ipv4: Fix bugs in fib_compute_spec_dst().

Based upon feedback from Julian Anastasov.

1) Use route flags to determine multicast/broadcast, not the
   packet flags.

2) Leave saddr unspecified in flow key.

3) Adjust how we invoke inet_select_addr().  Pass ip_hdr(skb)->saddr as
   second arg, and if it was zeronet use link scope.

4) Use loopback as input interface in flow key.
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/fib_frontend.c

@@ -185,28 +185,36 @@ __be32 fib_compute_spec_dst(struct sk_buff *skb)
 	struct net_device *dev = skb->dev;
 	struct in_device *in_dev;
 	struct fib_result res;
+	struct rtable *rt;
 	struct flowi4 fl4;
 	struct net *net;
+	int scope;
 
-	if (skb->pkt_type != PACKET_BROADCAST &&
-	    skb->pkt_type != PACKET_MULTICAST)
+	rt = skb_rtable(skb);
+	if (!(rt->rt_flags & (RTCF_BROADCAST | RTCF_MULTICAST)))
 		return ip_hdr(skb)->daddr;
 
 	in_dev = __in_dev_get_rcu(dev);
 	BUG_ON(!in_dev);
+
+	net = dev_net(dev);
+
+	scope = RT_SCOPE_UNIVERSE;
+	if (!ipv4_is_zeronet(ip_hdr(skb)->saddr)) {
 		fl4.flowi4_oif = 0;
-	fl4.flowi4_iif = 0;
+		fl4.flowi4_iif = net->loopback_dev->ifindex;
 		fl4.daddr = ip_hdr(skb)->saddr;
-	fl4.saddr = ip_hdr(skb)->daddr;
+		fl4.saddr = 0;
 		fl4.flowi4_tos = RT_TOS(ip_hdr(skb)->tos);
-	fl4.flowi4_scope = RT_SCOPE_UNIVERSE;
+		fl4.flowi4_scope = scope;
 		fl4.flowi4_mark = IN_DEV_SRC_VMARK(in_dev) ? skb->mark : 0;
-
-	net = dev_net(dev);
 		if (!fib_lookup(net, &fl4, &res))
 			return FIB_RES_PREFSRC(net, res);
-	else
-		return inet_select_addr(dev, 0, RT_SCOPE_UNIVERSE);
+	} else {
+		scope = RT_SCOPE_LINK;
+	}
+
+	return inet_select_addr(dev, ip_hdr(skb)->saddr, scope);
 }
 
 /* Given (packet source, input interface) and optional (dst, oif, tos):