Currently sk_rethink_txhash() re-rolls the socket's txhash on RTO,
PLB, and spurious-retransmission events, but the new hash is not
propagated into the IPv6 ECMP path selection logic.  The cached
route is reused and fib6_select_path() is never re-invoked, so
the connection uses the same local ECMP decision.

This series adds the two missing pieces:

1. __sk_dst_reset() alongside sk_rethink_txhash() so the cached dst
   is invalidated and the next transmit triggers a fresh route lookup.

2. fl6->mp_hash set from sk_txhash before each route lookup so
   fib6_select_path() picks a path based on the (potentially re-rolled)
   hash.  This only applies to fib_multipath_hash_policy 0 (the
   default L3 policy).  Policy 0 is already asymmetric for TCP: with
   auto_flowlabels enabled (the default), each socket gets a per-socket
   flowlabel, so the hash is already per-connection and unidirectional.
   For policy 0 the local ECMP path now follows sk_txhash even when a
   flow label is present (reflected REPFLOW or explicitly set); only
   local path selection changes, the on-wire flow label is unaffected.
   Policies 1-3 exist for operators who need deterministic symmetric
   hashing (e.g., for stateful middleboxes) and are left unchanged.

Patch 1 is the kernel change; patch 2 adds selftests covering SYN
rehash, SYN/ACK rehash, midstream RTO rehash, midstream ACK rehash
(spurious retransmission), PLB rehash, a policy 1 negative test,
a flowlabel leak regression test, two dst rebuild consistency
tests (normal and syncookie) verifying that natural route
invalidation does not cause unintended path changes, and a
syncookie server path consistency test verifying that the SYN-ACK
and post-cookie ACKs use the same ECMP nexthop.

Changes since v11: https://lore.kernel.org/netdev/20260602181428.2318919-1-ntspring@meta.com/ (local)
Patch 1:
- Fix the IPv6-only rule to exclude IPv4-mapped connections: key the
  cookie txhash on skb->protocol, not sk->sk_family (Sashiko AI review)
- Set fl6->mp_hash in tcp_v6_send_response() so RSTs and time-wait
  ACKs use the connection's ECMP path (Sashiko AI review)
- Remove the bpf_sk_assign_tcp_reqsk() txhash init added in v7; it is
  redundant, as cookie_tcp_reqsk_init() always sets txhash before the
  request socket is routed (verified by poisoning txhash and running
  the tcp_custom_syncookie BPF selftest: the route lookup never saw
  the poison)
- Document that policy 0 IPv6 TCP ECMP selection follows txhash over a
  reflected/explicit flow label (on-wire flow label unchanged)
Patch 2:
- Drain TCP teardown between rounds so late FIN/RST packets do not
  pollute the next round's tc filter counters (bot+bpf-ci)
- Skip the syncookie tests when CONFIG_SYN_COOKIES is unavailable;
  select it in selftests/net/config

Changes since v10: https://lore.kernel.org/netdev/20260529160136.1010064-1-ntspring@meta.com/ (local)
Patch 1:
- Fix build without CONFIG_SYN_COOKIES
- Leave IPv4 syncookie txhash unmodified (`net_tx_rndhash()`)
- Document the IPv6 TCP policy 0 behavior change in ip-sysctl.rst
Patch 2:
- Correct runtime estimate from ~15s to ~60s
- Build DCTCP as `=y` instead of `=m` to avoid module load races
- Fix false failure of the midstream ACK test by limiting the send
  buffer to avoid a closed receive window; window probes do not
  cause rehash

Changes since v9: https://lore.kernel.org/netdev/20260526203403.3517607-1-ntspring@meta.com/ (local)
Patch 1:
- Split cookie_init_sequence() into pure computation and a new
  cookie_record_sent() helper for the side effects; call
  cookie_record_sent() after route_req() succeeds so the overflow
  timestamp and SYNCOOKIESSENT counter are not bumped when no
  SYN-ACK is sent
Patch 2:
- Make midstream ACK rehash test more reliable by blocking the unused
  path first
- Fix port overlap when ECMP_REBUILD_ROUNDS exceeds the default

Changes since v8: https://lore.kernel.org/netdev/20260522215733.929238-1-ntspring@meta.com/ (local)
Patch 1:
- Fix REPFLOW flowlabel reflection for syncookie SYN-ACKs: pass 0 as
  tw_isn to route_req() so tcp_v6_init_req() saves ireq->pktopts
Patch 2:
- Give midstream and ACK rehash attempt helpers distinct failure
  messages (no TX activity vs no data on alternate path vs counter
  not incrementing) instead of a single generic error
- Drop unused ns_server parameter from ecmp_dst_rebuild_check()
- Clean up server socat before break on setup failure in the dst
  rebuild loop

Changes since v7: https://lore.kernel.org/netdev/20260520064310.4154268-1-ntspring@meta.com/ (local)
Patch 1:
- Remove #if IS_ENABLED(CONFIG_IPV6) guards around __sk_dst_reset()
  in tcp_plb.c and tcp_timer.c (Eric Dumazet)
- Guard mp_hash in inet6_csk_route_socket() on sk_protocol == IPPROTO_TCP
  instead of txhash != 0, since non-TCP callers like L2TP set sk_txhash
  in __ip6_datagram_connect() and should retain flow-key-based ECMP
- Use the syncookie (ISN) as txhash for both the SYN-ACK route lookup
  and cookie_v6_check() socket creation, so the server's ECMP selection is
  consistent across the stateless SYN-ACK and the subsequent full socket.
  Move cookie_init_sequence() before route_req() in tcp_conn_request()
  so the SYN-ACK dst is computed with the cookie-derived txhash; derive
  txhash from snt_isn in cookie_tcp_reqsk_init() to match
Patch 2:
- Invalidate dst via dummy route add/del instead of route replace to
  avoid a transient single-nexthop state during multipath replacement
- Add syncookie server path consistency test verifying the SYN-ACK and
  post-cookie ACKs use the same ECMP path
- Strengthen policy 1 negative test to wait for multiple rehash attempts
  and verify SYNs landed on exactly one interface

Changes since v6: https://lore.kernel.org/netdev/20260517174522.2232057-1-ntspring@meta.com/ (local)
- Guard mp_hash assignment so that non-TCP callers of
  inet6_csk_route_socket() fall through to rt6_multipath_hash()
  (superseded in v8 by sk_protocol == IPPROTO_TCP guard)
- Initialize txhash in bpf_sk_assign_tcp_reqsk() to avoid reading
  uninitialized slab memory in inet6_csk_route_req() (reverted in v12
  as redundant)
- Check post-rebuild busywait return status to avoid silent false pass

Changes since v5: https://lore.kernel.org/netdev/20260513204048.2721843-1-ntspring@meta.com/ (local)
- Improve selftest reliability: suppress __dst_negative_advice() via
  tcp_retries1=255 in dst rebuild tests so a real RTO cannot trigger
  an unintended rehash; add internal retry to midstream and ACK
  rehash tests to tolerate probabilistic ECMP path selection; fix
  midstream baseline capture to account for packets that bypass tc
  filters during the prio qdisc's TCQ_F_CAN_BYPASS window
- Increase ECMP_REBUILD_ROUNDS default to 10 for reliable regression
  detection with 2-way ECMP; replace sleep with busywait
- Use tcp_allowed_congestion_control instead of changing the host's
  default congestion control for PLB test
- Use (txhash >> 1) ?: 1 to guarantee non-zero mp_hash, since zero
  falls back to rt6_multipath_hash()

Changes since v4: https://lore.kernel.org/netdev/20260507171319.1259115-1-ntspring@meta.com/ (local)
- Condition fl6->mp_hash on fib_multipath_hash_policy == 0 to preserve
  deterministic hash policies 1-3 (e.g., symmetric 5-tuple for policy 1)
- Set fl6->mp_hash in tcp_v6_connect() and cookie_v6_check() for
  initial route lookup consistency; move sk_set_txhash() earlier
  (Jakub Kicinski)
- Add policy 1 negative test; improve sysctl save/restore
- Add flowlabel leak test confirming mp_hash does not alter the
  on-wire IPv6 flow label
- Add dst rebuild consistency tests (normal and syncookie) verifying
  that route table changes do not cause unintended ECMP path changes

Changes since v3: https://lore.kernel.org/netdev/20260505193824.2791642-1-ntspring@meta.com/ (local)
- Use __sk_dst_reset() instead of sk_dst_reset() since the socket lock
  is held in all three call sites (Eric Dumazet)
- Guard __sk_dst_reset() with sk->sk_family == AF_INET6 since IPv4 ECMP
  does not use sk_txhash for path selection
- Guard __sk_dst_reset() in tcp_plb_check_rehash() with the return value
  of sk_rethink_txhash()
- Move tcp_rsk(req)->txhash initialization before route_req() in
  tcp_conn_request() to avoid reading uninitialized memory
- Add CONFIG_TCP_CONG_DCTCP=m to selftests/net/config for PLB test
- Skip PLB test gracefully if DCTCP is not available
- Save and restore original congestion control algorithm in PLB test
- Default get_netstat_counter() to 0 when counter is not found
- Skip all tests if tcp_syn_linear_timeouts is not available
- Replace bash/pipe data sources with socat OPEN:/dev/zero for
  cleaner process cleanup
- Fix shellcheck warnings

Changes since v2: https://lore.kernel.org/netdev/20260408070514.1840227-1-ntspring@meta.com/ (local)
- Retitle "ECMP" to "local ECMP" to distinguish from remote ECMP
  (Neal Cardwell)
- Add fl6->mp_hash propagation in inet6_sk_rebuild_header() (af_inet6.c),
  covering the dst rebuild path used on established sockets
- Remove incorrect ir_iif update from tcp_check_req() in tcp_minisocks.c;
  the SYN/ACK rehash is already handled by tcp_rtx_synack() re-rolling
  txhash which feeds into inet6_csk_route_req()'s mp_hash
  (Eric Dumazet)
- Add ACK rehash and PLB rehash selftests
- Improve selftest reliability

Changes since v1: https://lore.kernel.org/netdev/20260408002802.2448424-1-ntspring@meta.com/ (local)
- Use tcp_rsk(req)->txhash instead of jhash_1word(req->num_retrans, ...)
  for ECMP path selection in inet6_csk_route_req(), making the request
  socket path consistent with the established socket path (Eric Dumazet)
- Add comments explaining the >> 1 shift for 31-bit mp_hash range
- Use socat -u (unidirectional) in selftest to avoid SIGPIPE race
- Increase tcp_syn_retries and tcp_syn_linear_timeouts to 25 for
  better rehash coverage

Neil Spring (2):
  tcp: rehash onto different local ECMP path on retransmit timeout
  selftests: net: add local ECMP rehash test

 Documentation/networking/ip-sysctl.rst     |    6 +-
 include/net/tcp.h                          |   20 +-
 net/ipv4/syncookies.c                      |   11 +-
 net/ipv4/tcp_input.c                       |   19 +-
 net/ipv4/tcp_plb.c                         |    5 +-
 net/ipv4/tcp_timer.c                       |    2 +
 net/ipv6/af_inet6.c                        |    3 +
 net/ipv6/inet6_connection_sock.c           |    8 +
 net/ipv6/syncookies.c                      |    4 +
 net/ipv6/tcp_ipv6.c                        |   21 +-
 tools/testing/selftests/net/Makefile       |    1 +
 tools/testing/selftests/net/config         |    2 +
 tools/testing/selftests/net/ecmp_rehash.sh | 1106 ++++++++++++++++++++
 13 files changed, 1193 insertions(+), 15 deletions(-)
 create mode 100755 tools/testing/selftests/net/ecmp_rehash.sh

-- 
2.53.0-Meta