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

["Zack Weinberg" <zack@owlfolio.org>]

On Fri, Jan 20, 2023, at 8:40 AM, Alejandro Colomar wrote:
> 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.

...

>     struct new_sockaddr_storage  nss;
>
>     // ... (initialize oss and nss)
>
>     inet_sockaddr2str(&nss.sa);  // correct (and has no casts)

I think we need to move slowly here and be _sure_ that any proposed change
does in fact reduce the amount of UB.  This construct, in particular, might
not actually be correct in practice: see https://godbolt.org/z/rn51cracn for
a case where, if I'm reading it right, the compiler assumes that a write
through a `struct fancy *` cannot alter the values accessible through a
`struct simple *` even though both pointers point into the same union.
(Test case provided by <https://stackoverflow.com/users/363751/supercat>;
I don't know any other identifier for them.)

zw