netdev - Re: [PATCH v2 net-next] tcp: Warn if sock_owned_by_user() is true in tcp_child

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Date:   Thu, 9 Dec 2021 00:00:35 -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 v2 net-next] tcp: Warn if sock_owned_by_user() is true in tcp_child_process().

On Wed, Dec 8, 2021 at 5:33 PM Kuniyuki Iwashima <kuniyu@...zon.co.jp> wrote:
>
> While creating a child socket from ACK (not TCP Fast Open case), 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().
>

I think you misunderstood me.

I think this code is not dead yet, so I would :

Not include a Fixes: tag to avoid unnecessary backports (of a patch
and its revert)

If you want to get syzbot coverage for few releases, especially with
MPTCP and synflood,
you  can then submit a patch like the following.

diff --git a/net/ipv4/tcp_minisocks.c b/net/ipv4/tcp_minisocks.c
index cf913a66df17..19da6e442fca 100644
--- a/net/ipv4/tcp_minisocks.c
+++ b/net/ipv4/tcp_minisocks.c
@@ -843,6 +843,9 @@ int tcp_child_process(struct sock *parent, struct
sock *child,
                 * in main socket hash table and lock on listening
                 * socket does not protect us more.
                 */
+
+               /* Check if this code path is obsolete ? */
+               WARN_ON_ONCE(1);
                __sk_add_backlog(child, skb);
        }