network: Allow af_packet to transmit +4 bytes for VLAN packets.

This allows user-space to send a '1500' MTU VLAN packet on a
1500 MTU ethernet frame.  The extra 4 bytes of a VLAN header is
not usually charged against the MTU when other parts of the
network stack is transmitting vlans...
Signed-off-by: Ben Greear <greearb@candelatech.com>
Reviewed-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/packet/af_packet.c

@@ -465,7 +465,7 @@ static int packet_sendmsg_spkt(struct kiocb *iocb, struct socket *sock,
 	 */
 
 	err = -EMSGSIZE;
-	if (len > dev->mtu + dev->hard_header_len)
+	if (len > dev->mtu + dev->hard_header_len + VLAN_HLEN)
 		goto out_unlock;
 
 	if (!skb) {
@@ -496,6 +496,19 @@ static int packet_sendmsg_spkt(struct kiocb *iocb, struct socket *sock,
 		goto retry;
 	}
 
+	if (len > (dev->mtu + dev->hard_header_len)) {
+		/* Earlier code assumed this would be a VLAN pkt,
+		 * double-check this now that we have the actual
+		 * packet in hand.
+		 */
+		struct ethhdr *ehdr;
+		skb_reset_mac_header(skb);
+		ehdr = eth_hdr(skb);
+		if (ehdr->h_proto != htons(ETH_P_8021Q)) {
+			err = -EMSGSIZE;
+			goto out_unlock;
+		}
+	}
 
 	skb->protocol = proto;
 	skb->dev = dev;
@@ -1199,7 +1212,7 @@ static int packet_snd(struct socket *sock,
 	}
 
 	err = -EMSGSIZE;
-	if (!gso_type && (len > dev->mtu+reserve))
+	if (!gso_type && (len > dev->mtu + reserve + VLAN_HLEN))
 		goto out_unlock;
 
 	err = -ENOBUFS;
@@ -1224,6 +1237,20 @@ static int packet_snd(struct socket *sock,
 	if (err < 0)
 		goto out_free;
 
+	if (!gso_type && (len > dev->mtu + reserve)) {
+		/* Earlier code assumed this would be a VLAN pkt,
+		 * double-check this now that we have the actual
+		 * packet in hand.
+		 */
+		struct ethhdr *ehdr;
+		skb_reset_mac_header(skb);
+		ehdr = eth_hdr(skb);
+		if (ehdr->h_proto != htons(ETH_P_8021Q)) {
+			err = -EMSGSIZE;
+			goto out_free;
+		}
+	}
+
 	skb->protocol = proto;
 	skb->dev = dev;
 	skb->priority = sk->sk_priority;