[PATCH mptcp-next v12 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
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 parent's sk_error_queue at
error-report time. All forwarded events go through sock_queue_err_skb(),
which re-homes skb->sk onto the parent and charges sk_rmem_alloc, so the
parent'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
parent'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 parent stores the bit so MPTCP-aware
helpers can branch on it.

Patch 4 is a selftest covering the propagation path.

Changes in v12 (addresses Paolo's v11 review):
 - reordered the series so the core MSG_ERRQUEUE infrastructure lands
   first, before the sockopt plumbing that exposes it to user space.
   The former patch 3/4 is now 1/4.
 - 1/4: narrow mptcp_errqueue_skb_forwardable() to
   SO_EE_ORIGIN_TIMESTAMPING. SO_EE_ORIGIN_ZEROCOPY and
   SO_EE_ORIGIN_LOCAL events cannot be generated on an MPTCP parent
   today, so the helper only needs extending if and when they can be.
 - 1/4: trim the block comment above sock_queue_err_skb() and the
   matching changelog paragraphs, now redundant with the narrower
   filter.
 - 2/4: reverse christmas tree declaration ordering in
   sync_socket_options().
 - dropped the Suggested-by: Paolo Abeni tags from patches 1-3.
 - rebased on current export.

v11: https://lore.kernel.org/mptcp/20260531145955.322337-1-devnexen@gmail.com/
v10: https://lore.kernel.org/mptcp/20260529174524.260199-1-devnexen@gmail.com/

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                          |  54 +++++--
 net/mptcp/sockopt.c                           | 153 +++++++++++++++---
 .../selftests/net/mptcp/mptcp_sockopt.c       |  55 +++++++
 3 files changed, 228 insertions(+), 34 deletions(-)


base-commit: 6201be839b36e8dd22a23b9b367388ed900a257f
-- 
2.53.0