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Date:	Mon, 10 Feb 2014 17:56:50 +0000
From:	Catalin Marinas <catalin.marinas@....com>
To:	Bernd Schubert <bernd.schubert@...m.fraunhofer.de>
Cc:	linux-kernel@...r.kernel.org, Vegard Nossum <vegardno@....uio.no>,
	Pekka Enberg <penberg@...nel.org>
Subject: Re: kmemleak or crc32_le  bug?

On Thu, Feb 06, 2014 at 05:04:49PM +0100, Bernd Schubert wrote:
> I'm frequently getting 
> 
> UG: unable to handle kernel paging request at ffff880f87550dc0
> IP: [<ffffffff813016d0>] crc32_le+0x30/0x110
> 
> called from kmemleak, see bottom of the message.
...
> With the "Cannot allocate a kmemleak_object structure" messages, 

Just curious, is the free memory low when this happens?

> somehow looks like object is not proper initialized, but update_checksum()
> checks for that. Hmm, I'm not sure about kmemcheck_shadow_lookup(), 
> especially about
> 
> > 	if (!virt_addr_valid(address))
> > 		return NULL;
> 
> So is the test 
> 
> > 	shadow = kmemcheck_shadow_lookup(addr);
> > 	if (!shadow)
> > 		return true;
> 
> right here? Shouldn't that be 'return false'?

Are you using kmemcheck and kmemleak together?

I don't think update_checksum() is called on the object being allocated
but possibly on an object being freed when kmemleak_scan() is running.
This is generally a safe operation because of the object locks in
kmemleak_scan() but when an error condition just occurred (like kmemleak
not being able to allocate memory), kmemleak gets disabled and
kmemleak_free() no longer passes the information down to update the
object's flags. At this point, the running kmemleak_scan() potentially
reads unmmapped objects.

I need to think a bit more about this. Thanks for reporting.

-- 
Catalin
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