Re: [PATCH net v2] ppp: defer channel free to an RCU grace period to fix pppol2tp RX UAF

On Wed, Jul 08, 2026 at 11:11:47AM +0200, Sebastian Andrzej Siewior wrote:
> On 2026-07-07 09:39:01 [-0700], Paul E. McKenney wrote:
> > Alternatives include:
> > 
> > o	Provide a patch like that above, but only execute the
> > 	rcu_barrier() in some debug mode.  If your code works when
> > 	that debug is enabled but does not otherwise, you add the
> > 	rcu_barrier().
> > 
> > o	If debug is enabled, make rcu_do_batch() check the function
> > 	before invoking it.  If the function is not mapped, issue a
> > 	diagnostic, and don't try to invoke the function.  (But is
> > 	there a sufficiently cheap way to check for the function not
> > 	being mapped?)
> 
> In both cases you would see a backtrace and the name of the last
> unloaded module. And since we don't see a lot of these reports, people
> either don't run into this because it does not exist or RCU is quick
> enough.

Good point, the splat from calling the no-longer-mapped function should
call out the offending module.  So maybe our debug code is good enough
already.

							Thanx, Paul