Hi Jiayuan,
On 11/11/2025 07:02, Jiayuan Chen wrote:
> The sockmap feature allows bpf syscall from userspace, or based
> on bpf sockops, replacing the sk_prot of sockets during protocol stack
> processing with sockmap's custom read/write interfaces.
> '''
> tcp_rcv_state_process()
> syn_recv_sock()/subflow_syn_recv_sock()
> tcp_init_transfer(BPF_SOCK_OPS_PASSIVE_ESTABLISHED_CB)
> bpf_skops_established <== sockops
> bpf_sock_map_update(sk) <== call bpf helper
> tcp_bpf_update_proto() <== update sk_prot
> '''
>
> When the server has MPTCP enabled but the client sends a TCP SYN
> without MPTCP, subflow_syn_recv_sock() performs a fallback on the
> subflow, replacing the subflow sk's sk_prot with the native sk_prot.
> '''
> subflow_syn_recv_sock()
> subflow_ulp_fallback()
> subflow_drop_ctx()
> mptcp_subflow_ops_undo_override()
> '''
>
> Then, this subflow can be normally used by sockmap, which replaces the
> native sk_prot with sockmap's custom sk_prot. The issue occurs when the
> user executes accept::mptcp_stream_accept::mptcp_fallback_tcp_ops().
> Here, it uses sk->sk_prot to compare with the native sk_prot, but this
> is incorrect when sockmap is used, as we may incorrectly set
> sk->sk_socket->ops.
>
> This fix uses the more generic sk_family for the comparison instead.
>
> Additionally, this also prevents a WARNING from occurring:
>
> result from ./scripts/decode_stacktrace.sh:
> ------------[ cut here ]------------
> WARNING: CPU: 0 PID: 337 at net/mptcp/protocol.c:68 mptcp_stream_accept \
> (net/mptcp/protocol.c:4005)
> Modules linked in:
> ...
>
> PKRU: 55555554
> Call Trace:
> <TASK>
> do_accept (net/socket.c:1989)
> __sys_accept4 (net/socket.c:2028 net/socket.c:2057)
> __x64_sys_accept (net/socket.c:2067)
> x64_sys_call (arch/x86/entry/syscall_64.c:41)
> do_syscall_64 (arch/x86/entry/syscall_64.c:63 arch/x86/entry/syscall_64.c:94)
> entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:130)
> RIP: 0033:0x7f87ac92b83d
>
> ---[ end trace 0000000000000000 ]---
>
> Fixes: cec37a6e41aa ("mptcp: Handle MP_CAPABLE options for outgoing
> connections")
The previous Fixes tag you used in the previous versions was correct for
this second patch:
Fixes: 0b4f33def7bb ("mptcp: fix tcp fallback crash")
But well, in terms of backports, that will be the same. So I guess we
don't need a new version just for that.
Also, I guess you missed my comments from the two previous versions:
> please remove the 'net,' prefix from the title. And maybe good
> to mention 'sockmap', e.g.
>
> mptcp: fix proto fallback detection with sockmap
But same here, fine like that.
> Cc: <[email protected]>
> Signed-off-by: Jiayuan Chen <[email protected]>
> Reviewed-by: Jakub Sitnicki <[email protected]>
> Reviewed-by: Matthieu Baerts (NGI0) <[email protected]>
For next time: the Signed-off-by of the sender should be at the end. So
here, your SoB should go to the end, after the already sent RvB tags.
But again, I don't think we need a v6 for that, except if net
maintainers prefer to.
Cheers,
Matt
--
Sponsored by the NGI0 Core fund.
