fix setting return values for htab batch ops and docs

include/uapi/linux/bpf.h

@@ -632,17 +632,19 @@ union bpf_iter_link_info {
  *			returning the lock. This must be specified if the
  *			elements contain a spinlock.
  *
- *		On success, *count* elements from the map are copied into the
- *		user buffer, with the keys copied into *keys* and the values
- *		copied into the corresponding indices in *values*.
- *
- *		If an error is returned and *errno* is not **EFAULT**, *count*
- *		is set to the number of successfully processed elements.
+ *		On success, up to *count* elements from the map are copied into
+ *		the user buffer, with the keys copied into *keys* and the
+ *		values copied into the corresponding indices in *values*.
  *
  *	Return
  *		Returns zero on success. On error, -1 is returned and *errno*
  *		is set appropriately.
  *
+ *		If an error is returned and *errno* is not **EFAULT**, then
+ *		*count* is set to the number of successfully processed
+ *		elements. In particular, the *errno* may be set to **ENOENT**
+ *		in case of success to indicate that the end of map is reached.
+ *
  *		May set *errno* to **ENOSPC** to indicate that *keys* or
  *		*values* is too small to dump an entire bucket during
  *		iteration of a hash-based map type.
@@ -655,15 +657,15 @@ union bpf_iter_link_info {
  *		**BPF_MAP_LOOKUP_BATCH** with two exceptions:
  *
  *		* Every element that is successfully returned is also deleted
- *		  from the map. This is at least *count* elements. Note that
- *		  *count* is both an input and an output parameter.
+ *		  from the map. The *count* parameter is set to the number of
+ *		  returned elements. This value can be less than the actual
+ *		  number of deleted elements, see the next item.
  *		* Upon returning with *errno* set to **EFAULT**, up to
  *		  *count* elements may be deleted without returning the keys
  *		  and values of the deleted elements.
  *
  *	Return
- *		Returns zero on success. On error, -1 is returned and *errno*
- *		is set appropriately.
+ *		Same as the BPF_MAP_LOOKUP_BATCH return values.
  *
  * BPF_MAP_UPDATE_BATCH
  *	Description

kernel/bpf/hashtab.c

@@ -1692,6 +1692,13 @@ __htab_map_lookup_and_delete_batch(struct bpf_map *map,
 	struct bucket *b;
 	int ret = 0;
 
+	max_count = attr->batch.count;
+	if (!max_count)
+		return 0;
+
+	if (put_user(0, &uattr->batch.count))
+		return -EFAULT;
+
 	elem_map_flags = attr->batch.elem_flags;
 	if ((elem_map_flags & ~BPF_F_LOCK) ||
 	    ((elem_map_flags & BPF_F_LOCK) && !btf_record_has_field(map->record, BPF_SPIN_LOCK)))
@@ -1701,13 +1708,6 @@ __htab_map_lookup_and_delete_batch(struct bpf_map *map,
 	if (map_flags)
 		return -EINVAL;
 
-	max_count = attr->batch.count;
-	if (!max_count)
-		return 0;
-
-	if (put_user(0, &uattr->batch.count))
-		return -EFAULT;
-
 	batch = 0;
 	if (ubatch && copy_from_user(&batch, ubatch, sizeof(batch)))
 		return -EFAULT;