netdev - Re: [bug, netconsole, SLUB] BUG skbuff_head

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Date:	Fri, 18 Jul 2008 20:07:47 +0400
From:	Evgeniy Polyakov <johnpol@....mipt.ru>
To:	Pekka Enberg <penberg@...helsinki.fi>
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 Pekka.

On Fri, Jul 18, 2008 at 05:44:44PM +0300, Pekka Enberg (penberg@...helsinki.fi) wrote:
> 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.

Yup, I see.

Actually with the network there will be no duuble free as long as uses
use kfree_skb(), since first freeing (with skb->users being equal to 1,
so user owns this skb) will fill skb with 0x6b, so any subsequent
kfree_skb() will try to decrement particulary huge reference counter
(0x6b6b6b6b) and actually will never reach 1 again, so things will look
quite clear, but if the same slub area will be reused for the next skb
before it is freed second time, very hard-to-debug data corruption will
happen, so I belive this kind of checks should be done into kfree_skb()
(of course under special debug compile option). I posted that patch just
for the reference and that Ingo would be able to detect where skb is
used after already being freed.

-- 
	Evgeniy Polyakov
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