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

[Denys Vlasenko <dvlasenk@redhat.com>]

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@redhat.com>
CC: Linus Torvalds <torvalds@linux-foundation.org>
CC: Steven Rostedt <rostedt@goodmis.org>
CC: Ingo Molnar <mingo@kernel.org>
CC: Borislav Petkov <bp@alien8.de>
CC: "H. Peter Anvin" <hpa@zytor.com>
CC: Andy Lutomirski <luto@amacapital.net>
CC: Oleg Nesterov <oleg@redhat.com>
CC: Frederic Weisbecker <fweisbec@gmail.com>
CC: Alexei Starovoitov <ast@plumgrid.com>
CC: Will Drewry <wad@chromium.org>
CC: Kees Cook <keescook@chromium.org>
CC: x86@kernel.org
CC: linux-kernel@vger.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