From: Joseph Myers <joseph@codesourcery.com>
To: Kees Cook <keescook@chromium.org>
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 22:52:25 +0000 [thread overview]
Message-ID: <a9f2739c-7582-853e-ea94-e13f5ea4698@codesourcery.com> (raw)
In-Reply-To: <202305111514.576EB7F@keescook>
On Thu, 11 May 2023, Kees Cook via Gcc wrote:
> Okay, understood. If this is a C-only thing, we can ignore the C++
> impact.
We're a lot more careful lately in WG14 about checking for C++
compatibility issues and expecting approval from the liaison group for
anything with possible compatibility concerns for syntax in the common
subset of C and C++. So, no, we can't ignore the C++ impact for adding
empty types; it would need careful consideration in the liaison group.
> 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?
"including a pointer to an object and a subobject at its beginning" and
"one is a pointer to one past the end of one array object and the other is
a pointer to the start of a different array object that happens to
immediately follow the first array object in the address space" are both
cases included in the semantics for comparison operators. If you allow
zero-size objects you get more special cases there (and quite possibly
affect optimizations based on points-to analysis that can determine
pointers are based on different objects, if an object is not known at
compile time to have nonzero size).
--
Joseph S. Myers
joseph@codesourcery.com
next prev parent reply other threads:[~2023-05-11 22:52 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 [this message]
2023-05-12 0:25 ` Alejandro Colomar
2023-05-12 7:49 ` Jonathan Wakely
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
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=a9f2739c-7582-853e-ea94-e13f5ea4698@codesourcery.com \
--to=joseph@codesourcery.com \
--cc=a.clayton@nginx.com \
--cc=alx.manpages@gmail.com \
--cc=alx@nginx.com \
--cc=andrew@digital-domain.net \
--cc=gcc@gcc.gnu.org \
--cc=keescook@chromium.org \
--cc=linux-hardening@vger.kernel.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).