Re: [XEN PATCH][for-4.19 v2 8/8] xen/compat: use BUILD_BUG_ON in CHECK_SIZE macros

[Jan Beulich <jbeulich@xxxxxxxx>]

On 12.10.2023 17:28, Nicola Vetrini wrote:
> BUILD_BUG_ON is the preferred way to induce a build error
> upon statically determined incorrect conditions.
> 
> This also fixes a MISRA C:2012 Rule 10.1 violation in the
> previous formulation.
> 
> Signed-off-by: Nicola Vetrini <nicola.vetrini@xxxxxxxxxxx>

Hmm, looking back it's indeed not clear why I didn't use BUILD_BUG_ON() right
away. Perhaps just to avoid inline functions when things can be done without.
And/or because originally the macros were intended to be usable in function
bodies, not (just) at file scope. However, ...

> --- a/xen/include/xen/compat.h
> +++ b/xen/include/xen/compat.h
> @@ -151,12 +151,20 @@ CHECK_NAME_(k, n, T)(k xen_ ## n *x, \
>      return x == c; \
>  }
>  
> -#define CHECK_SIZE(name) \
> -    typedef int CHECK_NAME(name, S)[1 - (sizeof(xen_ ## name ## _t) != \
> -                                         sizeof(compat_ ## name ## _t)) * 2]
> +#define CHECK_SIZE(name)                                  \
> +static inline void __maybe_unused CHECK_SIZE_##name(void) \
> +{                                                         \
> +    typedef int CHECK_NAME(name, S)[1];                   \

... what's this and ...

> +    BUILD_BUG_ON(sizeof(xen_ ## name ## _t) !=            \
> +                 sizeof(compat_ ## name ## _t));          \
> +}
>  #define CHECK_SIZE_(k, n) \
> -    typedef int CHECK_NAME_(k, n, S)[1 - (sizeof(k xen_ ## n) != \
> -                                          sizeof(k compat_ ## n)) * 2]
> +static inline void __maybe_unused CHECK_SIZE_##k_##n(void) \
> +{                                                          \
> +    typedef int CHECK_NAME_(k, n, S)[1];                   \

... this needed for? The types aren't used anywhere afaict.

Jan

> +    BUILD_BUG_ON(sizeof(k xen_ ## n) !=                    \
> +                 sizeof(k compat_ ## n));                  \
> +}
>  
>  #define CHECK_FIELD_COMMON(name, t, f) \
>  static inline int __maybe_unused name(xen_ ## t ## _t *x, compat_ ## t ## _t 
> *c) \