On Tue, Jun 06, 2023 at 09:17:24PM +0200, Mirsad Goran Todorovac wrote:
> On 6/6/23 20:50, Guillaume Nault wrote:
> > On Tue, Jun 06, 2023 at 04:28:02PM +0200, Mirsad Todorovac wrote:
> > > On 6/6/23 16:11, Guillaume Nault wrote:
> > > > On Tue, Jun 06, 2023 at 03:57:35PM +0200, Mirsad Todorovac wrote:
> > > > > + if (oif) {
> > > > > + rcu_read_lock();
> > > > > + dev = dev_get_by_index_rcu(net, oif);
> > > > > + rcu_read_unlock();
> > > >
> > > > You can't assume '*dev' is still valid after rcu_read_unlock() unless
> > > > you hold a reference on it.
> > > >
> > > > > + rtnl_lock();
> > > > > + mdev = netdev_master_upper_dev_get(dev);
> > > > > + rtnl_unlock();
> > > >
> > > > Because of that, 'dev' might have already disappeared at the time
> > > > netdev_master_upper_dev_get() is called. So it may dereference an
> > > > invalid pointer here.
> > >
> > > Good point, thanks. I didn't expect those to change.
> > >
> > > This can be fixed, provided that RCU and RTNL locks can be nested:
> >
> > Well, yes and no. You can call rcu_read_{lock,unlock}() while under the
> > rtnl protection, but not the other way around.
> >
> > > rcu_read_lock();
> > > if (oif) {
> > > dev = dev_get_by_index_rcu(net, oif);
> > > rtnl_lock();
> > > mdev = netdev_master_upper_dev_get(dev);
> > > rtnl_unlock();
> > > }
> >
> > This is invalid: rtnl_lock() uses a mutex, so it can sleep and that's
> > forbidden inside an RCU critical section.
>
> Obviously, that's bad. Mea culpa.
>
> > > if (sk->sk_bound_dev_if) {
> > > bdev = dev_get_by_index_rcu(net, sk->sk_bound_dev_if);
> > > }
> > >
> > > addr_type = ipv6_addr_type(daddr);
> > > if ((__ipv6_addr_needs_scope_id(addr_type) && !oif) ||
> > > (addr_type & IPV6_ADDR_MAPPED) ||
> > > (oif && sk->sk_bound_dev_if && oif != sk->sk_bound_dev_if &&
> > > !(mdev && sk->sk_bound_dev_if && bdev && mdev == bdev))) {
> > > rcu_read_unlock();
> > > return -EINVAL;
> > > }
> > > rcu_read_unlock();
> > >
> > > But again this is still probably not race-free (bdev might also disappear before
> > > the mdev == bdev test), even if it passed fcnal-test.sh, there is much duplication
> > > of code, so your one-line solution is obviously by far better. :-)
> >
> > The real problem is choosing the right function for getting the master
> > device. In particular netdev_master_upper_dev_get() was a bad choice.
> > It forces you to take the rtnl, which is unnatural here and obliges you
> > to add extra code, while all this shouldn't be necessary in the first
> > place.
>
> Thank you for the additional insight. I had poor luck with Googling on
> these.
>
> I made a blunder after blunder. But it was insightful and brainstorming.
> Good exercise for my little grey cells.
>
> However, learning without making any errors appears to be simply a lot
> of blunt memorising. :-/
>
> It's good to be in an environment when one can learn from errors.
>
> :-)
I'm happy you found this useful.
> Regards,
> Mirsad
>
