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 11:12:52 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Ingo Molnar <mingo@...nel.org>, Peter Anvin <hpa@...ux.intel.com>,
	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 11/19/2013 11:38 AM, Linus Torvalds wrote:
> On Mon, Nov 18, 2013 at 8:37 PM, H. Peter Anvin <hpa@...or.com> wrote:
>>
>> Do you have a preference:
>>
>> 1. Considering the 32-bit truncation incidental (take it or leave it);
>> 2. Require the 32-bit truncation, or
>> 3. Get rid of it completely?
> 
> I don't have a huge preference, but I hate the current situation (with
> Fenghua's patch) where it's not consistent. One path uses just 32-bits
> of the count (thanks to the "mov %edx,%ecx") while another path uses
> 64 bits.
> 
> One or the other, but not a mixture of both.
> 
> And only tangentially related to this: I do think that we could be
> stricter about the count. Make it oops if the high bits are set,
> rather than overwrite a lot of memory. So I would not be adverse to
> limiting the count to 31 bits (or even less) explicitly, and thus
> making the while 32-vs-64 bit issue moot.
> 

I guess the question is if we want to spend the extra cycles on a test
and branch.  For now I suggest that we just put back the truncation in
the form of a movl instruction.

	-hpa


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