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

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Message-ID: <CA+55aFywiJf1TiSaXSZNkUpGpgtJa1pmZa7E+BOJafvTQCuzdA@mail.gmail.com>
Date:	Tue, 19 Nov 2013 11:38:50 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	"H. Peter Anvin" <hpa@...or.com>
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 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.

             Linus
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