linux-kernel - Re: WARNING: kmemcheck: Caught 32-bit read from uninitialized memory (f6f6e1a4), by kmemleak's scan

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Message-Id: <1251191755.26351.4.camel@penberg-laptop>
Date:	Tue, 25 Aug 2009 12:15:55 +0300
From:	Pekka Enberg <penberg@...helsinki.fi>
To:	Catalin Marinas <catalin.marinas@....com>
Cc:	Vegard Nossum <vegard.nossum@...il.com>,
	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org
Subject: Re: WARNING: kmemcheck: Caught 32-bit read from uninitialized
 memory  (f6f6e1a4), by kmemleak's scan_block()

On Tue, 2009-08-25 at 10:11 +0100, Catalin Marinas wrote:
> Yes, that's possible.
> 
> Does kmemcheck work on a page-range basis. If an object doesn't cross
> page boundaries, would it be considered fully initialised after writing
> a single location?
> 
> > I think it would be better to ask kmemcheck on a per-pointer basis
> > (i.e. for each pointer-sized word in the object), whether it is
> > initialized or not.
> 
> This should work but how expensive is this check?

Everything in kmemcheck is expensive :-). kmemcheck_shadow_lookup()
needs to do PTE lookup to find the shadow page where we record object
states. I'm not sure why that's a problem, though. If you've enabled
kmemcheck, everything is already dead slow and kmemleak scanning happens
in background, no? Furthermore, I think we need to do it the way Vegard
suggested to avoid false positives.

			Pekka

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