From: Richard Biener <richard.guenther@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 08:16:28 +0200 [thread overview]
Message-ID: <CAFiYyc1JRm6gm8aw2ZQmcycN3AF3hu5wjwGeUgDCN6gRN7UbbA@mail.gmail.com> (raw)
In-Reply-To: <202305111410.CFE0875F@keescook>
On Thu, May 11, 2023 at 11:14 PM Kees Cook via Gcc <gcc@gcc.gnu.org> wrote:
>
> 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.
Hmm. but then the workaround
struct X {
int n;
union u {
char at_least_size_one;
int iarr[];
short sarr[];
};
};
doesn't work either. We could make that a GNU extension without
adverse effects?
Richard.
> 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
next prev parent reply other threads:[~2023-05-12 6:16 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
2023-05-12 6:16 ` Richard Biener [this message]
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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