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

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Date:   Thu, 11 May 2023 20:53:52 +0000
From:   Joseph Myers <joseph@...esourcery.com>
To:     Kees Cook <keescook@...omium.org>
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, 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.

-- 
Joseph S. Myers
joseph@...esourcery.com