sysfs: attribute_group: allow registration of const bin_attribute

To be able to constify instances of struct bin_attribute it has to be
possible to add them to string attribute_group.
The current type of the bin_attrs member however is not compatible with
that.
Introduce a union that allows registration of both const and non-const
attributes to enable a piecewise transition.
As both union member types are compatible no logic needs to be adapted.

Technically it is now possible register a const struct
bin_attribute and receive it as mutable pointer in the callbacks.
This is a soundness issue.
But this same soundness issue already exists today in
sysfs_create_bin_file().
Also the struct definition and callback implementation are always
closely linked and are meant to be moved to const in lockstep.

Signed-off-by: Thomas Weißschuh <[email protected]>
Link: https://lore.kernel.org/r/20241115-b4-sysfs-const-bin_attr-group-v1-1-2c9bb12dfc48@weissschuh.net
Signed-off-by: Greg Kroah-Hartman <[email protected]>

Author: t-8ch

include/linux/sysfs.h

@@ -106,7 +106,10 @@ struct attribute_group {
 					    const struct bin_attribute *,
 					    int);
 	struct attribute	**attrs;
-	struct bin_attribute	**bin_attrs;
+	union {
+		struct bin_attribute		**bin_attrs;
+		const struct bin_attribute	*const *bin_attrs_new;
+	};
 };
 
 #define SYSFS_PREALLOC		010000