From: Rich Felker <dalias@libc.org>
To: Xi Ruoyao <xry111@xry111.site>
Cc: "Alejandro Colomar" <alx.manpages@gmail.com>,
linux-man@vger.kernel.org, "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 06:20:20 -0500 [thread overview]
Message-ID: <20230206112019.GH3298@brightrain.aerifal.cx> (raw)
In-Reply-To: <0a9306fa37edeb4a989b2929de67fee8606a3d8a.camel@xry111.site>
On Mon, Feb 06, 2023 at 02:02:23PM +0800, 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).
>
> 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]]).
If you wanted to make this work, you can just memcpy sockaddr_storage
to a local object of the right declared type to access it. But this is
only relevant for a userspace implementation of bind() rather than one
that just marshalls it across some syscall boundary to a kernel.
Rich
next prev parent reply other threads:[~2023-02-06 11:20 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 [this message]
2023-02-06 11:55 ` Alejandro Colomar
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
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