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* [PATCH net v2 0/2] net: keep the TCP fixed-ID flags on the header they describe
@ 2026-11-14 18:27 Julius Bairaktaris
  2026-11-14 18:27 ` [PATCH net v2 1/2] " Julius Bairaktaris
                   ` (3 more replies)
  0 siblings, 4 replies; 5+ messages in thread
From: Julius Bairaktaris @ 2026-11-14 18:27 UTC (permalink / raw)
  To: netdev
  Cc: davem, edumazet, kuba, pabeni, horms, andrew+netdev, dsahern,
	idosch, ncardwell, kuniyu, richardbgobert, willemb, ecree.xilinx,
	linux-net-drivers, saeedm, tariqt, mbloch, leon, linux-rdma,
	alexei.starovoitov, ast, daniel, john.fastabend, sdf, martin.lau,
	andrii, eddyz87, memxor, song, yonghong.song, jolsa, emil,
	ihor.solodrai, corbet, rdunlap, skhan, shuah, bpf, linux-doc,
	linux-kselftest, linux-kernel

GRO sets SKB_GSO_TCP_FIXEDID for a TCP packet whose IPv4 IDs do not
change. Since commit 3271f19bf7b9 ("net: gso: restore ids of outer ip
headers correctly") the flag describes the outermost header and
SKB_GSO_TCP_FIXEDID_INNER the inner one, so every path that adds or
removes a header would have to move the flags. None does, and a TCP
stream that GRO merged and that crosses a tunnel boundary is segmented
with the fixed-ID state of the wrong header.

Patch 1 lets SKB_GSO_TCP_FIXEDID describe the innermost header again and
replaces the inner flag with SKB_GSO_TCP_FIXEDID_OUTER, as Alexei
suggested in review of v1. Patch 2 adds a selftest.

Tested in QEMU (KVM, KASAN and lockdep) with the selftest, 18 cases,
400 segments each:
- net (30c71476d842), 2 of 2 runs: 6 cases fail. Fixed inner IDs
  increment after IPIP over IPv4 and IPv6 and after the ESP tunnel (390
  of 400 segments carry an inner ID the sender did not use), the outer
  IDs repeat after GRE and VXLAN over IPv4 (10 distinct outer IDs for 400
  segments), and a VXLAN stream with fixed outer IDs keeps that state
  after decapsulation into IPIP (10 distinct outer IDs).
- this series, 2 of 2 runs: 18 of 18 pass, no KASAN or lockdep report.

With a tunnel inside a tunnel, such as GRE traffic that GRO merged and
that is routed into an IPIP tunnel without decapsulation, two flags still
describe three IPv4 headers: the new outermost header takes
SKB_GSO_TCP_FIXEDID_OUTER of the header below it, as it takes
SKB_GSO_TCP_FIXEDID on net. This series does not change that case; it
gets a separate fix.

Changes in v2:
- Keep SKB_GSO_TCP_FIXEDID on the innermost header and describe the
  outer header with SKB_GSO_TCP_FIXEDID_OUTER instead of moving the
  flag on encapsulation and decapsulation (Alexei). Encapsulation then
  needs no change, which also covers ESP tunnel mode with crypto
  offload, missed in v1 (Alexei, Sashiko), the Geneve GRO hint path,
  which sets skb->encapsulation again after iptunnel_pull_offloads()
  (Sashiko), and BPF encapsulation followed by an L3-only decapsulation
  (Sashiko). FOU, which pulls the offloads twice, clears the outer flag
  twice.
- Keep the encap_level reset in gre_gso_segment() and
  __skb_udp_tunnel_segment(): v1 undid commit 5197f3499c47 ("net: Reset
  encap_level to avoid resetting features on inner IP headers"), which
  with remote checksum offload masks NETIF_F_HW_CSUM out of the inner
  features on devices without it in hw_enc_features (Alexei, Sashiko).
- Drop the segment routing changes: the outer header there is IPv6
  (Alexei). This also drops the read of hdr->nexthdr after
  iptunnel_pull_offloads() in ipv6_srh_rcv() that Sashiko reported as a
  use-after-free; ipv6_srh_rcv() is unchanged.
- Convert mlx5 and sfc to the outer flag (Alexei).
- Selftest: add an ESP tunnel mode case offloaded to netdevsim; check
  the inner ID values, not only how many distinct IDs arrive; skip the
  decap cases when the IPIP device cannot be created; fail a case whose
  receiver reports no result; keep IPv6 link-local traffic off the
  tunnels, whose tx_packets decide the GRO skip; remove the temporary
  file on exit; sort the Makefile entry; wrap lines at 80 columns
  (Sashiko, NIPA).
- v1: https://lore.kernel.org/netdev/cover.1792857378.git.julius@bairaktaris.de/

I used Claude (Anthropic) to help write these patches.

Julius Bairaktaris (2):
  net: keep the TCP fixed-ID flags on the header they describe
  selftests: net: check TCP IP IDs across tunnels after GRO and GSO

 .../networking/segmentation-offloads.rst      |   4 +-
 .../net/ethernet/mellanox/mlx5/core/en_rx.c   |   8 +-
 drivers/net/ethernet/sfc/ef100_tx.c           |  12 +-
 include/linux/netdevice.h                     |   2 +-
 include/linux/skbuff.h                        |   4 +-
 include/net/ip_tunnels.h                      |   1 +
 net/core/filter.c                             |   1 +
 net/ipv4/af_inet.c                            |   8 +-
 net/ipv4/tcp_offload.c                        |   6 +-
 tools/testing/selftests/net/.gitignore        |   1 +
 tools/testing/selftests/net/Makefile          |   2 +
 tools/testing/selftests/net/tcp_fixedid.c     | 411 ++++++++++++++++++
 tools/testing/selftests/net/tcp_fixedid.sh    | 400 +++++++++++++++++
 13 files changed, 836 insertions(+), 24 deletions(-)
 create mode 100644 tools/testing/selftests/net/tcp_fixedid.c
 create mode 100755 tools/testing/selftests/net/tcp_fixedid.sh


base-commit: 30c71476d842b094651bd7712f6c8ee8716147d0
-- 
2.53.0


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2026-11-14 18:27 [PATCH net v2 0/2] net: keep the TCP fixed-ID flags on the header they describe Julius Bairaktaris
2026-11-14 18:27 ` [PATCH net v2 1/2] " Julius Bairaktaris
2026-11-14 18:27 ` [PATCH net v2 2/2] selftests: net: check TCP IP IDs across tunnels after GRO and GSO Julius Bairaktaris
2026-11-14 18:33 ` [PATCH net v2 0/2] net: keep the TCP fixed-ID flags on the header they describe netdev-bot+sinfo
2026-11-14 18:40 ` Alexei Starovoitov

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