Hi Jiayuan,
On 05/11/2025 12:36, Jiayuan Chen wrote:
The sockmap feature allows bpf syscall from userspace using , or based
(is there a word missing before the ','?)
on bpf sockops, replacing the sk_prot of sockets during protocol stack processing with sockmap's custom read/write interfaces. ''' tcp_rcv_state_process() 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 ''' Consider two scenarios:
When the server has MPTCP enabled and the client also requests MPTCP, the sk passed to the BPF program is a subflow sk. Since subflows only handle partial data, replacing their sk_prot is meaningless and will cause traffic disruption.
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_ulp_fallback() subflow_drop_ctx() mptcp_subflow_ops_undo_override() ''' Subsequently, accept::mptcp_stream_accept::mptcp_fallback_tcp_ops() converts the subflow to plain TCP.
For the first case, we should prevent it from being combined with sockmap by setting sk_prot->psock_update_sk_prot to NULL, which will be blocked by sockmap's own flow.
For the second case, since subflow_syn_recv_sock() has already restored sk_prot to native tcp_prot/tcpv6_prot, no further action is needed.
Fixes: 0b4f33def7bb ("mptcp: fix tcp fallback crash")
It should not change anything for the backport, but for this patch, the Fixes tag can be older I think, e.g.
Fixes: cec37a6e41aa ("mptcp: Handle MP_CAPABLE options for outgoing connections")
If you need to send a v5, please use this one.
Reviewed-by: Matthieu Baerts (NGI0) matttbe@kernel.org
Cheers, Matt