From a740576a4abf933de8f50787f24f24456cebd761 Mon Sep 17 00:00:00 2001 From: "H. Peter Anvin" Date: Fri, 27 Dec 2013 16:52:47 -0800 Subject: [PATCH] x86: Slightly tweak the access_ok() C variant for better code gcc can under very specific circumstances realize that the code sequence: foo += bar; if (foo < bar) ... ... is equivalent to a carry out from the addition. Tweak the implementation of access_ok() (specifically __chk_range_not_ok()) to make it more likely that gcc will make that connection. It isn't fool-proof (sometimes gcc seems to think it can make better code with lea, and ends up with a second comparison), still, but it seems to be able to connect the two more frequently this way. Cc: Linus Torvalds Link: http://lkml.kernel.org/r/CA%2B55aFzPBdbfKovMT8Edr4SmE2_=%2BOKJFac9XW2awegogTkVTA@mail.gmail.com Signed-off-by: H. Peter Anvin --- arch/x86/include/asm/uaccess.h | 8 +++++--- 1 file changed, 5 insertions(+), 3 deletions(-) diff --git a/arch/x86/include/asm/uaccess.h b/arch/x86/include/asm/uaccess.h index 84ecf1d..6f1bb74 100644 --- a/arch/x86/include/asm/uaccess.h +++ b/arch/x86/include/asm/uaccess.h @@ -41,7 +41,7 @@ * Test whether a block of memory is a valid user space address. * Returns 0 if the range is valid, nonzero otherwise. */ -static inline int __chk_range_not_ok(unsigned long addr, unsigned long size, unsigned long limit) +static inline bool __chk_range_not_ok(unsigned long addr, unsigned long size, unsigned long limit) { /* * If we have used "sizeof()" for the size, @@ -55,7 +55,9 @@ static inline int __chk_range_not_ok(unsigned long addr, unsigned long size, uns /* Arbitrary sizes? Be careful about overflow */ addr += size; - return (addr < size) || (addr > limit); + if (addr < size) + return true; + return addr > limit; } #define __range_not_ok(addr, size, limit) \ @@ -84,7 +86,7 @@ static inline int __chk_range_not_ok(unsigned long addr, unsigned long size, uns * this function, memory access functions may still return -EFAULT. */ #define access_ok(type, addr, size) \ - (likely(__range_not_ok(addr, size, user_addr_max()) == 0)) + likely(!__range_not_ok(addr, size, user_addr_max())) /* * The exception table consists of pairs of addresses relative to the -- 2.7.4