net/packet: Ask driver for protocol if not provided by user

If a socket was created with socket(AF_PACKET, SOCK_RAW, 0), the
protocol number is unavailable. Try to ask the driver to extract it from
the L2 header in order for skb_try_probe_transport_header to succeed.
Signed-off-by: Maxim Mikityanskiy <maximmi@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/packet/af_packet.c

@@ -1850,6 +1850,15 @@ static int packet_rcv_spkt(struct sk_buff *skb, struct net_device *dev,
 	return 0;
 }
 
+static void packet_parse_headers(struct sk_buff *skb, struct socket *sock)
+{
+	if (!skb->protocol && sock->type == SOCK_RAW) {
+		skb_reset_mac_header(skb);
+		skb->protocol = dev_parse_header_protocol(skb);
+	}
+
+	skb_probe_transport_header(skb);
+}
 
 /*
  *	Output a raw packet to a device layer. This bypasses all the other
@@ -1970,7 +1979,7 @@ static int packet_sendmsg_spkt(struct socket *sock, struct msghdr *msg,
 	if (unlikely(extra_len == 4))
 		skb->no_fcs = 1;
 
-	skb_probe_transport_header(skb);
+	packet_parse_headers(skb, sock);
 
 	dev_queue_xmit(skb);
 	rcu_read_unlock();
@@ -2519,7 +2528,7 @@ static int tpacket_fill_skb(struct packet_sock *po, struct sk_buff *skb,
 		len = ((to_write > len_max) ? len_max : to_write);
 	}
 
-	skb_probe_transport_header(skb);
+	packet_parse_headers(skb, sock);
 
 	return tp_len;
 }
@@ -2925,7 +2934,7 @@ static int packet_snd(struct socket *sock, struct msghdr *msg, size_t len)
 		virtio_net_hdr_set_proto(skb, &vnet_hdr);
 	}
 
-	skb_probe_transport_header(skb);
+	packet_parse_headers(skb, sock);
 
 	if (unlikely(extra_len == 4))
 		skb->no_fcs = 1;