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, 4 insertions, 2 deletions

diff --git a/net/ipv4/icmp.c b/net/ipv4/icmp.c
index 49a74cc79dc8..4bce5a2830aa 100644
--- a/net/ipv4/icmp.c
+++ b/net/ipv4/icmp.c
@@ -95,6 +95,7 @@
 #include <net/checksum.h>
 #include <net/xfrm.h>
 #include <net/inet_common.h>
+#include <net/ip_fib.h>
 
 /*
  *      Build xmit assembly blocks
@@ -333,7 +334,7 @@ static void icmp_reply(struct icmp_bxm *icmp_param, struct sk_buff *skb)
         struct flowi4 fl4;
         struct sock *sk;
         struct inet_sock *inet;
-        __be32 daddr;
+        __be32 daddr, saddr;
 
         if (ip_options_echo(&icmp_param->replyopts.opt.opt, skb))
                 return;
@@ -347,6 +348,7 @@ static void icmp_reply(struct icmp_bxm *icmp_param, struct sk_buff *skb)
 
         inet->tos = ip_hdr(skb)->tos;
         daddr = ipc.addr = ip_hdr(skb)->saddr;
+        saddr = fib_compute_spec_dst(skb);
         ipc.opt = NULL;
         ipc.tx_flags = 0;
         if (icmp_param->replyopts.opt.opt.optlen) {
@@ -356,7 +358,7 @@ static void icmp_reply(struct icmp_bxm *icmp_param, struct sk_buff *skb)
         }
         memset(&fl4, 0, sizeof(fl4));
         fl4.daddr = daddr;
-        fl4.saddr = rt->rt_spec_dst;
+        fl4.saddr = saddr;
         fl4.flowi4_tos = RT_TOS(ip_hdr(skb)->tos);
         fl4.flowi4_proto = IPPROTO_ICMP;
         security_skb_classify_flow(skb, flowi4_to_flowi(&fl4));