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

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Message-ID: <202305111514.576EB7F@keescook>
Date:   Thu, 11 May 2023 15:16:43 -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 09:43:49PM +0000, Joseph Myers wrote:
> On Thu, 11 May 2023, Kees Cook via Gcc wrote:
> 
> > 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?
> 
> Zero-sized objects break the principle that different objects have 
> different addresses, and the principle of being able to subtract pointers 
> to different elements of an array.  There would also be serious C++ 
> compatibility concerns, since C++ allows a struct with no members but it 
> has nonzero size, unlike the GNU C extension where a struct with no 
> members has size zero.

Okay, understood. If this is a C-only thing, we can ignore the C++
impact. What depends on the "different objects have different addresses"
principle? And why do unions not break this -- they could point to the
same locations within the object? And don't flexible arrays already need
special handling in this regard?

-- 
Kees Cook