Re: POSSIBLE BUG: selftests/net/fcnal-test.sh: [FAIL] in vrf "bind - ns-B IPv6 LLA" test

[Mirsad Goran Todorovac]

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.

:-)

Regards,
Mirsad