x86: Replace assembly access_ok() with a C variant (c5fe5d80) · Commits · 戴 / test

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:

H. Peter Anvin <hpa@zytor.com>

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); \
		})

		/**

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