Re: [XEN PATCH v2] x86: p2m-pod: address violation of MISRA C Rule 2.1

[Jan Beulich <jbeulich@xxxxxxxx>]

On 12.09.2024 03:05, Stefano Stabellini wrote:
> On Tue, 10 Sep 2024, Jan Beulich wrote:
>> On 10.09.2024 12:17, Nicola Vetrini wrote:
>>> On 2024-09-10 12:03, Jan Beulich wrote:
>>>> On 10.09.2024 11:53, Nicola Vetrini wrote:
>>>>> On 2024-09-10 11:08, Jan Beulich wrote:
>>>>>> On 10.09.2024 10:56, Nicola Vetrini wrote:
>>>>>>> On 2024-07-01 10:36, Jan Beulich wrote:
>>>>>>>> On 28.06.2024 08:30, Nicola Vetrini wrote:
>>>>>>>> This being about unreachable code, why are the domain_crash() not 
>>>>>>>> the
>>>>>>>> crucial points of "unreachability"? And even if they weren't there,
>>>>>>>> why
>>>>>>>> wouldn't it be the goto or ...
>>>>>>>>
>>>>>>>>> --- a/xen/arch/x86/mm/p2m-pod.c
>>>>>>>>> +++ b/xen/arch/x86/mm/p2m-pod.c
>>>>>>>>> @@ -1040,6 +1040,7 @@ out_unmap:
>>>>>>>>>       * Something went wrong, probably crashing the domain.  Unmap
>>>>>>>>>       * everything and return.
>>>>>>>>>       */
>>>>>>>>> +    /* SAF-8-safe Rule 2.1: defensive programming */
>>>>>>>>>      for ( i = 0; i < count; i++ )
>>>>>>>>>          if ( map[i] )
>>>>>>>>>              unmap_domain_page(map[i]);
>>>>>>>>
>>>>>>>> ... the label (just out of context) where the comment needs to go?
>>>>>>>
>>>>>>> Because of the way this rule's configuration work, deviations are
>>>>>>> placed
>>>>>>> on the construct that ends up being the target of the 
>>>>>>> unreachability,
>>>>>>
>>>>>> What's "target" here? What if this loop was removed from the 
>>>>>> function?
>>>>>> Then both the label and the domain_crash() invocations would still be
>>>>>> unreachable in debug builds. Are you telling me that this then 
>>>>>> wouldn't
>>>>>> be diagnosed by Eclair? Or that it would then consider the closing
>>>>>> figure brace of the function "the target of the unreachability"?
>>>>>
>>>>> Exactly, the end brace is a target to which the "function end" 
>>>>> construct
>>>>> is associated.
>>>>> It would be kind of strange, though: why not just doing 
>>>>> "domain_crash();
>>>>> return;" in that case?
>>>>
>>>> Sure, the question was theoretical. Now if "return" was used directly
>>>> there, what would then be the "target"? IOW - the more abstract 
>>>> question
>>>> of my earlier reply still wasn't answered.
>>>>
>>>
>>> The return statement in
>>>
>>> ...
>>> domain_crash();
>>> return;
>>> <~~~~~>
>>>
>>> Whichever statement is found to be unreachable in the current 
>>> preprocessed code.
>>
>> Yet then again: Why is it the return statement and not the function call
>> one (really, it being a macro invocation: the do/while one that the macro
>> expands to)? That's the first thing that won't be reached.
> 
> Are you trying to get clarity on the specific locations where the SAF
> deviations could be placed for the sake of understanding how the
> deviation system work?
> 
> Or are you asking for the SAF comment to be moved elsewhere because you
> don't like the SAF comment after the out_unmap macro?

The former, in order to make up my mind at all.

> I think that the position Nicola has used is better than any of the
> alternatives. It is clear and immediately obvious when you read it in
> context (I admit that looking at the patch alone, without applying it,
> it is a bit puzzling).

I disagree, but maybe the clarification asked for would change that.

Jan