bpf: lpm_trie: check left child of last leftmost node for NULL

If the leftmost parent node of the tree has does not have a child
on the left side, then trie_get_next_key (and bpftool map dump) will
not look at the child on the right.  This leads to the traversal
missing elements.

Lookup is not affected.

Update selftest to handle this case.

Reproducer:

 bpftool map create /sys/fs/bpf/lpm type lpm_trie key 6 \
     value 1 entries 256 name test_lpm flags 1
 bpftool map update pinned /sys/fs/bpf/lpm key  8 0 0 0  0   0 value 1
 bpftool map update pinned /sys/fs/bpf/lpm key 16 0 0 0  0 128 value 2
 bpftool map dump   pinned /sys/fs/bpf/lpm

Returns only 1 element. (2 expected)

Fixes: b471f2f1 ("bpf: implement MAP_GET_NEXT_KEY command for LPM_TRIE")
Signed-off-by: Jonathan Lemon <jonathan.lemon@gmail.com>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

kernel/bpf/lpm_trie.c

@@ -716,9 +716,14 @@ static int trie_get_next_key(struct bpf_map *map, void *_key, void *_next_key)
 	 * have exact two children, so this function will never return NULL.
 	 */
 	for (node = search_root; node;) {
-		if (!(node->flags & LPM_TREE_NODE_FLAG_IM))
+		if (node->flags & LPM_TREE_NODE_FLAG_IM) {
+			node = rcu_dereference(node->child[0]);
+		} else {
 			next_node = node;
 			node = rcu_dereference(node->child[0]);
+			if (!node)
+				node = rcu_dereference(next_node->child[1]);
+		}
 	}
 do_copy:
 	next_key->prefixlen = next_node->prefixlen;

tools/testing/selftests/bpf/test_lpm_map.c

@@ -573,13 +573,13 @@ static void test_lpm_get_next_key(void)
 
 	/* add one more element (total two) */
 	key_p->prefixlen = 24;
-	inet_pton(AF_INET, "192.168.0.0", key_p->data);
+	inet_pton(AF_INET, "192.168.128.0", key_p->data);
 	assert(bpf_map_update_elem(map_fd, key_p, &value, 0) == 0);
 
 	memset(key_p, 0, key_size);
 	assert(bpf_map_get_next_key(map_fd, NULL, key_p) == 0);
 	assert(key_p->prefixlen == 24 && key_p->data[0] == 192 &&
-	       key_p->data[1] == 168 && key_p->data[2] == 0);
+	       key_p->data[1] == 168 && key_p->data[2] == 128);
 
 	memset(next_key_p, 0, key_size);
 	assert(bpf_map_get_next_key(map_fd, key_p, next_key_p) == 0);
@@ -592,7 +592,7 @@ static void test_lpm_get_next_key(void)
 
 	/* Add one more element (total three) */
 	key_p->prefixlen = 24;
-	inet_pton(AF_INET, "192.168.128.0", key_p->data);
+	inet_pton(AF_INET, "192.168.0.0", key_p->data);
 	assert(bpf_map_update_elem(map_fd, key_p, &value, 0) == 0);
 
 	memset(key_p, 0, key_size);
@@ -643,6 +643,41 @@ static void test_lpm_get_next_key(void)
 	assert(bpf_map_get_next_key(map_fd, key_p, next_key_p) == -1 &&
 	       errno == ENOENT);
 
+	/* Add one more element (total five) */
+	key_p->prefixlen = 28;
+	inet_pton(AF_INET, "192.168.1.128", key_p->data);
+	assert(bpf_map_update_elem(map_fd, key_p, &value, 0) == 0);
+
+	memset(key_p, 0, key_size);
+	assert(bpf_map_get_next_key(map_fd, NULL, key_p) == 0);
+	assert(key_p->prefixlen == 24 && key_p->data[0] == 192 &&
+	       key_p->data[1] == 168 && key_p->data[2] == 0);
+
+	memset(next_key_p, 0, key_size);
+	assert(bpf_map_get_next_key(map_fd, key_p, next_key_p) == 0);
+	assert(next_key_p->prefixlen == 28 && next_key_p->data[0] == 192 &&
+	       next_key_p->data[1] == 168 && next_key_p->data[2] == 1 &&
+	       next_key_p->data[3] == 128);
+
+	memcpy(key_p, next_key_p, key_size);
+	assert(bpf_map_get_next_key(map_fd, key_p, next_key_p) == 0);
+	assert(next_key_p->prefixlen == 24 && next_key_p->data[0] == 192 &&
+	       next_key_p->data[1] == 168 && next_key_p->data[2] == 1);
+
+	memcpy(key_p, next_key_p, key_size);
+	assert(bpf_map_get_next_key(map_fd, key_p, next_key_p) == 0);
+	assert(next_key_p->prefixlen == 24 && next_key_p->data[0] == 192 &&
+	       next_key_p->data[1] == 168 && next_key_p->data[2] == 128);
+
+	memcpy(key_p, next_key_p, key_size);
+	assert(bpf_map_get_next_key(map_fd, key_p, next_key_p) == 0);
+	assert(next_key_p->prefixlen == 16 && next_key_p->data[0] == 192 &&
+	       next_key_p->data[1] == 168);
+
+	memcpy(key_p, next_key_p, key_size);
+	assert(bpf_map_get_next_key(map_fd, key_p, next_key_p) == -1 &&
+	       errno == ENOENT);
+
 	/* no exact matching key should return the first one in post order */
 	key_p->prefixlen = 22;
 	inet_pton(AF_INET, "192.168.1.0", key_p->data);