netdev - Re: [PATCH net] tcp: Remove sock_owned_by_user() test in tcp_child

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Date:   Wed, 8 Dec 2021 10:30:49 -0800
From:   Eric Dumazet <edumazet@...gle.com>
To:     Kuniyuki Iwashima <kuniyu@...zon.co.jp>
Cc:     "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Benjamin Herrenschmidt <benh@...zon.com>,
        Kuniyuki Iwashima <kuni1840@...il.com>, netdev@...r.kernel.org
Subject: Re: [PATCH net] tcp: Remove sock_owned_by_user() test in tcp_child_process().

On Tue, Dec 7, 2021 at 9:16 PM Kuniyuki Iwashima <kuniyu@...zon.co.jp> wrote:
>
> While creating a child socket, before v2.3.41, we used to call
> bh_lock_sock() later than now; it was called just before
> tcp_rcv_state_process().  The full socket was put into an accept queue
> and exposed to other CPUs before bh_lock_sock() so that process context
> might have acquired the lock by then.  Thus, we had to check if any
> process context was accessing the socket before tcp_rcv_state_process().
>
> We can see this code in tcp_v4_do_rcv() of v2.3.14. [0]
>
>         if (sk->state == TCP_LISTEN) {
>                 struct sock *nsk;
>
>                 nsk = tcp_v4_hnd_req(sk, skb);
>                 ...
>                 if (nsk != sk) {
>                         bh_lock_sock(nsk);
>                         if (nsk->lock.users != 0) {
>                                 ...
>                                 sk_add_backlog(nsk, skb);
>                                 bh_unlock_sock(nsk);
>                                 return 0;
>                         }
>                         ...
>                 }
>         }
>
>         if (tcp_rcv_state_process(sk, skb, skb->h.th, skb->len))
>                 goto reset;
>
> However, in 2.3.15, this lock.users test was replaced with BUG_TRAP() by
> mistake. [1]
>
>                 if (nsk != sk) {
>                         ...
>                         BUG_TRAP(nsk->lock.users == 0);
>                         ...
>                         ret = tcp_rcv_state_process(nsk, skb, skb->h.th, skb->len);
>                         ...
>                         bh_unlock_sock(nsk);
>                         ...
>                         return 0;
>                 }
>
> Fortunately, the test was back in 2.3.41. [2]  Then, related code was
> packed into tcp_child_process() with comments, which remains until now.
>
> What is interesting in v2.3.41 is that the bh_lock_sock() was moved to
> tcp_create_openreq_child() and placed just after sock_lock_init().
> Thus, the lock is never acquired until tcp_rcv_state_process() by process
> contexts.  The bh_lock_sock() is now in sk_clone_lock() and the rule does
> not change.
>
> As of now, alas, it is not possible to reach the commented path by the
> change.  Let's remove the remnant of the old days.
>
> [0]: https://cdn.kernel.org/pub/linux/kernel/v2.3/linux-2.3.14.tar.gz
> [1]: https://cdn.kernel.org/pub/linux/kernel/v2.3/patch-2.3.15.gz
> [2]: https://cdn.kernel.org/pub/linux/kernel/v2.3/patch-2.3.41.gz
>
> Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")

I do not think this patch qualifies as a stable candidate.

At best this is a cleanup.

At worst this could add a bug.

I would advise adding a WARN_ON_ONCE() there for at least one release
so that syzbot can validate for you if this is dead code or not.

TCP_SYN_RECV is not TCP_NEW_SYN_RECV

Thanks.

> Signed-off-by: Kuniyuki Iwashima <kuniyu@...zon.co.jp>
> ---
>  net/ipv4/tcp_minisocks.c | 18 ++++++------------
>  1 file changed, 6 insertions(+), 12 deletions(-)
>
> diff --git a/net/ipv4/tcp_minisocks.c b/net/ipv4/tcp_minisocks.c
> index 7c2d3ac2363a..b4a1f8728093 100644
> --- a/net/ipv4/tcp_minisocks.c
> +++ b/net/ipv4/tcp_minisocks.c
> @@ -833,18 +833,12 @@ int tcp_child_process(struct sock *parent, struct sock *child,
>         sk_mark_napi_id_set(child, skb);
>
>         tcp_segs_in(tcp_sk(child), skb);
> -       if (!sock_owned_by_user(child)) {
> -               ret = tcp_rcv_state_process(child, skb);
> -               /* Wakeup parent, send SIGIO */
> -               if (state == TCP_SYN_RECV && child->sk_state != state)
> -                       parent->sk_data_ready(parent);
> -       } else {
> -               /* Alas, it is possible again, because we do lookup
> -                * in main socket hash table and lock on listening
> -                * socket does not protect us more.
> -                */
> -               __sk_add_backlog(child, skb);
> -       }
> +
> +       ret = tcp_rcv_state_process(child, skb);
> +
> +       /* Wakeup parent, send SIGIO */
> +       if (state == TCP_SYN_RECV && child->sk_state != state)
> +               parent->sk_data_ready(parent);
>
>         bh_unlock_sock(child);
>         sock_put(child);
> --
> 2.30.2
>