[PATCH net v2 0/2] xsk: fix multi-buffer TX software checksum

[Rongzheng Wang <testwrzwrz@gmail.com>]

This is v2 of the AF_XDP multi-buffer TX software checksum fix.

v1 was a single patch containing both the kernel fix and a standalone
selftest.  Review feedback asked to split it into a 2-patch series and to
reuse the existing xdp_metadata selftest infrastructure.  v2 does that.

Patch 1 moves the software checksum calculation from xsk_skb_metadata() to
the point where the last descriptor has been added to the skb.  The old
code calculated the checksum after the first descriptor, so multi-buffer
packets only included the first fragment in the checksum.

On a checksum failure the packet is now dropped after the descriptors have
been consumed.  Metadata validation makes this path unreachable in the
current copy-mode implementation; keeping the defensive drop avoids a
retry path for an skb that is no longer complete.

Patch 2 extends test_xdp_metadata instead of adding another test harness.
It reuses the existing AF_XDP setup, packet generation, completion, and
verification helpers, and sends the same UDP packet both as one descriptor
and as two descriptors.

Testing:
- final exact-else patched kernel: xdp_metadata 1/0 PASSED
- same-config unpatched kernel: checksum mismatch, xdp_metadata FAILED
- checkpatch --strict: 0 errors / 0 warnings for both patches

Changes in v2:
- split the kernel fix and selftest into 1/2 and 2/2
- reuse test_xdp_metadata helpers and reduce the selftest patch to
  86 insertions
- use literal else and unlikely in the checksum error path
- document the drop decision and test it on patched and unpatched kernels


Rongzheng Wang (2):
  xsk: defer software checksum until the skb is complete
  selftests/bpf: cover AF_XDP multi-buffer TX software checksum

 net/xdp/xsk.c                                 |  22 +--
 .../selftests/bpf/prog_tests/xdp_metadata.c   | 126 ++++++++++++------
 2 files changed, 100 insertions(+), 48 deletions(-)

-- 
2.39.5 (Apple Git-154)