PACKET socket can retain its fanout membership through link down and up
and leave a fanout while closed regardless of link state.
However, socket was forbidden from joining a fanout while it was not
RUNNING.

This scenario was identified while studying DPDK pmd_af_packet_drv.
Since sockets are only created during initialization, there is no reason
to fail the initialization if a single link is temporarily down.

This patch allows PACKET socket to join a fanout while not RUNNING.

Selftest psock_fanout is extended to test this "fanout while link down"
scenario.

Selftest psock_fanout is also extended to test fanout create/join by
socket that did not bind or specified a protocol, which carries an
implicit bind.

This is the only test that was performed.

Changes:

V03:
* psock_fanout: add test for joining fanout with unbound socket.
* Test that socket can receive packets before adding it to a fanout match.
  This is kind of replaces the RUNNING test that was removed.
* Initialize po->ifindex in packet_create. To -1 if no protocol is specified
  and add an explicit initialization to 0 if protocol is specified.
* Refactor relevant code in fanout_add within bind_lock, as a sequence of
  if {} else if {}, in order to reduce indentation of nested if statements and
  provide specific error codes.

V02: https://lore.kernel.org/netdev/cover.1728382839.git.gur.stavi@huawei.com
* psock_fanout: use explicit loopback up/down instead of toggle.
* psock_fanout: don't try to restore loopback state on failure.
* Rephrase commit message about "leaving a fanout".

V01: https://lore.kernel.org/netdev/cover.1728303615.git.gur.stavi@huawei.com/

Gur Stavi (3):
  af_packet: allow fanout_add when socket is not RUNNING
  selftests: net/psock_fanout: socket joins fanout when link is down
  selftests: net/psock_fanout: unbound socket fanout

 net/packet/af_packet.c                     | 35 ++++++----
 tools/testing/selftests/net/psock_fanout.c | 78 +++++++++++++++++++++-
 2 files changed, 96 insertions(+), 17 deletions(-)


base-commit: 36efaca9cb28a893cad98f0448c39a8b698859e2
-- 
2.45.2