Overall, we encountered a warning [1] that can be triggered by running the
selftest I provided.

MPTCP creates subflows for data transmission between two endpoints.
However, BPF can use sockops to perform additional operations when TCP
completes the three-way handshake. The issue arose because we used sockmap
in sockops, which replaces sk->sk_prot and some handlers. Since subflows
also have their own specialized handlers, this creates a conflict and leads
to traffic failure. Therefore, we need to reject operations targeting
subflows.

This patchset simply prevents the combination of subflows and sockmap
without changing any functionality.

A complete integration of MPTCP and sockmap would require more effort, for
example, we would need to retrieve the parent socket from subflows in
sockmap and implement handlers like read_skb.

If maintainers don't object, we can further improve this in subsequent
work.

v1: https://lore.kernel.org/mptcp/a0a2b87119a06c5ffaa51427a0964a05534fe6f1@linux.dev/T/#t

[1] truncated warning:
[   18.234652] ------------[ cut here ]------------
[   18.234664] WARNING: CPU: 1 PID: 388 at net/mptcp/protocol.c:68 mptcp_stream_accept+0x34c/0x380
[   18.234726] Modules linked in:
[   18.234755] RIP: 0010:mptcp_stream_accept+0x34c/0x380
[   18.234762] RSP: 0018:ffffc90000cf3cf8 EFLAGS: 00010202
[   18.234800] PKRU: 55555554
[   18.234806] Call Trace:
[   18.234810]  <TASK>
[   18.234837]  do_accept+0xeb/0x190
[   18.234861]  ? __x64_sys_pselect6+0x61/0x80
[   18.234898]  ? _raw_spin_unlock+0x12/0x30
[   18.234915]  ? alloc_fd+0x11e/0x190
[   18.234925]  __sys_accept4+0x8c/0x100
[   18.234930]  __x64_sys_accept+0x1f/0x30
[   18.234933]  x64_sys_call+0x202f/0x20f0
[   18.234966]  do_syscall_64+0x72/0x9a0
[   18.234979]  ? switch_fpu_return+0x60/0xf0
[   18.234993]  ? irqentry_exit_to_user_mode+0xdb/0x1e0
[   18.235002]  ? irqentry_exit+0x3f/0x50
[   18.235005]  ? clear_bhb_loop+0x50/0xa0
[   18.235022]  ? clear_bhb_loop+0x50/0xa0
[   18.235025]  ? clear_bhb_loop+0x50/0xa0
[   18.235028]  entry_SYSCALL_64_after_hwframe+0x76/0x7e
[   18.235066]  </TASK>
[   18.235109] ---[ end trace 0000000000000000 ]---
[   18.235677] sockmap: MPTCP sockets are not supported

Jiayuan Chen (3):
  net,mptcp: fix incorrect IPv4/IPv6 fallback detection with BPF Sockmap
  bpf,sockmap: disallow MPTCP sockets from sockmap updates
  selftests/bpf: Add mptcp test with sockmap

 net/core/sock_map.c                           |   9 ++
 net/mptcp/protocol.c                          |   7 +-
 .../testing/selftests/bpf/prog_tests/mptcp.c  | 136 ++++++++++++++++++
 .../selftests/bpf/progs/mptcp_sockmap.c       |  43 ++++++
 4 files changed, 193 insertions(+), 2 deletions(-)
 create mode 100644 tools/testing/selftests/bpf/progs/mptcp_sockmap.c

-- 
2.43.0