[PATCH mptcp-next v7 0/4] mptcp: MSG_ERRQUEUE support on the parent socket

[David Carlier <devnexen@gmail.com>]

This series adds MSG_ERRQUEUE support on the MPTCP parent socket, so
poll() and recvmsg(MSG_ERRQUEUE) observe TX timestamps and MSG_ZEROCOPY
completion notifications through the standard inet ABI. IP_RECVERR /
IPV6_RECVERR (and their RFC4884 variants) are propagated to existing
and future subflows.

Patch 1 factors per-flag inet_assign_bit() calls in
sync_socket_options() into a mask-driven loop so future propagated
flags only need to extend MPTCP_INET_FLAGS_MASK.

Patch 2 wires up RECVERR setsockopt/getsockopt: snapshot the value,
apply it on the parent, and forward to every subflow under lock_sock()
so concurrent setsockopt callers cannot leave parent and subflows
desynchronized. Newly-joining subflows pick up the four RECVERR bits
through sync_socket_options().

Patch 3 splices forwardable err skbs (TIMESTAMPING / ZEROCOPY / LOCAL)
from each subflow's error queue onto the parent's, so pollers see
EPOLLERR and recvmsg(MSG_ERRQUEUE) on the parent drains them. Subflow
ICMP errors are dropped — they will be carried by a future
MPTCP_RECERR channel.

Patch 4 covers IP_RECVERR / IPV6_RECVERR propagation and the empty-
errqueue EAGAIN contract on MSG_ERRQUEUE | MSG_DONTWAIT in selftest.

v6 -> v7:
 - patch 2: gate SOL_IPV6 setsockopt/getsockopt dispatch on
   sk_family == AF_INET6, returning -ENOPROTOOPT otherwise, mirroring
   plain TCP. Addresses the sashiko Medium finding on v6 where
   IPV6_RECVERR silently succeeded on AF_INET MPTCP sockets.
 - patch 3: track moved skbs in mptcp_recv_error() and retry
   inet_recv_error() when ret == -EAGAIN && moved, so a successful
   subflow splice is not masked by the initial drain returning EAGAIN
   (sashiko High #2 on v6).
 - patch 3: add mptcp_subflow_errqueue_pending() and OR it into the
   EPOLLERR check in mptcp_poll(), so events stranded on a subflow
   when the parent is under rmem pressure still wake userspace
   (sashiko High #1 on v6).
 - rebased on current export.

Tested with KVM-validation auto-normal: 25/25 pass.

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                          |  92 ++++++++++-
 net/mptcp/sockopt.c                           | 146 ++++++++++++++----
 .../selftests/net/mptcp/mptcp_sockopt.c       |  55 +++++++
 3 files changed, 261 insertions(+), 32 deletions(-)


base-commit: 63b133728231ebba5167bd1e53dda9bcf0bee7c7
-- 
2.53.0