Re: [bug, netconsole, SLUB] BUG skbuff_head_cache: Poison overwritten

From: Pekka Enberg
Date: Fri Jul 18 2008 - 10:45:04 EST

Hi Evgeniy,

On Fri, Jul 18, 2008 at 12:02:26PM +0300, Pekka Enberg
(penberg@xxxxxxxxxxxxxx) 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@xxxxxxxxxxx> 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.

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