linux-kernel - Re: kmemcheck caught read from freed memory (cfq_free_io

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Date:	Thu, 3 Apr 2008 22:49:23 +0300 (EEST)
From:	Pekka J Enberg <penberg@...helsinki.fi>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
cc:	Vegard Nossum <vegard.nossum@...il.com>,
	Ingo Molnar <mingo@...e.hu>,
	Jens Axboe <jens.axboe@...cle.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: kmemcheck caught read from freed memory (cfq_free_io_context)

Hi Paul,

On Thu, 3 Apr 2008, Paul E. McKenney wrote:
> OK, so another approach would be to use a larger shadow block for
> SLAB_DESTROY_BY_RCU slabs, so that each shadow location would have enough
> room for an rcu_head and a size in addition to the flag.  That would
> trivialize tracking, or, more accurately, delegate such tracking to the
> RCU infrastructure.

Yeah, or just allocate some extra spaces for the RCU case and not 
overload the current shadow pages. But sounds good to me.

On Thu, 3 Apr 2008, Paul E. McKenney wrote:
> Of course, the case where the block gets reallocated before the RCU
> grace period ends would also need to be handled (which my rough sketch
> yesterday did -not- handle, by the way...).
> 
> There are a couple of ways of doing this.  Probably the easiest approach
> is to add more state to the flag, so that the RCU callback would check
> to see if reallocation had already happened.  If so, it would update the
> state to indicate that the rcu_head was again available, and would need to
> repost itself if the block had been freed again after being reallocated.
> 
> The other approach would be to defer actually adding the block to the
> freelist until the grace period expired.  This would be more accurate,
> but also quite a bit more intrusive.

We already talked about deferring the actual freeing in kmemcheck to 
better detect these use-after-free conditions with Vegard. So it's 
something that we probably want to do regardless of RCU.

			Pekka
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