ipv4: Create and use fib_compute_spec_dst() helper.

The specific destination is the host we direct unicast replies to.
Usually this is the original packet source address, but if we are
responding to a multicast or broadcast packet we have to use something
different.

Specifically we must use the source address we would use if we were to
send a packet to the unicast source of the original packet.

The routing cache precomputes this value, but we want to remove that
precomputation because it creates a hard dependency on the expensive
rpfilter source address validation which we'd like to make cheaper.

There are only three places where this matters:

1) ICMP replies.

2) pktinfo CMSG

3) IP options

Now there will be no real users of rt->rt_spec_dst and we can simply
remove it altogether.

Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 29 insertions, 0 deletions

diff --git a/net/ipv4/fib_frontend.c b/net/ipv4/fib_frontend.c
index 3854411..451939b 100644
--- a/net/ipv4/fib_frontend.c
+++ b/net/ipv4/fib_frontend.c
@@ -180,6 +180,35 @@ unsigned int inet_dev_addr_type(struct net *net, const struct net_device *dev,
 }
 EXPORT_SYMBOL(inet_dev_addr_type);
 
+__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 flowi4 fl4;
+	struct net *net;
+
+	if (skb->pkt_type != PACKET_BROADCAST &&
+	    skb->pkt_type != PACKET_MULTICAST)
+		return ip_hdr(skb)->daddr;
+
+	in_dev = __in_dev_get_rcu(dev);
+	BUG_ON(!in_dev);
+	fl4.flowi4_oif = 0;
+	fl4.flowi4_iif = 0;
+	fl4.daddr = ip_hdr(skb)->saddr;
+	fl4.saddr = ip_hdr(skb)->daddr;
+	fl4.flowi4_tos = RT_TOS(ip_hdr(skb)->tos);
+	fl4.flowi4_scope = RT_SCOPE_UNIVERSE;
+	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);
+}
+
 /* Given (packet source, input interface) and optional (dst, oif, tos):
  * - (main) check, that source is valid i.e. not broadcast or our local
  *   address.