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

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Date:   Thu, 11 May 2023 14:13:06 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Joseph Myers <joseph@...esourcery.com>
Cc:     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 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.

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