Re: [PATCH v2] socket: Implement sockaddr_storage with an anonymous union

["Bastien Roucariès" <rouca@debian.org>]

[-- Attachment #1: Type: text/plain, Size: 4638 bytes --]

Le vendredi 20 janvier 2023, 13:40:44 UTC Alejandro Colomar a écrit :
> 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.  */
no this is not correct you break ABI by reducing size
> +    union
> +      {
> +        __SOCKADDR_COMMON (ss_);	/* Address family, etc.  */
> +        struct sockaddr      sa;
> +        struct sockaddr_in   sin;
> +        struct sockaddr_in6  sin6;
> +        struct sockaddr_un   sun;
> +      };
>    };

Correct one structure is

struct __private_sock_storage {
    __SOCKADDR_COMMON (ssprivate_);   /* Address family, etc. */
    char __ss_padding[_SS_PADSIZE];
    __ss_aligntype __ss_align; /* Force desired alignment. */
}

 struct sockaddr_storage
   {
       union
      {
         __SOCKADDR_COMMON (ss_);       /* Address family, etc. */
        struct sockaddr      sa;
         struct sockaddr_in   sin;
        struct sockaddr_in6  sin6;
        struct sockaddr_un   sun;
        struct __private_sock_storage _private;
      };
};

May it could be dropped later using align construct for modern C and padding

Bastien
>  
>  
> 


[-- Attachment #2: This is a digitally signed message part. --]
[-- Type: application/pgp-signature, Size: 833 bytes --]