Re: [PATCH 1/2] automation/eclair_analysis: deviate MISRA C Rule 21.2

[Jan Beulich <jbeulich@xxxxxxxx>]

On 19.06.2024 19:09, Alessandro Zucchelli wrote:
> Rule 21.2 reports identifiers reserved for the C and POSIX standard
> libraries: all xen's translation units are compiled with option
> -nostdinc, this guarantees that these libraries are not used, therefore
> a justification is provided for allowing uses of such identifiers in
> the project.
> Builtins starting with "__builtin_" still remain available.
> 
> No functional change.
> 
> Signed-off-by: Alessandro Zucchelli <alessandro.zucchelli@xxxxxxxxxxx>
> ---
>  automation/eclair_analysis/ECLAIR/deviations.ecl | 11 +++++++++++
>  1 file changed, 11 insertions(+)
> 
> diff --git a/automation/eclair_analysis/ECLAIR/deviations.ecl 
> b/automation/eclair_analysis/ECLAIR/deviations.ecl
> index 447c1e6661..9fa9a7f01c 100644
> --- a/automation/eclair_analysis/ECLAIR/deviations.ecl
> +++ b/automation/eclair_analysis/ECLAIR/deviations.ecl
> @@ -487,6 +487,17 @@ leads to a violation of the Rule are deviated."
>  # Series 21.
>  #
>  
> +-doc_begin="Rules 21.1 and 21.2 report identifiers reserved for the C and 
> POSIX
> +standard libraries: if these libraries are not used there is no reason to 
> avoid such
> +identifiers. All xen's translation units are compiled with option -nostdinc,
> +this guarantees that these libraries are not used. Some compilers could 
> perform
> +optimization using built-in functions: this risk is partially addressed by
> +using the compilation option -fno-builtin. Builtins starting with 
> \"__builtin_\"
> +still remain available."

While the sub-section "Reserved Identifiers" is part of Section 7,
"Library", close coordination is needed between the library and the
compiler, which only together form an "implementation". Therefore any
use of identifiers beginning with two underscores or beginning with an
underscore and an upper case letter is at risk of colliding not only
with a particular library implementation (which we don't use), but
also of such with a particular compiler implementation (which we cannot
avoid to use). How can we permit use of such potentially problematic
identifiers?

Further, as to the rule mentioning file scope identifiers: Why is that?
The text in the C99 specification does not preclude their use, it merely
restricts what they may be used for. Why does Misra go yet farther?

Finally, why "partially addressed"? What part is unaddressed?

> +-config=MC3R1.R21.1,macros={safe , "!^__builtin_$" }
> +-config=MC3R1.R21.2,declarations+={safe, "!^__builtin_.*$"}

First: Why the differences in = vs += and in absence vs presence of .*

Second: The rules, according to my understanding, are about us defining
or declaring such identifiers, not about us using them. There shouldn't
be any #define, #undef, or declaration (let alone definition) of such
entities. All we do is _use_ them as e.g. expansion of #define-s. Thus:
Why is a deviation needed here in the first place? Then again - maybe
I'm reading this wrong, especially the leading ! may perhaps be some
form of negation.

Jan