linux-kernel - Re: [PATCH] kmemcheck: lazy checking for MOVS instructions

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Message-ID: <19f34abd0809121009o81f2705ve083209a4af23b5d@mail.gmail.com>
Date:	Fri, 12 Sep 2008 19:09:35 +0200
From:	"Vegard Nossum" <vegard.nossum@...il.com>
To:	"Ingo Molnar" <mingo@...e.hu>
Cc:	linux-kernel@...r.kernel.org,
	"Pekka Enberg" <penberg@...helsinki.fi>,
	"Andrew Morton" <akpm@...ux-foundation.org>
Subject: Re: [PATCH] kmemcheck: lazy checking for MOVS instructions

On Fri, Sep 12, 2008 at 11:06 AM, Ingo Molnar <mingo@...e.hu> wrote:
>> From: Vegard Nossum <vegard.nossum@...il.com>
>> Date: Thu, 11 Sep 2008 17:31:07 +0200
>> Subject: [PATCH] kmemcheck: lazy checking for MOVS instructions
>>
>> This patch adds the support for lazy (as opposed to eager) checking
>> for [REP] MOVS instructions (mostly used in memcpy()). This means that
>> if both the source and destination addresses are tracked by kmemcheck,
>> we copy the shadow memory instead of checking that it is initialized.
>>
>> In this way, we get rid of a few more false positives.
>
> looks good to me. I've applied it to tip/kmemcheck - but can zap it and
> pull your for-tip branch as well.

Please zap, I believe it contains an error :-)

In short, when reading/writing the shadow memory of the second page in
a page-boundary-crossing memory access, the offsets into the second
shadow page will be wrong (off by up to 8 bytes). It's a pretty
obscure case, but it would be nice to have it fixed. Will send a pull
request later.

Thanks,


Vegard

-- 
"The animistic metaphor of the bug that maliciously sneaked in while
the programmer was not looking is intellectually dishonest as it
disguises that the error is the programmer's own creation."
	-- E. W. Dijkstra, EWD1036
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