Re: [XEN PATCH] xen/include/xen/lib.h: avoid undefined behavior.

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 15/06/2023 11:30 am, Nicola Vetrini wrote:
> Redefine BUILD_BUG_ON_ZERO to fully comply with C99 avoiding
> undefined behavior 58 ("A structure or union is defined as
> containing no named members (6.7.2.1)."
> This also avoids a dependency on the compiler and its version.
>
> Signed-off-by: Nicola Vetrini <nicola.vetrini@xxxxxxxxxxx>
> ---
>  xen/include/xen/lib.h | 16 +---------------
>  1 file changed, 1 insertion(+), 15 deletions(-)
>
> diff --git a/xen/include/xen/lib.h b/xen/include/xen/lib.h
> index 67fc7c1d7e..a266159b9f 100644
> --- a/xen/include/xen/lib.h
> +++ b/xen/include/xen/lib.h
> @@ -40,22 +40,8 @@
>      unlikely(ret_warn_on_);             \
>  })
>  
> -/* All clang versions supported by Xen have _Static_assert. */
> -#if defined(__clang__) || \
> -    (__GNUC__ > 4 || (__GNUC__ == 4 && __GNUC_MINOR__ >= 6))
> -/* Force a compilation error if condition is true */
> -#define BUILD_BUG_ON(cond) ({ _Static_assert(!(cond), "!(" #cond ")"); })
> -
> -/* Force a compilation error if condition is true, but also produce a
> -   result (of value 0 and type size_t), so the expression can be used
> -   e.g. in a structure initializer (or where-ever else comma expressions
> -   aren't permitted). */
> -#define BUILD_BUG_ON_ZERO(cond) \
> -    sizeof(struct { _Static_assert(!(cond), "!(" #cond ")"); })
> -#else
> -#define BUILD_BUG_ON_ZERO(cond) sizeof(struct { int:-!!(cond); })
> +#define BUILD_BUG_ON_ZERO(cond) (sizeof(char[(cond)? -1 : 1]) - 1U)
>  #define BUILD_BUG_ON(cond) ((void)BUILD_BUG_ON_ZERO(cond))
> -#endif

Getting rid of memberless structs is fine.  Getting rid of
_Static_assert() is absolutely not, because this change massively
obfuscates build time error messages.

The MISRA work can do whatever is necessary to get _Static_assert()
permitted for use globally across Xen.

~Andrew