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Subject[tip:x86/asm] x86/asm/entry/64: Use local label to skip around sycall dispatch
Commit-ID:  a6de5a21fb25cdbbdf3c3e9afd8481581c4f2464
Gitweb: http://git.kernel.org/tip/a6de5a21fb25cdbbdf3c3e9afd8481581c4f2464
Author: Denys Vlasenko <dvlasenk@redhat.com>
AuthorDate: Tue, 31 Mar 2015 19:00:11 +0200
Committer: Ingo Molnar <mingo@kernel.org>
CommitDate: Wed, 1 Apr 2015 13:17:39 +0200

x86/asm/entry/64: Use local label to skip around sycall dispatch

Logically, we just want to jump around the following instruction
and its prologue/epilogue:

call *sys_call_table(,%rax,8)

if the syscall number is too big - we do not specifically target
the "int_ret_from_sys_call" label.

Use a local, numerical label for this jump, for more clarity.

This also makes the code smaller:

-ffffffff8187756b: 0f 87 0f 00 00 00 ja ffffffff81877580 <int_ret_from_sys_call>
+ffffffff8187756b: 77 0f ja ffffffff8187757c <int_ret_from_sys_call>

because jumps to global labels are never translated to short jump
instructions by GAS.

Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>
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/1427821211-25099-9-git-send-email-dvlasenk@redhat.com
[ Improved the changelog. ]
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
arch/x86/kernel/entry_64.S | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S
index bad285d..03c52e2 100644
--- a/arch/x86/kernel/entry_64.S
+++ b/arch/x86/kernel/entry_64.S
@@ -331,10 +331,11 @@ tracesys_phase2:
andl $__SYSCALL_MASK,%eax
cmpl $__NR_syscall_max,%eax
#endif
- ja int_ret_from_sys_call /* RAX(%rsp) is already set */
+ ja 1f /* return -ENOSYS (already in pt_regs->ax) */
movq %r10,%rcx /* fixup for C */
call *sys_call_table(,%rax,8)
movq %rax,RAX(%rsp)
+1:
/* Use IRET because user could have changed pt_regs->foo */

/*

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