linux-hardening - Re: [wish] Flexible array members in unions

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Date:   Fri, 12 May 2023 08:16:28 +0200
From:   Richard Biener <richard.guenther@...il.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     Joseph Myers <joseph@...esourcery.com>,
        Alejandro Colomar <alx.manpages@...il.com>,
        GCC <gcc@....gnu.org>, Alejandro Colomar <alx@...nx.com>,
        Andrew Clayton <a.clayton@...nx.com>,
        Andrew Clayton <andrew@...ital-domain.net>,
        linux-hardening@...r.kernel.org
Subject: Re: [wish] Flexible array members in unions

On Thu, May 11, 2023 at 11:14 PM Kees Cook via Gcc <gcc@....gnu.org> wrote:
>
> On Thu, May 11, 2023 at 08:53:52PM +0000, Joseph Myers wrote:
> > On Thu, 11 May 2023, Kees Cook via Gcc wrote:
> >
> > > On Thu, May 11, 2023 at 06:29:10PM +0200, Alejandro Colomar wrote:
> > > > On 5/11/23 18:07, Alejandro Colomar wrote:
> > > > [...]
> > > > > Would you allow flexible array members in unions?  Is there any
> > > > > strong reason to disallow them?
> > >
> > > Yes please!! And alone in a struct, too.
> > >
> > > AFAICT, there is no mechanical/architectural reason to disallow them
> > > (especially since they _can_ be constructed with some fancy tricks,
> > > and they behave as expected.) My understanding is that it's disallowed
> > > due to an overly strict reading of the very terse language that created
> > > flexible arrays in C99.
> >
> > Standard C has no such thing as a zero-size object or type, which would
> > lead to problems with a struct or union that only contains a flexible
> > array member there.
>
> Ah-ha, okay. That root cause makes sense now.

Hmm. but then the workaround

struct X {
  int n;
  union u {
      char at_least_size_one;
      int iarr[];
      short sarr[];
  };
};

doesn't work either.  We could make that a GNU extension without
adverse effects?

Richard.

> Why are zero-sized objects missing in Standard C? Or, perhaps, the better
> question is: what's needed to support the idea of a zero-sized object?
>
> --
> Kees Cook