[PATCH 1/4] container_of: add container_of_const() that preserves const-ness of the pointer

[Greg Kroah-Hartman]

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.

Co-developed-by: Jason Gunthorpe <jgg@xxxxxxxx>
Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Cc: Sakari Ailus <sakari.ailus@xxxxxxxxxxxxxxx>
Cc: Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx>
Cc: "Rafael J. Wysocki" <rafael@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 include/linux/container_of.h | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/include/linux/container_of.h b/include/linux/container_of.h
index 2008e9f4058c..3c290e865151 100644
--- a/include/linux/container_of.h
+++ b/include/linux/container_of.h
@@ -22,4 +22,18 @@
 		      "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_type:		the type of the pointer @ptr
+ * @ptr:		the pointer to the member
+ * @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, ptr, member_type, member)		\
+	_Generic(ptr,							\
+		const ptr_type *: ((const member_type *)container_of(ptr, member_type, member)),\
+		ptr_type *: ((member_type *)container_of(ptr, member_type, member))	\
+	)
+
 #endif	/* _LINUX_CONTAINER_OF_H */
-- 
2.38.1