[IPV4]: Fix the fib trie iterator to work with a single entry routing tables

In a kernel with trie routing enabled I had a simple routing setup
with only a single route to the outside world and no default
route. "ip route table list main" showed my the route just fine but
/proc/net/route was an empty file.  What was going on?

Thinking it was a bug in something I did and I looked deeper.  Eventually
I setup a second route and everything looked correct, huh?  Finally I
realized that the it was just the iterator pair in fib_trie_get_first,
fib_trie_get_next just could not handle a routing table with a single entry.

So to save myself and others further confusion, here is a simple fix for
the fib proc iterator so it works even when there is only a single route
in a routing table.

Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: Robert Olsson <robert.olsson@its.uu.se>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 16 insertions, 5 deletions

diff --git a/net/ipv4/fib_trie.c b/net/ipv4/fib_trie.c
index cfb249cc0a58..13307c04d5a1 100644
--- a/net/ipv4/fib_trie.c
+++ b/net/ipv4/fib_trie.c
@@ -1989,6 +1989,10 @@ static struct node *fib_trie_get_next(struct fib_trie_iter *iter)
         unsigned cindex = iter->index;
         struct tnode *p;
 
+        /* A single entry routing table */
+        if (!tn)
+                return NULL;
+
         pr_debug("get_next iter={node=%p index=%d depth=%d}\n",
                  iter->tnode, iter->index, iter->depth);
 rescan:
@@ -2037,11 +2041,18 @@ static struct node *fib_trie_get_first(struct fib_trie_iter *iter,
         if(!iter)
                 return NULL;
 
-        if (n && IS_TNODE(n)) {
-                iter->tnode = (struct tnode *) n;
-                iter->trie = t;
-                iter->index = 0;
-                iter->depth = 1;
+        if (n) {
+                if (IS_TNODE(n)) {
+                        iter->tnode = (struct tnode *) n;
+                        iter->trie = t;
+                        iter->index = 0;
+                        iter->depth = 1;
+                } else {
+                        iter->tnode = NULL;
+                        iter->trie  = t;
+                        iter->index = 0;
+                        iter->depth = 0;
+                }
                 return n;
         }
         return NULL;