net/packet: annotate accesses to po->xmit

po->xmit can be set from setsockopt(PACKET_QDISC_BYPASS),
while read locklessly.

Use READ_ONCE()/WRITE_ONCE() to avoid potential load/store
tearing issues.

Fixes: d346a3fa ("packet: introduce PACKET_QDISC_BYPASS socket option")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/packet/af_packet.c

@@ -307,7 +307,8 @@ static void packet_cached_dev_reset(struct packet_sock *po)
 
 static bool packet_use_direct_xmit(const struct packet_sock *po)
 {
-	return po->xmit == packet_direct_xmit;
+	/* Paired with WRITE_ONCE() in packet_setsockopt() */
+	return READ_ONCE(po->xmit) == packet_direct_xmit;
 }
 
 static u16 packet_pick_tx_queue(struct sk_buff *skb)
@@ -2867,7 +2868,8 @@ static int tpacket_snd(struct packet_sock *po, struct msghdr *msg)
 		packet_inc_pending(&po->tx_ring);
 
 		status = TP_STATUS_SEND_REQUEST;
-		err = po->xmit(skb);
+		/* Paired with WRITE_ONCE() in packet_setsockopt() */
+		err = READ_ONCE(po->xmit)(skb);
 		if (unlikely(err != 0)) {
 			if (err > 0)
 				err = net_xmit_errno(err);
@@ -3070,7 +3072,8 @@ static int packet_snd(struct socket *sock, struct msghdr *msg, size_t len)
 		virtio_net_hdr_set_proto(skb, &vnet_hdr);
 	}
 
-	err = po->xmit(skb);
+	/* Paired with WRITE_ONCE() in packet_setsockopt() */
+	err = READ_ONCE(po->xmit)(skb);
 	if (unlikely(err != 0)) {
 		if (err > 0)
 			err = net_xmit_errno(err);
@@ -4007,7 +4010,8 @@ packet_setsockopt(struct socket *sock, int level, int optname, sockptr_t optval,
 		if (copy_from_sockptr(&val, optval, sizeof(val)))
 			return -EFAULT;
 
-		po->xmit = val ? packet_direct_xmit : dev_queue_xmit;
+		/* Paired with all lockless reads of po->xmit */
+		WRITE_ONCE(po->xmit, val ? packet_direct_xmit : dev_queue_xmit);
 		return 0;
 	}
 	default: