[PATCH 1/1] linux/container_of.h: Warn about loss of constness

[Sakari Ailus]

container_of() casts the original type to another which leads to the loss
of the const qualifier if it is not specified in the caller-provided type.
This easily leads to container_of() returning a non-const pointer to a
const struct which the C compiler does not warn about.

Signed-off-by: Sakari Ailus <sakari.ailus@xxxxxxxxxxxxxxx>
---
 include/linux/container_of.h | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/include/linux/container_of.h b/include/linux/container_of.h
index 2f4944b791b81..c7c21d0f41a87 100644
--- a/include/linux/container_of.h
+++ b/include/linux/container_of.h
@@ -13,6 +13,10 @@
  * @type:	the type of the container struct this is embedded in.
  * @member:	the name of the member within the struct.
  *
+ * WARNING: as container_of() casts the given struct to another, also the
+ * possible const qualifier of @ptr is lost unless it is also specified in
+ * @type. This is not a problem if the containing object is not const. Use with
+ * care.
  */
 #define container_of(ptr, type, member) ({				\
 	void *__mptr = (void *)(ptr);					\
@@ -27,6 +31,11 @@
  * @type:	the type of the container struct this is embedded in.
  * @member:	the name of the member within the struct.
  *
+ * WARNING: as container_of() casts the given struct to another, also the
+ * possible const qualifier of @ptr is lost unless it is also specified in
+ * @type. This is not a problem if the containing object is not const. Use with
+ * care.
+ *
  * If IS_ERR_OR_NULL(ptr), ptr is returned unchanged.
  */
 #define container_of_safe(ptr, type, member) ({				\
-- 
2.30.2