From: Alejandro Colomar <alx.manpages@gmail.com>
To: Xi Ruoyao <xry111@xry111.site>,
linux-man@vger.kernel.org, Rich Felker <dalias@libc.org>
Cc: "Alejandro Colomar" <alx@kernel.org>, GCC <gcc@gcc.gnu.org>,
glibc <libc-alpha@sourceware.org>,
"Bastien Roucariès" <rouca@debian.org>,
"Stefan Puiu" <stefan.puiu@gmail.com>,
"Igor Sysoev" <igor@sysoev.ru>,
"Andrew Clayton" <a.clayton@nginx.com>,
"Richard Biener" <richard.guenther@gmail.com>,
"Zack Weinberg" <zack@owlfolio.org>,
"Florian Weimer" <fweimer@redhat.com>,
"Joseph Myers" <joseph@codesourcery.com>,
"Jakub Jelinek" <jakub@redhat.com>,
"Eric Blake" <eblake@redhat.com>
Subject: Re: [PATCH] sockaddr.3type: BUGS: Document that libc should be fixed using a union
Date: Mon, 6 Feb 2023 12:55:12 +0100 [thread overview]
Message-ID: <ae73337b-7b37-9c94-e5e0-d6ebbf2c7ffb@gmail.com> (raw)
In-Reply-To: <0a9306fa37edeb4a989b2929de67fee8606a3d8a.camel@xry111.site>
[-- Attachment #1.1: Type: text/plain, Size: 3028 bytes --]
Hi Xi,
On 2/6/23 07:02, Xi Ruoyao wrote:
> On Sun, 2023-02-05 at 16:31 +0100, Alejandro Colomar via Libc-alpha wrote:
>
>> The only correct way to use different types in an API is
>> through a union.
>
> I don't think this statement is true (in general). Technically we can
> write something like this:
>
> struct sockaddr { ... };
> struct sockaddr_in { ... };
> struct sockaddr_in6 { ... };
>
> int bind(int fd, const struct sockaddr *addr, socklen_t addrlen)
> {
> if (addrlen < sizeof(struct sockaddr) {
> errno = EINVAL;
> return -1;
> }
>
> /* cannot use "addr->sa_family" directly: it will be an UB */
> sa_family_t sa_family;
> memcpy(&sa_family, addr, sizeof(sa_family));
>
> switch (sa_family) {
> case AF_INET:
> return _do_bind_in(fd, (struct sockaddr_in *)addr, addrlen);
> case AF_INET6:
> return _do_bind_in6(fd, (struct sockaddr_in6 *)addr, addrlen);
> /* more cases follow here */
> default:
> errno = EINVAL;
> return -1;
> }
> }
> }
>
> In this way we can use sockaddr_{in,in6,...} for bind() safely, as long
> as we can distinguish the "real" type of addr using the leading byte
> sequence (and the caller uses it carefully).
True; I hadn't thought of memcpy()ing the first member of the struct. That's
valid; overcomplicated but valid.
>
> But obviously sockaddr_storage can't be distinguished here, so casting a
> struct sockaddr_stroage * to struct sockaddr * and passing it to bind()
> will still be wrong (unless we make sockaddr_storage an union or add
> [[gnu::may_alias]]).
But as you say, it still leaves us with a question. What should one declare for
passing to the standard APIs? It can only be a union. So we can either tell
users to each create their own union, or we can make sockaddr_storage be a
union. The latter slightly violates POSIX due to namespaces as Rich noted, but
that's a minor violation, and POSIX can be changed to accomodate for that.
If we change sockaddr_storage to be a union, we have two benefits:
- Old code which uses sockaddr_storage is made conforming (non-UB) without
modifying the source.
- Users can inspect the structures.
If we don't, and deprecate sockaddr_storage, we should tell users to declare
their own unions _and_ treat all these structures as black boxes which can only
be read by memcpy()ing their contents.
Which of the two do we want? I think fixing sockaddr_storage is simpler, and
allows existing practice of reading these structures. The other one just makes
(or rather acknowledges, since it has always been like that) a lot of existing
code invoke UB, and acknowledges that you can't safely use these structures
without a lot of workarounding.
Cheers,
Alex
--
<http://www.alejandro-colomar.es/>
GPG key fingerprint: A9348594CE31283A826FBDD8D57633D441E25BB5
[-- Attachment #2: OpenPGP digital signature --]
[-- Type: application/pgp-signature, Size: 833 bytes --]
next prev parent reply other threads:[~2023-02-06 11:55 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-02-05 15:28 Alejandro Colomar
2023-02-05 15:31 ` Alejandro Colomar
2023-02-06 6:02 ` Xi Ruoyao
2023-02-06 11:20 ` Rich Felker
2023-02-06 11:55 ` Alejandro Colomar [this message]
2023-02-06 13:38 ` Rich Felker
2023-02-06 14:11 ` Alejandro Colomar
2023-02-06 17:21 ` Zack Weinberg
2023-02-06 17:48 ` Rich Felker
2023-02-05 23:43 ` Rich Felker
2023-02-05 23:59 ` Alejandro Colomar
2023-02-06 0:15 ` Rich Felker
2023-02-06 18:45 ` Eric Blake
2023-02-07 1:21 ` Alejandro Colomar
2023-03-18 7:54 ` roucaries bastien
2023-03-20 10:49 ` Alejandro Colomar
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=ae73337b-7b37-9c94-e5e0-d6ebbf2c7ffb@gmail.com \
--to=alx.manpages@gmail.com \
--cc=a.clayton@nginx.com \
--cc=alx@kernel.org \
--cc=dalias@libc.org \
--cc=eblake@redhat.com \
--cc=fweimer@redhat.com \
--cc=gcc@gcc.gnu.org \
--cc=igor@sysoev.ru \
--cc=jakub@redhat.com \
--cc=joseph@codesourcery.com \
--cc=libc-alpha@sourceware.org \
--cc=linux-man@vger.kernel.org \
--cc=richard.guenther@gmail.com \
--cc=rouca@debian.org \
--cc=stefan.puiu@gmail.com \
--cc=xry111@xry111.site \
--cc=zack@owlfolio.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).