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

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

October 28, 2025 at 19:47, "Paolo Abeni" <pabeni@redhat.com mailto:pabeni@redhat.com?to=%22Paolo%20Abeni%22%20%3Cpabeni%40redhat.com%3E > wrote:


> 
> On 10/28/25 12:30 PM, Paolo Abeni wrote:
> 
> > 
> > On 10/23/25 2:54 PM, 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.
> > > 
> >  
> >  I don't see how sockmap could modify the to-be-accepted socket sk_prot
> >  before mptcp_fallback_tcp_ops(), as such call happens before the fd is
> >  installed, and AFAICS sockmap can only fetch sockets via fds.
> >  
> >  Is this patch needed?
> > 
> Matttbe explained off-list the details of how that could happen. I think
> the commit message here must be more verbose to explain clearly the
> whys, even to those non proficient in sockmap like me.
> 
> Thanks,
> 
> Paolo
>

Thanks, I will add more details into commit message :).