[PATCH 0/3] mptcp: add RECVERR and MSG_ERRQUEUE support

[David Carlier <devnexen@gmail.com>]

MPTCP already advertises IP_RECVERR/IPV6_RECVERR as supported, but the
parent socket does not currently provide usable MSG_ERRQUEUE handling.

This series wires the MPTCP socket up to the IPv4/IPv6 error queue
paths. It propagates RECVERR-related sockopts to existing and future
subflows, makes poll() report pending errqueue activity through the
parent socket, and allows recvmsg(MSG_ERRQUEUE) on the MPTCP socket to
consume queued errors with the parent socket ABI.

The series also handles mixed-family subflows by applying the matching
sockopt according to each subflow family, and avoids silently losing an
error skb if requeueing to the parent socket fails under rmem pressure.

Patch 1 propagates the RECVERR sockopts to subflows.
Patch 2 implements parent-socket MSG_ERRQUEUE handling and poll()
reporting.
Patch 3 adds selftest coverage for RECVERR sockopt round-trips and
timestamping-driven MSG_ERRQUEUE delivery on the MPTCP parent socket.

Testing:
- make -C tools/testing/selftests/net/mptcp mptcp_sockopt
- git diff --check

David Carlier (3):
  mptcp: propagate RECVERR sockopts to subflows
  mptcp: support MSG_ERRQUEUE on the parent socket
  selftests: mptcp: cover RECVERR and MSG_ERRQUEUE

 net/mptcp/protocol.c                          | 121 +++++++++++---
 net/mptcp/sockopt.c                           | 125 ++++++++++++++
 .../selftests/net/mptcp/mptcp_sockopt.c       | 152 ++++++++++++++++++
 3 files changed, 380 insertions(+), 18 deletions(-)

-- 
2.53.0