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/20250520205920.2134829-9-anthony.l.nguyen@intel.com [1]
Fixes: 47e36ed78406 ("overflow: Fix direct struct member initialization in _DEFINE_FLEX()")
Signed-off-by: Kees Cook <kees@kernel.org>
---
Cc: "Gustavo A. R. Silva" <gustavoars@kernel.org>
Cc: Alexander Lobakin <aleksander.lobakin@intel.com>
Cc: <linux-hardening@vger.kernel.org>
---
 include/linux/overflow.h | 25 +++++++++++++++++++------
 1 file changed, 19 insertions(+), 6 deletions(-)

diff --git a/include/linux/overflow.h b/include/linux/overflow.h
index 7b7be27ca113..154ed0dbb43f 100644
--- a/include/linux/overflow.h
+++ b/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
-- 
2.34.1

From: Kees Cook <kees@kernel.org>
Date: Fri, 30 May 2025 12:13:16 -0700

> 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/20250520205920.2134829-9-anthony.l.nguyen@intel.com [1]
> Fixes: 47e36ed78406 ("overflow: Fix direct struct member initialization in _DEFINE_FLEX()")
> Signed-off-by: Kees Cook <kees@kernel.org>

Looks great to me, thanks!

Also, double thanks for this hint about using __uninitialized to
mitigate perf hits with automatic stack initialization -- will
definitely help :)

Olek