On Fri, Sep 08, 2023 at 07:03:17PM +0300, Alexey Dobriyan wrote:
> 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?
:( Oops. I'm shocked this hasn't caused bigger problems.
>
> > > 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")
Okay, can you please split the patch so they can be backported
separately? Then I'll get them landed, etc.
Thanks for fixing these!
-Kees
--
Kees Cook
