[PATCH net-next 0/4] mptcp: support MSG_ERRQUEUE

[Matthieu Baerts (NGI0)]

This series lets MPTCP applications use poll(EPOLLERR) and
recvmsg(MSG_ERRQUEUE) on the MPTCP socket to drain TX timestamps
through the standard inet ABI, the same way they would on a plain TCP
socket. ICMP-derived errors stay on the subflow queue: the legacy
RECVERR ABI cannot convey their per-subflow peer identity, and they
are intended for a future MPTCP_RECERR channel.

- Patch 1 splices subflow err-skbs onto the MPTCP's sk_error_queue at
  error-report time. All forwarded events go through sock_queue_err_skb,
  which re-homes skb->sk onto the MPTCP and charges sk_rmem_alloc, so
  the MPTCP's error queue stays bounded by sk_rcvbuf and is dropped under
  rmem pressure, matching tcp's tx-timestamp path and ip_icmp_error() /
  ipv6_icmp_error(). mptcp_recvmsg(MSG_ERRQUEUE) forwards directly to
  inet_recv_error(), and mptcp_poll() advertises EPOLLERR purely on the
  MPTCP's sk_err / sk_error_queue, matching tcp_poll().

- Patch 2 factors the existing inet_flags subflow-propagation hard-coded
  list into a mask, so the next patch can extend it without churn.

- Patch 3 makes IP_RECVERR / IPV6_RECVERR (and the RFC4884 variants)
  propagate to the subflows. The MPTCP stores the bit so MPTCP-aware
  helpers can branch on it.

- Patch 4 is a selftest covering the propagation path.

Signed-off-by: Matthieu Baerts (NGI0) <matttbe@xxxxxxxxxx>
---
David Carlier (4):
      mptcp: support MSG_ERRQUEUE on the parent socket
      mptcp: sockopt: factor inet_flags propagation into a mask
      mptcp: propagate RECVERR sockopts to subflows
      selftests: mptcp: cover IP_RECVERR sockopt propagation

 net/mptcp/protocol.c                              |  55 ++++++--
 net/mptcp/sockopt.c                               | 155 ++++++++++++++++++----
 tools/testing/selftests/net/mptcp/mptcp_sockopt.c |  70 ++++++++++
 3 files changed, 246 insertions(+), 34 deletions(-)
---
base-commit: 4bb9710c6a68d35207f123aef55dcd50e7195ec5
change-id: 20260918-net-next-mptcp-msg_errqueue-0e2049a30061

Best regards,
--  
Matthieu Baerts (NGI0) <matttbe@xxxxxxxxxx>