Re: [PATCH net 1/1] mptcp: hold subflow request owners when cloning reqsk

[Matthieu Baerts <matttbe@kernel.org>]

Hi Kuniyuki,

Thank you for your reply!

16 Apr 2026 20:48:58 Kuniyuki Iwashima <kuniyu@google.com>:

> On Thu, Apr 16, 2026 at 10:45 AM Matthieu Baerts <matttbe@kernel.org> wrote:
>>
>> Hi Ren,
>>
>> On 15/04/2026 11:31, Ren Wei wrote:
>>> From: Ruide Cao <caoruide123@gmail.com>
>>>
>>> TCP request migration clones pending request sockets with
>>> inet_reqsk_clone(). For MPTCP MP_JOIN requests this raw-copies
>>> subflow_req->msk, but the cloned request does not take a new reference.
>>>
>>> Both the original and the cloned request can later drop the same msk in
>>> subflow_req_destructor(), and a migrated request may keep a dangling msk
>>> pointer after the original owner has already been released.
>>>
>>> Add a request_sock clone callback and let MPTCP grab a reference for cloned
>>> subflow requests that carry an msk. This keeps ownership balanced across
>>> both successful migrations and failed clone/insert paths without changing
>>> other protocols.

(...)

>>> diff --git a/net/ipv4/inet_connection_sock.c b/net/ipv4/inet_connection_sock.c
>>> index e961936b6be7..140a9e96ad58 100644
>>> --- a/net/ipv4/inet_connection_sock.c
>>> +++ b/net/ipv4/inet_connection_sock.c
>>> @@ -954,6 +954,9 @@ static struct request_sock *inet_reqsk_clone(struct request_sock *req,
>>>       if (sk->sk_protocol == IPPROTO_TCP && tcp_rsk(nreq)->tfo_listener)
>>>               rcu_assign_pointer(tcp_sk(nreq->sk)->fastopen_rsk, nreq);
>>
>> (Maybe TCP with fastopen could be this other user to call
>> rcu_assign_pointer()? (net-next material))
>>
>>> +     if (req->rsk_ops->init_clone)
>>> +             req->rsk_ops->init_clone(req, nreq);
>
> I think a simple direct call is better.
>
> #ifdef CONFIG_MPTCP
>     if (tcp_rsk(req)->is_mptcp)
>         mptcp_reqsk_clone(nreq);
> #endif

Fine by me!

I guess it is needed to check the protocol, similar to what is fine with
TFO above:

  if (sk->sk_protocol == IPPROTO_TCP) {
      if TFO
          ...
      if MPTCP (+ifdef)
          ...
  }

Cheers,
Matt