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

[Alexey Dobriyan]

On Fri, Sep 08, 2023 at 08:53:12AM -0700, Kees Cook wrote:
> 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++?

Sort of, but it can't be fixed.

> > 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?

By your last commit?

> > Signed-off-by: Alexey Dobriyan <adobriyan@xxxxxxxxx>
> 
> Probably a Fixes: tag would be nice too.

OK

Fixes: c8248faf3ca2 ("Compiler Attributes: counted_by: Adjust name and identifier expansion")
Fixes: 3080ea5553cc ("stddef: Introduce DECLARE_FLEX_ARRAY() helper")

> >  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