From: Jonathan Wakely <jwakely.gcc@gmail.com>
To: Kees Cook <keescook@chromium.org>
Cc: Joseph Myers <joseph@codesourcery.com>,
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: Fri, 12 May 2023 08:49:43 +0100 [thread overview]
Message-ID: <CAH6eHdQNdpBTtbJ6Sxnxaq-2U=BsU52bDzjvkUEqJiCnJN4Vwg@mail.gmail.com> (raw)
In-Reply-To: <202305111514.576EB7F@keescook>
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On Thu, 11 May 2023, 23:17 Kees Cook via Gcc, <gcc@gcc.gnu.org> wrote:
> 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?
You don't have two different objects with the same address in a union,
because only one of them "exists" at any time. You're reusing the storage
for one object at a time, not for all of the union members.
next prev parent reply other threads:[~2023-05-12 7:49 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-05-11 16:07 Alejandro Colomar
2023-05-11 16:29 ` Alejandro Colomar
2023-05-11 19:07 ` 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
2023-05-11 22:52 ` Joseph Myers
2023-05-12 0:25 ` Alejandro Colomar
2023-05-12 7:49 ` Jonathan Wakely [this message]
2023-05-12 6:16 ` Richard Biener
2023-05-12 12:32 ` David Brown
2023-05-15 19:58 ` Qing Zhao
2023-05-18 16:25 ` Martin Uecker
2023-05-18 20:59 ` Qing Zhao
2023-05-19 12:08 ` Martin Uecker
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