Currently, to statically initialize the struct members of the `type`
object created by _DEFINE_FLEX(), the internal object `obj` must be
explicitly referenced at the call site. See:

struct flex {
        int a;
        int b;
        struct foo flex_array[];
};

_DEFINE_RAW_FLEX(struct flex, instance, flex_array,
                 FIXED_SIZE, = {
                        .obj = {
                                .a = 13,
                                .b = 37,
                        },
                });

This leaks _DEFINE_FLEX() internal implementation details and make
the helper harder to use and read.

Fix this and allow for a more natural and intuitive C99 init-style:

_DEFINE_RAW_FLEX(struct flex, instance, flex_array,
                 FIXED_SIZE, = {
                        .a = 13,
                        .b = 37,
                });

Note that before these changes, the `initializer` argument was optional,
but now it's required.

Also, update "counter" member initialization in DEFINE_FLEX().

Fixes: 26dd68d293fd ("overflow: add DEFINE_FLEX() for on-stack allocs")
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
---
Changes in v2:
 - Update kernel-doc block for _DEFINE_FLEX(). (Kees)
 - Update changelog text - `initializer` argument is now required.

v1:
 - Link: https://lore.kernel.org/linux-hardening/aBK2TUEeQfCFop9Y@kspp/

 include/linux/overflow.h | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/include/linux/overflow.h b/include/linux/overflow.h
index 69533e703be5..b50c8b30a14a 100644
--- a/include/linux/overflow.h
+++ b/include/linux/overflow.h
@@ -396,7 +396,7 @@ static inline size_t __must_check size_sub(size_t minuend, size_t subtrahend)
  * @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 (could be empty for no init).
+ * @initializer: Initializer expression (e.g., pass `= { }` at minimum).
  */
 #define _DEFINE_FLEX(type, name, member, count, initializer...)			\
 	_Static_assert(__builtin_constant_p(count),				\
@@ -404,7 +404,7 @@ static inline size_t __must_check size_sub(size_t minuend, size_t subtrahend)
 	union {									\
 		u8 bytes[struct_size_t(type, member, count)];			\
 		type obj;							\
-	} name##_u initializer;							\
+	} name##_u = { .obj initializer };					\
 	type *name = (type *)&name##_u
 
 /**
@@ -444,7 +444,7 @@ static inline size_t __must_check size_sub(size_t minuend, size_t subtrahend)
  * elements in array @member.
  */
 #define DEFINE_FLEX(TYPE, NAME, MEMBER, COUNTER, COUNT)	\
-	_DEFINE_FLEX(TYPE, NAME, MEMBER, COUNT, = { .obj.COUNTER = COUNT, })
+	_DEFINE_FLEX(TYPE, NAME, MEMBER, COUNT, = { .COUNTER = COUNT, })
 
 /**
  * STACK_FLEX_ARRAY_SIZE() - helper macro for DEFINE_FLEX() family.
-- 
2.43.0

On Thu, 01 May 2025 16:23:43 -0600, Gustavo A. R. Silva wrote:
> Currently, to statically initialize the struct members of the `type`
> object created by _DEFINE_FLEX(), the internal object `obj` must be
> explicitly referenced at the call site. See:
> 
> struct flex {
>         int a;
>         int b;
>         struct foo flex_array[];
> };
> 
> [...]

Applied to for-next/hardening, thanks!

[1/1] overflow: Fix direct struct member initialization in _DEFINE_FLEX()
      https://git.kernel.org/kees/c/d2e95bf39497

Take care,

-- 
Kees Cook