From: Kees Cook <keescook@chromium.org>
To: Joseph Myers <joseph@codesourcery.com>
Cc: Alejandro Colomar <alx.manpages@gmail.com>, GCC <gcc@gcc.gnu.org>,
Alejandro Colomar <alx@nginx.com>,
Andrew Clayton <a.clayton@nginx.com>,
Andrew Clayton <andrew@digital-domain.net>,
linux-hardening@vger.kernel.org
Subject: Re: [wish] Flexible array members in unions
Date: Thu, 11 May 2023 15:16:43 -0700 [thread overview]
Message-ID: <202305111514.576EB7F@keescook> (raw)
In-Reply-To: <bb3aac3c-f377-5598-cbf6-47d0be448f3@codesourcery.com>
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
next prev parent reply other threads:[~2023-05-11 22:16 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <ac2550e0-2f5c-3680-08e6-0c224d043036@gmail.com>
[not found] ` <44940599-7b43-99f6-5b09-4f050d645c7b@gmail.com>
2023-05-11 19:07 ` [wish] Flexible array members in unions Kees Cook
2023-05-11 20:53 ` Joseph Myers
2023-05-11 21:13 ` Kees Cook
2023-05-11 21:43 ` Joseph Myers
2023-05-11 22:16 ` Kees Cook [this message]
2023-05-11 22:52 ` Joseph Myers
2023-05-12 0:25 ` Alejandro Colomar
2023-05-12 6:16 ` Richard Biener
2023-05-15 19:58 ` Qing Zhao
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