[PATCH net-next v3 0/4] tcp: fix tcp_base_mss on passive connections and test MTU probing

[Yongjian Wang via B4 Relay]

With tcp_mtu_probing=2, passive TCP connections can start at the full
MSS because they sync MSS before initializing MTU probing. Patch 1
moves initialization next to the existing MSS sync in the passive
paths and in the active SYN-ACK path. Patch 2 adds a regression test.

Patches 3 and 4 add coverage for black hole detection and the MTU
search on active connections.

On x86_64 KVM, the revised MTU search test passed in IPv4, IPv6 and
IPv4-mapped IPv6 with patch 1 applied. Without patch 1, it failed in
all three modes. The first TSO packet carried 2072 bytes of TCP
payload, and the test expected 2048.

---
Changes in v3:
- Document how the passive MSS issue was discovered (patch 1).
- Let the SYN offer timestamps in the MTU search test so it also
  covers the active-open change in patch 1 (Neal, patch 4).
- Link to v2: https://lore.kernel.org/r/20261108-mtu-probing-series-v2-0-355d6490c3db@xxxxxxxxx

Changes in v2:
- Move tcp_mtup_init() before the existing MSS sync in IPv4 and IPv6
  passive opens and in the active SYN-ACK path. Remove the late
  initialization from tcp_init_transfer() (Kuniyuki and Neal).
- Keep the three packetdrill patches unchanged.
- Link to v1: https://lore.kernel.org/netdev/20261103-mtu-probing-series-v1-0-a2809e725aaa@xxxxxxxxx/

---
Yongjian Wang (4):
      tcp: initialize MTU probing before syncing MSS at connection setup
      selftests/net: packetdrill: test tcp_base_mss on passive connections
      selftests/net: packetdrill: add TCP black hole detection test
      selftests/net: packetdrill: add TCP MTU probing test

 net/ipv4/tcp_input.c                               |  2 +-
 net/ipv4/tcp_ipv4.c                                |  1 +
 net/ipv6/tcp_ipv6.c                                |  1 +
 .../net/packetdrill/tcp_mtu_probing_blackhole.pkt  | 37 +++++++++++++++++++
 .../net/packetdrill/tcp_mtu_probing_passive.pkt    | 27 ++++++++++++++
 .../net/packetdrill/tcp_mtu_probing_search.pkt     | 42 ++++++++++++++++++++++
 6 files changed, 109 insertions(+), 1 deletion(-)
---
base-commit: fa5196392691051963726787944db872420dfa75
change-id: 20261103-mtu-probing-series-a87b39fdbedf

Best regards,
--  
Yongjian Wang <echoowall@xxxxxxxxx>