From: Alejandro Colomar <alx.manpages@gmail.com>
To: libc-alpha@sourceware.org
Cc: "Alejandro Colomar" <alx@kernel.org>,
linux-man@vger.kernel.org, "Bastien Roucariès" <rouca@debian.org>,
"Eric Blake" <eblake@redhat.com>,
"Zack Weinberg" <zack@owlfolio.org>,
"Stefan Puiu" <stefan.puiu@gmail.com>,
"Igor Sysoev" <igor@sysoev.ru>
Subject: [PATCH v2] socket: Implement sockaddr_storage with an anonymous union
Date: Fri, 20 Jan 2023 14:40:44 +0100 [thread overview]
Message-ID: <20230120134043.10247-1-alx@kernel.org> (raw)
The historical design of `sockaddr_storage` makes it impossible to use
without breaking strict aliasing rules. Not only this type is unusable,
but even the use of other `sockaddr_*` structures directly (when the
programmer only cares about a single address family) is also incorrect,
since at some point the structure will be accessed as a `sockaddr`, and
that breaks strict aliasing rules too.
So, the only way for a programmer to not invoke Undefined Behavior is to
declare a union that includes `sockaddr` and any `sockaddr_*` structures
that are of interest, which allows later accessing as either the correct
structure or plain `sockaddr` for the sa_family.
This patch fixes sockaddr_storage to remove UB on its uses and make it
that structure that everybody should be using. It also allows removing
many casts in code that needs to pass a sockaddr as a side effect.
The following is an example of how this improves both existing code and
new code:
void
foo(foo)
{
struct old_sockaddr_storage oss;
struct new_sockaddr_storage nss;
// ... (initialize oss and nss)
inet_sockaddr2str(&nss.sa); // correct (and has no casts)
inet_sockaddr2str((struct sockaddr *)&oss); // UB
inet_sockaddr2str((struct sockaddr *)&nss); // correct
}
/* This function is correct, as far as the accessed object has the
* type we're using. That's only possible through a `union`, since
* we're accessing it with 2 different types: `sockaddr` for the
* `sa_family` and then the appropriate subtype for the address
* itself.
*/
const char *
inet_sockaddr2str(const struct sockaddr *sa)
{
struct sockaddr_in *sin;
struct sockaddr_in6 *sin6;
static char buf[MAXHOSTNAMELEN];
switch (sa->sa_family) {
case AF_INET:
sin = (struct sockaddr_in *) sa;
inet_ntop(AF_INET, &sin->sin_addr, buf, NITEMS(buf));
return buf;
case AF_INET6:
sin6 = (struct sockaddr_in6 *) sa;
inet_ntop(AF_INET6, &sin6->sin6_addr, buf, NITEMS(buf));
return buf;
default:
errno = EAFNOSUPPORT;
return NULL;
}
}
While it's not necessary to do the same for `sockaddr`, it might still
be interesting to so, since it will allow removing many casts in the
implementation of many libc functions.
Link: <https://lore.kernel.org/linux-man/2860541.uBSZ6KuyZf@portable-bastien/T/>
Link: <https://inbox.sourceware.org/libc-alpha/0f25d60f-f183-b518-b6c1-6d46aa63ee57@gmail.com/T/>
Link: <https://stackoverflow.com/a/42190913/6872717>
Link: <https://software.codidact.com/posts/287748>
Cc: Bastien Roucariès <rouca@debian.org>
Cc: Eric Blake <eblake@redhat.com>
Cc: Zack Weinberg <zack@owlfolio.org>
Cc: Stefan Puiu <stefan.puiu@gmail.com>
Cc: Igor Sysoev <igor@sysoev.ru>
Signed-off-by: Alejandro Colomar <alx@kernel.org>
---
v2:
- Fix incorrect cast in commit message.
bits/socket.h | 11 ++++++++---
1 file changed, 8 insertions(+), 3 deletions(-)
diff --git a/bits/socket.h b/bits/socket.h
index aac8c49b00..c0c23b4e84 100644
--- a/bits/socket.h
+++ b/bits/socket.h
@@ -168,9 +168,14 @@ struct sockaddr
struct sockaddr_storage
{
- __SOCKADDR_COMMON (ss_); /* Address family, etc. */
- char __ss_padding[_SS_PADSIZE];
- __ss_aligntype __ss_align; /* Force desired alignment. */
+ union
+ {
+ __SOCKADDR_COMMON (ss_); /* Address family, etc. */
+ struct sockaddr sa;
+ struct sockaddr_in sin;
+ struct sockaddr_in6 sin6;
+ struct sockaddr_un sun;
+ };
};
--
2.39.0
next reply other threads:[~2023-01-20 13:41 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-01-20 13:40 Alejandro Colomar [this message]
2023-01-20 17:49 ` Joseph Myers
2023-01-20 19:26 ` Alejandro Colomar
2023-01-20 18:04 ` Zack Weinberg
2023-01-20 19:25 ` Alejandro Colomar
2023-01-21 2:38 ` Alejandro Colomar
2023-01-21 3:17 ` Alejandro Colomar
2023-01-21 13:30 ` Bastien Roucariès
2023-01-21 14:30 ` Alejandro Colomar
2023-01-22 14:12 ` Bastien Roucariès
2023-01-20 20:32 ` Bastien Roucariès
2023-01-20 20:38 ` Alejandro Colomar
2023-01-20 20:46 ` Bastien Roucariès
2023-01-20 20:51 ` Alejandro Colomar
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