linux-kernel - [GIT PULL 2/4] x86: vdso32/syscall.S: do not load __USER32

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Message-Id: <63da6d778f69fd0f1345d9287f6764d58be519fa.1427482063.git.luto@kernel.org>
Date:	Fri, 27 Mar 2015 11:47:41 -0700
From:	Andy Lutomirski <luto@...nel.org>
To:	Ingo Molnar <mingo@...nel.org>, X86 ML <x86@...nel.org>
Cc:	lkml@...r.kernel.org, Denys Vlasenko <dvlasenk@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Borislav Petkov <bp@...en8.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Andy Lutomirski <luto@...capital.net>,
	Oleg Nesterov <oleg@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Alexei Starovoitov <ast@...mgrid.com>,
	Will Drewry <wad@...omium.org>,
	Kees Cook <keescook@...omium.org>,
	linux-kernel@...r.kernel.org, Andy Lutomirski <luto@...nel.org>
Subject: [GIT PULL 2/4] x86: vdso32/syscall.S: do not load __USER32_DS to %ss

From: Denys Vlasenko <dvlasenk@...hat.com>

This vDSO code only gets used by 64-bit kernel,
not 32-bit. In 64-bit kernels, data segment is the same
for 32-bit and 64-bit userspace, and SYSRET insn does load %ss
with its selector. No need to repeat it by hand. Segment loads
are somewhat expensive: tens of cycles.

Signed-off-by: Denys Vlasenko <dvlasenk@...hat.com>
CC: Linus Torvalds <torvalds@...ux-foundation.org>
CC: Steven Rostedt <rostedt@...dmis.org>
CC: Ingo Molnar <mingo@...nel.org>
CC: Borislav Petkov <bp@...en8.de>
CC: "H. Peter Anvin" <hpa@...or.com>
CC: Andy Lutomirski <luto@...capital.net>
CC: Oleg Nesterov <oleg@...hat.com>
CC: Frederic Weisbecker <fweisbec@...il.com>
CC: Alexei Starovoitov <ast@...mgrid.com>
CC: Will Drewry <wad@...omium.org>
CC: Kees Cook <keescook@...omium.org>
CC: x86@...nel.org
CC: linux-kernel@...r.kernel.org
Message-Id: <1427129240-15543-1-git-send-email-dvlasenk@...hat.com>
[removed unnecessary comment]
Signed-off-by: Andy Lutomirski <luto@...nel.org>
---
 arch/x86/vdso/vdso32/syscall.S | 2 --
 1 file changed, 2 deletions(-)

diff --git a/arch/x86/vdso/vdso32/syscall.S b/arch/x86/vdso/vdso32/syscall.S
index 5415b5613d55..6b286bb5251c 100644
--- a/arch/x86/vdso/vdso32/syscall.S
+++ b/arch/x86/vdso/vdso32/syscall.S
@@ -19,8 +19,6 @@ __kernel_vsyscall:
 .Lpush_ebp:
 	movl	%ecx, %ebp
 	syscall
-	movl	$__USER32_DS, %ecx
-	movl	%ecx, %ss
 	movl	%ebp, %ecx
 	popl	%ebp
 .Lpop_ebp:
-- 
2.3.0

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