x86_32: Migrate to new entry/exit paths
Subsystems (3)
Andy Lutomirski
maintainercover x86_32: Migrate to new entry/exit paths
This is a review version of the 32-bit asm-to-C migration. I think it works, but it's not yet well enough tested. I'm a lot more familiar with the 64-bit asm than the 32-bit asm.
Al is cc'd, because some of this partially reverts some of his old changes.
The vm86 stuff especially needs much more careful testing. Brian, since you're playing with vm86 now, can you take a look?
NB: Even if this code turns out to be perfect, patches 3 and 4 need to be squashed together. Patch 3 is a hack to temporarily add an assertion that the existing asm works the way I think it does. The assertion doesn't apprear to fire but, again, I need to pound on it harder.
Andy Lutomirski (7):
x86/entry/32: Remove 32-bit syscall audit optimizations x86/entry/32: Fix an incorrect comment for work_notifysig_v86 [TEMPORARY] x86/entry/32: Sanity check for work_notifysig x86/entry/32: Finish removing bogus kernel-mode check x86/vm86: Teach handle_vm86_trap to return to 32bit mode directly x86/entry/32: Use prepare_exit_to_usermode and syscall_return_slowpath x86/entry: Remove do_notify_resume, syscall_trace_leave, and their TIF
masks
arch/x86/entry/common.c | 57 ---------------- arch/x86/entry/entry_32.S | 130 ++++--------------------------------- arch/x86/include/asm/ptrace.h | 1 - arch/x86/include/asm/signal.h | 1 - arch/x86/include/asm/thread_info.h | 16 ----- arch/x86/kernel/traps.c | 12 ++++ arch/x86/kernel/vm86_32.c | 8 +-- 7 files changed, 26 insertions(+), 199 deletions(-)
Signature · 1 line
2.4.3
Andy Lutomirski
maintainerPATCH 2/7 x86/entry/32: Fix an incorrect comment for work_notifysig_v86
This code path is for returns to user mode only.
--- arch/x86/entry/entry_32.S | 3 +-- 1 file changed, 1 insertion(+), 2 deletions(-)
arch/x86/entry/entry_32.S +1 -2
@@ -484,8 +484,7 @@ work_notifysig: # deal with pending signals and #ifdef CONFIG_VM86 testl $X86_EFLAGS_VM, PT_EFLAGS(%esp) movl %esp, %eax - jnz work_notifysig_v86 # returning to kernel-space or - # vm86-space + jnz work_notifysig_v86 # special case for v86 1: #else movl %esp, %eax
Signature · 1 line
2.4.3
Andy Lutomirski
maintainerPATCH 7/7 x86/entry: Remove do_notify_resume, syscall_trace_leave, and their TIF masks
In reply to Andy LutomirskiThey are no longer used. Good riddance!
Deleting the TIF_ macros is really nice. It was never clear why there were so many variants.
--- arch/x86/entry/common.c | 57 -------------------------------------- arch/x86/include/asm/ptrace.h | 1 - arch/x86/include/asm/signal.h | 1 - arch/x86/include/asm/thread_info.h | 16 ----------- 4 files changed, 75 deletions(-)
arch/x86/entry/common.c +0 -57
@@ -207,37 +207,6 @@ long syscall_trace_enter(struct pt_regs *regs)return syscall_trace_enter_phase2 (regs, arch, phase1_result); } -/* Deprecated. */ -void syscall_trace_leave (struct pt_regs *regs) -{ -bool step; - -/* -* We may come here right after calling schedule_user() -* or do_notify_resume(), in which case we can be in RCU -* user mode. -*/ -user_exit (); - -audit_syscall_exit (regs); - -if (unlikely (test_thread_flag (TIF_SYSCALL_TRACEPOINT ))) -trace_sys_exit (regs, regs->ax); - -/* -* If TIF_SYSCALL_EMU is set, we only get here because of -* TIF_SINGLESTEP (i.e. this is PTRACE_SYSEMU_SINGLESTEP). -* We already reported this syscall instruction in -* syscall_trace_enter(). -*/ - step =unlikely (test_thread_flag (TIF_SINGLESTEP )) && - !test_thread_flag (TIF_SYSCALL_EMU ); -if (step ||test_thread_flag (TIF_SYSCALL_TRACE )) -tracehook_report_syscall_exit (regs, step); - -user_enter (); -} -static struct thread_info *pt_regs_to_thread_info (struct pt_regs *regs) { unsigned long top_of_stack =
@@ -346,29 +315,3 @@ __visible void syscall_return_slowpath(struct pt_regs *regs)local_irq_disable ();prepare_exit_to_usermode (regs); } - -/* -* Deprecated notification of userspace execution resumption -* - triggered by the TIF_WORK_MASK flags -*/ -__visible void -do_notify_resume (struct pt_regs *regs,void *unused,__u32 thread_info_flags) -{ -user_exit (); - -if (thread_info_flags & _TIF_UPROBE) -uprobe_notify_resume (regs); - -/* deal with pending signal delivery */ -if (thread_info_flags & _TIF_SIGPENDING) -do_signal (regs); - -if (thread_info_flags & _TIF_NOTIFY_RESUME) { -clear_thread_flag (TIF_NOTIFY_RESUME ); -tracehook_notify_resume (regs); - } -if (thread_info_flags & _TIF_USER_RETURN_NOTIFY) -fire_user_return_notifiers (); - -user_enter (); -}
arch/x86/include/asm/ptrace.h +0 -1
@@ -88,7 +88,6 @@ extern long syscall_trace_enter_phase2(struct pt_regs *, u32 arch, unsigned long phase1_result);extern long syscall_trace_enter (struct pt_regs *); -extern void syscall_trace_leave (struct pt_regs *);static inline unsigned long regs_return_value (struct pt_regs *regs) {
arch/x86/include/asm/signal.h +0 -1
@@ -31,7 +31,6 @@ typedef sigset_t compat_sigset_t;#include <uapi/asm/signal.h> #ifndef __ASSEMBLY__extern void do_signal (struct pt_regs *regs); -extern void do_notify_resume (struct pt_regs *,void *,__u32 );#define __ARCH_HAS_SA_RESTORER
arch/x86/include/asm/thread_info.h +0 -16
@@ -140,27 +140,11 @@ struct thread_info { _TIF_SECCOMP | _TIF_SINGLESTEP | _TIF_SYSCALL_TRACEPOINT | \ _TIF_NOHZ) -/* work to do in syscall_trace_leave() */ -#define _TIF_WORK_SYSCALL_EXIT \ - (_TIF_SYSCALL_TRACE | _TIF_SYSCALL_AUDIT | _TIF_SINGLESTEP | \ - _TIF_SYSCALL_TRACEPOINT | _TIF_NOHZ) - -/* work to do on interrupt/exception return */ -#define _TIF_WORK_MASK \ - (0x0000FFFF & \ - ~(_TIF_SYSCALL_TRACE|_TIF_SYSCALL_AUDIT| \ - _TIF_SINGLESTEP|_TIF_SECCOMP|_TIF_SYSCALL_EMU)) -/* work to do on any return to user space */ #define _TIF_ALLWORK_MASK \ ((0x0000FFFF & ~_TIF_SECCOMP) | _TIF_SYSCALL_TRACEPOINT | \ _TIF_NOHZ) -/* Only used for 64 bit */ -#define _TIF_DO_NOTIFY_MASK \ - (_TIF_SIGPENDING | _TIF_NOTIFY_RESUME | \ - _TIF_USER_RETURN_NOTIFY | _TIF_UPROBE) -/* flags to check in __switch_to() */ #define _TIF_WORK_CTXSW \ (_TIF_IO_BITMAP|_TIF_NOTSC|_TIF_BLOCKSTEP)
Signature · 1 line
2.4.3
Andy Lutomirski
maintainerPATCH 6/7 x86/entry/32: Use prepare_exit_to_usermode and syscall_return_slowpath
In reply to Andy LutomirskiThis removes the hybrid asm-and-C implementation of exit work.
--- arch/x86/entry/entry_32.S | 62 +++++++++-------------------------------------- 1 file changed, 11 insertions(+), 51 deletions(-)
arch/x86/entry/entry_32.S +11 -51
@@ -256,14 +256,10 @@ ret_from_intr: ENTRY(resume_userspace) LOCKDEP_SYS_EXIT - DISABLE_INTERRUPTS(CLBR_ANY) # make sure we don't miss an interrupt - # setting need_resched or sigpending - # between sampling and the iret + DISABLE_INTERRUPTS(CLBR_ANY) TRACE_IRQS_OFF - movl TI_flags(%ebp), %ecx - andl $_TIF_WORK_MASK, %ecx # is there any work to be done on - # int/exception return? - jne work_pending + movl %esp, %eax + call prepare_exit_to_usermode jmp restore_all END(ret_from_exception)
@@ -341,7 +337,7 @@ sysenter_after_call: TRACE_IRQS_OFF movl TI_flags(%ebp), %ecx testl $_TIF_ALLWORK_MASK, %ecx - jnz syscall_exit_work + jnz syscall_exit_work_irqs_off sysenter_exit: /* if something modifies registers it must also disable sysexit */ movl PT_EIP(%esp), %edx
@@ -377,13 +373,7 @@ syscall_after_call: movl %eax, PT_EAX(%esp) # store the return value syscall_exit: LOCKDEP_SYS_EXIT - DISABLE_INTERRUPTS(CLBR_ANY) # make sure we don't miss an interrupt - # setting need_resched or sigpending - # between sampling and the iret - TRACE_IRQS_OFF - movl TI_flags(%ebp), %ecx - testl $_TIF_ALLWORK_MASK, %ecx # current->work - jnz syscall_exit_work + jmp syscall_exit_work restore_all: TRACE_IRQS_IRET
@@ -460,35 +450,6 @@ ldt_ss: #endif ENDPROC(entry_INT80_32) - # perform work that needs to be done immediately before resumption - ALIGN -work_pending: - testb $_TIF_NEED_RESCHED, %cl - jz work_notifysig -work_resched: - call schedule - LOCKDEP_SYS_EXIT - DISABLE_INTERRUPTS(CLBR_ANY) # make sure we don't miss an interrupt - # setting need_resched or sigpending - # between sampling and the iret - TRACE_IRQS_OFF - movl TI_flags(%ebp), %ecx - andl $_TIF_WORK_MASK, %ecx # is there any work to be done other - # than syscall tracing? - jz restore_all - testb $_TIF_NEED_RESCHED, %cl - jnz work_resched - -work_notifysig: # deal with pending signals and - # notify-resume requests - movl %esp, %eax - TRACE_IRQS_ON - ENABLE_INTERRUPTS(CLBR_NONE) - xorl %edx, %edx - call do_notify_resume - jmp resume_userspace -END(work_pending) - # perform syscall exit tracing ALIGN syscall_trace_entry:
@@ -503,15 +464,14 @@ END(syscall_trace_entry) # perform syscall exit tracing ALIGN -syscall_exit_work: - testl $_TIF_WORK_SYSCALL_EXIT, %ecx - jz work_pending +syscall_exit_work_irqs_off: TRACE_IRQS_ON - ENABLE_INTERRUPTS(CLBR_ANY) # could let syscall_trace_leave() call - # schedule() instead + ENABLE_INTERRUPTS(CLBR_ANY) + +syscall_exit_work: movl %esp, %eax - call syscall_trace_leave - jmp resume_userspace + call syscall_return_slowpath + jmp restore_all END(syscall_exit_work) syscall_fault:
Signature · 1 line
2.4.3
Andy Lutomirski
maintainerPATCH 5/7 x86/vm86: Teach handle_vm86_trap to return to 32bit mode directly
In reply to Andy LutomirskiThe TIF_NOTIFY_RESUME hack it was using was buggy and unsupportable. vm86 mode was completely broken under ptrace, for example, because we'd never make it to v8086 mode.
This code is still a huge, scary mess, but at least it's no longer tangled with the exit-to-userspace loop.
--- arch/x86/entry/entry_32.S | 17 ----------------- arch/x86/kernel/traps.c | 12 ++++++++++++ arch/x86/kernel/vm86_32.c | 8 ++------ 3 files changed, 14 insertions(+), 23 deletions(-)
arch/x86/entry/entry_32.S +0 -17
@@ -481,29 +481,12 @@ work_resched: work_notifysig: # deal with pending signals and # notify-resume requests -#ifdef CONFIG_VM86 - testl $X86_EFLAGS_VM, PT_EFLAGS(%esp) - movl %esp, %eax - jnz work_notifysig_v86 # special case for v86 -1: -#else movl %esp, %eax -#endif TRACE_IRQS_ON ENABLE_INTERRUPTS(CLBR_NONE) xorl %edx, %edx call do_notify_resume jmp resume_userspace - -#ifdef CONFIG_VM86 - ALIGN -work_notifysig_v86: - pushl %ecx # save ti_flags for do_notify_resume - call save_v86_state # %eax contains pt_regs pointer - popl %ecx - movl %eax, %esp - jmp 1b -#endif END(work_pending) # perform syscall exit tracing
arch/x86/kernel/traps.c +12 -0
@@ -190,6 +190,13 @@ do_trap_no_signal(struct task_struct *tsk, int trapnr, char *str,* On nmi (interrupt 2), do_trap should not be called. */ if (trapnr <X86_TRAP_UD ) { +/* +* handle_vm86_trap may not return. If that +* happens, then there is no debug stack counter +* (it's a 32-bit kernel) and handle_vm86_trap +* will clear the preempt counter. +*/ +if (!handle_vm86_trap ((struct kernel_vm86_regs *) regs, error_code, trapnr))return 0 ;
@@ -650,6 +657,11 @@ dotraplinkage void do_debug(struct pt_regs *regs, long error_code)preempt_conditional_sti (regs);if (v8086_mode (regs)) { +/* +* handle_vm86_trap may not return. If that happens, then +* there is no debug stack counter (it's a 32-bit kernel) +* and handle_vm86_trap will clear the preempt counter. +*/ handle_vm86_trap ((struct kernel_vm86_regs *) regs, error_code,X86_TRAP_DB );preempt_conditional_cli (regs);
arch/x86/kernel/vm86_32.c +2 -6
@@ -351,6 +351,7 @@ static inline void return_to_32bit(struct kernel_vm86_regs *regs16, int retval) {struct pt_regs *regs32; +preempt_count_set (0 ); regs32 =save_v86_state (regs16); regs32->ax = retval; __asm__ __volatile__("movl %0,%%esp\n\t"
@@ -549,12 +550,7 @@ int handle_vm86_trap(struct kernel_vm86_regs *regs, long error_code, int trapno) {if (VMPI .is_vm86pus) {if ((trapno ==3 ) || (trapno ==1 )) { -KVM86 ->regs32->ax =VM86_TRAP + (trapno <<8 ); -/* setting this flag forces the code in entry_32.S to -the path where we call save_v86_state() and change -the stack pointer to KVM86->regs32 */ -set_thread_flag (TIF_NOTIFY_RESUME ); -return 0 ; +return_to_32bit (regs,VM86_TRAP + (trapno <<8 )); }do_int (regs, trapno, (unsigned char __user *) (regs->pt.ss <<4 ),SP (regs));return 0 ;
Signature · 1 line
2.4.3
Andy Lutomirski
maintainerPATCH 4/7 x86/entry/32: Finish removing bogus kernel-mode check
In reply to Andy Lutomirski--- arch/x86/entry/entry_32.S | 7 ------- 1 file changed, 7 deletions(-)
arch/x86/entry/entry_32.S +0 -7
@@ -491,13 +491,6 @@ work_notifysig: # deal with pending signals and #endif TRACE_IRQS_ON ENABLE_INTERRUPTS(CLBR_NONE) - /* Temporary sanity check */ - movb PT_CS(%esp), %bl - andb $SEGMENT_RPL_MASK, %bl - cmpb $USER_RPL, %bl - jnb 2f - ud2 -2: xorl %edx, %edx call do_notify_resume jmp resume_userspace
Signature · 1 line
2.4.3
Andy Lutomirski
maintainerPATCH 3/7 x86/entry/32: Sanity check for work_notifysig
In reply to Andy Lutomirski44fbbb3dc687c added an unnecessary check. Add a temporary assertion to confirm that it's unnecessary.
--- arch/x86/entry/entry_32.S | 5 ++++- 1 file changed, 4 insertions(+), 1 deletion(-)
arch/x86/entry/entry_32.S +4 -1
@@ -491,10 +491,13 @@ work_notifysig: # deal with pending signals and #endif TRACE_IRQS_ON ENABLE_INTERRUPTS(CLBR_NONE) + /* Temporary sanity check */ movb PT_CS(%esp), %bl andb $SEGMENT_RPL_MASK, %bl cmpb $USER_RPL, %bl - jb resume_kernel + jnb 2f + ud2 +2: xorl %edx, %edx call do_notify_resume jmp resume_userspace
Signature · 1 line
2.4.3
Re: [RFC/PATCH 3/7] [TEMPORARY] x86/entry/32: Sanity check for work_notifysig
[cc: Al -- saying I'm cc-ing you in the patch 0 description doesn't make it so.]
Quoted 30 lines · Andy Lutomirski
44fbbb3dc687cadded an unnecessary check. Add a temporary assertion to confirm that it's unnecessary.Signed-off-by Andy Lutomirski <[email protected]>--- arch/x86/entry/entry_32.S | 5 ++++- 1 file changed, 4 insertions(+), 1 deletion(-)arch/x86/entry/entry_32.S +4
-2@@ -491,10 +491,13 @@ work_notifysig: # deal with pending signals and #endif TRACE_IRQS_ON ENABLE_INTERRUPTS(CLBR_NONE) + /* Temporary sanity check */ movb PT_CS(%esp), %bl andb $SEGMENT_RPL_MASK, %bl cmpb $USER_RPL, %bl - jb resume_kernel + jnb 2f + ud2 +2: xorl %edx, %edx call do_notify_resume jmp resume_userspace --2.4.3
Signature · 2 lines
Andy Lutomirski AMA Capital Management, LLC
Re: [RFC/PATCH 4/7] x86/entry/32: Finish removing bogus kernel-mode check
In reply to Andy Lutomirski[cc: Al -- saying I'm cc-ing you in the patch 0 description doesn't make it so.]
Quoted 26 lines · Andy Lutomirski
Signed-off-by Andy Lutomirski <[email protected]>--- arch/x86/entry/entry_32.S | 7 ------- 1 file changed, 7 deletions(-)arch/x86/entry/entry_32.S +0
-8@@ -491,13 +491,6 @@ work_notifysig: # deal with pending signals and #endif TRACE_IRQS_ON ENABLE_INTERRUPTS(CLBR_NONE) - /* Temporary sanity check */ - movb PT_CS(%esp), %bl - andb $SEGMENT_RPL_MASK, %bl - cmpb $USER_RPL, %bl - jnb 2f - ud2 -2: xorl %edx, %edx call do_notify_resume jmp resume_userspace --2.4.3
Signature · 2 lines
Andy Lutomirski AMA Capital Management, LLC
Andy Lutomirski
maintainerPATCH 1/7 x86/entry/32: Remove 32-bit syscall audit optimizations
In reply to Andy LutomirskiThe asm audit optimizations are ugly and obfuscate the code too much. Remove them.
This will regress performance if syscall auditing is enabled on 32-bit kernels and sysenter is in use. If this becomes a problem, interested parties are encouraged to implement the equivalent of the 64-bit opportunistic sysret optimization.
Alternatively, a case could be made that, on 32-bit kernels, a less messy asm audit optimization could be done. 32-bit kernels don't have the complicated partial register saving tricks that 64-bit kernels have, so the sysenter post-syscall path could just call the audit hooks directly. Any reimplementation of this ought to demonstrate that it only calls the audit hook once per syscall, though, which does not currently appear to be true. Someone would have to make the case that doing so would be better than implementing opportunistic sysexit, though.
--- arch/x86/entry/entry_32.S | 48 ++--------------------------------------------- 1 file changed, 2 insertions(+), 46 deletions(-)
arch/x86/entry/entry_32.S +2 -46
@@ -45,16 +45,6 @@ #include <asm/asm.h> #include <asm/smap.h> -/* Avoid __ASSEMBLER__'ifying <linux/audit.h> just for this. */ -#include <linux/elf-em.h> -#define AUDIT_ARCH_I386 (EM_386|__AUDIT_ARCH_LE) -#define __AUDIT_ARCH_LE 0x40000000 - -#ifndef CONFIG_AUDITSYSCALL -# define sysenter_audit syscall_trace_entry -# define sysexit_audit syscall_exit_work -#endif - .section .entry.text, "ax" /*
@@ -339,7 +329,7 @@ sysenter_past_esp: GET_THREAD_INFO(%ebp) testl $_TIF_WORK_SYSCALL_ENTRY, TI_flags(%ebp) - jnz sysenter_audit + jnz syscall_trace_entry sysenter_do_call: cmpl $(NR_syscalls), %eax jae sysenter_badsys
@@ -351,7 +341,7 @@ sysenter_after_call: TRACE_IRQS_OFF movl TI_flags(%ebp), %ecx testl $_TIF_ALLWORK_MASK, %ecx - jnz sysexit_audit + jnz syscall_exit_work sysenter_exit: /* if something modifies registers it must also disable sysexit */ movl PT_EIP(%esp), %edx
@@ -362,40 +352,6 @@ sysenter_exit: PTGS_TO_GS ENABLE_INTERRUPTS_SYSEXIT -#ifdef CONFIG_AUDITSYSCALL -sysenter_audit: - testl $(_TIF_WORK_SYSCALL_ENTRY & ~_TIF_SYSCALL_AUDIT), TI_flags(%ebp) - jnz syscall_trace_entry - /* movl PT_EAX(%esp), %eax already set, syscall number: 1st arg to audit */ - movl PT_EBX(%esp), %edx /* ebx/a0: 2nd arg to audit */ - /* movl PT_ECX(%esp), %ecx already set, a1: 3nd arg to audit */ - pushl PT_ESI(%esp) /* a3: 5th arg */ - pushl PT_EDX+4(%esp) /* a2: 4th arg */ - call __audit_syscall_entry - popl %ecx /* get that remapped edx off the stack */ - popl %ecx /* get that remapped esi off the stack */ - movl PT_EAX(%esp), %eax /* reload syscall number */ - jmp sysenter_do_call - -sysexit_audit: - testl $(_TIF_ALLWORK_MASK & ~_TIF_SYSCALL_AUDIT), %ecx - jnz syscall_exit_work - TRACE_IRQS_ON - ENABLE_INTERRUPTS(CLBR_ANY) - movl %eax, %edx /* second arg, syscall return value */ - cmpl $-MAX_ERRNO, %eax /* is it an error ? */ - setbe %al /* 1 if so, 0 if not */ - movzbl %al, %eax /* zero-extend that */ - call __audit_syscall_exit - DISABLE_INTERRUPTS(CLBR_ANY) - TRACE_IRQS_OFF - movl TI_flags(%ebp), %ecx - testl $(_TIF_ALLWORK_MASK & ~_TIF_SYSCALL_AUDIT), %ecx - jnz syscall_exit_work - movl PT_EAX(%esp), %eax /* reload syscall return value */ - jmp sysenter_exit -#endif - .pushsection .fixup, "ax" 2: movl $0, PT_FS(%esp) jmp 1b
Signature · 1 line
2.4.3
Re: [RFC/PATCH 5/7] x86/vm86: Teach handle_vm86_trap to return to 32bit mode directly
In reply to Andy LutomirskiQuoted 6 lines · Andy Lutomirski
The
TIF_NOTIFY_RESUMEhack it was using was buggy and unsupportable. vm86 mode was completely broken under ptrace, for example, because we'd never make it to v8086 mode.This code is still a huge, scary mess, but at least it's no longer tangled with the exit-to-userspace loop.
This patch is incorrect. Brian, what's the ETA for your vm86 cleanup?
If it's very soon, then I'll see if I can rely on it. If not, I'll
have to come up with a way to fix this patch.
Grr. The kernel state when handle_vm86_trap is called is absurd right now. Somehow we're supposed to survive do_trap, send a signal corresponding to the outside-vm86 state, and exit vm86 cleanly (with ax = 0), all before returning to user mode. I doubt these semantics are even intentional.
This code sucks.
--Andy
Re: [RFC/PATCH 5/7] x86/vm86: Teach handle_vm86_trap to return to 32bit mode directly
Quoted 12 lines · Andy Lutomirski
Quoted 6 lines · Andy Lutomirski
The
TIF_NOTIFY_RESUMEhack it was using was buggy and unsupportable. vm86 mode was completely broken under ptrace, for example, because we'd never make it to v8086 mode.This code is still a huge, scary mess, but at least it's no longer tangled with the exit-to-userspace loop.
This patch is incorrect. Brian, what's the ETA for your vm86 cleanup?
If it's very soon, then I'll see if I can rely on it. If not, I'll
have to come up with a way to fix this patch.
Quoted 7 lines
Grr. The kernel state when
handle_vm86_trapis called is absurd right now. Somehow we're supposed to survivedo_trap, send a signal corresponding to the outside-vm86 state, and exit vm86 cleanly (with ax = 0), all before returning to user mode. I doubt these semantics are even intentional.This code sucks.
OK, I have a version that seems to work. It comes with a much better selftest, too. I'll send it shortly.
Brian, would it make sense to base your work on top of it?
Now that I've looked at this stuff, if I were designing Linux support for v8086 mode, I'd do it very differently. There wouldn't be a vm86 syscall at all. Instead you'd call sigaltstack, then raise a signal, set X86_EFLAGS_VM, and return.
The kernel would handle X86_EFLAGS_VM being set by setting TIF_V8086 and adjusting sp0. On entry, TIF_V8086 would move the segment registers from the hardware frame into pt_regs and, on exit, TIF_V8086 would move them back. Clearing X86_EFLAGS_VM (via ptrace, signal delivery, or sigreturn) would sanitize the segment registers.
SYSENTER would be safe, so the SYSENTER_CS hack wouldn't be needed. Of course, we'd lose the CPU state, so the user would have to be careful.
And that's it. There wouldn't be any emulation -- user code could emulate syscalls all by itself in a signal handler. Exiting v8086 mode would be straightforward -- just do anything that would raise a signal.
Of course, this isn't at all ABI-compatible with the current turd, and v8086 mode isn't really that useful, so this is just idle retroactive speculation. But the TIF_V8086 trick would still be useful to let us get rid of all the awful hacks in the trap and exit code.
--Andy
Re: [RFC/PATCH 5/7] x86/vm86: Teach handle_vm86_trap to return to 32bit mode directly
Quoted 46 lines · Andy Lutomirski
Quoted 12 lines · Andy Lutomirski
Quoted 6 lines · Andy Lutomirski
The
TIF_NOTIFY_RESUMEhack it was using was buggy and unsupportable. vm86 mode was completely broken under ptrace, for example, because we'd never make it to v8086 mode.This code is still a huge, scary mess, but at least it's no longer tangled with the exit-to-userspace loop.
This patch is incorrect. Brian, what's the ETA for your vm86 cleanup?
If it's very soon, then I'll see if I can rely on it. If not, I'll
have to come up with a way to fix this patch.Quoted 7 lines
Grr. The kernel state when
handle_vm86_trapis called is absurd right now. Somehow we're supposed to survivedo_trap, send a signal corresponding to the outside-vm86 state, and exit vm86 cleanly (with ax = 0), all before returning to user mode. I doubt these semantics are even intentional.This code sucks.
OK, I have a version that seems to work. It comes with a much better selftest, too. I'll send it shortly.
Brian, would it make sense to base your work on top of it?
Now that I've looked at this stuff, if I were designing Linux support for v8086 mode, I'd do it very differently. There wouldn't be a vm86 syscall at all. Instead you'd call sigaltstack, then raise a signal, set
X86_EFLAGS_VM, and return.The kernel would handle
X86_EFLAGS_VMbeing set by settingTIF_V8086and adjusting sp0. On entry,TIF_V8086would move the segment registers from the hardware frame intopt_regsand, on exit,TIF_V8086would move them back. ClearingX86_EFLAGS_VM(via ptrace, signal delivery, or sigreturn) would sanitize the segment registers.SYSENTER would be safe, so the
SYSENTER_CShack wouldn't be needed. Of course, we'd lose the CPU state, so the user would have to be careful.And that's it. There wouldn't be any emulation -- user code could emulate syscalls all by itself in a signal handler. Exiting v8086 mode would be straightforward -- just do anything that would raise a signal.
Quoted 4 lines
Of course, this isn't at all ABI-compatible with the current turd, and v8086 mode isn't really that useful, so this is just idle retroactive speculation. But the
TIF_V8086trick would still be useful to let us get rid of all the awful hacks in the trap and exit code.
I'll post my patches tonight when I get home. It would probably make more sense for you to base off mine, since it should eliminate the need for you to touch any vm86 code.
I fixed the signal issue by checking if the VM flag is set in handle_signal(), and swap the register state there before pushing the signal frame, but that is only possible after removing the need to jump back into the exit asm routines. work_notifysig_v86 is gone too as a result.
--
Brian Gerst