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Commit c5fe5d80 authored by Linus Torvalds's avatar Linus Torvalds Committed by H. Peter Anvin
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x86: Replace assembly access_ok() with a C variant 

It turns out that the assembly variant doesn't actually produce that
good code, presumably partly because it creates a long dependency
chain with no scheduling, and partly because we cannot get a flags
result out of gcc (which could be fixed with asm goto, but it turns
out not to be worth it.)

The C code allows gcc to schedule and generate multiple (easily
predictable) branches, and as a side benefit we can really optimize
the case where the size is constant.

Link: http://lkml.kernel.org/r/CA%2B55aFzPBdbfKovMT8Edr4SmE2_=%2BOKJFac9XW2awegogTkVTA@mail.gmail.com


Signed-off-by: default avatarH. Peter Anvin <hpa@zytor.com>
parent 661c8019

arch/x86/include/asm/uaccess.h

+17 −11
Original line number Diff line number Diff line
@@ -40,22 +40,28 @@ 
/*

 * Test whether a block of memory is a valid user space address.

 * Returns 0 if the range is valid, nonzero otherwise.

 *

 * This is equivalent to the following test:

 * (u33)addr + (u33)size > (u33)current->addr_limit.seg (u65 for x86_64)

 *

 * This needs 33-bit (65-bit for x86_64) arithmetic. We have a carry...

 */

static inline int __chk_range_not_ok(unsigned long addr, unsigned long size, unsigned long limit)

{

	/*

	 * If we have used "sizeof()" for the size,

	 * we know it won't overflow the limit (but

	 * it might overflow the 'addr', so it's

	 * important to subtract the size from the

	 * limit, not add it to the address).

	 */

	if (__builtin_constant_p(size))

		return addr > limit - size;



	/* Arbitrary sizes? Be careful about overflow */

	addr += size;

	return (addr < size) || (addr > limit);

}



#define __range_not_ok(addr, size, limit)				\

({									\

	unsigned long flag, roksum;					\

	__chk_user_ptr(addr);						\

	asm("add %3,%1 ; sbb %0,%0 ; cmp %1,%4 ; sbb $0,%0"		\

	    : "=&r" (flag), "=r" (roksum)				\

	    : "1" (addr), "g" ((long)(size)),				\

	      "rm" (limit));						\

	flag;								\

	__chk_range_not_ok((unsigned long __force)(addr), size, limit); \

})



/**