erspan: auto detect truncated ipv6 packets.

Currently the truncated bit is set only when 1) the mirrored packet
is larger than mtu and 2) the ipv4 packet tot_len is larger than
the actual skb->len.  This patch adds another case for detecting
whether ipv6 packet is truncated or not, by checking the ipv6 header
payload_len and the skb->len.
Reported-by: NXiaoyan Jin <xiaoyanj@vmware.com>
Signed-off-by: NWilliam Tu <u9012063@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/ip_gre.c

@@ -579,6 +579,7 @@ static void erspan_fb_xmit(struct sk_buff *skb, struct net_device *dev,
 	int version;
 	__be16 df;
 	int nhoff;
+	int thoff;
 
 	tun_info = skb_tunnel_info(skb);
 	if (unlikely(!tun_info || !(tun_info->mode & IP_TUNNEL_INFO_TX) ||
@@ -611,6 +612,11 @@ static void erspan_fb_xmit(struct sk_buff *skb, struct net_device *dev,
 	    (ntohs(ip_hdr(skb)->tot_len) > skb->len - nhoff))
 		truncate = true;
 
+	thoff = skb_transport_header(skb) - skb_mac_header(skb);
+	if (skb->protocol == htons(ETH_P_IPV6) &&
+	    (ntohs(ipv6_hdr(skb)->payload_len) > skb->len - thoff))
+		truncate = true;
+
 	if (version == 1) {
 		erspan_build_header(skb, ntohl(tunnel_id_to_key32(key->tun_id)),
 				    ntohl(md->u.index), truncate, true);

net/ipv6/ip6_gre.c

@@ -897,6 +897,7 @@ static netdev_tx_t ip6erspan_tunnel_xmit(struct sk_buff *skb,
 	int err = -EINVAL;
 	__u32 mtu;
 	int nhoff;
+	int thoff;
 
 	if (!ip6_tnl_xmit_ctl(t, &t->parms.laddr, &t->parms.raddr))
 		goto tx_err;
@@ -914,6 +915,11 @@ static netdev_tx_t ip6erspan_tunnel_xmit(struct sk_buff *skb,
 	    (ntohs(ip_hdr(skb)->tot_len) > skb->len - nhoff))
 		truncate = true;
 
+	thoff = skb_transport_header(skb) - skb_mac_header(skb);
+	if (skb->protocol == htons(ETH_P_IPV6) &&
+	    (ntohs(ipv6_hdr(skb)->payload_len) > skb->len - thoff))
+		truncate = true;
+
 	if (skb_cow_head(skb, dev->needed_headroom))
 		goto tx_err;