net: gre: provide multicast mappings for ipv4 and ipv6

My commit 6d55cb91a0020ac0 (gre: fix hard header destination
address checking) broke multicast.

The reason is that ip_gre used to get ipgre_header() calls with
zero destination if we have NOARP or multicast destination. Instead
the actual target was decided at ipgre_tunnel_xmit() time based on
per-protocol dissection.

Instead of allowing the "abuse" of ->header() calls with invalid
destination, this creates multicast mappings for ip_gre. This also
fixes "ip neigh show nud noarp" to display the proper multicast
mappings used by the gre device.

Reported-by: Doug Kehn <[e-mail]>
Signed-off-by: Timo Teräs <[e-mail]>
Acked-by: Doug Kehn <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

include/net/if_inet6.h

···
 	buf[9]  = broadcast[9];
 	memcpy(buf + 10, addr->s6_addr + 6, 10);
 }
+
+static inline int ipv6_ipgre_mc_map(const struct in6_addr *addr,
+				    const unsigned char *broadcast, char *buf)
+{
+	if ((broadcast[0] | broadcast[1] | broadcast[2] | broadcast[3]) != 0) {
+		memcpy(buf, broadcast, 4);
+	} else {
+		/* v4mapped? */
+		if ((addr->s6_addr32[0] | addr->s6_addr32[1] |
+		     (addr->s6_addr32[2] ^ htonl(0x0000ffff))) != 0)
+			return -EINVAL;
+		memcpy(buf, &addr->s6_addr32[3], 4);
+	}
+	return 0;
+}
+
 #endif
 #endif

include/net/ip.h

···
 	buf[16] = addr & 0x0f;
 }
 
+static inline void ip_ipgre_mc_map(__be32 naddr, const unsigned char *broadcast, char *buf)
+{
+	if ((broadcast[0] | broadcast[1] | broadcast[2] | broadcast[3]) != 0)
+		memcpy(buf, broadcast, 4);
+	else
+		memcpy(buf, &naddr, sizeof(naddr));
+}
+
 #if defined(CONFIG_IPV6) || defined(CONFIG_IPV6_MODULE)
 #include <linux/ipv6.h>
 #endif

net/ipv4/arp.c

···
 	case ARPHRD_INFINIBAND:
 		ip_ib_mc_map(addr, dev->broadcast, haddr);
 		return 0;
+	case ARPHRD_IPGRE:
+		ip_ipgre_mc_map(addr, dev->broadcast, haddr);
+		return 0;
 	default:
 		if (dir) {
 			memcpy(haddr, dev->broadcast, dev->addr_len);

net/ipv6/ndisc.c

···
 	case ARPHRD_INFINIBAND:
 		ipv6_ib_mc_map(addr, dev->broadcast, buf);
 		return 0;
+	case ARPHRD_IPGRE:
+		return ipv6_ipgre_mc_map(addr, dev->broadcast, buf);
 	default:
 		if (dir) {
 			memcpy(buf, dev->broadcast, dev->addr_len);