erspan: auto detect truncated packets.

Currently the truncated bit is set only when the mirrored packet
is larger than mtu.  For certain cases, the packet might already
been truncated before sending to the erspan tunnel.  In this case,
the patch detect whether the IP header's total length is larger
than the actual skb->len.  If true, this indicated that the
mirrored packet is truncated and set the erspan truncate bit.

I tested the patch using bpf_skb_change_tail helper function to
shrink the packet size and send to erspan tunnel.

Reported-by: Xiaoyan Jin <xiaoyanj@vmware.com>
Signed-off-by: William Tu <u9012063@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/ip_gre.c

@@ -578,6 +578,7 @@ static void erspan_fb_xmit(struct sk_buff *skb, struct net_device *dev,
 	int tunnel_hlen;
 	int version;
 	__be16 df;
+	int nhoff;
 
 	tun_info = skb_tunnel_info(skb);
 	if (unlikely(!tun_info || !(tun_info->mode & IP_TUNNEL_INFO_TX) ||
@@ -605,6 +606,11 @@ static void erspan_fb_xmit(struct sk_buff *skb, struct net_device *dev,
 		truncate = true;
 	}
 
+	nhoff = skb_network_header(skb) - skb_mac_header(skb);
+	if (skb->protocol == htons(ETH_P_IP) &&
+	    (ntohs(ip_hdr(skb)->tot_len) > skb->len - nhoff))
+		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

@@ -896,6 +896,7 @@ static netdev_tx_t ip6erspan_tunnel_xmit(struct sk_buff *skb,
 	struct flowi6 fl6;
 	int err = -EINVAL;
 	__u32 mtu;
+	int nhoff;
 
 	if (!ip6_tnl_xmit_ctl(t, &t->parms.laddr, &t->parms.raddr))
 		goto tx_err;
@@ -908,6 +909,11 @@ static netdev_tx_t ip6erspan_tunnel_xmit(struct sk_buff *skb,
 		truncate = true;
 	}
 
+	nhoff = skb_network_header(skb) - skb_mac_header(skb);
+	if (skb->protocol == htons(ETH_P_IP) &&
+	    (ntohs(ip_hdr(skb)->tot_len) > skb->len - nhoff))
+		truncate = true;
+
 	if (skb_cow_head(skb, dev->needed_headroom))
 		goto tx_err;