af-packet: Add flag to distinguish VID 0 from no-vlan.

Currently, user-space cannot determine if a 0 tcp_vlan_tci
means there is no VLAN tag or the VLAN ID was zero.

Add flag to make this explicit.  User-space can check for
TP_STATUS_VLAN_VALID || tp_vlan_tci > 0, which will be backwards
compatible. Older could would have just checked for tp_vlan_tci,
so it will work no worse than before.
Signed-off-by: Ben Greear <greearb@candelatech.com>
Acked-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/if_packet.h

@@ -70,6 +70,7 @@ struct tpacket_auxdata {
 #define TP_STATUS_COPY		0x2
 #define TP_STATUS_LOSING	0x4
 #define TP_STATUS_CSUMNOTREADY	0x8
+#define TP_STATUS_VLAN_VALID   0x10 /* auxdata has valid tp_vlan_tci */
 
 /* Tx ring - header status */
 #define TP_STATUS_AVAILABLE	0x0

net/packet/af_packet.c

@@ -798,7 +798,12 @@ static int tpacket_rcv(struct sk_buff *skb, struct net_device *dev,
 			getnstimeofday(&ts);
 		h.h2->tp_sec = ts.tv_sec;
 		h.h2->tp_nsec = ts.tv_nsec;
-		h.h2->tp_vlan_tci = vlan_tx_tag_get(skb);
+		if (vlan_tx_tag_present(skb)) {
+			h.h2->tp_vlan_tci = vlan_tx_tag_get(skb);
+			status |= TP_STATUS_VLAN_VALID;
+		} else {
+			h.h2->tp_vlan_tci = 0;
+		}
 		hdrlen = sizeof(*h.h2);
 		break;
 	default:
@@ -1725,8 +1730,12 @@ static int packet_recvmsg(struct kiocb *iocb, struct socket *sock,
 		aux.tp_snaplen = skb->len;
 		aux.tp_mac = 0;
 		aux.tp_net = skb_network_offset(skb);
-		aux.tp_vlan_tci = vlan_tx_tag_get(skb);
-
+		if (vlan_tx_tag_present(skb)) {
+			aux.tp_vlan_tci = vlan_tx_tag_get(skb);
+			aux.tp_status |= TP_STATUS_VLAN_VALID;
+		} else {
+			aux.tp_vlan_tci = 0;
+		}
 		put_cmsg(msg, SOL_PACKET, PACKET_AUXDATA, sizeof(aux), &aux);
 	}