FIB recusrion loop checks traverse midchain adjacencies

if a tunnel's destination address is reachable through the tunnel
(see example config belwo) then search for and detect a recursion
loop and don't stack the adjacency. Otherwise this results in a
nasty surprise.

DBGvpp# loop cre
DBGvpp# set int state loop0 up
DBGvpp# set int ip addr loop0 10.0.0.1/24
DBGvpp# create gre tunnel src 10.0.0.1 dst 1.1.1.1
DBGvpp# set int state gre0 up
DBGvpp# set int unnum gre0 use loop0
DBGvpp# ip route 1.1.1.1/32 via gre0

DBGvpp# sh ip fib 1.1.1.1
ipv4-VRF:0, fib_index:0, flow hash:[src dst sport dport proto ] locks:[src:plugin-hi:2, src:default-route:1, ]
1.1.1.1/32 fib:0 index:11 locks:4   <<< this is entry #11
  src:CLI refs:1 entry-flags:attached, src-flags:added,contributing,active,
    path-list:[14] locks:2 flags:shared,looped, uPRF-list:12 len:1 itfs:[2, ]
      path:[14] pl-index:14 ip4 weight=1 pref=0 attached-nexthop:  oper-flags:recursive-loop,resolved, cfg-flags:attached,
        1.1.1.1 gre0 (p2p)
          [@0]: ipv4 via 0.0.0.0 gre0: mtu:9000 4500000000000000fe2fb0cc0a0000010101010100000800
             stacked-on entry:11:  <<<< and the midchain forwards via entry #11
               [@2]: dpo-drop ip4

  src:recursive-resolution refs:1 src-flags:added, cover:-1

 forwarding:   unicast-ip4-chain
  [@0]: dpo-load-balance: [proto:ip4 index:13 buckets:1 uRPF:12 to:[0:0]]
    [0] [@6]: ipv4 via 0.0.0.0 gre0: mtu:9000 4500000000000000fe2fb0cc0a0000010101010100000800
        stacked-on entry:11:
          [@2]: dpo-drop ip4

DBGvpp# sh adj 1
[@1] ipv4 via 0.0.0.0 gre0: mtu:9000 4500000000000000fe2fb0cc0a0000010101010100000800
  stacked-on entry:11:
    [@2]: dpo-drop ip4
   flags:midchain-ip-stack midchain-looped  <<<<< this is a loop
   counts:[0:0]
   locks:4
 delegates:

 children:
  {path:14}

Change-Id: I39b82bd1ea439be4611c88b130d40289fa0c1b59
Signed-off-by: Neale Ranns <nranns@cisco.com>

2 files changed, 12 insertions, 39 deletions

diff --git a/src/vnet/gre/gre.c b/src/vnet/gre/gre.c
index 449968c1be0..e30319f5f99 100644
--- a/src/vnet/gre/gre.c
+++ b/src/vnet/gre/gre.c
@@ -301,17 +301,20 @@ gre_update_adj (vnet_main_t * vnm, u32 sw_if_index, adj_index_t ai)
 {
   gre_main_t *gm = &gre_main;
   gre_tunnel_t *t;
-  u32 ti;
+  adj_flags_t af;
   u8 is_ipv6;
+  u32 ti;
 
   ti = gm->tunnel_index_by_sw_if_index[sw_if_index];
   t = pool_elt_at_index (gm->tunnels, ti);
   is_ipv6 = t->tunnel_dst.fp_proto == FIB_PROTOCOL_IP6 ? 1 : 0;
+  af = ADJ_FLAG_MIDCHAIN_IP_STACK;
+
+  if (VNET_LINK_ETHERNET == adj_get_link_type (ai))
+    af |= ADJ_FLAG_MIDCHAIN_NO_COUNT;
 
   adj_nbr_midchain_update_rewrite
-    (ai, !is_ipv6 ? gre4_fixup : gre6_fixup, NULL,
-     (VNET_LINK_ETHERNET == adj_get_link_type (ai) ?
-      ADJ_FLAG_MIDCHAIN_NO_COUNT : ADJ_FLAG_NONE),
+    (ai, !is_ipv6 ? gre4_fixup : gre6_fixup, NULL, af,
      gre_build_rewrite (vnm, sw_if_index, adj_get_link_type (ai), NULL));
 
   gre_tunnel_stack (ai);
diff --git a/src/vnet/gre/interface.c b/src/vnet/gre/interface.c
index 6be934af56c..b9bfb79c172 100644
--- a/src/vnet/gre/interface.c
+++ b/src/vnet/gre/interface.c
@@ -128,9 +128,7 @@ gre_tunnel_from_fib_node (fib_node_t * node)
 void
 gre_tunnel_stack (adj_index_t ai)
 {
-  fib_forward_chain_type_t fib_fwd;
   gre_main_t *gm = &gre_main;
-  dpo_id_t tmp = DPO_INVALID;
   ip_adjacency_t *adj;
   gre_tunnel_t *gt;
   u32 sw_if_index;
@@ -149,42 +147,14 @@ gre_tunnel_stack (adj_index_t ai)
        VNET_HW_INTERFACE_FLAG_LINK_UP) == 0)
     {
       adj_nbr_midchain_unstack (ai);
-      return;
     }
-
-  fib_fwd = fib_forw_chain_type_from_fib_proto (gt->tunnel_dst.fp_proto);
-
-  fib_entry_contribute_forwarding (gt->fib_entry_index, fib_fwd, &tmp);
-  if (DPO_LOAD_BALANCE == tmp.dpoi_type)
+  else
     {
-      /*
-       * post GRE rewrite we will load-balance. However, the GRE encap
-       * is always the same for this adjacency/tunnel and hence the IP/GRE
-       * src,dst hash is always the same result too. So we do that hash now and
-       * stack on the choice.
-       * If the choice is an incomplete adj then we will need a poke when
-       * it becomes complete. This happens since the adj update walk propagates
-       * as far a recursive paths.
-       */
-      const dpo_id_t *choice;
-      load_balance_t *lb;
-      int hash;
-
-      lb = load_balance_get (tmp.dpoi_index);
-
-      if (fib_fwd == FIB_FORW_CHAIN_TYPE_UNICAST_IP4)
-	hash = ip4_compute_flow_hash ((ip4_header_t *) adj_get_rewrite (ai),
-				      lb->lb_hash_config);
-      else
-	hash = ip6_compute_flow_hash ((ip6_header_t *) adj_get_rewrite (ai),
-				      lb->lb_hash_config);
-      choice =
-	load_balance_get_bucket_i (lb, hash & lb->lb_n_buckets_minus_1);
-      dpo_copy (&tmp, choice);
+      adj_nbr_midchain_stack_on_fib_entry (ai,
+					   gt->fib_entry_index,
+					   fib_forw_chain_type_from_fib_proto
+					   (gt->tunnel_dst.fp_proto));
     }
-
-  adj_nbr_midchain_stack (ai, &tmp);
-  dpo_reset (&tmp);
 }
 
 /**