add vif using local interface index instead of IP

When routing daemon wants to enable forwarding of multicast traffic it
performs something like:

       struct vifctl vc = {
               .vifc_vifi  = 1,
               .vifc_flags = 0,
               .vifc_threshold = 1,
               .vifc_rate_limit = 0,
               .vifc_lcl_addr = ip, /* <--- ip address of physical
interface, e.g. eth0 */
               .vifc_rmt_addr.s_addr = htonl(INADDR_ANY),
         };
       setsockopt(fd, IPPROTO_IP, MRT_ADD_VIF, &vc, sizeof(vc));

This leads (in the kernel) to calling  vif_add() function call which
search the (physical) device using assigned IP address:
       dev = ip_dev_find(net, vifc->vifc_lcl_addr.s_addr);

The current API (struct vifctl) does not allow to specify an
interface other way than using it's IP, and if there are more than a
single interface with specified IP only the first one will be found.

The attached patch (against 2.6.30.4) allows to specify an interface
by its index, instead of IP address:

       struct vifctl vc = {
               .vifc_vifi  = 1,
               .vifc_flags = VIFF_USE_IFINDEX,   /* NEW */
               .vifc_threshold = 1,
               .vifc_rate_limit = 0,
               .vifc_lcl_ifindex = if_nametoindex("eth0"),   /* NEW */
               .vifc_rmt_addr.s_addr = htonl(INADDR_ANY),
         };
       setsockopt(fd, IPPROTO_IP, MRT_ADD_VIF, &vc, sizeof(vc));
Signed-off-by: Ilia K. <mail4ilia@gmail.com>

=== modified file 'include/linux/mroute.h'
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/mroute.h

@@ -59,13 +59,18 @@ struct vifctl {
 	unsigned char vifc_flags;	/* VIFF_ flags */
 	unsigned char vifc_threshold;	/* ttl limit */
 	unsigned int vifc_rate_limit;	/* Rate limiter values (NI) */
-	struct in_addr vifc_lcl_addr;	/* Our address */
+	union {
+		struct in_addr vifc_lcl_addr;     /* Local interface address */
+		int            vifc_lcl_ifindex;  /* Local interface index   */
+	};
 	struct in_addr vifc_rmt_addr;	/* IPIP tunnel addr */
 };
 
-#define VIFF_TUNNEL	0x1	/* IPIP tunnel */
-#define VIFF_SRCRT	0x2	/* NI */
-#define VIFF_REGISTER	0x4	/* register vif	*/
+#define VIFF_TUNNEL		0x1	/* IPIP tunnel */
+#define VIFF_SRCRT		0x2	/* NI */
+#define VIFF_REGISTER		0x4	/* register vif	*/
+#define VIFF_USE_IFINDEX	0x8	/* use vifc_lcl_ifindex instead of
+					   vifc_lcl_addr to find an interface */
 
 /*
  *	Cache manipulation structures for mrouted and PIMd

net/ipv4/ipmr.c

@@ -469,8 +469,18 @@ static int vif_add(struct net *net, struct vifctl *vifc, int mrtsock)
 			return err;
 		}
 		break;
+
+	case VIFF_USE_IFINDEX:
 	case 0:
-		dev = ip_dev_find(net, vifc->vifc_lcl_addr.s_addr);
+		if (vifc->vifc_flags == VIFF_USE_IFINDEX) {
+			dev = dev_get_by_index(net, vifc->vifc_lcl_ifindex);
+			if (dev && dev->ip_ptr == NULL) {
+				dev_put(dev);
+				return -EADDRNOTAVAIL;
+			}
+		} else
+			dev = ip_dev_find(net, vifc->vifc_lcl_addr.s_addr);
+
 		if (!dev)
 			return -EADDRNOTAVAIL;
 		err = dev_set_allmulti(dev, 1);