linux-kernel - Re: [tip:x86/asm] x86-64, copy_user: Remove zero byte check before copy user buffer.

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Date:	Wed, 20 Nov 2013 12:36:20 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>,
	"H. Peter Anvin" <hpa@...ux.intel.com>
CC:	Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-tip-commits@...r.kernel.org,
	Fenghua Yu <fenghua.yu@...el.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [tip:x86/asm] x86-64, copy_user: Remove zero byte check before
 copy user buffer.

On 11/20/2013 12:13 PM, Linus Torvalds wrote:
> On Wed, Nov 20, 2013 at 11:28 AM, H. Peter Anvin <hpa@...ux.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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