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
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 is conditioned on fib_multipath_hash_policy == 0 (L3)
   because policies 1-3 compute a deterministic hash from the flow
   keys which must not be overridden.

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.

I'd like guidance on whether to use the ISN as txhash when using
syncookies; it keeps the SYN-ACK and subsequent data path consistent,
but one could argue that this consistency doesn't matter because no
reordering is possible.

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()
- 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

 net/core/filter.c                          |   1 +
 net/ipv4/tcp_input.c                       |   6 +-
 net/ipv4/tcp_plb.c                         |   5 +-
 net/ipv4/tcp_timer.c                       |   2 +
 net/ipv6/af_inet6.c                        |   3 +
 net/ipv6/inet6_connection_sock.c           |   7 +
 net/ipv6/syncookies.c                      |   4 +
 net/ipv6/tcp_ipv6.c                        |  13 +-
 tools/testing/selftests/net/Makefile       |   1 +
 tools/testing/selftests/net/config         |   1 +
 tools/testing/selftests/net/ecmp_rehash.sh | 952 +++++++++++++++++++++
 11 files changed, 990 insertions(+), 5 deletions(-)
 create mode 100755 tools/testing/selftests/net/ecmp_rehash.sh

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2.53.0-Meta