Messages in this thread | | | Date | Tue, 6 Jun 2023 20:50:52 +0200 | From | Guillaume Nault <> | Subject | Re: POSSIBLE BUG: selftests/net/fcnal-test.sh: [FAIL] in vrf "bind - ns-B IPv6 LLA" test |
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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.
> 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.
> Much obliged. > > Best regards, > Mirsad
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