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

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