Re: [PATCH v3] Linux: Simplify __opensock and fix race condition [BZ #28353]

[Adhemerval Zanella <adhemerval.zanella@linaro.org>]

On 27/09/2021 15:53, Florian Weimer via Libc-alpha wrote:
> AF_NETLINK support is not quite optional on modern Linux systems
> anymore, so it is likely that the first attempt will always succeed.
> Consequently, there is no need to cache the result.  Keep AF_UNIX
> and the Internet address families as a fallback, for the rare case
> that AF_NETLINK is missing.  The other address families previously
> probed are totally obsolete be now, so remove them.
> 
> Use this simplified version as the generic implementation, disabling
> Netlink support as needed.

It seems that defaulting to AF_NETLINKS fails for some operations on
older kernels:

$ uname -a
Linux ubuntu16 4.4.0-210-generic #242-Ubuntu SMP Fri Apr 16 09:57:56 UTC 2021 x86_64 x86_64 x86_64 GNU/Linux
$ ./testrun.sh --tool=strace inet/test_ifindex --direct
[...]
socket(PF_NETLINK, SOCK_DGRAM|SOCK_CLOEXEC, NETLINK_ROUTE) = 3
ioctl(3, SIOCGIFINDEX, {ifr_name="lo"}) = -1 EOPNOTSUPP (Operation not supported)
close(3)                                = 0

I am not sure if this something related to the system configuration or
some security features, but AF_UNIX does work in this case.