[PATCH net v5 0/2] xsk: fix multi-buffer TX software checksum

[Rongzheng Wang]

This is v5 of the AF_XDP multi-buffer TX software checksum fix.

I found this while testing multi-buffer TX with XDP_UMEM_TX_SW_CSUM: a
two-descriptor packet had a bad wire checksum, while a single descriptor
was correct. There was no production or syzbot report.

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.

Patch 2 sends the packet using every descriptor count from 1 through
MAX_SKB_FRAGS + 1, and runs the existing metadata and checksum verification
for each case.  The last case is the copy-mode frame limit documented in
af_xdp.rst.

The updated selftest passes in a local QEMU run.

v5:
- remove the unnecessary else branch after the multi-buffer check
- carry all review tags received on v3 and v4

v4: https://lore.kernel.org/all/20261030023248.78802-1-testwrzwrz@xxxxxxxxx/
v3: https://lore.kernel.org/all/20261028025026.11401-1-testwrzwrz@xxxxxxxxx/
v2: https://lore.kernel.org/all/20261024141157.59301-1-testwrzwrz@xxxxxxxxx/
v1: https://lore.kernel.org/all/20261024091256.17918-1-testwrzwrz@xxxxxxxxx/

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                                 |  23 ++-
 .../selftests/bpf/prog_tests/xdp_metadata.c   | 147 +++++++++++-------
 2 files changed, 107 insertions(+), 63 deletions(-)

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