Subject: [PATCH bpf v2 0/4] bpf: Update transport_header when encapsulating UDP tunnel in lwt

From: Leon Hwang <[email-masked]>
Date: Fri, 29 May 2026 23:13:47 +0800

Currently, bpf_lwt_push_ip_encap() does not update skb->transport_header.
When a driver, e.g. ice, reuses the stale skb->transport_header to
offload checksum computation to NIC hardware, VxLAN packets encapsulated
by bpf_lwt_push_encap() helper may be dropped due to incorrect checksum.

Update skb->transport_header in bpf_lwt_push_ip_encap() whenever the
encapsulated packet uses UDP, so checksum offload works correctly.

Fix these two issues reported by sashiko:

1. memcpy() hdr to a local buffer to avoid TOCTOU issue.
1. "iph->ihl < 5" was missing to avoid infinite-loop in MIPS driver.

Changes:
v1 -> v2:
* Address sashiko's reviews:
  * Fix TOCTOU issue in lwt to avoid changing hdr after checks.
  * Add check iph->ihl < 5 in lwt to avoid infinite-loop in MIPS driver.
  * Update comment style in selftests with BPF comment style.
* v1: https://lore.kernel.org/bpf/[email-masked]/

Leon Hwang (4):
  bpf: Fix TOCTOU issue in lwt
  bpf: Add check iph->ihl < 5 in lwt
  bpf: Update transport_header when encapsulating UDP tunnel in lwt
  selftests/bpf: Add tests to verify the fix of encapsulating VxLAN in
    lwt

 net/core/lwt_bpf.c                            |  20 ++-
 .../selftests/bpf/prog_tests/lwt_ip_encap.c   | 158 ++++++++++++++++++
 .../selftests/bpf/progs/test_lwt_ip_encap.c   | 112 +++++++++++++
 .../bpf/progs/test_lwt_ip_encap_fix.c         |  36 ++++
 4 files changed, 323 insertions(+), 3 deletions(-)
 create mode 100644 tools/testing/selftests/bpf/progs/test_lwt_ip_encap_fix.c

-- 
2.54.0