[PATCH] x86/asm/entry/32: Slightly better handling of syscall errors in auditing

[Denys Vlasenko <dvlasenk@redhat.com>]

"setbe %al" insn has a register merge stall: it needs to combine
previous %eax value with new value for the lowest byte.
Subsequent "movzbl %al,%edi" in turn depends on its completion.

This patch replaces "setbe %al + movzbl %al,%edi" pair of insns
with "xor %edi,%edi" before the comparison, and conditional "inc %edi".

This results in the same value of %edi as produced by old code,
but first insn has no dependencies, and we end up with having
only one insn with deps which executes only if %eax contains error
return, and both insns are shorter: 2 bytes each versus 3 bytes each.

(The old code was inherited from 32-bit code, where it allowed to avoid
a conditional jump. Here we have to use a jump anyway).

Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>
CC: Linus Torvalds <torvalds@linux-foundation.org>
CC: Steven Rostedt <rostedt@goodmis.org>
CC: Ingo Molnar <mingo@kernel.org>
CC: Borislav Petkov <bp@alien8.de>
CC: "H. Peter Anvin" <hpa@zytor.com>
CC: Andy Lutomirski <luto@amacapital.net>
CC: Oleg Nesterov <oleg@redhat.com>
CC: Frederic Weisbecker <fweisbec@gmail.com>
CC: Alexei Starovoitov <ast@plumgrid.com>
CC: Will Drewry <wad@chromium.org>
CC: Kees Cook <keescook@chromium.org>
CC: x86@kernel.org
CC: linux-kernel@vger.kernel.org
---
 arch/x86/entry/entry_64_compat.S | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/arch/x86/entry/entry_64_compat.S b/arch/x86/entry/entry_64_compat.S
index bb187a6..96f33a4 100644
--- a/arch/x86/entry/entry_64_compat.S
+++ b/arch/x86/entry/entry_64_compat.S
@@ -213,12 +213,13 @@ sysexit_from_sys_call:
 	jnz	ia32_ret_from_sys_call
 	TRACE_IRQS_ON
 	ENABLE_INTERRUPTS(CLBR_NONE)
+	xor	%edi, %edi
 	movl	%eax, %esi		/* second arg, syscall return value */
 	cmpl	$-MAX_ERRNO, %eax	/* is it an error ? */
 	jbe	1f
 	movslq	%eax, %rsi		/* if error sign extend to 64 bits */
-1:	setbe	%al			/* 1 if error, 0 if not */
-	movzbl	%al, %edi		/* zero-extend that into %edi */
+	inc	%edi
+1:	/* edi: 1 if error, 0 if not */
 	call	__audit_syscall_exit
 	movq	RAX(%rsp), %rax		/* reload syscall return value */
 	movl	$(_TIF_ALLWORK_MASK & ~_TIF_SYSCALL_AUDIT), %edi
-- 
1.8.1.4