fs/configfs: Add a callback to determine attribute visibility

In order to support dynamic decisions as to whether an attribute should be
created, add a callback that returns a bool to indicate whether the
attribute should be displayed. If no callback is registered, the attribute
is displayed by default.

Co-developed-by: Dan Williams <[e-mail]>
Signed-off-by: Dan Williams <[e-mail]>
Signed-off-by: Tom Lendacky <[e-mail]>
Signed-off-by: Borislav Petkov (AMD) <[e-mail]>
Link: https://lore.kernel.org/r/e555c87[e-mail]

fs/configfs/dir.c

···
 static int populate_attrs(struct config_item *item)
 {
 	const struct config_item_type *t = item->ci_type;
+	struct configfs_group_operations *ops;
 	struct configfs_attribute *attr;
 	struct configfs_bin_attribute *bin_attr;
 	int error = 0;
···
 
 	if (!t)
 		return -EINVAL;
+
+	ops = t->ct_group_ops;
+
 	if (t->ct_attrs) {
 		for (i = 0; (attr = t->ct_attrs[i]) != NULL; i++) {
+			if (ops && ops->is_visible && !ops->is_visible(item, attr, i))
+				continue;
+
 			if ((error = configfs_create_file(item, attr)))
 				break;
 		}
 	}
 	if (t->ct_bin_attrs) {
 		for (i = 0; (bin_attr = t->ct_bin_attrs[i]) != NULL; i++) {
+			if (ops && ops->is_bin_visible && !ops->is_bin_visible(item, bin_attr, i))
+				continue;
+
 			error = configfs_create_bin_file(item, bin_attr);
 			if (error)
 				break;

include/linux/configfs.h

···
 	struct config_group *(*make_group)(struct config_group *group, const char *name);
 	void (*disconnect_notify)(struct config_group *group, struct config_item *item);
 	void (*drop_item)(struct config_group *group, struct config_item *item);
+	bool (*is_visible)(struct config_item *item, struct configfs_attribute *attr, int n);
+	bool (*is_bin_visible)(struct config_item *item, struct configfs_bin_attribute *attr,
+			       int n);
 };
 
 struct configfs_subsystem {