linux-kernel - Re: [bug, netconsole, SLUB] BUG skbuff_head

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Date:	Fri, 18 Jul 2008 17:44:44 +0300
From:	"Pekka Enberg" <penberg@...helsinki.fi>
To:	"Evgeniy Polyakov" <johnpol@....mipt.ru>
Cc:	"Ingo Molnar" <mingo@...e.hu>, linux-kernel@...r.kernel.org,
	netdev@...r.kernel.org, "Vegard Nossum" <vegard.nossum@...il.com>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	"Christoph Lameter" <cl@...ux-foundation.org>
Subject: Re: [bug, netconsole, SLUB] BUG skbuff_head_cache: Poison overwritten

Hi Evgeniy,

On Fri, Jul 18, 2008 at 12:02:26PM +0300, Pekka Enberg
(penberg@...helsinki.fi) wrote:
>> > Out of curiosity, why does it scream at allocation time?
>>
>> Because it's checking for use-after-free errors. The object is
>> poisoned with POISON_FREE when it's free'd and we verify the poison
>> values at allocation time.

On Fri, Jul 18, 2008 at 1:16 PM, Evgeniy Polyakov <johnpol@....mipt.ru> wrote:
> Does it also scream on double free event? Just to closer guilty
> circles... 0x9c offset is somewhere at the very end of the skbuff
> structure, likely skb->users.

Yeah. See the free_debug_processing() function in mm/slub.c for
details (the on_freelist() part). However, if you look at slab_free()
you can see that in the SLUB fast-path we don't do any of these
debugging checks. So you can end up with slab corruption without a
nice error message.

                                  Pekka
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