[PATCH mptcp-next v4 0/4] mptcp: MSG_ERRQUEUE support on the parent socket

[David Carlier <devnexen@gmail.com>]

MPTCP already advertises IP_RECVERR/IPV6_RECVERR as supported, but the
parent socket does not currently provide usable MSG_ERRQUEUE handling.

This series wires the MPTCP socket up to the IPv4/IPv6 error queue
paths. It propagates RECVERR-related sockopts to existing and future
subflows, makes poll() report pending errqueue activity through the
parent socket, and lets recvmsg(MSG_ERRQUEUE) on the MPTCP socket
consume queued errors with the parent socket ABI.

A new prerequisite patch factors the per-flag inet_flags propagation
in sync_socket_options() into a single masked word copy, so further
inet_flags propagated by MPTCP can be added by extending the mask
rather than touching the call site.

Patch 2 then leverages the existing mptcp_setsockopt_all_sf() helper
for the setsockopt path and extends MPTCP_INET_FLAGS_MASK with the
four RECVERR bits, dropping the family-specific helpers from v3.

Based-on: <20260424-mptcp-pm-sockopt-set-all-sf-v1-1-38e7023822f8@kernel.org>

v3 -> v4:
  - New patch 1/4: factor inet_flags propagation in
    sync_socket_options() through MPTCP_INET_FLAGS_MASK, per Paolo's
    review.
  - Patch 2/4 (was 1/3): drop the mptcp_recverr_enabled() and
    mptcp_subflow_set_recverr() helpers; route the setsockopt path
    through mptcp_setsockopt_all_sf(). Inherit the four RECVERR bits
    via MPTCP_INET_FLAGS_MASK in sync_socket_options() instead of
    explicit inet[6]_assign_bit() calls.
  - Patch 3/4 (was 2/3): rework the MSG_ERRQUEUE plumbing per Paolo's
    review. Subflow err skbs are now spliced onto the parent msk's
    sk_error_queue from __mptcp_subflow_error_report() via the new
    __mptcp_subflow_splice_errqueue() helper. recvmsg(MSG_ERRQUEUE)
    on the parent reverts to plain inet_recv_error(), and mptcp_poll()
    only inspects the parent's sk_error_queue -- no more on-demand
    subflow walks, no extra lock_sock() / data_lock() in the poll or
    recv paths. Keep the original early-return structure of
    __mptcp_subflow_error_report() and fix the reverse christmas-tree
    variable order Paolo flagged.

v2 -> v3:
  - Only consume ssk->sk_err in the fallback / MPC-connect branch of
    __mptcp_subflow_error_report(). Steady-state MPTCP now leaves
    TCP's one-shot sk_err to TCP's own consumer instead of silently
    draining it via sock_error().
  - In mptcp_recv_error(), also route to inet_recv_error() when
    sk->sk_err is set, so a fallback-propagated error reaches userspace
    even when the parent errqueue is empty.
  - Scope the new selftest to IP_RECVERR sockopt propagation only.
    End-to-end errqueue delivery (TX timestamps, ICMP, zerocopy)
    depends on subflow-side producers that are out of scope for this
    series and will be covered by follow-up work. Fixes the
    mptcp_sockopt selftest timeout reported by the MPTCP CI on v2.

v1 -> v2:
  - Retargeted to mptcp-next per Matthieu Baerts' feedback (net-next
    closed during the merge window; iterate on the MPTCP tree).
  - Guard mptcp_setsockopt_v6_recverr() and its dispatch cases in
    mptcp_setsockopt_v6() with #if IS_ENABLED(CONFIG_IPV6) to fix
    the MPTCP CI link break on without_ipv6/with_mptcp configs
    (undefined reference to ipv6_setsockopt).

v1: https://lore.kernel.org/mptcp/20260421152216.38127-1-devnexen@gmail.com/
v2: https://lore.kernel.org/mptcp/20260421191337.58341-1-devnexen@gmail.com/
v3: https://lore.kernel.org/mptcp/20260421223338.52743-1-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                          |  33 +++++-
 net/mptcp/sockopt.c                           | 107 ++++++++++++++----
 .../selftests/net/mptcp/mptcp_sockopt.c       |  55 +++++++++
 3 files changed, 170 insertions(+), 25 deletions(-)

--
2.53.0