October 23, 2025 at 22:10, "Matthieu Baerts" <[email protected]
mailto:[email protected]?to=%22Matthieu%20Baerts%22%20%3Cmatttbe%40kernel.org%3E
> wrote:
>
> Hi Jiayuan,
>
> On 23/10/2025 14:54, Jiayuan Chen wrote:
>
> >
> > When the server has MPTCP enabled but receives a non-MP-capable request
> > from a client, it calls mptcp_fallback_tcp_ops().
> >
> > Since non-MPTCP connections are allowed to use sockmap, which replaces
> > sk->sk_prot, using sk->sk_prot to determine the IP version in
> > mptcp_fallback_tcp_ops() becomes unreliable. This can lead to assigning
> > incorrect ops to sk->sk_socket->ops.
> >
> > Additionally, when BPF Sockmap modifies the protocol handlers, the
> > original WARN_ON_ONCE(sk->sk_prot != &tcp_prot) check would falsely
> > trigger warnings.
> >
> > Fix this by using the more stable sk_family to distinguish between IPv4
> > and IPv6 connections, ensuring correct fallback protocol operations are
> > selected even when BPF Sockmap has modified the socket protocol handlers.
> >
> > Fixes: 0b4f33def7bb ("mptcp: fix tcp fallback crash")
> > Cc: <[email protected]>
> > Signed-off-by: Jiayuan Chen <[email protected]>
> > Reviewed-by: Jakub Sitnicki <[email protected]>
> > ---
> > net/mptcp/protocol.c | 9 +++++++--
> > 1 file changed, 7 insertions(+), 2 deletions(-)
> >
> > diff --git a/net/mptcp/protocol.c b/net/mptcp/protocol.c
> > index 0292162a14ee..2393741bc310 100644
> > --- a/net/mptcp/protocol.c
> > +++ b/net/mptcp/protocol.c
> > @@ -61,11 +61,16 @@ static u64 mptcp_wnd_end(const struct mptcp_sock *msk)
> >
> > static const struct proto_ops *mptcp_fallback_tcp_ops(const struct sock
> > *sk)
> > {
> > + /* When BPF sockmap is used, it may replace sk->sk_prot.
> > + * Using sk_family is a reliable way to determine the IP version.
> > + */
> > + unsigned short family = READ_ONCE(sk->sk_family);
> > +
> > #if IS_ENABLED(CONFIG_MPTCP_IPV6)
> > - if (sk->sk_prot == &tcpv6_prot)
> > + if (family == AF_INET6)
> > return &inet6_stream_ops;
> > #endif
> > - WARN_ON_ONCE(sk->sk_prot != &tcp_prot);
> > + WARN_ON_ONCE(family != AF_INET);
> > return &inet_stream_ops;
> >
> Just to be sure: is there anything in BPF modifying sk->sk_socket->ops?
> Because that's what mptcp_fallback_tcp_ops() will do somehow.
>
> In other words, is it always fine to set inet(6)_stream_ops? (I guess
> yes, but better to be sure while we are looking at that :) )
Hi Matt,
I can confirm that on the BPF side, the only special operations targeting
sockets currently are sockmap/sockhash. Their implementations do not modify
sk->sk_socket->ops. Currently, they only modify sk->prot, because the BPF
side typically operates on 'struct sock' and does not concern itself with
'struct socket'.
Therefore, setting inet(6)_stream_ops is fine.
Thanks,
Jiayuan
> >
> > }
> >
> Cheers,
> Matt
> --
> Sponsored by the NGI0 Core fund.
>
