[EVPN] BIRD segfaults when I add interface to a bridge

[Tomáš Matuš]

Hello BIRDs! And hello Ondřej.

As a part of my thesis I've been toying with EVPN in BIRD on the oz-evpn branch and I managed to run into a segfault
when I add ethernet interface into a bridge. I am using a couple fedora 42 VMs and I have this running in pnetlab
but I hope virtualization is not a problem here.

I narrowed it down to this very minimal bird.conf:
​```
log stderr all;
log "/var/log/bird.log" all;

router id 10.10.10.3;

ipv4 table master4;
protocol device {
scan time 10;
}​```

and a reproducer is basically what is in the README in [cf-evpn-bgp-tags](https://gitlab.nic.cz/labs/bird-tools/-/tree/master/netlab/cf-evpn-bgp-tags) test:
```
[root at debug ~]# ip link add name br0 type bridge vlan_filtering 1
[root at debug ~]# ip link add name vxlan0 type vxlan id 100 local 10.10.10.3 dstport 4789 nolearning
[root at debug ~]# ip link set vxlan0 master br0[root at debug ~]# bridge link set dev vxlan0 learning off
[root at debug ~]# ip link set dev eth1 master br0
[root at debug ~]# bird -fSegmentation fault (core dumped)

```

I spent some time debugging this, although I am not too familiar with netlink I hope my description will be useful :)
Segfault happens in "sysdep/linux/netlink.c" because [this strcmp](https://gitlab.nic.cz/labs/bird/-/blob/oz-evpn/sysdep/linux/netlink.c?ref_type=heads#L1037) on line 1037 gets a NULL in the first parameter "kind" which seems
expected based on that "kind" is set conditionally. Looking at the [array li](https://gitlab.nic.cz/labs/bird/-/blob/oz-evpn/sysdep/linux/netlink.c?ref_type=heads#L1030) and its size == 3 and into the "nl_parse_attrs" function
I understand that you are only trying to get the first three (?) IFLA_INFO_x items which are
IFLA_INFO_UNSPEC, IFLA_INFO_KIND, IFLA_INFO_DATA.
The first "a->rta_type" that I see on my eth1 interface has a value 4 which is the IFLA_INFO_SLAVE_KIND
and reading the data from it I indeed get "bridge" as a result.
All of these get skipped by the first [continue](https://gitlab.nic.cz/labs/bird/-/blob/oz-evpn/sysdep/linux/netlink.c?ref_type=heads#L538) so "nl_parse_attrs" does not return anything useful.
Is it possible that netlink is just not sending the data you need?

Bug most likely comes from [66edbe43](https://gitlab.nic.cz/labs/bird/-/commit/66edbe43ce1fec8465577637c359d0294f580055) commit which is specific to the oz-evpn branch so other non-evpn branches are unaffected.
I'm also attaching a patch which fixes it for me and I can have EVPN up and running.

Thank you for your time and I hope you'll have a great day!
Tomáš
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