[PATCH bpf-next] bpf, lpm: fix check prefixlen before walking trie

[Florian Lehner <dev@der-flo.net>]

When looking up an element in LPM trie, the condition 'matchlen ==
trie->max_prefixlen' will never return true, if key->prefixlen is larger
than trie->max_prefixlen. Consequently all elements in the LPM trie will
be visited and no element is returned in the end.

To resolve this, check key->prefixlen first before walking the LPM trie.

Fixes: b95a5c4db09b ("bpf: add a longest prefix match trie map implementation")
Signed-off-by: Florian Lehner <dev@der-flo.net>
---
 kernel/bpf/lpm_trie.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/kernel/bpf/lpm_trie.c b/kernel/bpf/lpm_trie.c
index 17c7e7782a1f..b32be680da6c 100644
--- a/kernel/bpf/lpm_trie.c
+++ b/kernel/bpf/lpm_trie.c
@@ -231,6 +231,9 @@ static void *trie_lookup_elem(struct bpf_map *map, void *_key)
 	struct lpm_trie_node *node, *found = NULL;
 	struct bpf_lpm_trie_key *key = _key;
 
+	if (key->prefixlen > trie->max_prefixlen)
+		return NULL;
+
 	/* Start walking the trie from the root node ... */
 
 	for (node = rcu_dereference_check(trie->root, rcu_read_lock_bh_held());
-- 
2.39.2