Re: [PATCH v2] uapi: fix __DECLARE_FLEX_ARRAY for C++

[Kees Cook]

On Fri, Sep 08, 2023 at 06:14:38PM +0300, Alexey Dobriyan wrote:
> __DECLARE_FLEX_ARRAY(T, member) macro expands to
> 
> 	struct {
> 		struct {} __empty_member;
> 		T member[];
> 	};
> 
> which is subtly wrong in C++ because sizeof(struct{}) is 1 not 0,

Ewwww. Isn't this a bug in C++?

> changing UAPI structures layouts.
> 
> This can be fixed by expanding to
> 
> 	T member[];
> 
> Now g++ doesn't like "T member[]" either throwing errors on code like
> this:
> 
> 	struct S {
> 		union {
> 			T1 member1[];
> 			T2 member2[];
> 		};
> 	};
> 
> or
> 
> 	struct S {
> 		T member[];
> 	};
> So use
> 
> 	T member[0];
> 
> which seems to work and does the right thing wrt structure layout.

It seems sad to leave C++ broken, but I guess we have to do this.

Acked-by: Kees Cook <keescook@xxxxxxxxxxxx>

> Fix header guard while I'm at it.

Hm, when did that get broken? Maybe that should be fixed separately?

> 
> Signed-off-by: Alexey Dobriyan <adobriyan@xxxxxxxxx>

Probably a Fixes: tag would be nice too.

-Kees

> ---
> 
>  include/uapi/linux/stddef.h |    6 ++++++
>  1 file changed, 6 insertions(+)
> 
> --- a/include/uapi/linux/stddef.h
> +++ b/include/uapi/linux/stddef.h
> @@ -39,6 +39,10 @@
>   * struct, it needs to be wrapped in an anonymous struct with at least 1
>   * named member, but that member can be empty.
>   */
> +#ifdef __cplusplus
> +#define __DECLARE_FLEX_ARRAY(T, member)		\
> +	T member[0]
> +#else
>  #define __DECLARE_FLEX_ARRAY(TYPE, NAME)	\
>  	struct { \
>  		struct { } __empty_ ## NAME; \
> @@ -49,3 +53,5 @@
>  #ifndef __counted_by
>  #define __counted_by(m)
>  #endif
> +
> +#endif

-- 
Kees Cook