Re: BUG: unable to handle kernel NULL pointer dereference in qfq_dequeue()

[Eric Dumazet <eric.dumazet@gmail.com>]

On Thu, 2012-10-11 at 17:05 +0200, Eric Dumazet wrote:
> On Thu, 2012-10-11 at 16:38 +0800, Cong Wang wrote:
> > On Mon, 2012-10-08 at 17:15 +0800, Cong Wang wrote:
> > > Hi, all,
> > > 
> > > We got the following kernel crash on RHEL6 and I confirmed upstream has
> > > the same problem (I didn't save this kernel log though):
> > 
> > Ok, I got the backtrace of the latest kernel, see below. Seems
> > qfq_slot_scan() in qfq_dequeue() returns something bad, 'cl' becomes
> > '0x10'.
> 
> not exactly, cl is -0x50
> 

> > 
> 
> 
> static struct qfq_class *qfq_slot_head(struct qfq_group *grp)
> {
>         return hlist_entry(grp->slots[grp->front].first,
>                            struct qfq_class, next);
> }
> 
> 
> problem is : grp->slots[grp->front].first is NULL here,
> 
> so we return RAX = -offsetof(struct qfq_class, next)
> 
> (ie -0x50 : ffffffffffffffb0)
> 
> 
> So one bit is set in full_slots while the corresponding slots[] is
> empty.
> 
> I wonder if qfq_slot_remove() is correct ?


I just realize its a 2.6.32 redhat kernel, while QFQ is a 3.0 addition.

Can you reproduce the bug on current kernel (3.6 or git tree)