Re: [PATCH 6.1.y v1 2/2] net,mptcp: fix proto fallback detection with BPF

[Matthieu Baerts <matttbe@kernel.org>]

Hi Jiayuan,

On 30/11/2025 04:23, 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.

(...)

> diff --git a/net/mptcp/protocol.c b/net/mptcp/protocol.c
> index 1dbc62537259..13e3510e6c8f 100644
> --- a/net/mptcp/protocol.c
> +++ b/net/mptcp/protocol.c
> @@ -79,8 +79,9 @@ static u64 mptcp_wnd_end(const struct mptcp_sock *msk)
>  static bool mptcp_is_tcpsk(struct sock *sk)
>  {
>  	struct socket *sock = sk->sk_socket;
> +	unsigned short family = READ_ONCE(sk->sk_family);
>  
> -	if (unlikely(sk->sk_prot == &tcp_prot)) {
> +	if (unlikely(family == AF_INET)) {
>  		/* we are being invoked after mptcp_accept() has
>  		 * accepted a non-mp-capable flow: sk is a tcp_sk,
>  		 * not an mptcp one.
> @@ -91,7 +92,7 @@ static bool mptcp_is_tcpsk(struct sock *sk)
>  		sock->ops = &inet_stream_ops;
>  		return true;
>  #if IS_ENABLED(CONFIG_MPTCP_IPV6)
> -	} else if (unlikely(sk->sk_prot == &tcpv6_prot)) {
> +	} else if (unlikely(family == AF_INET6)) {

These modifications here break MPTCP: this function (mptcp_is_tcpsk) is
there to check if the socket is a "plain" TCP one (return "true") or an
MPTCP one (return "false"). If it is not an MPTCP one, the sock ops is
modified.

Here, you are saying: any IPv4 or IPv6 socket is a "plain" TCP one,
never an MPTCP socket then.

I suggest adding ...

  if (sk->sk_protocol == IPPROTO_MPTCP)
          return false;

... at the beginning of this function. I'm planning to send a patch
later on including this check. Once it is sent, do you mind checking it
with sockmap if you have the setup available, please?

Cheers,
Matt
-- 
Sponsored by the NGI0 Core fund.