x86/vdso32/syscall.S: Do not load __USER32_DS to %ss

This vDSO code only gets used by 64-bit kernels, not 32-bit ones.

On 64-bit kernels, the data segment is the same for 32-bit and
64-bit userspace, and the SYSRET instruction loads %ss with its
selector.

So there's no need to repeat it by hand. Segment loads are somewhat
expensive: tens of cycles.

Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>
[ Removed unnecessary comment. ]
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Cc: Alexei Starovoitov <ast@plumgrid.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Will Drewry <wad@chromium.org>
Link: http://lkml.kernel.org/r/63da6d778f69fd0f1345d9287f6764d58be519fa.1427482099.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/arch/x86/vdso/vdso32/syscall.S b/arch/x86/vdso/vdso32/syscall.S
index 5415b5613d5545b68dc88ee08de03f1061c48f8c..6b286bb5251ce1d15cc00299ae096e7518f03986 100644 (file)
--- 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: