sym { };struct vdso_sym required_syms[] = { - {"__kernel_vsyscall" , true}, - {"__kernel_sigreturn" , true}, - {"__kernel_rt_sigreturn" , true}, - {"int80_landing_pad" , true}, - {"vdso32_rt_sigreturn_landing_pad" , true}, - {"vdso32_sigreturn_landing_pad" , true}, + {"__kernel_vsyscall" , true}, + {"__kernel_sigreturn" , true}, + {"__kernel_rt_sigreturn" , true}, + {"int80_landing_pad" , true}, + {"vdso32_rt_sigreturn_landing_pad" , true}, + {"vdso32_sigreturn_landing_pad" , true}, + {"__futex_list64_try_unlock_cs_start" , true}, + {"__futex_list64_try_unlock_cs_end" , true}, + {"__futex_list32_try_unlock_cs_start" , true}, + {"__futex_list32_try_unlock_cs_end" , true}, }; __attribute__((format(printf, 1, 2))) __attribute__((noreturn))
Thomas Gleixner
maintainerPATCH v5 13/16 x86/vdso: Implement __vdso_futex_robust_try_unlock()
In reply to Thomas GleixnerWhen the FUTEX_ROBUST_UNLOCK mechanism is used for unlocking (PI-)futexes, then the unlock sequence in userspace looks like this:
1) robust_list_set_op_pending(mutex); 2) robust_list_remove(mutex); lval = gettid(); 3) if (atomic_try_cmpxchg(&mutex->lock, lval, 0)) 4) robust_list_clear_op_pending(); else 5) sys_futex(OP,...FUTEX_ROBUST_UNLOCK);
That still leaves a minimal race window between #3 and #4 where the mutex could be acquired by some other task which observes that it is the last user and:
1) unmaps the mutex memory
2) maps a different file, which ends up covering the same address
When then the original task exits before reaching #5 then the kernel robust list handling observes the pending op entry and tries to fix up user space.
In case that the newly mapped data contains the TID of the exiting thread at the address of the mutex/futex the kernel will set the owner died bit in that memory and therefore corrupt unrelated data.
Provide a VDSO function which exposes the critical section window in the VDSO symbol table. The resulting addresses are updated in the task's mm when the VDSO is (re)map()'ed.
The core code detects when a task was interrupted within the critical section and is about to deliver a signal. It then invokes an architecture specific function which determines whether the pending op pointer has to be cleared or not. The unlock assembly sequence on 64-bit is:
mov %esi,%eax // Load TID into EAX
xor %ecx,%ecx // Set ECX to 0
lock cmpxchg %ecx,(%rdi) // Try the TID -> 0 transition
.Lstart:
jnz .Lend
movq %rcx,(%rdx) // Clear list_op_pending .Lend:
ret
So the decision can be simply based on the ZF state in regs->flags. The pending op pointer is always in DX independent of the build mode (32/64-bit) to make the pending op pointer retrieval uniform. The size of the pointer is stored in the matching criticial section range struct and the core code retrieves it from there. So the pointer retrieval function does not have to care. It is bit-size independent:
return regs->flags & X86_EFLAGS_ZF ? regs->dx : NULL;
There are two entry points to handle the different robust list pending op pointer size:
__vdso_futex_robust_list64_try_unlock() __vdso_futex_robust_list32_try_unlock()
The 32-bit VDSO provides only __vdso_futex_robust_list32_try_unlock().
The 64-bit VDSO provides always __vdso_futex_robust_list64_try_unlock() and when COMPAT is enabled also the list32 variant, which is required to support multi-size robust list pointers used by gaming emulators.
The unlock function is inspired by an idea from Mathieu Desnoyers.
--
V3: Use 'r' for the zero register - Uros
V2: Provide different entry points - Florian
Use __u32 and __x86_64__ - Thomas
Use private labels - Thomas
Optimize assembly - Uros
Split the functions up now that ranges are supported in the core and document the actual assembly.
--- arch/x86/Kconfig | 1 arch/x86/entry/vdso/common/vfutex.c | 71 +++++++++++++++++++++++++++++++ arch/x86/entry/vdso/vdso32/Makefile | 5 +- arch/x86/entry/vdso/vdso32/vdso32.lds.S | 3 + arch/x86/entry/vdso/vdso32/vfutex.c | 1 arch/x86/entry/vdso/vdso64/Makefile | 7 +-- arch/x86/entry/vdso/vdso64/vdso64.lds.S | 7 +++ arch/x86/entry/vdso/vdso64/vdsox32.lds.S | 7 +++ arch/x86/entry/vdso/vdso64/vfutex.c | 1 arch/x86/include/asm/futex_robust.h | 19 ++++++++ 10 files changed, 117 insertions(+), 5 deletions(-)
arch/x86/Kconfig +1 -0
@@ -239,6 +239,7 @@ config X86 select HAVE_EFFICIENT_UNALIGNED_ACCESS select HAVE_EISA if X86_32 select HAVE_EXIT_THREAD + select HAVE_FUTEX_ROBUST_UNLOCK select HAVE_GENERIC_TIF_BITS select HAVE_GUP_FAST select HAVE_FENTRY if X86_64 || DYNAMIC_FTRACE
arch/x86/entry/vdso/common/vfutex.c new file +71 -0
71 lines not on this page · raw message
arch/x86/entry/vdso/vdso32/Makefile +3 -2
@@ -7,8 +7,9 @@ vdsos-y := 32 # Files to link into the vDSO: -vobjs-y := note.o vclock_gettime.o vgetcpu.o -vobjs-y += system_call.o sigreturn.o +vobjs-y := note.o vclock_gettime.o vgetcpu.o +vobjs-y += system_call.o sigreturn.o +vobjs-$(CONFIG_FUTEX_ROBUST_UNLOCK) += vfutex.o # Compilation flags flags-y := -DBUILD_VDSO32 -m32 -mregparm=0
arch/x86/entry/vdso/vdso32/vdso32.lds.S +3 -0
@@ -30,6 +30,9 @@ VERSION __vdso_clock_gettime64; __vdso_clock_getres_time64; __vdso_getcpu; +#ifdef CONFIG_FUTEX_ROBUST_UNLOCK + __vdso_futex_robust_list32_try_unlock; +#endif }; LINUX_2.5 {
arch/x86/entry/vdso/vdso32/vfutex.c new file +1 -0
@@ -0,0 +1 @@ +#include "common/vfutex.c"
arch/x86/entry/vdso/vdso64/Makefile +4 -3
@@ -8,9 +8,10 @@ vdsos-y := 64 vdsos-$(CONFIG_X86_X32_ABI) += x32 # Files to link into the vDSO: -vobjs-y := note.o vclock_gettime.o vgetcpu.o -vobjs-y += vgetrandom.o vgetrandom-chacha.o -vobjs-$(CONFIG_X86_SGX) += vsgx.o +vobjs-y := note.o vclock_gettime.o vgetcpu.o +vobjs-y += vgetrandom.o vgetrandom-chacha.o +vobjs-$(CONFIG_X86_SGX) += vsgx.o +vobjs-$(CONFIG_FUTEX_ROBUST_UNLOCK) += vfutex.o # Compilation flags flags-y := -DBUILD_VDSO64 -m64 -mcmodel=small
arch/x86/entry/vdso/vdso64/vdso64.lds.S +7 -0
@@ -32,6 +32,13 @@ VERSION { #endif getrandom; __vdso_getrandom; + +#ifdef CONFIG_FUTEX_ROBUST_UNLOCK + __vdso_futex_robust_list64_try_unlock; +#ifdef CONFIG_COMPAT + __vdso_futex_robust_list32_try_unlock; +#endif +#endif local: *; }; }
arch/x86/entry/vdso/vdso64/vdsox32.lds.S +7 -0
@@ -22,6 +22,13 @@ VERSION { __vdso_getcpu; __vdso_time; __vdso_clock_getres; + +#ifdef CONFIG_FUTEX_ROBUST_UNLOCK + __vdso_futex_robust_list64_try_unlock; +#ifdef CONFIG_COMPAT + __vdso_futex_robust_list32_try_unlock; +#endif +#endif local: *; }; }
arch/x86/entry/vdso/vdso64/vfutex.c new file +1 -0
@@ -0,0 +1 @@ +#include "common/vfutex.c"
arch/x86/include/asm/futex_robust.h new file +19 -0
@@ -0,0 +1,19 @@ +/* SPDX-License-Identifier: GPL-2.0 */ +#ifndef _ASM_X86_FUTEX_ROBUST_H +#define _ASM_X86_FUTEX_ROBUST_H + +#include <asm/ptrace.h> + +static __always_inline void __user *x86_futex_robust_unlock_get_pop(struct pt_regs *regs) +{ +/* +* If ZF is set then the cmpxchg succeeded and the pending op pointer +* needs to be cleared. +*/ +return regs->flags &X86_EFLAGS_ZF ? (void __user *)regs->dx :NULL ; +} + +#define arch_futex_robust_unlock_get_pop (regs) \ + x86_futex_robust_unlock_get_pop(regs) + +#endif /* _ASM_X86_FUTEX_ROBUST_H */
[tip: locking/core] x86/vdso: Implement __vdso_futex_robust_try_unlock()
The following commit has been merged into the locking/core branch of tip:
The following commit has been merged into the locking/core branch of tip:
Commit-ID: a2274cc0091ed4fdce10fad68d08c529b8d3e7dd Gitweb: https://git.kernel.org/tip/a2274cc0091ed4fdce10fad68d08c529b8d3e7dd Author: Thomas Gleixner <[email protected]> AuthorDate: Tue, 02 Jun 2026 11:10:12 +02:00 Committer: Peter Zijlstra <[email protected]> CommitterDate: Wed, 03 Jun 2026 11:38:52 +02:00
x86/vdso: Implement __vdso_futex_robust_try_unlock()
When the FUTEX_ROBUST_UNLOCK mechanism is used for unlocking (PI-)futexes, then the unlock sequence in userspace looks like this:
1) robust_list_set_op_pending(mutex); 2) robust_list_remove(mutex); lval = gettid(); 3) if (atomic_try_cmpxchg(&mutex->lock, lval, 0)) 4) robust_list_clear_op_pending(); else 5) sys_futex(OP,...FUTEX_ROBUST_UNLOCK);
That still leaves a minimal race window between #3 and #4 where the mutex could be acquired by some other task which observes that it is the last user and:
1) unmaps the mutex memory
2) maps a different file, which ends up covering the same address
When then the original task exits before reaching #5 then the kernel robust list handling observes the pending op entry and tries to fix up user space.
In case that the newly mapped data contains the TID of the exiting thread at the address of the mutex/futex the kernel will set the owner died bit in that memory and therefore corrupt unrelated data.
Provide a VDSO function which exposes the critical section window in the VDSO symbol table. The resulting addresses are updated in the task's mm when the VDSO is (re)map()'ed.
The core code detects when a task was interrupted within the critical section and is about to deliver a signal. It then invokes an architecture specific function which determines whether the pending op pointer has to be cleared or not. The unlock assembly sequence on 64-bit is:
mov %esi,%eax // Load TID into EAX
xor %ecx,%ecx // Set ECX to 0
lock cmpxchg %ecx,(%rdi) // Try the TID -> 0 transition
.Lstart:
jnz .Lend
movq %rcx,(%rdx) // Clear list_op_pending .Lend:
ret
So the decision can be simply based on the ZF state in regs->flags. The pending op pointer is always in DX independent of the build mode (32/64-bit) to make the pending op pointer retrieval uniform. The size of the pointer is stored in the matching criticial section range struct and the core code retrieves it from there. So the pointer retrieval function does not have to care. It is bit-size independent:
return regs->flags & X86_EFLAGS_ZF ? regs->dx : NULL;
There are two entry points to handle the different robust list pending op pointer size:
__vdso_futex_robust_list64_try_unlock() __vdso_futex_robust_list32_try_unlock()
The 32-bit VDSO provides only __vdso_futex_robust_list32_try_unlock().
The 64-bit VDSO provides always __vdso_futex_robust_list64_try_unlock() and when COMPAT is enabled also the list32 variant, which is required to support multi-size robust list pointers used by gaming emulators.
The unlock function is inspired by an idea from Mathieu Desnoyers.
--- arch/x86/Kconfig | 1 +- arch/x86/entry/vdso/common/vfutex.c | 71 +++++++++++++++++++++++- arch/x86/entry/vdso/vdso32/Makefile | 5 +- arch/x86/entry/vdso/vdso32/vdso32.lds.S | 3 +- arch/x86/entry/vdso/vdso32/vfutex.c | 1 +- arch/x86/entry/vdso/vdso64/Makefile | 7 +- arch/x86/entry/vdso/vdso64/vdso64.lds.S | 7 ++- arch/x86/entry/vdso/vdso64/vdsox32.lds.S | 7 ++- arch/x86/entry/vdso/vdso64/vfutex.c | 1 +- arch/x86/include/asm/futex_robust.h | 19 ++++++- 10 files changed, 117 insertions(+), 5 deletions(-) create mode 100644 arch/x86/entry/vdso/common/vfutex.c create mode 100644 arch/x86/entry/vdso/vdso32/vfutex.c create mode 100644 arch/x86/entry/vdso/vdso64/vfutex.c create mode 100644 arch/x86/include/asm/futex_robust.h
arch/x86/Kconfig +1 -0
@@ -239,6 +239,7 @@ config X86 select HAVE_EFFICIENT_UNALIGNED_ACCESS select HAVE_EISA if X86_32 select HAVE_EXIT_THREAD + select HAVE_FUTEX_ROBUST_UNLOCK select HAVE_GENERIC_TIF_BITS select HAVE_GUP_FAST select HAVE_FENTRY if X86_64 || DYNAMIC_FTRACE
arch/x86/entry/vdso/common/vfutex.c new file +71 -0
71 lines not on this page · raw message
arch/x86/entry/vdso/vdso32/Makefile +3 -2
@@ -7,8 +7,9 @@ vdsos-y := 32 # Files to link into the vDSO: -vobjs-y := note.o vclock_gettime.o vgetcpu.o -vobjs-y += system_call.o sigreturn.o +vobjs-y := note.o vclock_gettime.o vgetcpu.o +vobjs-y += system_call.o sigreturn.o +vobjs-$(CONFIG_FUTEX_ROBUST_UNLOCK) += vfutex.o # Compilation flags flags-y := -DBUILD_VDSO32 -m32 -mregparm=0
arch/x86/entry/vdso/vdso32/vdso32.lds.S +3 -0
@@ -30,6 +30,9 @@ VERSION __vdso_clock_gettime64; __vdso_clock_getres_time64; __vdso_getcpu; +#ifdef CONFIG_FUTEX_ROBUST_UNLOCK + __vdso_futex_robust_list32_try_unlock; +#endif }; LINUX_2.5 {
arch/x86/entry/vdso/vdso32/vfutex.c new file +1 -0
@@ -0,0 +1 @@ +#include "common/vfutex.c"
arch/x86/entry/vdso/vdso64/Makefile +4 -3
@@ -8,9 +8,10 @@ vdsos-y := 64 vdsos-$(CONFIG_X86_X32_ABI) += x32 # Files to link into the vDSO: -vobjs-y := note.o vclock_gettime.o vgetcpu.o -vobjs-y += vgetrandom.o vgetrandom-chacha.o -vobjs-$(CONFIG_X86_SGX) += vsgx.o +vobjs-y := note.o vclock_gettime.o vgetcpu.o +vobjs-y += vgetrandom.o vgetrandom-chacha.o +vobjs-$(CONFIG_X86_SGX) += vsgx.o +vobjs-$(CONFIG_FUTEX_ROBUST_UNLOCK) += vfutex.o # Compilation flags flags-y := -DBUILD_VDSO64 -m64 -mcmodel=small
arch/x86/entry/vdso/vdso64/vdso64.lds.S +7 -0
@@ -32,6 +32,13 @@ VERSION { #endif getrandom; __vdso_getrandom; + +#ifdef CONFIG_FUTEX_ROBUST_UNLOCK + __vdso_futex_robust_list64_try_unlock; +#ifdef CONFIG_COMPAT + __vdso_futex_robust_list32_try_unlock; +#endif +#endif local: *; }; }
arch/x86/entry/vdso/vdso64/vdsox32.lds.S +7 -0
@@ -22,6 +22,13 @@ VERSION { __vdso_getcpu; __vdso_time; __vdso_clock_getres; + +#ifdef CONFIG_FUTEX_ROBUST_UNLOCK + __vdso_futex_robust_list64_try_unlock; +#ifdef CONFIG_COMPAT + __vdso_futex_robust_list32_try_unlock; +#endif +#endif local: *; }; }
arch/x86/entry/vdso/vdso64/vfutex.c new file +1 -0
@@ -0,0 +1 @@ +#include "common/vfutex.c"
arch/x86/include/asm/futex_robust.h new file +19 -0
@@ -0,0 +1,19 @@ +/* SPDX-License-Identifier: GPL-2.0 */ +#ifndef _ASM_X86_FUTEX_ROBUST_H +#define _ASM_X86_FUTEX_ROBUST_H + +#include <asm/ptrace.h> + +static __always_inline void __user *x86_futex_robust_unlock_get_pop(struct pt_regs *regs) +{ +/* +* If ZF is set then the cmpxchg succeeded and the pending op pointer +* needs to be cleared. +*/ +return regs->flags &X86_EFLAGS_ZF ? (void __user *)regs->dx :NULL ; +} + +#define arch_futex_robust_unlock_get_pop (regs) \ + x86_futex_robust_unlock_get_pop(regs) + +#endif /* _ASM_X86_FUTEX_ROBUST_H */
Thomas Gleixner
maintainerPATCH v5 14/16 Documentation: futex: Add a note about robust list race condition
In reply to Thomas GleixnerFrom: André Almeida <[email protected]>
Add a note to the documentation giving a brief explanation why doing a robust futex release in userspace is racy, what should be done to avoid it and provide links to read more.
[ tglx: Fixed a few typos ]
--- Documentation/locking/robust-futex-ABI.rst | 44 +++++++++++++++++++++++++++++ 1 file changed, 44 insertions(+)
Documentation/locking/robust-futex-ABI.rst +44 -0
@@ -153,6 +153,9 @@ manipulating this list), the user code m 3) release the futex lock, and 4) clear the 'lock_op_pending' word. +Please note that the removal of a robust futex purely in userspace is +racy. Refer to the next chapter to learn more and how to avoid this. + On exit, the kernel will consider the address stored in 'list_op_pending' and the address of each 'lock word' found by walking the list starting at 'head'. For each such address, if the bottom 30
@@ -182,3 +185,44 @@ The kernel exit code will silently stop When the kernel sees a list entry whose 'lock word' doesn't have the current threads TID in the lower 30 bits, it does nothing with that entry, and goes on to the next entry. + +Robust release is racy +---------------------- + +The removal of a robust futex from the list is racy when doing it solely in +userspace. Quoting Thomas Gleixner for the explanation: + + The robust futex unlock mechanism is racy in respect to the clearing of the + robust_list_head::list_op_pending pointer because unlock and clearing the + pointer are not atomic. The race window is between the unlock and clearing + the pending op pointer. If the task is forced to exit in this window, exit + will access a potentially invalid pending op pointer when cleaning up the + robust list. That happens if another task manages to unmap the object + containing the lock before the cleanup, which results in an UAF. In the + worst case this UAF can lead to memory corruption when unrelated content + has been mapped to the same address by the time the access happens. + +A full in-depth analysis can be read at +https://lore.kernel.org/lkml/[email protected]/ + +To overcome that, the kernel needs to participate in the lock release operation. +This ensures that the release happens "atomically" with regard to releasing +the lock and removing the address from ``list_op_pending``. If the release is +interrupted by a signal, the kernel will also verify if it interrupted the +release operation. + +For the contended unlock case, where other threads are waiting for the lock +release, there's the ``FUTEX_ROBUST_UNLOCK`` operation feature flag for the +``futex()`` system call, which must be used with one of the following +operations: ``FUTEX_WAKE``, ``FUTEX_WAKE_BITSET`` or ``FUTEX_UNLOCK_PI``. +The kernel will release the lock (set the futex word to zero), clean the +``list_op_pending`` field. Then, it will proceed with the normal wake path. + +For the non-contended path, there's still a race between checking the futex word +and clearing the ``list_op_pending`` field. To solve this without the need of a +complete system call, userspace should call the virtual syscall +``__vdso_futex_robust_listXX_try_unlock()`` (where XX is either 32 or 64, +depending on the size of the pointer). If the vDSO call succeeds, it means that +it released the lock and cleared ``list_op_pending``. If it fails, that means +that there are waiters for this lock and a call to ``futex()`` syscall with +``FUTEX_ROBUST_UNLOCK`` is needed.
[tip: locking/core] Documentation: futex: Add a note about robust list race condition
The following commit has been merged into the locking/core branch of tip:
The following commit has been merged into the locking/core branch of tip:
Commit-ID: 3f63e2545978abda58f2cf7ff0d7a2942965e8cb Gitweb: https://git.kernel.org/tip/3f63e2545978abda58f2cf7ff0d7a2942965e8cb Author: André Almeida <[email protected]> AuthorDate: Tue, 02 Jun 2026 11:10:16 +02:00 Committer: Peter Zijlstra <[email protected]> CommitterDate: Wed, 03 Jun 2026 11:38:52 +02:00
Documentation: futex: Add a note about robust list race condition
Add a note to the documentation giving a brief explanation why doing a robust futex release in userspace is racy, what should be done to avoid it and provide links to read more.
[ tglx: Fixed a few typos ]
--- Documentation/locking/robust-futex-ABI.rst | 44 +++++++++++++++++++++- 1 file changed, 44 insertions(+)
Documentation/locking/robust-futex-ABI.rst +44 -0
@@ -153,6 +153,9 @@ On removal: 3) release the futex lock, and 4) clear the 'lock_op_pending' word. +Please note that the removal of a robust futex purely in userspace is +racy. Refer to the next chapter to learn more and how to avoid this. + On exit, the kernel will consider the address stored in 'list_op_pending' and the address of each 'lock word' found by walking the list starting at 'head'. For each such address, if the bottom 30
@@ -182,3 +185,44 @@ any point: When the kernel sees a list entry whose 'lock word' doesn't have the current threads TID in the lower 30 bits, it does nothing with that entry, and goes on to the next entry. + +Robust release is racy +---------------------- + +The removal of a robust futex from the list is racy when doing it solely in +userspace. Quoting Thomas Gleixner for the explanation: + + The robust futex unlock mechanism is racy in respect to the clearing of the + robust_list_head::list_op_pending pointer because unlock and clearing the + pointer are not atomic. The race window is between the unlock and clearing + the pending op pointer. If the task is forced to exit in this window, exit + will access a potentially invalid pending op pointer when cleaning up the + robust list. That happens if another task manages to unmap the object + containing the lock before the cleanup, which results in an UAF. In the + worst case this UAF can lead to memory corruption when unrelated content + has been mapped to the same address by the time the access happens. + +A full in-depth analysis can be read at +https://lore.kernel.org/lkml/[email protected]/ + +To overcome that, the kernel needs to participate in the lock release operation. +This ensures that the release happens "atomically" with regard to releasing +the lock and removing the address from ``list_op_pending``. If the release is +interrupted by a signal, the kernel will also verify if it interrupted the +release operation. + +For the contended unlock case, where other threads are waiting for the lock +release, there's the ``FUTEX_ROBUST_UNLOCK`` operation feature flag for the +``futex()`` system call, which must be used with one of the following +operations: ``FUTEX_WAKE``, ``FUTEX_WAKE_BITSET`` or ``FUTEX_UNLOCK_PI``. +The kernel will release the lock (set the futex word to zero), clean the +``list_op_pending`` field. Then, it will proceed with the normal wake path. + +For the non-contended path, there's still a race between checking the futex word +and clearing the ``list_op_pending`` field. To solve this without the need of a +complete system call, userspace should call the virtual syscall +``__vdso_futex_robust_listXX_try_unlock()`` (where XX is either 32 or 64, +depending on the size of the pointer). If the vDSO call succeeds, it means that +it released the lock and cleared ``list_op_pending``. If it fails, that means +that there are waiters for this lock and a call to ``futex()`` syscall with +``FUTEX_ROBUST_UNLOCK`` is needed.
Thomas Gleixner
maintainerPATCH v5 15/16 selftests: futex: Add tests for robust release operations
In reply to Thomas GleixnerFrom: André Almeida <[email protected]>
Add tests for __vdso_futex_robust_listXX_try_unlock() and for the futex() op FUTEX_ROBUST_UNLOCK.
Test the contended and uncontended cases for the vDSO functions and all ops combinations for FUTEX_ROBUST_UNLOCK.
[ tglx: Replace the VDSO function lookup ]
---
V3:
Replaced the VDSO lookup
Change from v2:
- Add test variants for FUTEX_ROBUST_LIST32
- Skip 64 bit tests for 32 bit builds
--- tools/testing/selftests/futex/functional/robust_list.c | 239 +++++++++++++++++ tools/testing/selftests/futex/include/futextest.h | 6 2 files changed, 245 insertions(+)
tools/testing/selftests/futex/functional/robust_list.c +239 -0
@@ -27,12 +27,15 @@#include "futextest.h" #include "../../kselftest_harness.h" +#include <dlfcn.h> #include <errno.h> #include <pthread.h> #include <signal.h> +#include <stdint.h> #include <stdatomic.h> #include <stdbool.h> #include <stddef.h> +#include <sys/auxv.h> #include <sys/mman.h> #include <sys/wait.h>
@@ -42,6 +45,10 @@#define SLEEP_US 100 +#if __SIZEOF_LONG__ ==8 +# define BUILD_64 +#endif +static pthread_barrier_t barrier, barrier2;static int set_robust_list (struct robust_list_head *head,size_t len)
@@ -54,6 +61,12 @@ static int get_robust_list(int pid, strureturn syscall (SYS_get_robust_list, pid, head, len_ptr); } +static int sys_futex_robust_unlock(_Atomic(uint32_t) *uaddr, unsigned int op, int val, +void *list_op_pending,unsigned int val3) +{ +return syscall (SYS_futex, uaddr, op, val,NULL , list_op_pending, val3,0 ); +} +/* * Basic lock struct, contains just the futex word and the robust list element * Real implementations have also a *prev to easily walk in the list
@@ -549,4 +562,230 @@ TEST(test_circular_list)ksft_test_result_pass ("%s\n" , __func__); } +/* +* Below are tests for the fix of robust release race condition. Please read the following +* thread to learn more about the issue in the first place and why the following functions fix it: +* https://lore.kernel.org/lkml/[email protected]/ +*/ + +/* +* Auxiliary code for binding the vDSO functions +*/ +static void *get_vdso_func_addr (const char *function) +{ +const char *vdso_names[] = { +"linux-vdso.so.1" ,"linux-gate.so.1" ,"linux-vdso32.so.1" ,"linux-vdso64.so.1" , + }; + +for (int i =0 ; i <ARRAY_SIZE (vdso_names); i++) { +void *vdso =dlopen (vdso_names[i],RTLD_LAZY |RTLD_LOCAL |RTLD_NOLOAD ); + +if (vdso) +return dlsym (vdso, function); + } +return NULL ; +} + +/* +* These are the real vDSO function signatures: +* +* __vdso_futex_robust_list64_try_unlock(__u32 *lock, __u32 tid, __u64 *pop) +* __vdso_futex_robust_list32_try_unlock(__u32 *lock, __u32 tid, __u32 *pop) +* +* So for the generic entry point we need to use a void pointer as the last argument +*/ +FIXTURE (vdso_unlock) +{ + uint32_t (*vdso)(_Atomic(uint32_t) *lock, uint32_t tid, void *pop); +}; + +FIXTURE_VARIANT (vdso_unlock) +{ +bool is_32; +char func_name[]; +}; + +FIXTURE_SETUP (vdso_unlock) +{ + self->vdso =get_vdso_func_addr (variant->func_name); +} + +FIXTURE_TEARDOWN (vdso_unlock) {} + +FIXTURE_VARIANT_ADD (vdso_unlock,32 ) +{ + .func_name = "__vdso_futex_robust_list32_try_unlock", + .is_32 = true, +}; + +FIXTURE_VARIANT_ADD (vdso_unlock,64 ) +{ + .func_name ="__vdso_futex_robust_list64_try_unlock" , + .is_32 = false, +}; + +/* +* Test the vDSO robust_listXX_try_unlock() for the uncontended case. The virtual syscall should +* return the thread ID of the lock owner, the lock word must be 0 and the list_op_pending should +* be NULL. +*/ +TEST_F(vdso_unlock, test_robust_try_unlock_uncontended) +{ +struct lock_struct lock = { .futex =0 }; + _Atomic(unsigned int) *futex = &lock.futex; +struct robust_list_head head; +uintptr_t exp = (uintptr_t )NULL ; +pid_t tid =gettid (); +int ret; + +if (!self->vdso) { +ksft_test_result_skip ("%s not found\n" , variant->func_name); +return ; + } + + *futex = tid; + + ret =set_list (&head); +if (ret) +ksft_test_result_fail ("set_robust_list error\n" ); + + head.list_op_pending = &lock.list; + + ret = self->vdso (futex, tid, &head.list_op_pending); + +ASSERT_EQ (ret, tid); +ASSERT_EQ (*futex,0 ); + +/* Check only the lower 32 bits for the 32-bit entry point */ +if (variant->is_32) { + exp = (uintptr_t )(unsigned long )&lock.list; + exp &= ~0xFFFFFFFFULL ; + } + +ASSERT_EQ ((uintptr_t )(unsigned long )head.list_op_pending, exp); +} + +/* +* If the lock is contended, the operation fails. The return value is the value found at the +* futex word (tid | FUTEX_WAITERS), the futex word is not modified and the list_op_pending is_32 +* not cleared. +*/ +TEST_F(vdso_unlock, test_robust_try_unlock_contended) +{ +struct lock_struct lock = { .futex =0 }; + _Atomic(unsigned int) *futex = &lock.futex; +struct robust_list_head head; +pid_t tid =gettid (); +int ret; + +if (!self->vdso) { +ksft_test_result_skip ("%s not found\n" , variant->func_name); +return ; + } + + *futex = tid |FUTEX_WAITERS ; + + ret =set_list (&head); +if (ret) +ksft_test_result_fail ("set_robust_list error\n" ); + + head.list_op_pending = &lock.list; + + ret = self->vdso (futex, tid, &head.list_op_pending); + +ASSERT_EQ (ret, tid |FUTEX_WAITERS ); +ASSERT_EQ (*futex, tid |FUTEX_WAITERS ); +ASSERT_EQ (head.list_op_pending, &lock.list); +} + +FIXTURE (futex_op) {}; + +FIXTURE_VARIANT (futex_op) +{ +unsigned int op; +unsigned int val3; +}; + +FIXTURE_SETUP (futex_op) {} + +FIXTURE_TEARDOWN (futex_op) {} + +FIXTURE_VARIANT_ADD (futex_op, wake) +{ + .op = FUTEX_WAKE, + .val3 =0 , +}; + +FIXTURE_VARIANT_ADD (futex_op, wake_bitset) +{ + .op =FUTEX_WAKE_BITSET , + .val3 = FUTEX_BITSET_MATCH_ANY, +}; + +FIXTURE_VARIANT_ADD (futex_op, unlock_pi) +{ + .op =FUTEX_UNLOCK_PI , + .val3 = 0, +}; + +FIXTURE_VARIANT_ADD (futex_op, wake32) +{ + .op =FUTEX_WAKE |FUTEX_ROBUST_LIST32 , + .val3 = 0, +}; + +FIXTURE_VARIANT_ADD (futex_op, wake_bitset32) +{ + .op =FUTEX_WAKE_BITSET |FUTEX_ROBUST_LIST32 , + .val3 = FUTEX_BITSET_MATCH_ANY, +}; + +FIXTURE_VARIANT_ADD (futex_op, unlock_pi32) +{ + .op =FUTEX_UNLOCK_PI |FUTEX_ROBUST_LIST32 , + .val3 = 0, +}; + +/* +* The syscall should return the number of tasks waken (for this test, 0), clear the futex word and +* clear list_op_pending +*/ +TEST_F(futex_op, test_futex_robust_unlock) +{ +struct lock_struct lock = { .futex =0 }; + _Atomic(unsigned int) *futex = &lock.futex; +uintptr_t exp = (uintptr_t )NULL ; +struct robust_list_head head; +pid_t tid =gettid (); +int ret; + +#ifndef BUILD_64 +if (!(variant->op &FUTEX_ROBUST_LIST32 )) { +ksft_test_result_skip ("Not supported for 32 bit build\n" ); +return ; + } +#endif + + *futex = tid |FUTEX_WAITERS ; + + ret =set_list (&head); +if (ret) +ksft_test_result_fail ("set_robust_list error\n" ); + + head.list_op_pending = &lock.list; + + ret =sys_futex_robust_unlock (futex,FUTEX_ROBUST_UNLOCK | variant->op, tid, + &head.list_op_pending, variant->val3); + +ASSERT_EQ (ret,0 ); +ASSERT_EQ (*futex,0 ); + +if (variant->op &FUTEX_ROBUST_LIST32 ) { + exp = (uint64_t )(unsigned long )&lock.list; + exp &= ~0xFFFFFFFFULL ; + } + +ASSERT_EQ ((uintptr_t )(unsigned long )head.list_op_pending, exp); +} +TEST_HARNESS_MAIN
tools/testing/selftests/futex/include/futextest.h +6 -0
@@ -38,6 +38,12 @@ typedef volatile u_int32_t futex_t;#ifndef FUTEX_CMP_REQUEUE_PI #define FUTEX_CMP_REQUEUE_PI 12#endif +#ifndef FUTEX_ROBUST_UNLOCK +#define FUTEX_ROBUST_UNLOCK 512 +#endif +#ifndef FUTEX_ROBUST_LIST32 +#define FUTEX_ROBUST_LIST32 1024 +#endif #ifndef FUTEX_WAIT_REQUEUE_PI_PRIVATE #define FUTEX_WAIT_REQUEUE_PI_PRIVATE (FUTEX_WAIT_REQUEUE_PI | \ FUTEX_PRIVATE_FLAG)
[tip: locking/core] selftests: futex: Add tests for robust release operations
The following commit has been merged into the locking/core branch of tip:
The following commit has been merged into the locking/core branch of tip:
Commit-ID: 608323bf7bb85bbb647eca4373acef247f105e67 Gitweb: https://git.kernel.org/tip/608323bf7bb85bbb647eca4373acef247f105e67 Author: André Almeida <[email protected]> AuthorDate: Tue, 02 Jun 2026 11:10:21 +02:00 Committer: Peter Zijlstra <[email protected]> CommitterDate: Wed, 03 Jun 2026 11:38:53 +02:00
selftests: futex: Add tests for robust release operations
Add tests for __vdso_futex_robust_listXX_try_unlock() and for the futex() op FUTEX_ROBUST_UNLOCK.
Test the contended and uncontended cases for the vDSO functions and all ops combinations for FUTEX_ROBUST_UNLOCK.
[ tglx: Replace the VDSO function lookup ]
--- tools/testing/selftests/futex/functional/robust_list.c | 239 ++++++++- tools/testing/selftests/futex/include/futextest.h | 6 +- 2 files changed, 245 insertions(+)
tools/testing/selftests/futex/functional/robust_list.c +239 -0
@@ -27,12 +27,15 @@#include "futextest.h" #include "../../kselftest_harness.h" +#include <dlfcn.h> #include <errno.h> #include <pthread.h> #include <signal.h> +#include <stdint.h> #include <stdatomic.h> #include <stdbool.h> #include <stddef.h> +#include <sys/auxv.h> #include <sys/mman.h> #include <sys/wait.h>
@@ -42,6 +45,10 @@#define SLEEP_US 100 +#if __SIZEOF_LONG__ ==8 +# define BUILD_64 +#endif +static pthread_barrier_t barrier, barrier2;static int set_robust_list (struct robust_list_head *head,size_t len)
@@ -54,6 +61,12 @@ static int get_robust_list(int pid, struct robust_list_head **head, size_t *len_return syscall (SYS_get_robust_list, pid, head, len_ptr); } +static int sys_futex_robust_unlock(_Atomic(uint32_t) *uaddr, unsigned int op, int val, +void *list_op_pending,unsigned int val3) +{ +return syscall (SYS_futex, uaddr, op, val,NULL , list_op_pending, val3,0 ); +} +/* * Basic lock struct, contains just the futex word and the robust list element * Real implementations have also a *prev to easily walk in the list
@@ -549,4 +562,230 @@ TEST(test_circular_list)ksft_test_result_pass ("%s\n" , __func__); } +/* +* Below are tests for the fix of robust release race condition. Please read the following +* thread to learn more about the issue in the first place and why the following functions fix it: +* https://lore.kernel.org/lkml/[email protected]/ +*/ + +/* +* Auxiliary code for binding the vDSO functions +*/ +static void *get_vdso_func_addr (const char *function) +{ +const char *vdso_names[] = { +"linux-vdso.so.1" ,"linux-gate.so.1" ,"linux-vdso32.so.1" ,"linux-vdso64.so.1" , + }; + +for (int i =0 ; i <ARRAY_SIZE (vdso_names); i++) { +void *vdso =dlopen (vdso_names[i],RTLD_LAZY |RTLD_LOCAL |RTLD_NOLOAD ); + +if (vdso) +return dlsym (vdso, function); + } +return NULL ; +} + +/* +* These are the real vDSO function signatures: +* +* __vdso_futex_robust_list64_try_unlock(__u32 *lock, __u32 tid, __u64 *pop) +* __vdso_futex_robust_list32_try_unlock(__u32 *lock, __u32 tid, __u32 *pop) +* +* So for the generic entry point we need to use a void pointer as the last argument +*/ +FIXTURE (vdso_unlock) +{ + uint32_t (*vdso)(_Atomic(uint32_t) *lock, uint32_t tid, void *pop); +}; + +FIXTURE_VARIANT (vdso_unlock) +{ +bool is_32; +char func_name[]; +}; + +FIXTURE_SETUP (vdso_unlock) +{ + self->vdso =get_vdso_func_addr (variant->func_name); +} + +FIXTURE_TEARDOWN (vdso_unlock) {} + +FIXTURE_VARIANT_ADD (vdso_unlock,32 ) +{ + .func_name = "__vdso_futex_robust_list32_try_unlock", + .is_32 = true, +}; + +FIXTURE_VARIANT_ADD (vdso_unlock,64 ) +{ + .func_name ="__vdso_futex_robust_list64_try_unlock" , + .is_32 = false, +}; + +/* +* Test the vDSO robust_listXX_try_unlock() for the uncontended case. The virtual syscall should +* return the thread ID of the lock owner, the lock word must be 0 and the list_op_pending should +* be NULL. +*/ +TEST_F(vdso_unlock, test_robust_try_unlock_uncontended) +{ +struct lock_struct lock = { .futex =0 }; + _Atomic(unsigned int) *futex = &lock.futex; +struct robust_list_head head; +uintptr_t exp = (uintptr_t )NULL ; +pid_t tid =gettid (); +int ret; + +if (!self->vdso) { +ksft_test_result_skip ("%s not found\n" , variant->func_name); +return ; + } + + *futex = tid; + + ret =set_list (&head); +if (ret) +ksft_test_result_fail ("set_robust_list error\n" ); + + head.list_op_pending = &lock.list; + + ret = self->vdso (futex, tid, &head.list_op_pending); + +ASSERT_EQ (ret, tid); +ASSERT_EQ (*futex,0 ); + +/* Check only the lower 32 bits for the 32-bit entry point */ +if (variant->is_32) { + exp = (uintptr_t )(unsigned long )&lock.list; + exp &= ~0xFFFFFFFFULL ; + } + +ASSERT_EQ ((uintptr_t )(unsigned long )head.list_op_pending, exp); +} + +/* +* If the lock is contended, the operation fails. The return value is the value found at the +* futex word (tid | FUTEX_WAITERS), the futex word is not modified and the list_op_pending is_32 +* not cleared. +*/ +TEST_F(vdso_unlock, test_robust_try_unlock_contended) +{ +struct lock_struct lock = { .futex =0 }; + _Atomic(unsigned int) *futex = &lock.futex; +struct robust_list_head head; +pid_t tid =gettid (); +int ret; + +if (!self->vdso) { +ksft_test_result_skip ("%s not found\n" , variant->func_name); +return ; + } + + *futex = tid |FUTEX_WAITERS ; + + ret =set_list (&head); +if (ret) +ksft_test_result_fail ("set_robust_list error\n" ); + + head.list_op_pending = &lock.list; + + ret = self->vdso (futex, tid, &head.list_op_pending); + +ASSERT_EQ (ret, tid |FUTEX_WAITERS ); +ASSERT_EQ (*futex, tid |FUTEX_WAITERS ); +ASSERT_EQ (head.list_op_pending, &lock.list); +} + +FIXTURE (futex_op) {}; + +FIXTURE_VARIANT (futex_op) +{ +unsigned int op; +unsigned int val3; +}; + +FIXTURE_SETUP (futex_op) {} + +FIXTURE_TEARDOWN (futex_op) {} + +FIXTURE_VARIANT_ADD (futex_op, wake) +{ + .op = FUTEX_WAKE, + .val3 =0 , +}; + +FIXTURE_VARIANT_ADD (futex_op, wake_bitset) +{ + .op =FUTEX_WAKE_BITSET , + .val3 = FUTEX_BITSET_MATCH_ANY, +}; + +FIXTURE_VARIANT_ADD (futex_op, unlock_pi) +{ + .op =FUTEX_UNLOCK_PI , + .val3 = 0, +}; + +FIXTURE_VARIANT_ADD (futex_op, wake32) +{ + .op =FUTEX_WAKE |FUTEX_ROBUST_LIST32 , + .val3 = 0, +}; + +FIXTURE_VARIANT_ADD (futex_op, wake_bitset32) +{ + .op =FUTEX_WAKE_BITSET |FUTEX_ROBUST_LIST32 , + .val3 = FUTEX_BITSET_MATCH_ANY, +}; + +FIXTURE_VARIANT_ADD (futex_op, unlock_pi32) +{ + .op =FUTEX_UNLOCK_PI |FUTEX_ROBUST_LIST32 , + .val3 = 0, +}; + +/* +* The syscall should return the number of tasks waken (for this test, 0), clear the futex word and +* clear list_op_pending +*/ +TEST_F(futex_op, test_futex_robust_unlock) +{ +struct lock_struct lock = { .futex =0 }; + _Atomic(unsigned int) *futex = &lock.futex; +uintptr_t exp = (uintptr_t )NULL ; +struct robust_list_head head; +pid_t tid =gettid (); +int ret; + +#ifndef BUILD_64 +if (!(variant->op &FUTEX_ROBUST_LIST32 )) { +ksft_test_result_skip ("Not supported for 32 bit build\n" ); +return ; + } +#endif + + *futex = tid |FUTEX_WAITERS ; + + ret =set_list (&head); +if (ret) +ksft_test_result_fail ("set_robust_list error\n" ); + + head.list_op_pending = &lock.list; + + ret =sys_futex_robust_unlock (futex,FUTEX_ROBUST_UNLOCK | variant->op, tid, + &head.list_op_pending, variant->val3); + +ASSERT_EQ (ret,0 ); +ASSERT_EQ (*futex,0 ); + +if (variant->op &FUTEX_ROBUST_LIST32 ) { + exp = (uint64_t )(unsigned long )&lock.list; + exp &= ~0xFFFFFFFFULL ; + } + +ASSERT_EQ ((uintptr_t )(unsigned long )head.list_op_pending, exp); +} +TEST_HARNESS_MAIN
tools/testing/selftests/futex/include/futextest.h +6 -0
@@ -38,6 +38,12 @@ typedef volatile u_int32_t futex_t;#ifndef FUTEX_CMP_REQUEUE_PI #define FUTEX_CMP_REQUEUE_PI 12#endif +#ifndef FUTEX_ROBUST_UNLOCK +#define FUTEX_ROBUST_UNLOCK 512 +#endif +#ifndef FUTEX_ROBUST_LIST32 +#define FUTEX_ROBUST_LIST32 1024 +#endif #ifndef FUTEX_WAIT_REQUEUE_PI_PRIVATE #define FUTEX_WAIT_REQUEUE_PI_PRIVATE (FUTEX_WAIT_REQUEUE_PI | \ FUTEX_PRIVATE_FLAG)
Thomas Gleixner
maintainerRFC PATCH v5 16/16 vdso, x86: Expose vdso.so.dbg through sysfs
In reply to Thomas GleixnerFinding the debug version of the VDSO is not trivial as there is no common scheme where it is placed. That's especially problematic for CI testing.
The VDSO futex unlock mechanism requires for testing to have access to the inner labels of the unlock assembly, which are only accessible via the debug so.
Also for general debugging purposes it's conveniant to have access to the debug VDSO at a well defined place.
The files are placed in /sys/kernel/vdso/ and named vdso32.so.dbg, vdso64.so.dbg, vdsox32.so.dbg.
--- arch/x86/Kconfig | 1 + arch/x86/include/asm/vdso.h | 3 +++ arch/x86/tools/vdso2c.c | 15 ++++++++++----- arch/x86/tools/vdso2c.h | 32 ++++++++++++++++++++++++++++++-- include/vdso/sysfs.h | 7 +++++++ lib/vdso/Kconfig | 6 ++++++ lib/vdso/Makefile | 3 ++- lib/vdso/sysfs.c | 44 ++++++++++++++++++++++++++++++++++++++++++++ 8 files changed, 103 insertions(+), 8 deletions(-)
arch/x86/Kconfig +1 -0
@@ -306,6 +306,7 @@ config X86 select HAVE_UNWIND_USER_FP if X86_64 select HAVE_USER_RETURN_NOTIFIER select HAVE_GENERIC_VDSO + select HAVE_VDSO_DEBUG_SYSFS select VDSO_GETRANDOM if X86_64 select HOTPLUG_PARALLEL if SMP && X86_64 select HOTPLUG_SMT if SMP
arch/x86/include/asm/vdso.h +3 -0
@@ -14,6 +14,9 @@ struct vdso_image {void *data;unsigned long size;/* Always a multiple of PAGE_SIZE */ +void *dbg_data; +unsigned long dbg_size;/* Always a multiple of PAGE_SIZE */ +unsigned long alt, alt_len;unsigned long extable_base, extable_len;const void *extable;
arch/x86/tools/vdso2c.c +10 -5
@@ -150,16 +150,16 @@ extern void bad_put_le(void);static void go (void *raw_addr,size_t raw_len,void *stripped_addr,size_t stripped_len, -FILE *outfile,const char *name) +FILE *outfile,const char *name,const char *dbg_name) {Elf64_Ehdr *hdr = (Elf64_Ehdr *)raw_addr;if (hdr->e_ident[EI_CLASS ] ==ELFCLASS64 ) {go64 (raw_addr, raw_len, stripped_addr, stripped_len, - outfile, name); + outfile, name, dbg_name); }else if (hdr->e_ident[EI_CLASS ] ==ELFCLASS32 ) {go32 (raw_addr, raw_len, stripped_addr, stripped_len, - outfile, name); + outfile, name, dbg_name); }else {fail ("unknown ELF class\n" ); }
@@ -189,8 +189,8 @@ int main(int argc, char **argv) {size_t raw_len, stripped_len;void *raw_addr, *stripped_addr; +char *name, *tmp, *dbg_name;FILE *outfile; -char *name, *tmp;int namelen;if (argc !=4 ) {
@@ -226,7 +226,12 @@ int main(int argc, char **argv)if (!outfile)err (1 ,"fopen(%s)" , outfilename); -go (raw_addr, raw_len, stripped_addr, stripped_len, outfile, name); + dbg_name =strdup (argv[1 ]); + tmp =strrchr (dbg_name,'/' ); +if (tmp) + dbg_name = tmp +1 ; + +go (raw_addr, raw_len, stripped_addr, stripped_len, outfile, name, dbg_name);munmap (raw_addr, raw_len);munmap (stripped_addr, stripped_len);
arch/x86/tools/vdso2c.h +30 -2
@@ -42,11 +42,12 @@ static void BITSFUNC(extract)(const unsistatic void BITSFUNC (go )(void *raw_addr,size_t raw_len,void *stripped_addr,size_t stripped_len, -FILE *outfile,const char *image_name) +FILE *outfile,const char *image_name, +const char *dbg_name) {int found_load =0 ;unsigned long load_size =-1 ;/* Work around bogus warning */ -unsigned long mapping_size; +unsigned long mapping_size, dbg_size;ELF (Ehdr) *hdr = (ELF (Ehdr) *)raw_addr;unsigned long i, syms_nr; ELF(Shdr) *symtab_hdr = NULL, *strtab_hdr, *secstrings_hdr,
@@ -160,6 +161,7 @@ static void BITSFUNC(go)(void *raw_addr,fprintf (outfile,"/* AUTOMATICALLY GENERATED -- DO NOT EDIT */\n\n" );fprintf (outfile,"#include <linux/linkage.h>\n" );fprintf (outfile,"#include <linux/init.h>\n" ); +fprintf (outfile,"#include <vdso/sysfs.h>\n" );fprintf (outfile,"#include <asm/page_types.h>\n" );fprintf (outfile,"#include <asm/vdso.h>\n" );fprintf (outfile,"\n" );
@@ -173,6 +175,21 @@ static void BITSFUNC(go)(void *raw_addr, (int)((unsigned char *)stripped_addr)[i]); }fprintf (outfile,"\n};\n\n" ); + + dbg_size = (raw_len +4095 ) /4096 *4096 ; + +fprintf (outfile,"#ifdef CONFIG_VDSO_DEBUG_SYSFS\n" ); +fprintf (outfile, +"static unsigned char dbg_data[%lu] __ro_after_init __aligned(PAGE_SIZE) = {" , + dbg_size); +for (i =0 ; i < raw_len; i++) { +if (i %10 ==0 ) +fprintf (outfile,"\n\t" ); +fprintf (outfile,"0x%02X, " , (int )((unsigned char *)raw_addr)[i]); + } +fprintf (outfile,"\n};\n" ); +fprintf (outfile,"#endif\n\n" ); +if (extable_sec)BITSFUNC (extract)(raw_addr, raw_len, outfile, extable_sec,"extable" );
@@ -180,6 +197,10 @@ static void BITSFUNC(go)(void *raw_addr,fprintf (outfile,"const struct vdso_image %s = {\n" , image_name);fprintf (outfile,"\t.data = raw_data,\n" );fprintf (outfile,"\t.size = %lu,\n" , mapping_size); +fprintf (outfile,"#ifdef CONFIG_VDSO_DEBUG_SYSFS\n" ); +fprintf (outfile,"\t.dbg_data = dbg_data,\n" ); +fprintf (outfile,"\t.dbg_size = %lu,\n" , dbg_size); +fprintf (outfile,"#endif\n" );if (alt_sec) {fprintf (outfile,"\t.alt = %lu,\n" , (unsigned long )GET_LE (&alt_sec->sh_offset));
@@ -205,4 +226,11 @@ static void BITSFUNC(go)(void *raw_addr,fprintf (outfile,"};\n" );fprintf (outfile,"subsys_initcall(init_%s);\n" , image_name); +fprintf (outfile,"\n#ifdef CONFIG_VDSO_DEBUG_SYSFS\n" ); +fprintf (outfile,"static __init int sysfs_init_%s(void) {\n" , image_name); +fprintf (outfile,"\treturn vdso_sysfs_init_image(\"%s\", (void *)%s.dbg_data, %lu);\n" , + dbg_name, image_name, raw_len); +fprintf (outfile,"};\n" ); +fprintf (outfile,"late_initcall(sysfs_init_%s);\n" , image_name); +fprintf (outfile,"#endif\n" ); }
include/vdso/sysfs.h new file +7 -0
@@ -0,0 +1,7 @@ +/* SPDX-License-Identifier: GPL-2.0 */ +#ifndef __VDSO_SYSFS_H +#define __VDSO_SYSFS_H + +int vdso_sysfs_init_image (const char *name,void *addr,unsigned int size); + +#endif /* __VDSO_SYSFS_H */
lib/vdso/Kconfig +6 -0
@@ -3,6 +3,9 @@ config HAVE_GENERIC_VDSO bool +config HAVE_VDSO_DEBUG_SYSFS + bool + if HAVE_GENERIC_VDSO config GENERIC_GETTIMEOFDAY
@@ -24,4 +27,7 @@ config VDSO_GETRANDOM help Selected by architectures that support vDSO getrandom(). +config VDSO_DEBUG_SYSFS + def_bool y if SYSFS && HAVE_VDSO_DEBUG_SYSFS + endif
lib/vdso/Makefile +2 -1
@@ -1,3 +1,4 @@ # SPDX-License-Identifier: GPL-2.0-only -obj-$(CONFIG_HAVE_GENERIC_VDSO) += datastore.o +obj-$(CONFIG_HAVE_GENERIC_VDSO) += datastore.o +obj-$(CONFIG_VDSO_DEBUG_SYSFS) += sysfs.o
lib/vdso/sysfs.c new file +44 -0
44 lines not on this page · raw message
Re: [patch V5 16/16] [RFC] vdso, x86: Expose vdso.so.dbg through sysfs
Quoted 7 lines · Thomas Gleixner
Finding the debug version of the VDSO is not trivial as there is no common scheme where it is placed. That's especially problematic for CI testing.
The VDSO futex unlock mechanism requires for testing to have access to the inner labels of the unlock assembly, which are only accessible via the debug so.
Quoted 5 lines
Also for general debugging purposes it's conveniant to have access to the debug VDSO at a well defined place.
The files are placed in
/sys/kernel/vdso/and named vdso32.so.dbg, vdso64.so.dbg, vdsox32.so.dbg.
How is a user supposed to find the correct one for a given task? As currently proposed that requires architecture-specific logic.
What about mirroring CONFIG_IKCONFIG and CONFIG_IKHEADERS, packaging the output of 'make vdso_install' as an archive and embedding that into the kernel.
It has the following advantages:
* Can be loaded on-demand from a module.
* Does not require additional (per-architecture) code.
* Contains build-id symlinks which can be followed automatically to find a task's debug vDSO.
Currently CONFIG_IKHEADERS only provide the compressed archives, and not a directly usable directory, though. That could be fine here, too.
Quoted 11 lines
Signed-off-by Thomas Gleixner <[email protected]>--- arch/x86/Kconfig | 1 + arch/x86/include/asm/vdso.h | 3 +++ arch/x86/tools/vdso2c.c | 15 ++++++++++----- arch/x86/tools/vdso2c.h | 32 ++++++++++++++++++++++++++++++-- include/vdso/sysfs.h | 7 +++++++ lib/vdso/Kconfig | 6 ++++++ lib/vdso/Makefile | 3 ++- lib/vdso/sysfs.c | 44 ++++++++++++++++++++++++++++++++++++++++++++ 8 files changed, 103 insertions(+), 8 deletions(-)
(...)
include/vdso/sysfs.h line 7
Quoted 10 lines
include/vdso/sysfs.h new file +7
-0@@ -0,0 +1,7 @@ +/* SPDX-License-Identifier: GPL-2.0 */ +#ifndef __VDSO_SYSFS_H +#define __VDSO_SYSFS_H + +int vdso_sysfs_init_image (const char *name,void *addr,unsigned int size); + +#endif /* __VDSO_SYSFS_H */
This breaks the vdso/ header namespace.
For the datastore I went with include/linux/vdso_datastore.h. Maye we can use include/linux/vdso/ ?
(...)
Thomas
Thomas Gleixner
maintainerRe: [patch V5 16/16] [RFC] vdso, x86: Expose vdso.so.dbg through sysfs
Quoted 21 lines · Thomas Weißschuh
Quoted 7 lines · Thomas Gleixner
Finding the debug version of the VDSO is not trivial as there is no common scheme where it is placed. That's especially problematic for CI testing.
The VDSO futex unlock mechanism requires for testing to have access to the inner labels of the unlock assembly, which are only accessible via the debug so.
Quoted 5 lines
Also for general debugging purposes it's conveniant to have access to the debug VDSO at a well defined place.
The files are placed in
/sys/kernel/vdso/and named vdso32.so.dbg, vdso64.so.dbg, vdsox32.so.dbg.How is a user supposed to find the correct one for a given task? As currently proposed that requires architecture-specific logic.
What about mirroring
CONFIG_IKCONFIGandCONFIG_IKHEADERS, packaging the output of 'makevdso_install' as an archive and embedding that into the kernel.
Quoted 8 lines
It has the following advantages:
* Can be loaded on-demand from a module.
* Does not require additional (per-architecture) code.
* Contains build-id symlinks which can be followed automatically to find a task's debug vDSO.Currently
CONFIG_IKHEADERSonly provide the compressed archives, and not a directly usable directory, though. That could be fine here, too.
I'm fine with any sensible solution. This one just worked for my nefarious purposes and that's why it's marked RFC. :)