bpf: map_get_next_key to return first key on NULL

commit 8fe45924 upstream.

When iterating through a map, we need to find a key that does not exist
in the map so map_get_next_key will give us the first key of the map.
This often requires a lot of guessing in production systems.

This patch makes map_get_next_key return the first key when the key
pointer in the parameter is NULL.
Signed-off-by: Teng Qin <qinteng@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Chenbo Feng <fengc@google.com>
Cc: Lorenzo Colitti <lorenzo@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

kernel/bpf/arraymap.c

@@ -190,7 +190,7 @@ int bpf_percpu_array_copy(struct bpf_map *map, void *key, void *value)
 static int array_map_get_next_key(struct bpf_map *map, void *key, void *next_key)
 {
 	struct bpf_array *array = container_of(map, struct bpf_array, map);
-	u32 index = *(u32 *)key;
+	u32 index = key ? *(u32 *)key : U32_MAX;
 	u32 *next = (u32 *)next_key;
 
 	if (index >= array->map.max_entries) {

kernel/bpf/hashtab.c

@@ -326,12 +326,15 @@ static int htab_map_get_next_key(struct bpf_map *map, void *key, void *next_key)
 	struct hlist_head *head;
 	struct htab_elem *l, *next_l;
 	u32 hash, key_size;
-	int i;
+	int i = 0;
 
 	WARN_ON_ONCE(!rcu_read_lock_held());
 
 	key_size = map->key_size;
 
+	if (!key)
+		goto find_first_elem;
+
 	hash = htab_map_hash(key, key_size);
 
 	head = select_bucket(htab, hash);
@@ -339,10 +342,8 @@ static int htab_map_get_next_key(struct bpf_map *map, void *key, void *next_key)
 	/* lookup the key */
 	l = lookup_elem_raw(head, hash, key, key_size);
 
-	if (!l) {
-		i = 0;
+	if (!l)
 		goto find_first_elem;
-	}
 
 	/* key was found, get next key in the same bucket */
 	next_l = hlist_entry_safe(rcu_dereference_raw(hlist_next_rcu(&l->hash_node)),

kernel/bpf/syscall.c

@@ -508,6 +508,7 @@ static int map_get_next_key(union bpf_attr *attr)
 	if (IS_ERR(map))
 		return PTR_ERR(map);
 
+	if (ukey) {
 		err = -ENOMEM;
 		key = kmalloc(map->key_size, GFP_USER);
 		if (!key)
@@ -516,6 +517,9 @@ static int map_get_next_key(union bpf_attr *attr)
 		err = -EFAULT;
 		if (copy_from_user(key, ukey, map->key_size) != 0)
 			goto free_key;
+	} else {
+		key = NULL;
+	}
 
 	err = -ENOMEM;
 	next_key = kmalloc(map->key_size, GFP_USER);