Commit 539f5113 authored by Andy Lutomirski's avatar Andy Lutomirski Committed by Ingo Molnar
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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: default avatarAndy 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: default avatarIngo Molnar <mingo@kernel.org>
parent a92fde25

arch/x86/entry/entry_64.S

+31 −8
Original line number Diff line number Diff line
@@ -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

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