container_of: add container_of_const() that preserves const-ness of the pointer

container_of does not preserve the const-ness of a pointer that is
passed into it, which can cause C code that passes in a const pointer to
get a pointer back that is not const and then scribble all over the data
in it.  To prevent this, container_of_const() will preserve the const
status of the pointer passed into it using the newly available _Generic()
method.

Suggested-by: Jason Gunthorpe <[email protected]>
Suggested-by: Sakari Ailus <[email protected]>
Reviewed-by: Matthew Wilcox (Oracle) <[email protected]>
Reviewed-by: Jason Gunthorpe <[email protected]>
Reviewed-by: Andy Shevchenko <[email protected]>
Reviewed-by: Sakari Ailus <[email protected]>
Acked-by: Rafael J. Wysocki <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Greg Kroah-Hartman <[email protected]>

Author: gregkh

include/linux/container_of.h

@@ -22,4 +22,17 @@
 		      "pointer type mismatch in container_of()");	\
 	((type *)(__mptr - offsetof(type, member))); })
 
+/**
+ * container_of_const - cast a member of a structure out to the containing
+ *			structure and preserve the const-ness of the pointer
+ * @ptr:		the pointer to the member
+ * @type:		the type of the container struct this is embedded in.
+ * @member:		the name of the member within the struct.
+ */
+#define container_of_const(ptr, type, member)				\
+	_Generic(ptr,							\
+		const typeof(*(ptr)) *: ((const type *)container_of(ptr, type, member)),\
+		default: ((type *)container_of(ptr, type, member))	\
+	)
+
 #endif	/* _LINUX_CONTAINER_OF_H */