[PATCH bpf v2 0/2] bpf: test_run: Fix false -EBADMSG with BPF_F_TEST_SKB_CHECKSUM_COMPLETE

BPF_PROG_TEST_RUN with BPF_F_TEST_SKB_CHECKSUM_COMPLETE returns -EBADMSG
for correct tc programs that call bpf_skb_change_tail(). The helper
moves the skb to CHECKSUM_NONE and leaves skb->csum stale, but the
post-run check added by commit a3cfe84cca28 ("bpf: Add CHECKSUM_COMPLETE
to bpf test progs") still compares against it.

Patch 1 only does the check while the skb is still CHECKSUM_COMPLETE.
Patch 2 adds selftests for a trim, a grow followed by a write, and a
negative case that keeps the skb CHECKSUM_COMPLETE with a stale
skb->csum.

Tested in a VM on bpf, with two kernels that differ only by patch 1
(subtest results):

                           without patch 1    with patch 1
  change_tail_trim         FAIL (-EBADMSG)    OK
  change_tail_grow_write   FAIL (-EBADMSG)    OK
  store_no_recompute       OK                 OK
  test_skb_pkt_end         OK                 OK

Changes in v2:
- Put the opening /* of multi-line comments on its own line (bpf-ci,
  Sashiko).

v1: https://lore.kernel.org/bpf/20261007181249.352831-1-skokovmaksimevg@gmail.com/ (local)

Maxim Skokov (2):
  bpf: test_run: Fix false -EBADMSG after skb leaves CHECKSUM_COMPLETE
  selftests/bpf: Cover test_run csum validation after
    bpf_skb_change_tail()

 net/bpf/test_run.c                            |  8 +++-
 .../bpf/prog_tests/skb_csum_complete.c        | 39 ++++++++++++++++
 .../selftests/bpf/progs/skb_csum_complete.c   | 46 +++++++++++++++++++
 3 files changed, 92 insertions(+), 1 deletion(-)
 create mode 100644 tools/testing/selftests/bpf/prog_tests/skb_csum_complete.c
 create mode 100644 tools/testing/selftests/bpf/progs/skb_csum_complete.c


base-commit: ff47652a4b66c067c765a7ad464d930b5a9367cc
-- 
2.47.3