bpf: sockmap: write_space events need to be passed to TCP handler

When sockmap code is using the stream parser it also handles the write
space events in order to handle the case where (a) verdict redirects
skb to another socket and (b) the sockmap then sends the skb but due
to memory constraints (or other EAGAIN errors) needs to do a retry.

But the initial code missed a third case where the
skb_send_sock_locked() triggers an sk_wait_event(). A typically case
would be when sndbuf size is exceeded. If this happens because we
do not pass the write_space event to the lower layers we never wake
up the event and it will wait for sndtimeo. Which as noted in ktls
fix may be rather large and look like a hang to the user.

To reproduce the best test is to reduce the sndbuf size and send
1B data chunks to stress the memory handling. To fix this pass the
event from the upper layer to the lower layer.
Signed-off-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

kernel/bpf/sockmap.c

@@ -1427,12 +1427,15 @@ static void smap_tx_work(struct work_struct *w)
 static void smap_write_space(struct sock *sk)
 {
 	struct smap_psock *psock;
+	void (*write_space)(struct sock *sk);
 
 	rcu_read_lock();
 	psock = smap_psock_sk(sk);
 	if (likely(psock && test_bit(SMAP_TX_RUNNING, &psock->state)))
 		schedule_work(&psock->tx_work);
+	write_space = psock->save_write_space;
 	rcu_read_unlock();
+	write_space(sk);
 }
 
 static void smap_stop_sock(struct smap_psock *psock, struct sock *sk)