stddef: Introduce __TRAILING_OVERLAP()

Introduce underlying __TRAILING_OVERLAP() macro to let callers apply
atributes to trailing overlapping members.

For instance, the code below:

| struct flex {
| 	size_t count;
| 	int data[];
| };

| struct {
| 	struct flex f;
| 	struct foo a;
| 	struct boo b;
| } __packed instance;

can now be changed to the following, and preserve the __packed
attribute:

| __TRAILING_OVERLAP(struct flex, f, data, __packed,
| 	struct foo a;
| 	struct boo b;
| ) instance;

Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Link: https://lore.kernel.org/r/f80c529b239ce11f0a51f714fe00ddf839e05f5e.1758115257.git.gustavoars@kernel.org
Signed-off-by: Kees Cook <kees@kernel.org>

Author: GustavoARSilva

include/linux/stddef.h

@@ -94,23 +94,39 @@ enum {
 	__DECLARE_FLEX_ARRAY(TYPE, NAME)
 
 /**
- * TRAILING_OVERLAP() - Overlap a flexible-array member with trailing members.
+ * __TRAILING_OVERLAP() - Overlap a flexible-array member with trailing
+ *			  members.
  *
  * Creates a union between a flexible-array member (FAM) in a struct and a set
  * of additional members that would otherwise follow it.
  *
  * @TYPE: Flexible structure type name, including "struct" keyword.
  * @NAME: Name for a variable to define.
  * @FAM: The flexible-array member within @TYPE
+ * @ATTRS: Any struct attributes (usually empty)
  * @MEMBERS: Trailing overlapping members.
  */
-#define TRAILING_OVERLAP(TYPE, NAME, FAM, MEMBERS)				\
+#define __TRAILING_OVERLAP(TYPE, NAME, FAM, ATTRS, MEMBERS)			\
 	union {									\
 		TYPE NAME;							\
 		struct {							\
 			unsigned char __offset_to_FAM[offsetof(TYPE, FAM)];	\
 			MEMBERS							\
-		};								\
+		} ATTRS;							\
 	}
 
+/**
+ * TRAILING_OVERLAP() - Overlap a flexible-array member with trailing members.
+ *
+ * Creates a union between a flexible-array member (FAM) in a struct and a set
+ * of additional members that would otherwise follow it.
+ *
+ * @TYPE: Flexible structure type name, including "struct" keyword.
+ * @NAME: Name for a variable to define.
+ * @FAM: The flexible-array member within @TYPE
+ * @MEMBERS: Trailing overlapping members.
+ */
+#define TRAILING_OVERLAP(TYPE, NAME, FAM, MEMBERS)				\
+	__TRAILING_OVERLAP(TYPE, NAME, FAM, /* no attrs */, MEMBERS)
+
 #endif