Support direct writes to nf_conn:mark from TC and XDP prog types. This
is useful when applications want to store per-connection metadata. This
is also particularly useful for applications that run both bpf and
iptables/nftables because the latter can trivially access this metadata.

One example use case would be if a bpf prog is responsible for advanced
packet classification and iptables/nftables is later used for routing
due to pre-existing/legacy code.

Past discussion:
- v1: https://lore.kernel.org/bpf/cover.1660592020.git.dxu@dxuuu.xyz/

Changes since v1:
- Add unimplemented stub for when !CONFIG_BPF_SYSCALL

Daniel Xu (4):
  bpf: Remove duplicate PTR_TO_BTF_ID RO check
  bpf: Add stub for btf_struct_access()
  bpf: Add support for writing to nf_conn:mark
  selftests/bpf: Add tests for writing to nf_conn:mark

 include/linux/bpf.h                           |  9 ++++
 include/net/netfilter/nf_conntrack_bpf.h      | 18 +++++++
 kernel/bpf/verifier.c                         |  3 --
 net/core/filter.c                             | 34 +++++++++++++
 net/netfilter/nf_conntrack_bpf.c              | 50 +++++++++++++++++++
 .../testing/selftests/bpf/prog_tests/bpf_nf.c |  1 +
 .../testing/selftests/bpf/progs/test_bpf_nf.c |  6 ++-
 .../selftests/bpf/progs/test_bpf_nf_fail.c    | 14 ++++++
 8 files changed, 130 insertions(+), 5 deletions(-)

-- 
2.37.1