linux-kernel - Re: [PATCH 2/2] x86: fix incomplete clear by clear

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Date:	Sun, 21 Jun 2015 08:55:23 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Alexey Dobriyan <adobriyan@...il.com>
Cc:	hpa@...or.com, x86@...nel.org, linux-kernel@...r.kernel.org,
	Thomas Gleixner <tglx@...utronix.de>,
	Borislav Petkov <bp@...en8.de>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Andy Lutomirski <luto@...nel.org>,
	Brian Gerst <brgerst@...il.com>
Subject: Re: [PATCH 2/2] x86: fix incomplete clear by clear_user()


* Alexey Dobriyan <adobriyan@...il.com> wrote:

> clear_user() used MOVQ+MOVB and if MOVQ faults, code simply exits and
> honestly returns remaining length. In case of unaligned area, unaligned
> remainder would count towards return value (correctly) but not cleared
> (lazy code at least):
> 
> 	clear_user(p + 4096 - 4, 8) = 8
> 
> No one would have noticed but REP MOVSB addition to clear_user()
> repertoire creates a problem: REP MOVSB does everything correctly,
> clears and counts to the last possible byte, but REP STOSQ and MOVQ
> variants DO NOT:
> 
> 	MOVQ		clear_user(p + 4096 - 4, 8) = 8
> 	REP STOSQ	clear_user(p + 4096 - 4, 8) = 8
> 	REP STOSB	clear_user(p + 4096 - 4, 8) = 4
> 
> Patch fixes incomplete clear on 32-bit and 64-bit REP STOSQ, MOVQ.

So please flip the order of the changes around so that we never have this 
inconsistency observable: i.e. first update the existing clearing method, then 
move it and introduce the new variants without having to patch them.

Thanks,

	Ingo
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