Mention the use of __member_size() for DEFINE_FLEX variables as a hint
for getting at the compile-time size of the resulting flexible array
member.

Signed-off-by: Kees Cook <kees@kernel.org>
---
Cc: "Gustavo A. R. Silva" <gustavoars@kernel.org>
Cc: linux-hardening@vger.kernel.org
---
 include/linux/overflow.h | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/include/linux/overflow.h b/include/linux/overflow.h
index 0c7e3dcfe867..6ee67c20b575 100644
--- a/include/linux/overflow.h
+++ b/include/linux/overflow.h
@@ -419,6 +419,7 @@ static inline size_t __must_check size_sub(size_t minuend, size_t subtrahend)
  * Define a zeroed, on-stack, instance of @type structure with a trailing
  * flexible array member.
  * Use __struct_size(@name) to get compile-time size of it afterwards.
+ * Use __member_size(@name->member) to get compile-time size of @name members.
  */
 #define DEFINE_RAW_FLEX(type, name, member, count)	\
 	_DEFINE_FLEX(type, name, member, count, = {})
@@ -436,6 +437,7 @@ static inline size_t __must_check size_sub(size_t minuend, size_t subtrahend)
  * Define a zeroed, on-stack, instance of @TYPE structure with a trailing
  * flexible array member.
  * Use __struct_size(@NAME) to get compile-time size of it afterwards.
+ * Use __member_size(@NAME->member) to get compile-time size of @NAME members.
  */
 #define DEFINE_FLEX(TYPE, NAME, MEMBER, COUNTER, COUNT)	\
 	_DEFINE_FLEX(TYPE, NAME, MEMBER, COUNT, = { .obj.COUNTER = COUNT, })
-- 
2.34.1

On Wed, 16 Apr 2025 10:29:15 -0700, Kees Cook wrote:
> Mention the use of __member_size() for DEFINE_FLEX variables as a hint
> for getting at the compile-time size of the resulting flexible array
> member.
> 
> 

Applied to for-next/hardening, thanks!

[1/1] overflow: Clarify expectations for getting DEFINE_FLEX variable sizes
      https://git.kernel.org/kees/c/2122b4571745

Take care,

-- 
Kees Cook

On 16/04/25 11:29, Kees Cook wrote:
> Mention the use of __member_size() for DEFINE_FLEX variables as a hint
> for getting at the compile-time size of the resulting flexible array
> member.
> 
> Signed-off-by: Kees Cook <kees@kernel.org>

Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>

Thanks!
--
Gustavo

> ---
> Cc: "Gustavo A. R. Silva" <gustavoars@kernel.org>
> Cc: linux-hardening@vger.kernel.org
> ---
>   include/linux/overflow.h | 2 ++
>   1 file changed, 2 insertions(+)
> 
> diff --git a/include/linux/overflow.h b/include/linux/overflow.h
> index 0c7e3dcfe867..6ee67c20b575 100644
> --- a/include/linux/overflow.h
> +++ b/include/linux/overflow.h
> @@ -419,6 +419,7 @@ static inline size_t __must_check size_sub(size_t minuend, size_t subtrahend)
>    * Define a zeroed, on-stack, instance of @type structure with a trailing
>    * flexible array member.
>    * Use __struct_size(@name) to get compile-time size of it afterwards.
> + * Use __member_size(@name->member) to get compile-time size of @name members.
>    */
>   #define DEFINE_RAW_FLEX(type, name, member, count)	\
>   	_DEFINE_FLEX(type, name, member, count, = {})
> @@ -436,6 +437,7 @@ static inline size_t __must_check size_sub(size_t minuend, size_t subtrahend)
>    * Define a zeroed, on-stack, instance of @TYPE structure with a trailing
>    * flexible array member.
>    * Use __struct_size(@NAME) to get compile-time size of it afterwards.
> + * Use __member_size(@NAME->member) to get compile-time size of @NAME members.
>    */
>   #define DEFINE_FLEX(TYPE, NAME, MEMBER, COUNTER, COUNT)	\
>   	_DEFINE_FLEX(TYPE, NAME, MEMBER, COUNT, = { .obj.COUNTER = COUNT, })