Re: [PATCH v1] misra: add deviation for rules 21.1 and 21.2

[Nicola Vetrini <nicola.vetrini@bugseng.com>]

On 2025-04-25 10:07, Jan Beulich wrote:
> On 24.04.2025 23:45, Stefano Stabellini wrote:
>> On Thu, 24 Apr 2025, Jan Beulich wrote:
>>> On 23.04.2025 19:54, victorm.lira@amd.com wrote:
>>>> From: Nicola Vetrini <nicola.vetrini@bugseng.com>
>>>> 
>>>> MISRA C Rules 21.1 ("#define and #undef shall not be used on a
>>>> reserved identifier or reserved macro name") and R21.2 ("A reserved
>>>> identifier or reserved macro name shall not be declared") violations
>>>> are not problematic for Xen, as it does not use the C or POSIX
>>>> libraries.
>>>> 
>>>> Xen uses -fno-builtin and -nostdinc to ensure this, but there are 
>>>> still
>>>> __builtin_* functions from the compiler that are available so
>>>> a deviation is formulated for all identifiers not starting with
>>>> "__builtin_".
>>>> 
>>>> The missing text of a deviation for Rule 21.2 is added to
>>>> docs/misra/deviations.rst.
>>>> 
>>>> To avoid regressions, tag both rules as clean and add them to the
>>>> monitored set.
>>>> 
>>>> Signed-off-by: Nicola Vetrini <nicola.vetrini@bugseng.com>
>>>> Signed-off-by: Federico Serafini <federico.serafini@bugseng.com>
>>>> Signed-off-by: Victor Lira <victorm.lira@amd.com>
>>> 
>>> While the rule is in the library section, ...
>>> 
>>>> --- a/docs/misra/deviations.rst
>>>> +++ b/docs/misra/deviations.rst
>>>> @@ -587,7 +587,31 @@ Deviations related to MISRA C:2012 Rules:
>>>>         construct is deviated only in Translation Units that present 
>>>> a violation
>>>>         of the Rule due to uses of this macro.
>>>>       - Tagged as `deliberate` for ECLAIR.
>>>> -
>>>> +
>>>> +   * - R21.1
>>>> +     - Rule 21.1 reports identifiers reserved for the C and POSIX 
>>>> standard
>>>> +       libraries. Xen does not use such libraries and all 
>>>> translation units
>>>> +       are compiled with option '-nostdinc', therefore there is no 
>>>> reason to
>>>> +       avoid to use `#define` or `#undef` on such identifiers 
>>>> except for those
>>>> +       beginning with `__builtin_` for which compilers may perform 
>>>> (wrong)
>>>> +       optimizations.
>>>> +     - Tagged as `safe` for ECLAIR.
>>> 
>>> ... I'd like to ask that it be explicitly clarified here that it's 
>>> solely
>>> the library (and not e.g. the compiler itself) that are of concern 
>>> here.
>> 
>> The language can be clarified:
>> 
>> - Rule 21.1 reports identifiers reserved for the C and POSIX standard
>>   libraries. Xen does not use such libraries and all translation units
>>   are compiled with option '-nostdinc', therefore there is no reason 
>> to
>>   avoid to use `#define` or `#undef` on C and POSIX standard libraries
>>   identifiers except for those beginning with `__builtin_` for which
>>   compilers may perform (wrong) optimizations.
> 
> Which makes it more apparent that there is a gap: What about e.g. 
> __x86_64__?
> That falls within what the rules cover, is not a C or POSIX standard 
> library
> identifier, yet very clearly must not be fiddled with. Whereas the text
> above deviates it.
> 

Hi Jan,

that is true, even if unlikely: one approach could be to avoid deviating 
predefined macros for all CUs as -nostdinc and -fno-builtins should take 
care of the rest; this kind of deviation is not currently possible in 
ECLAIR, but it might be in the future. I think this could be 
accomplished also via some gcc trickery on each CU, though I'm not sure 
how valued that is for Xen.

-- 
Nicola Vetrini, B.Sc.
Software Engineer
BUGSENG (https://bugseng.com)
LinkedIn: https://www.linkedin.com/in/nicola-vetrini-a42471253