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

["Jiayuan Chen" <jiayuan.chen@linux.dev>]

2025/12/1 24:21, "Matthieu Baerts" <matttbe@kernel.org mailto:matttbe@kernel.org?to=%22Matthieu%20Baerts%22%20%3Cmatttbe%40kernel.org%3E > wrote:


> 
> 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?

Yes, of course. I can test it once I receive the patch.

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