linux-kernel - Re: POSSIBLE BUG: selftests/net/fcnal-test.sh: [FAIL] in vrf "bind

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Date:   Tue, 6 Jun 2023 21:17:24 +0200
From:   Mirsad Goran Todorovac <mirsad.todorovac@....unizg.hr>
To:     Guillaume Nault <gnault@...hat.com>
Cc:     netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>, Shuah Khan <shuah@...nel.org>,
        linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org
Subject: Re: POSSIBLE BUG: selftests/net/fcnal-test.sh: [FAIL] in vrf "bind -
 ns-B IPv6 LLA" test

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