Messages in this thread | | | Date | Wed, 20 Nov 2013 12:36:20 -0800 | From | "H. Peter Anvin" <> | Subject | Re: [tip:x86/asm] x86-64, copy_user: Remove zero byte check before copy user buffer. |
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On 11/20/2013 12:13 PM, Linus Torvalds wrote: > On Wed, Nov 20, 2013 at 11:28 AM, H. Peter Anvin <hpa@linux.intel.com> wrote: >>> >>> .section .fixup,"ax" >>> 11: lea (%rdx,%rcx,8),%rcx >>> 12: movl %ecx,%edx /* ecx is zerorest also */ >> >> -> Even if %rdx+%rcx*8 > 2^32 we end up truncating at 12: -- not that it >> matters, since both arguments are prototyped as "unsigned" and therefore >> the C compiler is supposed to guarantee the upper 32 bits are ignored. > > Ahh. That was the one I thought was broken, but yes, while the upper > bits of %rcx are calculated and not zeroed, they end up not actually > getting used. So yeah, I'll believe it's correct. >
That being said, "lea (%rdx,%rcx,8),%ecx" (leal, as opposed to leaq) is a perfectly legitimate instruction and actually one byte shorter. The big question is if some broken version of gas will choke on it.
-hpa
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