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
