Re: [XEN PATCH v2 1/3] EFI: address a violation of MISRA C Rule 13.6

[Jan Beulich <jbeulich@xxxxxxxx>]

On 01.10.2024 23:36, Stefano Stabellini wrote:
> On Tue, 1 Oct 2024, Jan Beulich wrote:
>> On 01.10.2024 07:25, Roberto Bagnara wrote:
>>> On 2024-09-30 15:07, Jan Beulich wrote:
>>>> On 30.09.2024 14:49, Federico Serafini wrote:
>>>>> guest_handle_ok()'s expansion contains a sizeof() involving its
>>>>> first argument which is guest_handle_cast().
>>>>> The expansion of the latter, in turn, contains a variable
>>>>> initialization.
>>>>>
>>>>> Since MISRA considers the initialization (even of a local variable)
>>>>> a side effect, the chain of expansions mentioned above violates
>>>>> MISRA C:2012 Rule 13.6 (The operand of the `sizeof' operator shall not
>>>>> contain any expression which has potential side effect).
>>>>
>>>> I'm afraid I need to ask for clarification of terminology and alike here.
>>>> While the Misra doc has a section on Persistent Side Effects in its
>>>> Glossary appendix, what constitutes a side effect from its pov isn't
>>>> really spelled out anywhere. Which in turn raises the question whether it
>>>> is indeed Misra (and not just Eclair) which deems initialization a side
>>>> effect. This is even more so relevant as 13.6 talks of only expressions,
>>>> yet initializers fall under declarations (in turn involving an expression
>>>> on the rhs of the equal sign).
>>>>
>>>> All the same of course affects patch 2 then, too.
>>>
>>> MISRA C leaves the definition of "side effect" to the C Standard.
>>> E.g., C18 5.1.2.3p2:
>>>
>>>    Accessing a volatile object, modifying an object, modifying a file,
>>>    or calling a function that does any of those operations are all
>>>    side effects,[omitted irrelevant footnote reference] which are
>>>    changes in the state of the execution environment.
>>>
>>> The MISRA C:2012/2023 Glossary entry for "Persistent side effect"
>>> indirectly confirms that initialization is always a side effect.
>>
>> Hmm, that's interesting: There's indeed an example with an initializer
>> there. Yet to me the text you quote from the C standard does not say
>> that initialization is a side effect - it would be "modifying an
>> object" aiui, yet ahead of initialization being complete the object
>> doesn't "exist" imo, and hence can be "modified" only afterwards.
> 
> I feel it's becoming a bit too philosophical. Since there's some room
> for interpretation and only two violations left to address, I believe
> it's best to stick with the stricter interpretation of the definition.
> Therefore, I'd proceed with this series in its current form.

Proceeding with the series in its current form may be okay (as you say,
you view the changes as readability improvements anyway), but imo the
interpretation needs settling on no matter what. In fact even for these
two patches it may affect what their descriptions ought to say (would
be nice imo to avoid permanently recording potentially misleading
information by committing as is). And of course clarity would help
dealing with future instances that might appear. I take it you realize
that if someone had submitted a patch adding code similar to the
original forms of what's being altered here, it would be relatively
unlikely for a reviewer to spot the issue. IOW here we're making
ourselves heavily dependent upon Eclair spotting (supposed) issues,
adding extra work and delays for such changes to go in.

Jan