linux-kernel - Re: PROBLEM: null pointer dereference in cfq_dispatch

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Date:	Thu, 22 Mar 2007 20:22:34 +0100
From:	Johannes Weiner <hannes-kernel@...urebad.de>
To:	Jens Axboe <jens.axboe@...cle.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: PROBLEM: null pointer dereference in cfq_dispatch_requests (2.6.21-rc2 and 2.6.20)

Hi,

On Thu, Mar 22, 2007 at 07:42:35PM +0100, Jens Axboe wrote:
> > diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
> > index b6491c0..ca84f0b 100644
> > --- a/block/cfq-iosched.c
> > +++ b/block/cfq-iosched.c
> > @@ -961,8 +961,8 @@ __cfq_dispatch_requests(struct cfq_data *cfqd, struct cfq_queue *cfqq,
> >  		/*
> >  		 * follow expired path, else get first next available
> >  		 */
> > -		if ((rq = cfq_check_fifo(cfqq)) == NULL)
> > -			rq = cfqq->next_rq;
> > +		if (!(rq = cfq_check_fifo(cfqq)) && !(rq = cfqq->next_rq))
> > +			break;
> 
> That still only hides a bug. It is illegal for ->next_rq to be NULL
> while the RB tree is non-empty.

As I noticed afterwards this isn't even the point where the NULL ptr is
dereferenced.  It must be in the next code-line, cfqd->queue or
cfqd->queue->elevator was NULL when the oops occured or am I wrong?

'Hannes
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