linux-kernel - [PATCH 5/9] x86/asm/entry/64: if execve fails, no need to use IRET return

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Message-Id: <1427985378-4287-5-git-send-email-dvlasenk@redhat.com>
Date:	Thu,  2 Apr 2015 16:36:14 +0200
From:	Denys Vlasenko <dvlasenk@...hat.com>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	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>, x86@...nel.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH 5/9] x86/asm/entry/64: if execve fails, no need to use IRET return

This makes failed execve's faster.

This also enables a future possible optimization: SAVE_EXTRA_REGS can be avoided
in execve's.

Run-tested.

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
---
 arch/x86/kernel/entry_64.S | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S
index 060cb2e..e8f2aeb 100644
--- a/arch/x86/kernel/entry_64.S
+++ b/arch/x86/kernel/entry_64.S
@@ -421,6 +421,11 @@ ENTRY(stub_execve)
 	DEFAULT_FRAME 0, 8
 	SAVE_EXTRA_REGS 8
 	call sys_execve
+return_from_exec:
+	testl	%eax,%eax
+	jz	return_from_stub
+	/* exec failed, can use fast SYSRET code path in this case */
+	ret
 return_from_stub:
 	addq $8, %rsp
 	movq %rax,RAX(%rsp)
@@ -434,7 +439,7 @@ ENTRY(stub_execveat)
 	DEFAULT_FRAME 0, 8
 	SAVE_EXTRA_REGS 8
 	call sys_execveat
-	jmp  return_from_stub
+	jmp  return_from_exec
 	CFI_ENDPROC
 END(stub_execveat)
 
@@ -471,7 +476,7 @@ ENTRY(stub_x32_execve)
 	DEFAULT_FRAME 0, 8
 	SAVE_EXTRA_REGS 8
 	call compat_sys_execve
-	jmp  return_from_stub
+	jmp  return_from_exec
 	CFI_ENDPROC
 END(stub_x32_execve)
 
@@ -480,7 +485,7 @@ ENTRY(stub_x32_execveat)
 	DEFAULT_FRAME 0, 8
 	SAVE_EXTRA_REGS 8
 	call compat_sys_execveat
-	jmp  return_from_stub
+	jmp  return_from_exec
 	CFI_ENDPROC
 END(stub_x32_execveat)
 
-- 
1.8.1.4

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