[PATCH net v3 0/2] xsk: fix multi-buffer TX software checksum

[Rongzheng Wang]

This is v3 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 extends test_xdp_metadata to cover the single-descriptor and
multi-buffer TX cases.

The updated selftest passes on the v3 kernel in a local QEMU run.

v3:
- preserve unlikely() around pool->tx_sw_csum (Maciej)
- restore the binary MAC literals in the selftest (Sashiko)
- keep a distinct malformed-packet counter out of this stable fix; if it is
  useful, it can be added separately for net-next later (Magnus)

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   | 122 ++++++++++++------
 2 files changed, 99 insertions(+), 46 deletions(-)

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