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>

2 files changed, 13 insertions, 3 deletions

diff --git a/include/linux/if_packet.h b/include/linux/if_packet.h
index 72bfa5a034dd..6d66ce1791a9 100644
--- a/include/linux/if_packet.h
+++ b/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
diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
index 925f715686a5..ba248d93399a 100644
--- a/net/packet/af_packet.c
+++ b/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);
         }