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Commit a740576a authored by H. Peter Anvin's avatar H. Peter Anvin
Browse files

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 <torvalds@linux-foundation.org>
Link: http://lkml.kernel.org/r/CA%2B55aFzPBdbfKovMT8Edr4SmE2_=%2BOKJFac9XW2awegogTkVTA@mail.gmail.com


Signed-off-by: default avatarH. Peter Anvin <hpa@zytor.com>
parent c5fe5d80

arch/x86/include/asm/uaccess.h

+5 −3
Original line number Diff line number Diff line
@@ -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