Re: [PATCH v2] misra: consider conversion from UL or (void*) to function pointer as safe

[Jan Beulich <jbeulich@suse.com>]

On 25.09.2025 10:04, Dmytro Prokopchuk1 wrote:
> --- a/docs/misra/deviations.rst
> +++ b/docs/misra/deviations.rst
> @@ -366,11 +366,22 @@ Deviations related to MISRA C:2012 Rules:
>       - Tagged as `safe` for ECLAIR.
>  
>     * - R11.1
> -     - The conversion from a function pointer to unsigned long or (void \*) does
> +     - The conversion from a function pointer to unsigned long or '(void *)' does
>         not lose any information, provided that the target type has enough bits
>         to store it.
>       - Tagged as `safe` for ECLAIR.
>  
> +   * - R11.1
> +     - The conversion from unsigned long or '(void *)' to a function pointer is
> +       safe because it relies on both ABI definitions and compiler implementations
> +       supported by Xen which define consistent and compatible representations
> +       (i.e., having the same size and memory layout) for '(void *)', unsigned
> +       long, and function pointers, enabling safe conversions between these types
> +       without data loss or corruption. The compile-time assertions (BUILD_BUG_ON
> +       macro) is integrated into 'xen/common/version.c' to confirm conversions
> +       compatibility across all target platforms.

As you use (and mean) plural, s/is/are/ ? I also think the "The" at the start
of the sentence wants dropping.

Further, why this very dissimilar wording compared to what's said about
conversions _from_ function pointer types?

And then ...

> --- a/xen/common/version.c
> +++ b/xen/common/version.c
> @@ -217,6 +217,17 @@ void __init xen_build_init(void)
>  #endif /* CONFIG_X86 */
>  }
>  #endif /* BUILD_ID */
> +
> +static void __init __maybe_unused build_assertions(void)
> +{
> +    /*
> +     * To confirm conversion compatibility between unsigned long, (void *)
> +     * and function pointers for all supported architectures.
> +     */
> +    BUILD_BUG_ON(sizeof(unsigned long) != sizeof(void (*)(void)));
> +    BUILD_BUG_ON(sizeof(void *) != sizeof(void (*)(void)));
> +}

... I'm unconvinced checking merely the sizes is sufficient. On architectures
involving function descriptors (e.g. ia64) converting in this direction is
safe only if earlier on the value was obtained as the result of a conversion
in the opposite direction (and all of this within a single component, which
of course is guaranteed for Xen).

Jan