linux-kernel - [PATCH v2 01/36] x86/uaccess: Tell the compiler that uaccess is unlikely to fault

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Date:	Mon,  5 Oct 2015 17:47:49 -0700
From:	Andy Lutomirski <luto@...nel.org>
To:	x86@...nel.org, linux-kernel@...r.kernel.org
Cc:	Brian Gerst <brgerst@...il.com>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Borislav Petkov <bp@...en8.de>,
	Andy Lutomirski <luto@...nel.org>
Subject: [PATCH v2 01/36] x86/uaccess: Tell the compiler that uaccess is unlikely to fault

GCC doesn't realize that get_user, put_user, and their __ variants
are unlikely to fail.  Tell it.

I noticed this while playing with the C entry code.

Before:
   text	   data	    bss	    dec	    hex	filename
21828763	5194760	1277952	28301475	1afd8a3	baseline

After:
   text	   data	    bss	    dec	    hex	filename
21828379	5194760	1277952	28301091	1afd723	vmlinux

Signed-off-by: Andy Lutomirski <luto@...nel.org>
---
 arch/x86/include/asm/uaccess.h | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/arch/x86/include/asm/uaccess.h b/arch/x86/include/asm/uaccess.h
index a8df874f3e88..3e911c68876e 100644
--- a/arch/x86/include/asm/uaccess.h
+++ b/arch/x86/include/asm/uaccess.h
@@ -182,7 +182,7 @@ __typeof__(__builtin_choose_expr(sizeof(x) > sizeof(0UL), 0ULL, 0UL))
 		     : "=a" (__ret_gu), "=r" (__val_gu)			\
 		     : "0" (ptr), "i" (sizeof(*(ptr))));		\
 	(x) = (__force __typeof__(*(ptr))) __val_gu;			\
-	__ret_gu;							\
+	__builtin_expect(__ret_gu, 0);					\
 })
 
 #define __put_user_x(size, x, ptr, __ret_pu)			\
@@ -278,7 +278,7 @@ extern void __put_user_8(void);
 		__put_user_x(X, __pu_val, ptr, __ret_pu);	\
 		break;						\
 	}							\
-	__ret_pu;						\
+	__builtin_expect(__ret_pu, 0);				\
 })
 
 #define __put_user_size(x, ptr, size, retval, errret)			\
@@ -401,7 +401,7 @@ do {									\
 ({								\
 	int __pu_err;						\
 	__put_user_size((x), (ptr), (size), __pu_err, -EFAULT);	\
-	__pu_err;						\
+	__builtin_expect(__pu_err, 0);				\
 })
 
 #define __get_user_nocheck(x, ptr, size)				\
@@ -410,7 +410,7 @@ do {									\
 	unsigned long __gu_val;						\
 	__get_user_size(__gu_val, (ptr), (size), __gu_err, -EFAULT);	\
 	(x) = (__force __typeof__(*(ptr)))__gu_val;			\
-	__gu_err;							\
+	__builtin_expect(__gu_err, 0);					\
 })
 
 /* FIXME: this hack is definitely wrong -AK */
-- 
2.4.3

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