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x86/asm/entry/64: Disentangle error_entry/exit gsbase/ebx/usermode code 

The error_entry/error_exit code to handle gsbase and whether we
return to user mdoe was a mess:

 - error_sti was misnamed.  In particular, it did not enable interrupts.

 - Error handling for gs_change was hopelessly tangled the normal
   usermode path.  Separate it out.  This saves a branch in normal
   entries from kernel mode.

 - The comments were bad.

Fix it up.  As a nice side effect, there's now a code path that
happens on error entries from user mode.  We'll use it soon.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/f1be898ab93360169fb845ab85185948832209ee.1433878454.git.luto@kernel.org
[ Prettified it, clarified comments some more. ]
Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Andy Lutomirski 2015-06-09 12:36:01 -07:00 committed by Ingo Molnar
parent a92fde2523
commit 539f511365
1 changed files with 31 additions and 8 deletions
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39
arch/x86/entry/entry_64.S
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@ -1135,7 +1135,7 @@ END(paranoid_exit)
/*
* Save all registers in pt_regs, and switch gs if needed.
* Return: ebx=0: need swapgs on exit, ebx=1: otherwise
* Return: EBX=0: came from user mode; EBX=1: otherwise
*/
ENTRY(error_entry)
cld
@ -1144,9 +1144,11 @@ ENTRY(error_entry)
xorl %ebx, %ebx
testb $3, CS+8(%rsp)
jz error_kernelspace
error_swapgs:
/* We entered from user mode */
SWAPGS
error_sti:
error_entry_done:
TRACE_IRQS_OFF
ret
@ -1165,8 +1167,15 @@ error_kernelspace:
cmpq %rax, RIP+8(%rsp)
je bstep_iret
cmpq $gs_change, RIP+8(%rsp)
je error_swapgs
jmp error_sti
jne error_entry_done
/*
* hack: gs_change can fail with user gsbase. If this happens, fix up
* gsbase and proceed. We'll fix up the exception and land in
* gs_change's error handler with kernel gsbase.
*/
SWAPGS
jmp error_entry_done
bstep_iret:
/* Fix truncated RIP */
@ -1174,16 +1183,30 @@ bstep_iret:
/* fall through */
error_bad_iret:
/*
* We came from an IRET to user mode, so we have user gsbase.
* Switch to kernel gsbase:
*/
SWAPGS
/*
* Pretend that the exception came from user mode: set up pt_regs
* as if we faulted immediately after IRET and clear EBX so that
* error_exit knows that we will be returning to user mode.
*/
mov %rsp, %rdi
call fixup_bad_iret
mov %rax, %rsp
decl %ebx /* Return to usergs */
jmp error_sti
decl %ebx
jmp error_entry_done
END(error_entry)
/* On entry, ebx is "no swapgs" flag (1: don't need swapgs, 0: need it) */
/*
* On entry, EBS is a "return to kernel mode" flag:
* 1: already in kernel mode, don't need SWAPGS
* 0: user gsbase is loaded, we need SWAPGS and standard preparation for return to usermode
*/
ENTRY(error_exit)
movl %ebx, %eax
RESTORE_EXTRA_REGS