linux-kernel - [tip:x86/asm] x86/asm/entry/64: Use local label to skip around sycall dispatch

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Date:	Thu, 2 Apr 2015 05:26:45 -0700
From:	tip-bot for Denys Vlasenko <tipbot@...or.com>
To:	linux-tip-commits@...r.kernel.org
Cc:	dvlasenk@...hat.com, luto@...capital.net, rostedt@...dmis.org,
	keescook@...omium.org, fweisbec@...il.com, ast@...mgrid.com,
	tglx@...utronix.de, hpa@...or.com, mingo@...nel.org,
	linux-kernel@...r.kernel.org, bp@...en8.de, oleg@...hat.com,
	torvalds@...ux-foundation.org, wad@...omium.org
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@...hat.com>
AuthorDate: Tue, 31 Mar 2015 19:00:11 +0200
Committer:  Ingo Molnar <mingo@...nel.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@...hat.com>
Cc: Alexei Starovoitov <ast@...mgrid.com>
Cc: Andy Lutomirski <luto@...capital.net>
Cc: Borislav Petkov <bp@...en8.de>
Cc: Frederic Weisbecker <fweisbec@...il.com>
Cc: H. Peter Anvin <hpa@...or.com>
Cc: Kees Cook <keescook@...omium.org>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Oleg Nesterov <oleg@...hat.com>
Cc: Steven Rostedt <rostedt@...dmis.org>
Cc: Will Drewry <wad@...omium.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@...nel.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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