af_packet: fix the SO_SNDTIMEO constraint not effective on tpacked_snd()

Due to the changes in commit 581073f ("af_packet: do not call
packet_read_pending() from tpacket_destruct_skb()"), every time
tpacket_destruct_skb() is executed, the skb_completion is marked as
completed. When wait_for_completion_interruptible_timeout() returns
completed, the pending_refcnt has not yet been reduced to zero.
Therefore, when ph is NULL, the wait function may need to be called
multiple times until packet_read_pending() finally returns zero.

We should call sock_sndtimeo() only once, otherwise the SO_SNDTIMEO
constraint could be way off.

Fixes: 581073f ("af_packet: do not call packet_read_pending() from tpacket_destruct_skb()")
Cc: stable@kernel.org
Suggested-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Yun Lu <luyun@kylinos.cn>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Author: Yun Lu

net/packet/af_packet.c

@@ -2785,7 +2785,7 @@ static int tpacket_snd(struct packet_sock *po, struct msghdr *msg)
 	int len_sum = 0;
 	int status = TP_STATUS_AVAILABLE;
 	int hlen, tlen, copylen = 0;
-	long timeo = 0;
+	long timeo;
 
 	mutex_lock(&po->pg_vec_lock);
 
@@ -2839,14 +2839,14 @@ static int tpacket_snd(struct packet_sock *po, struct msghdr *msg)
 	if ((size_max > dev->mtu + reserve + VLAN_HLEN) && !vnet_hdr_sz)
 		size_max = dev->mtu + reserve + VLAN_HLEN;
 
+	timeo = sock_sndtimeo(&po->sk, msg->msg_flags & MSG_DONTWAIT);
 	reinit_completion(&po->skb_completion);
 
 	do {
 		ph = packet_current_frame(po, &po->tx_ring,
 					  TP_STATUS_SEND_REQUEST);
 		if (unlikely(ph == NULL)) {
 			if (need_wait && skb) {
-				timeo = sock_sndtimeo(&po->sk, msg->msg_flags & MSG_DONTWAIT);
 				timeo = wait_for_completion_interruptible_timeout(&po->skb_completion, timeo);
 				if (timeo <= 0) {
 					err = !timeo ? -ETIMEDOUT : -ERESTARTSYS;