sysfs: introduce callback attribute_group::bin_size

Several drivers need to dynamically calculate the size of an binary
attribute. Currently this is done by assigning attr->size from the
is_bin_visible() callback.

This has drawbacks:
* It is not documented.
* A single attribute can be instantiated multiple times, overwriting the
  shared size field.
* It prevents the structure to be moved to read-only memory.

Introduce a new dedicated callback to calculate the size of the
attribute.

Signed-off-by: Thomas Weißschuh <[email protected]>
Acked-by: Krzysztof Wilczyński <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Greg Kroah-Hartman <[email protected]>

Author: t-8ch

fs/sysfs/group.c

@@ -98,6 +98,8 @@ static int create_files(struct kernfs_node *parent, struct kobject *kobj,
 				if (!mode)
 					continue;
 			}
+			if (grp->bin_size)
+				size = grp->bin_size(kobj, *bin_attr, i);
 
 			WARN(mode & ~(SYSFS_PREALLOC | 0664),
 			     "Attribute %s: Invalid permissions 0%o\n",

include/linux/sysfs.h

@@ -87,6 +87,11 @@ do {							\
  *		SYSFS_GROUP_VISIBLE() when assigning this callback to
  *		specify separate _group_visible() and _attr_visible()
  *		handlers.
+ * @bin_size:
+ *		Optional: Function to return the size of a binary attribute
+ *		of the group. Will be called repeatedly for each binary
+ *		attribute in the group. Overwrites the size field embedded
+ *		inside the attribute itself.
  * @attrs:	Pointer to NULL terminated list of attributes.
  * @bin_attrs:	Pointer to NULL terminated list of binary attributes.
  *		Either attrs or bin_attrs or both must be provided.
@@ -97,6 +102,9 @@ struct attribute_group {
 					      struct attribute *, int);
 	umode_t			(*is_bin_visible)(struct kobject *,
 						  struct bin_attribute *, int);
+	size_t			(*bin_size)(struct kobject *,
+					    const struct bin_attribute *,
+					    int);
 	struct attribute	**attrs;
 	struct bin_attribute	**bin_attrs;
 };