Socket error queue with timestamping and SOF_TIMESTAMPING_OPT_CMSG

[Miroslav Lichvar <mlichvar@redhat.com>]

I came across an interesting issue with error messages in sockets with
enabled timestamping using the SOF_TIMESTAMPING_OPT_CMSG option. When
the socket is connected and there is an error (e.g. due to destination
unreachable ICMP), select() indicates there is an exception on the
socket, but recvmsg() reading from the error queue returns with EAGAIN
and the application gets stuck in an infinite loop.

Some observations:
- it happens on both AF_INET and AF_INET6 SOCK_DGRAM sockets
- enabling the IP_RECVERR option avoids getting EAGAIN
- using recvmmsg() instead of recvmsg() avoids getting EAGAIN
  (that is why I didn't notice it earlier)
- disabling TX timestamping doesn't prevent the socket from having an
  exception
- reading from the non-error queue stops the loop

Is this a bug?

It looks to me like SOF_TIMESTAMPING_OPT_CMSG implicitly, but only
partially, enables IP_RECVERR. Are applications required to use
IP_RECVERR in this case? My expectation was that without IP_RECVERR
the error queue would only have messages with transmit timestamps, and
nothing would change with reporting of real errors. Also, from the
documentation I had an impression that SOF_TIMESTAMPING_OPT_CMSG is a
no-op on AF_INET6 sockets.

-- 
Miroslav Lichvar