Subject: Re: [BUG] NULL pointer dereference in skb_dequeue

From: "Paul E. McKenney" <[email-masked]>
Date: Tue, 12 Aug 2008 06:42:24 -0700

On Tue, Aug 12, 2008 at 06:36:22AM +0000, Jarek Poplawski wrote:
> On Mon, Aug 11, 2008 at 04:26:57PM -0700, Paul E. McKenney wrote:
> > On Mon, Aug 11, 2008 at 10:01:26AM +0000, Jarek Poplawski wrote:
> > > On 10-08-2008 21:04, Jarek Poplawski wrote:
> > > ...
> > > > Hmm.. Actually, it's completely unreasonable. Let's forget this.
> > > 
> > > But accidentally it might even sometimes work here...
> > > 
> > > Currently, the most suspicious place to me seems to be
> > > __netif_schedule(). Is it legal to store RCU protected pointers out of
> > > rcu_read_lock() sections?
> > 
> > Yes, but:
> > 
> > 1.	You need to use one of the update-side primitives to do the
> > 	store: rcu_assign_pointer(), list_add_rcu(), etc.
> > 
> > 2.	There has to be some way for multiple updaters to coordinate,
> > 	for example:
> > 
> > 	a.	Only a single task is permitted to update.
> > 
> > 	b.	Locking is used to coordinate among multiple updaters
> > 		(so that only one such updater may proceed at a given
> > 		time).
> > 
> > 	c.	Atomic operations are used to coordinate multiple
> > 		updaters.  Here be dragons, go for (a) or (b)
> > 		instead unless you have an extremely good reason
> > 		-and- you have both a proof of correctness and
> > 		a totally brutal and malign test suite.
> > 
> > The main reason to update RCU-protected pointers within rcu_read_lock()
> > regions is when sharing code between RCU readers and updaters, or when
> > an RCU reader can see the need to do an update.
> 
> Sure, but I'm concerned here with pure RCU reading:
> 
> >From net/sched/sch_generic.c:
> 
> void __qdisc_run(struct Qdisc *q)
> {
>         unsigned long start_time = jiffies;
> 
>         while (qdisc_restart(q)) {
>                 /*
>                  * Postpone processing if
>                  * 1. another process needs the CPU;
>                  * 2. we've been doing it for too long.
>                  */
>                 if (need_resched() || jiffies != start_time) {
>                         __netif_schedule(q);
> 
> This function is run from dev_queue_xmit() (net/core/dev.c) under
> rcu_read_lock_bh(), and this "q" pointer is passed here for later use
> (reading) by softirq run net_tx_action(). Alas in net/ RCU primitives
> are probably omitted in a few places...

If I understand this code, one way to handle it would be to increment
q->refcnt before passing to netif_schedule(), then decrementing it
(within an RCU read-side critical section) in the softirq handler.

There are probably other ways to handle this as well.

							Thanx, Paul

> Thanks for the explanation,
> Jarek P.
> 
>                         break;
>                 }
>         }
> 
>         clear_bit(__QDISC_STATE_RUNNING, &q->state);
> }