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

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Date:	Fri, 18 Jul 2008 11:09:51 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Pekka Enberg <penberg@...helsinki.fi>
Cc:	Evgeniy Polyakov <johnpol@....mipt.ru>,
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
	Vegard Nossum <vegard.nossum@...il.com>,
	"Rafael J. Wysocki" <rjw@...k.pl>
Subject: Re: [bug, netconsole, SLUB] BUG skbuff_head_cache: Poison
	overwritten

* Pekka Enberg <penberg@...helsinki.fi> wrote:

> On Fri, Jul 18, 2008 at 8:46 AM, Evgeniy Polyakov <johnpol@....mipt.ru> wrote:
> > Does SLUB have a debug check at freeing time? If so, how does it work
> > and why didn't it caught use after free there?
> 
> You can't detect use after free before the object is actually free'd 
> ;-)

yeah, we want to check use-after free at the next allocation point - 
i.e. as late as possible to gather all corruptions that happened 
meanwhile.

We could in theory have a SLUB debug mode where a SCHED_IDLE kernel 
thread would periodically check all free objects (of that CPU) in the 
background to ensure their integrity. That would catch corruptions 
sooner, with a possibly still meaningful context to print out. [right 
after the IRQ or process that corrupts them finishes running]

It could also be hooked into ftrace to print out the last few hundred 
kernel function calls executed prior any corruption. ftrace/slub-debug 
plugin perhaps?

	Ingo
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