[PATCH v2][next] stddef: Introduce TRAILING_OVERLAP() helper macro

["Gustavo A. R. Silva" <gustavoars@kernel.org>]

Add new TRAILING_OVERLAP() helper macro to create a union between
a flexible-array member (FAM) and a set of members that would
otherwise follow it. This overlays the trailing members onto the
FAM while preserving the original memory layout.

Co-developed-by: Kees Cook <kees@kernel.org>
Signed-off-by: Kees Cook <kees@kernel.org>
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
---
Changes in v2:
 - Use `__offset_of_##FAM` instead of `__offset_of_fam` for the
   offset variable.
 - Update changelog text.

v1:
 - Link: https://lore.kernel.org/linux-hardening/aFDIBBeYgRSx1ekO@kspp/

 include/linux/stddef.h | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/include/linux/stddef.h b/include/linux/stddef.h
index 929d67710cc5..dab49e2ec8c0 100644
--- a/include/linux/stddef.h
+++ b/include/linux/stddef.h
@@ -93,4 +93,24 @@ enum {
 #define DECLARE_FLEX_ARRAY(TYPE, NAME) \
 	__DECLARE_FLEX_ARRAY(TYPE, NAME)
 
+/**
+ * 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)				\
+	union {									\
+		TYPE NAME;							\
+		struct {							\
+			unsigned char __offset_to_##FAM[offsetof(TYPE, FAM)];	\
+			MEMBERS							\
+		};								\
+	}
+
 #endif
-- 
2.43.0