[PATCH net 0/2] Revert advertising the TCP MSS from the configured MTU

[Esko Mobius via B4 Relay <devnull+echoowall.gmail.com@kernel.org>]

Commit 2640e6419594 ("net: advertise TCP MSS from the configured MTU,
not the learned PMTU") breaks TCP for clients behind networks that
filter inbound ICMP, when the server reaches them through a hop with a
smaller MTU: the handshake completes, then the connection stalls as
soon as the client sends full-size segments. This was reported in [1]
and confirmed on 7.3-rc7; reverting the commit on top of 7.3-rc7 fixes
it. Details and numbers are in patch 2.

The asymmetric (DSR-style) case that commit targets can still be served
by the existing per-route advmss metric, which overrides the default
after the revert; patch 2 includes a test of that as well. If you would
rather keep the new behaviour as an explicit opt-in some other way, I
am happy to test or rework this.

Patch 1 removes the packetdrill tests that assert the reverted
behaviour, patch 2 is the revert itself.

[1] https://lore.kernel.org/regressions/4F0FAEB6-A13B-47F3-BFF1-C706686C6F64@gmail.com/

Signed-off-by: Esko Mobius <echoowall@gmail.com>
---
Esko Mobius (2):
      Revert "selftests: net: packetdrill: add tests for advertised MSS with PMTU exceptions"
      Revert "net: advertise TCP MSS from the configured MTU, not the learned PMTU"

 include/net/ip.h                                   | 25 --------
 include/net/ip6_route.h                            | 37 ------------
 net/ipv4/route.c                                   |  4 +-
 net/ipv6/route.c                                   |  2 +-
 .../net/packetdrill/tcp_advmss_pmtu_ipv4.pkt       | 67 ----------------------
 .../net/packetdrill/tcp_advmss_pmtu_ipv6.pkt       | 67 ----------------------
 6 files changed, 3 insertions(+), 199 deletions(-)
---
base-commit: 7076f684d5132e6a6145e1da2fa74771ecd052e5
change-id: 20261026-advmss-revert-5805de17369f

Best regards,
--  
Esko Mobius <echoowall@gmail.com>