Re: [PATCH v2] automation/eclair: deviate intentionally unreachable code

[Jan Beulich <jbeulich@xxxxxxxx>]

On 30.07.2025 23:01, Dmytro Prokopchuk1 wrote:
> From: Nicola Vetrini <nicola.vetrini@xxxxxxxxxxx>
> 
> MISRA C Rule 2.1 states: "A project shall not contain unreachable code".
> Functions that are non-returning and are not explicitly annotated with
> the `noreturn' attribute are considered a violation of this rule.
> 
> In certain cases, some functions might be non-returning in specific build
> configurations (when assertions are enabled, i.e., when `NDEBUG' is not 
> defined).
> This is due to calls to `__builtin_unreachable()' in the expansion of the
> macro `ASSERT_UNREACHABLE()'.
> 
> Conversely, in builds where `NDEBUG' is defined (assertions are disabled),
> the macro `ASSERT_UNREACHABLE()' expands to an empty construct
> (`do { } while (0)'), which does not affect the execution flow.
> This allows such functions to return normally in such builds,
> avoiding unreachable code.
> 
> To account for that in specific builds, the `noreturn` property of
> `__builtin_unreachable()` is overridden in the ECLAIR configuration
> to deviate these violations.
> 
> Signed-off-by: Nicola Vetrini <nicola.vetrini@xxxxxxxxxxx>
> Signed-off-by: Dmytro Prokopchuk <dmytro_prokopchuk1@xxxxxxxx>

Wording-wise I'm okay now (one further nit below), but I don't feel
capable of ack-ing. I'd like to remind you though that commit messages
want to be limited to 75(?) chars per line, so "git log" output wouldn't
go beyond 80 chars.

> --- a/docs/misra/deviations.rst
> +++ b/docs/misra/deviations.rst
> @@ -86,6 +86,18 @@ Deviations related to MISRA C:2012 Rules:
>         generate definitions for asm modules.
>       - Tagged as `deliberate` for ECLAIR.
>  
> +   * - R2.1
> +     - Calls to the `__builtin_unreachable()` function inside the expansion 
> of
> +       the `ASSERT_UNREACHABLE()` macro may cause a function to be marked as
> +       non-returning. This behavior occurs only in configurations where
> +       assertions are enabled. To address this, the `noreturn` property
> +       for `__builtin_unreachable()` is overridden in these contexts,
> +       resulting in the absence of reports that do not have an impact on
> +       safety, despite being true positives.
> +       Xen expects developers to ensure code remains safe and reliable in
> +       builds, even when debug-only assertions like `ASSERT_UNREACHABLE()
> +       are removed.

Formatting nit: I think it would be nice if lines were wrapped consistently,
such that available space is actually properly used. (Right here this means
the "for" on the 5th line could move up, which likely would call for further
re-wrapping of subsequent text. The "are" on the last line apparently also
would still fit on the earlier line.)

> --- a/docs/misra/rules.rst
> +++ b/docs/misra/rules.rst
> @@ -124,6 +124,15 @@ maintainers if you want to suggest a change.
>             they are used to generate definitions for asm modules
>           - Declarations without initializer are safe, as they are not
>             executed
> +         - Functions that are noreturn due to calls to `ASSERT_UNREACHABLE()`
> +           macro in debug build configurations are not deemed as violations::

I'm not a native speaker, but "deemed as" feels wrong to me; I would expect
the "as" wants simply dropping (or replacing by "to be").

Adjustments could likely be done while committing, assuming the necessary
ack appears.

Jan