[PATCH mptcp-next v8 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 v8 (addresses Matt v7 review on patch 2 + sashiko v7
findings on patch 3,
https://sashiko.dev/#/patchset/20260509211651.104934-1-devnexen@gmail.com):
 - patch 2/4: drop the unnecessary <net/ip.h> include (transitively
   pulled in).
 - patch 2/4: explain the v4-on-AF_INET6-msk skip in
   mptcp_setsockopt_all_sf() inline and in the commit message; note
   the case was unreachable before this patch.
 - patch 2/4: drop the unreachable default: case in
   mptcp_setsockopt_recverr().
 - patch 2/4: drop the unrelated sk_family != AF_INET6 checks added
   to mptcp_setsockopt() / mptcp_getsockopt() — the v6 dispatch path
   already errors out further down.
 - patch 3/4: drop the offending skb when sock_queue_err_skb() fails
   on the parent under rmem pressure, instead of re-queueing it on
   the subflow. (sashiko v7 #2, High)
 - patch 3/4: remove mptcp_subflow_errqueue_pending() and its
   reference in mptcp_poll(). No skbs are retained on subflow queues
   anymore, so the lockless conn_list walk is no longer needed.
   (sashiko v7 #1, Critical)
 - patch 3/4: drop mptcp_recv_error() — with no on-subflow retention,
   the MSG_ERRQUEUE branch in mptcp_recvmsg() forwards directly to
   inet_recv_error().

v7: https://lore.kernel.org/mptcp/20260509211651.104934-1-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                          |  47 +++++-
 net/mptcp/sockopt.c                           | 136 ++++++++++++++----
 .../selftests/net/mptcp/mptcp_sockopt.c       |  55 +++++++
 3 files changed, 209 insertions(+), 29 deletions(-)


base-commit: 8b808b715575367c7e00bcd76a597b7b57d674b8
--
2.53.0