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

From: Greg Kroah-Hartman
Date: Thu Dec 01 2022 - 14:31:59 EST


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