[PATCH mptcp-next v9 0/4] mptcp: MSG_ERRQUEUE support on the parent socket

[David Carlier <devnexen@gmail.com>]

This series lets MPTCP applications use poll(EPOLLERR) and
recvmsg(MSG_ERRQUEUE) on the parent socket to drain TX timestamps,
MSG_ZEROCOPY completion notifications and SO_EE_ORIGIN_LOCAL events
that are produced by the subflows, 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 factors the existing inet_flags subflow-propagation hard-coded
list into a mask, so subsequent patches can extend it without churn.

Patch 2 makes IP_RECVERR / IPV6_RECVERR (and the RFC4884 variants)
propagate to the subflows. The parent stores the bit so MPTCP-aware
helpers can branch on it.

Patch 3 splices subflow err-skbs onto the parent's sk_error_queue at
error-report time. When the parent's err queue is full, the splice
drops the offending skb (matching ip_icmp_error() / ipv6_icmp_error()
behaviour). mptcp_recvmsg(MSG_ERRQUEUE) forwards directly to
inet_recv_error(), and mptcp_poll() advertises EPOLLERR purely on the
parent's sk_err / sk_error_queue, matching tcp_poll().

Patch 4 is a selftest covering the propagation path.

Changes in v9 (addresses sashiko v8 review,
https://sashiko.dev/#/patchset/cover.1779890764.git.devnexen@gmail.com):
 - patch 2/4: don't propagate the v6-only RECVERR6 bits onto v4
   subflows in sync_socket_options(); matches the SOL_IPV6 skip already
   performed in mptcp_setsockopt_all_sf(). The bits were never read on
   an AF_INET subflow, but left existing and newly-joined subflows
   inconsistent. (sashiko v8, Low)
 - patch 3/4: don't break the __mptcp_error_report() subflow scan after
   a bare errqueue splice; only break on a real sk_err. Splicing one
   subflow's error queue no longer short-circuits the loop and leaves
   the remaining subflows' pending notifications undrained. (sashiko
   v8, High)

v8: https://lore.kernel.org/mptcp/cover.1779890764.git.devnexen@gmail.com/

David Carlier (4):
  mptcp: sockopt: factor inet_flags propagation into a mask
  mptcp: propagate RECVERR sockopts to subflows
  mptcp: support MSG_ERRQUEUE on the parent socket
  selftests: mptcp: cover IP_RECVERR sockopt propagation

 net/mptcp/protocol.c                          |  55 +++++--
 net/mptcp/sockopt.c                           | 144 +++++++++++++++---
 .../selftests/net/mptcp/mptcp_sockopt.c       |  55 +++++++
 3 files changed, 221 insertions(+), 33 deletions(-)


base-commit: 6ae41d6df6c12883277b2c9ab490b5c8b1a4fc85
-- 
2.53.0