XDP metadata support for tun driver

Hi all,

this v5 of the patch series is very similar to v4, but rebased onto the
bpf-next/net branch instead of bpf-next/master.
Because the commit c047e0e0e435 ("selftests/bpf: Optionally open a
dedicated namespace to run test in it") is not yet included in this branch,
I changed the xdp_context_tuntap test to manually create a namespace to run
the test in.

Not so successful pipeline: https://github.com/kernel-patches/bpf/actions/runs/136824...

The CI pipeline failed because of veristat changes in seemingly unrelated
eBPF programs. I don't think this has to do with the changes from this
patch series, but if it does, please let me know what I may have to do
different to make the CI pass.

---

v5:
- rebase onto bpf-next/net
- resolve rebase conflicts
- change xdp_context_tuntap test to manually create and open a network
  namespace using netns_new

v4: https://lore.kernel.org/bpf/20250227142330.1605996-1-marc...
- strip unrelated changes from the selftest patches
- extend commit message for "selftests/bpf: refactor xdp_context_functional
  test and bpf program"
- the NOARP flag was not effective to prevent other packets from
  interfering with the tests, add a filter to the XDP program instead
- run xdp_context_tuntap in a separate namespace to avoid conflicts with
  other tests

v3: https://lore.kernel.org/bpf/20250224152909.3911544-1-marc...
- change the condition to handle xdp_buffs without metadata support, as
  suggested by Willem de Bruijn <willemb@google.com>
- add clarifying comment why that condition is needed
- set NOARP flag in selftests to ensure that the kernel does not send
  packets on the test interfaces that may interfere with the tests

v2: https://lore.kernel.org/bpf/20250217172308.3291739-1-marc...
- submit against bpf-next subtree
- split commits and improved commit messages
- remove redundant metasize check and add clarifying comment instead
- use max() instead of ternary operator
- add selftest for metadata support in the tun driver

v1: https://lore.kernel.org/all/20250130171614.1657224-1-marc...

Marcus Wichelmann (6):
  net: tun: enable XDP metadata support
  net: tun: enable transfer of XDP metadata to skb
  selftests/bpf: move open_tuntap to network helpers
  selftests/bpf: refactor xdp_context_functional test and bpf program
  selftests/bpf: add test for XDP metadata support in tun driver
  selftests/bpf: fix file descriptor assertion in open_tuntap helper

 drivers/net/tun.c                             |  28 +++-
 tools/testing/selftests/bpf/network_helpers.c |  28 ++++
 tools/testing/selftests/bpf/network_helpers.h |   3 +
 .../selftests/bpf/prog_tests/lwt_helpers.h    |  29 ----
 .../bpf/prog_tests/xdp_context_test_run.c     | 145 +++++++++++++++++-
 .../selftests/bpf/progs/test_xdp_meta.c       |  53 +++++--
 6 files changed, 230 insertions(+), 56 deletions(-)

-- 
2.43.0