overflow: Introduce __DEFINE_FLEX for having no initializer

While not yet in the tree, there is a proposed patch[1] that was
depending on the prior behavior of _DEFINE_FLEX, which did not have an
explicit initializer. Provide this via __DEFINE_FLEX now, which can also
have attributes applied (e.g. __uninitialized).

Examples of the resulting initializer behaviors can be seen here:
https://godbolt.org/z/P7Go8Tr33

Link: https://lore.kernel.org/netdev/[email protected] [1]
Fixes: 47e36ed ("overflow: Fix direct struct member initialization in _DEFINE_FLEX()")
Signed-off-by: Kees Cook <[email protected]>

Author: kees

include/linux/overflow.h

@@ -389,24 +389,37 @@ static inline size_t __must_check size_sub(size_t minuend, size_t subtrahend)
 	struct_size((type *)NULL, member, count)
 
 /**
- * _DEFINE_FLEX() - helper macro for DEFINE_FLEX() family.
- * Enables caller macro to pass (different) initializer.
+ * __DEFINE_FLEX() - helper macro for DEFINE_FLEX() family.
+ * Enables caller macro to pass arbitrary trailing expressions
  *
  * @type: structure type name, including "struct" keyword.
  * @name: Name for a variable to define.
  * @member: Name of the array member.
  * @count: Number of elements in the array; must be compile-time const.
- * @initializer: Initializer expression (e.g., pass `= { }` at minimum).
+ * @trailer: Trailing expressions for attributes and/or initializers.
  */
-#define _DEFINE_FLEX(type, name, member, count, initializer...)			\
+#define __DEFINE_FLEX(type, name, member, count, trailer...)			\
 	_Static_assert(__builtin_constant_p(count),				\
 		       "onstack flex array members require compile-time const count"); \
 	union {									\
 		u8 bytes[struct_size_t(type, member, count)];			\
 		type obj;							\
-	} name##_u = { .obj initializer };					\
+	} name##_u trailer;							\
 	type *name = (type *)&name##_u
 
+/**
+ * _DEFINE_FLEX() - helper macro for DEFINE_FLEX() family.
+ * Enables caller macro to pass (different) initializer.
+ *
+ * @type: structure type name, including "struct" keyword.
+ * @name: Name for a variable to define.
+ * @member: Name of the array member.
+ * @count: Number of elements in the array; must be compile-time const.
+ * @initializer: Initializer expression (e.g., pass `= { }` at minimum).
+ */
+#define _DEFINE_FLEX(type, name, member, count, initializer...)			\
+	__DEFINE_FLEX(type, name, member, count, = { .obj initializer })
+
 /**
  * DEFINE_RAW_FLEX() - Define an on-stack instance of structure with a trailing
  * flexible array member, when it does not have a __counted_by annotation.
@@ -424,7 +437,7 @@ static inline size_t __must_check size_sub(size_t minuend, size_t subtrahend)
  * elements in array @member.
  */
 #define DEFINE_RAW_FLEX(type, name, member, count)	\
-	_DEFINE_FLEX(type, name, member, count, = {})
+	__DEFINE_FLEX(type, name, member, count, = { })
 
 /**
  * DEFINE_FLEX() - Define an on-stack instance of structure with a trailing