net: do not leave an empty skb in write queue

Under memory stress conditions, tcp_sendmsg_locked()
might call sk_stream_wait_memory(), thus releasing the socket lock.

If a fresh skb has been allocated prior to this,
we should not leave it in the write queue otherwise
tcp_write_xmit() could panic.

This apparently does not happen often, but a future change
in __sk_mem_raise_allocated() that Shakeel and others are
considering would increase chances of being hurt.

Under discussion is to remove this controversial part:

    /* Fail only if socket is _under_ its sndbuf.
     * In this case we cannot block, so that we have to fail.
     */
    if (sk->sk_wmem_queued + size >= sk->sk_sndbuf) {
        /* Force charge with __GFP_NOFAIL */
        if (memcg_charge && !charged) {
            mem_cgroup_charge_skmem(sk->sk_memcg, amt,
                gfp_memcg_charge() | __GFP_NOFAIL);
        }
        return 1;
    }

Fixes: fdfc5c85 ("tcp: remove empty skb from write queue in error cases")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Shakeel Butt <shakeelb@google.com>
Link: https://lore.kernel.org/r/20231019112457.1190114-1-edumazet@google.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/ipv4/tcp.c

@@ -927,10 +927,11 @@ int tcp_send_mss(struct sock *sk, int *size_goal, int flags)
 	return mss_now;
 }
 
-/* In some cases, both sendmsg() could have added an skb to the write queue,
+/* In some cases, sendmsg() could have added an skb to the write queue,
  * but failed adding payload on it. We need to remove it to consume less
  * memory, but more importantly be able to generate EPOLLOUT for Edge Trigger
- * epoll() users.
+ * epoll() users. Another reason is that tcp_write_xmit() does not like
+ * finding an empty skb in the write queue.
  */
 void tcp_remove_empty_skb(struct sock *sk)
 {
@@ -1289,6 +1290,7 @@ int tcp_sendmsg_locked(struct sock *sk, struct msghdr *msg, size_t size)
 
 wait_for_space:
 		set_bit(SOCK_NOSPACE, &sk->sk_socket->flags);
+		tcp_remove_empty_skb(sk);
 		if (copied)
 			tcp_push(sk, flags & ~MSG_MORE, mss_now,
 				 TCP_NAGLE_PUSH, size_goal);