Subject: Re: [PATCH net v3 1/3] net,mptcp: fix proto fallback detection with BPF sockmap

From: "Jiayuan Chen" <[email-masked]>
Date: Thu, 23 Oct 2025 14:38:48 +0000

October 23, 2025 at 22:10, "Matthieu Baerts" <[email-masked] mailto:[email-masked]?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-masked]>
> >  Signed-off-by: Jiayuan Chen <[email-masked]>
> >  Reviewed-by: Jakub Sitnicki <[email-masked]>
> >  ---
> >  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.
>