ipmr: Add high byte of VIF ID to igmpmsg

Use the unused3 byte in struct igmpmsg to hold the high 8 bits of the
VIF ID.

If using more than 255 IPv4 multicast interfaces it is necessary to have
access to a VIF ID for cache reports that is wider than 8 bits, the VIF
ID present in the igmpmsg reports sent to mroute_sk was only 8 bits wide
in the igmpmsg header.  Adding the high 8 bits of the 16 bit VIF ID in
the unused byte allows use of more than 255 IPv4 multicast interfaces.
Signed-off-by: Paul Davey <paul.davey@alliedtelesis.co.nz>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/uapi/linux/mroute.h

@@ -113,8 +113,8 @@ struct igmpmsg {
 	__u32 unused1,unused2;
 	unsigned char im_msgtype;		/* What is this */
 	unsigned char im_mbz;			/* Must be zero */
-	unsigned char im_vif;			/* Interface (this ought to be a vifi_t!) */
-	unsigned char unused3;
+	unsigned char im_vif;			/* Low 8 bits of Interface */
+	unsigned char im_vif_hi;		/* High 8 bits of Interface */
 	struct in_addr im_src,im_dst;
 };
 

net/ipv4/ipmr.c

@@ -1038,10 +1038,13 @@ static int ipmr_cache_report(struct mr_table *mrt,
 		memcpy(msg, skb_network_header(pkt), sizeof(struct iphdr));
 		msg->im_msgtype = assert;
 		msg->im_mbz = 0;
-		if (assert == IGMPMSG_WRVIFWHOLE)
+		if (assert == IGMPMSG_WRVIFWHOLE) {
 			msg->im_vif = vifi;
-		else
+			msg->im_vif_hi = vifi >> 8;
+		} else {
 			msg->im_vif = mrt->mroute_reg_vif_num;
+			msg->im_vif_hi = mrt->mroute_reg_vif_num >> 8;
+		}
 		ip_hdr(skb)->ihl = sizeof(struct iphdr) >> 2;
 		ip_hdr(skb)->tot_len = htons(ntohs(ip_hdr(pkt)->tot_len) +
 					     sizeof(struct iphdr));
@@ -1054,6 +1057,7 @@ static int ipmr_cache_report(struct mr_table *mrt,
 		ip_hdr(skb)->protocol = 0;
 		msg = (struct igmpmsg *)skb_network_header(skb);
 		msg->im_vif = vifi;
+		msg->im_vif_hi = vifi >> 8;
 		skb_dst_set(skb, dst_clone(skb_dst(pkt)));
 		/* Add our header */
 		igmp = skb_put(skb, sizeof(struct igmphdr));