Re: [PATCH] public/domctl: Fix the struct xen_domctl ABI in 32bit builds

[Jan Beulich <jbeulich@xxxxxxxx>]

On 28.07.2020 12:15, Andrew Cooper wrote:
> The Xen domctl ABI currently relies on the union containing a field with
> alignment of 8.
>
> 32bit projects which only copy the used subset of functionality end up with an
> ABI breakage if they don't have at least one uint64_aligned_t field copied.
I'm entirely find with the change, but I'm struggling with this justification:
How can _any_ change to the header in our tree help people using custom
derivations (i.e. including those defining only some of the union members)?

> Further proof that C isn't an appropriate way to desribe an ABI...
Further proof that it requires more careful writing in C to serve as an ABI
description, I would say.

> --- a/xen/common/domctl.c
> +++ b/xen/common/domctl.c
> @@ -959,6 +959,14 @@ long do_domctl(XEN_GUEST_HANDLE_PARAM(xen_domctl_t) 
> u_domctl)
>       return ret;
>   }
>   
> +static void __init __maybe_unused build_assertions(void)
> +{
> +    struct xen_domctl d;
Doesn't this also need __maybe_unused? Afaik ...

> +    BUILD_BUG_ON(sizeof(d) != 16 /* header */ + 128 /* union */);
> +    BUILD_BUG_ON(offsetof(typeof(d), u) != 16);
> +}
... neither sizeof() nor typeof() count as actual uses.

Jan