x86: Enable the deferred unwinding infrastructure

From: Josh Poimboeuf <[email protected]>

Use ARCH_INIT_USER_FP_FRAME to describe how frame pointers are unwound on x86, and enable CONFIG_HAVE_UNWIND_USER_FP accordingly so the unwind_user interfaces can be used.

Signed-off-by Josh Poimboeuf <[email protected]>

Signed-off-by Steven Rostedt (Google) <[email protected]>

---
 arch/x86/Kconfig                   |  1 +
 arch/x86/include/asm/unwind_user.h | 11 +++++++++++
 2 files changed, 12 insertions(+)
 create mode 100644 arch/x86/include/asm/unwind_user.h

arch/x86/Kconfig +1 -0

index 58d890fe2100..8f94c58d4de8 100644
@@ -299,6 +299,7 @@ config X86
 	select HAVE_SYSCALL_TRACEPOINTS
 	select HAVE_UACCESS_VALIDATION		if HAVE_OBJTOOL
 	select HAVE_UNSTABLE_SCHED_CLOCK
+	select HAVE_UNWIND_USER_FP		if X86_64
 	select HAVE_USER_RETURN_NOTIFIER
 	select HAVE_GENERIC_VDSO
 	select VDSO_GETRANDOM			if X86_64

arch/x86/include/asm/unwind_user.h new file +1 -0

new file mode 100644
index 000000000000..8597857bf896
@@ -0,0 +1,11 @@
+/* SPDX-License-Identifier: GPL-2.0 */

Moo, and now you have me look at unwind/user.c:

        /* Make sure that the address is word aligned */
        shift = sizeof(long) == 4 ? 2 : 3;
        if (cfa & ((1 << shift) - 1))
                return -EINVAL;

Isn't that just:

	if (cfa & (sizeof(long) - 1))

?

Let me go add a patch to clean that up...

From: Steven Rostedt <[email protected]>

Currently compat tasks are not supported. If a deferred user space stack trace is requested on a compat task, it should fail and return an error so that the profiler can use an alternative approach (whatever it uses today).

Add a arch_unwind_can_defer() macro that is called in unwind_deferred_request(). Have x86 define it to a function that makes sure that the current task is running in 64bit mode, and if it is not, it returns false. This will cause unwind_deferred_request() to error out and the caller can use the current method of user space stack tracing.

Signed-off-by Steven Rostedt (Google) <[email protected]>

---
Changes since v1: https://lore.kernel.org/[email protected]

- Added #include <linux/compat.h> to use user_64bit_mode() in unwind_user.h. When adding this to the ftrace code, it failed to build due to the missing header.

 arch/x86/include/asm/unwind_user.h | 12 ++++++++++++
 include/linux/unwind_deferred.h    |  5 +++++
 kernel/unwind/deferred.c           |  3 +++
 3 files changed, 20 insertions(+)

arch/x86/include/asm/unwind_user.h +12 -0

index 8597857bf896..0cef2e3b08c8 100644
@@ -2,6 +2,18 @@
 #ifndef _ASM_X86_UNWIND_USER_H
 #define _ASM_X86_UNWIND_USER_H
 
+#ifdef CONFIG_IA32_EMULATION
+#include <linux/compat.h>
+/* Currently compat mode is not supported for deferred stack trace */
+static inline bool arch_unwind_can_defer(void)
+{
+	struct pt_regs *regs = task_pt_regs(current);
+
+	return user_64bit_mode(regs);
+}
+# define arch_unwind_can_defer	arch_unwind_can_defer
+#endif /* CONFIG_IA32_EMULATION */
+
 #define ARCH_INIT_USER_FP_FRAME							\
 	.cfa_off	= (s32)sizeof(long) *  2,				\
 	.ra_off		= (s32)sizeof(long) * -1,				\

include/linux/unwind_deferred.h +5 -0

index 26122d00708a..0124865aaab4 100644
@@ -16,6 +16,11 @@ struct unwind_work {
 	int				bit;
 };
 
+/* Architectures can add a test to not defer unwinding */
+#ifndef arch_unwind_can_defer
+# define arch_unwind_can_defer()	(true)
+#endif
+
 #ifdef CONFIG_UNWIND_USER
 
 enum {

kernel/unwind/deferred.c +0 -0

index dc6040aae3ee..3601b2efe48d 100644

Quoted 7 lines · Steven Rostedt

From: Steven Rostedt <[email protected]>

Currently compat tasks are not supported. If a deferred user space stack trace is requested on a compat task, it should fail and return an error so that the profiler can use an alternative approach (whatever it uses today).

Quoted 5 lines

Add a arch_unwind_can_defer() macro that is called in unwind_deferred_request(). Have x86 define it to a function that makes sure that the current task is running in 64bit mode, and if it is not, it returns false. This will cause unwind_deferred_request() to error out and the caller can use the current method of user space stack tracing.

Changelog seems to forget mentioning *why* we can't unwind compat.

I'm sure I've seen compat FP unwind support at some point in this series. Did that go missing somewhere?

Changelog seems to forget mentioning *why* we can't unwind compat.

I'm sure I've seen compat FP unwind support at some point in this series. Did that go missing somewhere?

Also, these two patches are in the wrong order, first you enable things, including compat tasks, and then you go 'whoopsie, no compats'.