Re: struct mctelem_cookie missing definition

[Jan Beulich <jbeulich@xxxxxxxx>]

On 14.02.2025 04:00, Stefano Stabellini wrote:
> On Thu, 13 Feb 2025, Stefano Stabellini wrote:
>>>> diff --git a/xen/arch/x86/cpu/mcheck/mctelem.h 
>>>> b/xen/arch/x86/cpu/mcheck/mctelem.h
>>>> index f4c5ff848d..2ccd490e5d 100644
>>>> --- a/xen/arch/x86/cpu/mcheck/mctelem.h
>>>> +++ b/xen/arch/x86/cpu/mcheck/mctelem.h
>>>> @@ -52,7 +52,7 @@
>>>>   * the element from the processing list.
>>>>   */
>>>>  
>>>> -typedef struct mctelem_cookie *mctelem_cookie_t;
>>>> +typedef uint64_t *mctelem_cookie_t;
>>>
>>> Yet that makes it possible to de-reference the pointer. Which, as Andrew
>>> explained, is intended to be impossible. If this could be properly
>>> replaced (not exactly what Andrew indicated by "file it in /dev/null"),
>>> fine. Truly purging the code (i.e. as Andrew suggests) may still be an
>>> option, with appropriate justification. But simply adjusting the type
>>> and then moving on is too little, imo. Even if you used void * (to make
>>> de-referencing impossible) I'd view it as largely papering over an issue;
>>> then converting to other pointers (without explicit cast, and hence
>>> without making apparent the badness of doing so) would become possible.
>>
>> What about converting to uintptr_t (not a pointer)?
>>
>>
>> In general, there are quite a few MISRA rules that we could mark as
>> blocking (clean) in our GitLab scan with just a few code changes like
>> this one. My goal is to make these rules blocking as soon as possible.
>> If I can improve the code in the process, that is even better, but it is
>> not mandatory. And I would rather spend one more hour marking a second
>> rule as blocking instead. 
>>
>> What I mean is that I believe it would be acceptable to make some
>> compromises and accept non-perfect changes to the code if it helps us
>> enforce more rules as blocking in GitLab CI.
> 
> After briefly speaking with Andrew about this, and re-reading Jan's
> email above, I think it is best to resolve this as a deviation.
> 
> Would this deviation work for you? Please suggest a better wording if
> you prefer.

Sounds reasonable to me; one nit below.

> Nicola, in reality I think it would be better to use deviations.rst
> because the SAF comment below would need to be replicated at every call
> side, if I am not mistaken.

If replication indeed would be needed, I agree doing it the other way
might be better.

> --- a/docs/misra/safe.json
> +++ b/docs/misra/safe.json
> @@ -92,6 +92,14 @@
>          },
>          {
>              "id": "SAF-11-safe",
> +            "analyser": {
> +                "eclair": "MC3A2.R11.2"
> +            },
> +            "name": "Rule 11.2: purposely impossible to dereference",
> +            "text": "Certain pointers points to incomplete types purposely 
> so that they are impossible to dereference."

Nit: s/ points / point /

Jan