Merge branch 'mptcp-fix-conflicts-between-mptcp-and-sockmap'

Jiayuan Chen says:

====================
mptcp: Fix conflicts between MPTCP and sockmap

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

sockmap works by replacing sk_data_ready, recvmsg, sendmsg operations and
implementing fast socket-level forwarding logic:
1. Users can obtain file descriptors through userspace socket()/accept()
   interfaces, then call BPF syscall to perform these replacements.
2. Users can also use the bpf_sock_hash_update helper (in sockops programs)
   to replace handlers when TCP connections enter ESTABLISHED state
  (BPF_SOCK_OPS_PASSIVE_ESTABLISHED_CB/BPF_SOCK_OPS_ACTIVE_ESTABLISHED_CB)

However, when combined with MPTCP, an issue arises: MPTCP creates subflow
sk's and performs TCP handshakes, so the BPF program obtains subflow sk's
and may incorrectly replace their sk_prot. We need to reject such
operations. In patch 1, we set psock_update_sk_prot to NULL in the
subflow's custom sk_prot.

Additionally, if the server's listening socket has MPTCP enabled and the
client's TCP also uses MPTCP, we should allow the combination of subflow
and sockmap. This is because the latest Golang programs have enabled MPTCP
for listening sockets by default [2]. For programs already using sockmap,
upgrading Golang should not cause sockmap functionality to fail.

Patch 2 prevents the WARNING from occurring.

Despite these patches fixing stream corruption, users of sockmap must set
GODEBUG=multipathtcp=0 to disable MPTCP until sockmap fully supports it.

[1] truncated warning:
------------[ cut here ]------------
WARNING: CPU: 1 PID: 388 at net/mptcp/protocol.c:68 mptcp_stream_accept+0x34c/0x380
Modules linked in:
RIP: 0010:mptcp_stream_accept+0x34c/0x380
RSP: 0018:ffffc90000cf3cf8 EFLAGS: 00010202
PKRU: 55555554
Call Trace:
 <TASK>
 do_accept+0xeb/0x190
 ? __x64_sys_pselect6+0x61/0x80
 ? _raw_spin_unlock+0x12/0x30
 ? alloc_fd+0x11e/0x190
 __sys_accept4+0x8c/0x100
 __x64_sys_accept+0x1f/0x30
 x64_sys_call+0x202f/0x20f0
 do_syscall_64+0x72/0x9a0
 ? switch_fpu_return+0x60/0xf0
 ? irqentry_exit_to_user_mode+0xdb/0x1e0
 ? irqentry_exit+0x3f/0x50
 ? clear_bhb_loop+0x50/0xa0
 ? clear_bhb_loop+0x50/0xa0
 ? clear_bhb_loop+0x50/0xa0
 entry_SYSCALL_64_after_hwframe+0x76/0x7e
 </TASK>
---[ end trace 0000000000000000 ]---

[2]: https://go-review.googlesource.com/c/go/+/607715
====================

Link: https://patch.msgid.link/20251111060307.194196-1-jiayuan.chen@linux.dev
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>