Re: [RFC PATCH 2/4] xen/arm64: bitops: justify uninitialized variable inside a macro

[Jan Beulich <jbeulich@xxxxxxxx>]

On 17.07.2023 17:28, Nicola Vetrini wrote:
> 
> 
> On 17/07/23 15:59, Jan Beulich wrote:
>> On 14.07.2023 16:20, Luca Fancellu wrote:
>>>
>>>
>>>> On 14 Jul 2023, at 12:49, Nicola Vetrini <nicola.vetrini@xxxxxxxxxxx> 
>>>> wrote:
>>>>
>>>> The macro 'testop' expands to a function that declares the local
>>>> variable 'oldbit', which is written before being set, but is such a
>>>> way that is not amenable to automatic checking.
>>>>
>>>> Therefore, a deviation comment, is introduced to document this situation.
>>>>
>>>> A similar reasoning applies to macro 'guest_testop'.
>>>>
>>>> Signed-off-by: Nicola Vetrini <nicola.vetrini@xxxxxxxxxxx>
>>>> ---
>>>> docs/misra/safe.json                     | 16 ++++++++++++++++
>>>> xen/arch/arm/arm64/lib/bitops.c          |  3 +++
>>>> xen/arch/arm/include/asm/guest_atomics.h |  3 +++
>>>> 3 files changed, 22 insertions(+)
>>>>
>>>> diff --git a/docs/misra/safe.json b/docs/misra/safe.json
>>>> index 244001f5be..4cf7cbf57b 100644
>>>> --- a/docs/misra/safe.json
>>>> +++ b/docs/misra/safe.json
>>>> @@ -20,6 +20,22 @@
>>>>          },
>>>>          {
>>>>              "id": "SAF-2-safe",
>>>> +            "analyser": {
>>>> +                "eclair": "MC3R1.R9.1"
>>>> +            },
>>>> +            "name": "Rule 9.1: initializer not needed",
>>>> +            "text": "The following local variables are possibly subject 
>>>> to being read before being written, but code inspection ensured that the 
>>>> control flow in the construct where they appear ensures that no such event 
>>>> may happen."
>>>> +        },
>>>> +        {
>>>> +            "id": "SAF-3-safe",
>>>> +            "analyser": {
>>>> +                "eclair": "MC3R1.R9.1"
>>>> +            },
>>>> +            "name": "Rule 9.1: initializer not needed",
>>>> +            "text": "The following local variables are possibly subject 
>>>> to being read before being written, but code inspection ensured that the 
>>>> control flow in the construct where they appear ensures that no such event 
>>>> may happen."
>>>> +        },
>>>
>>> Since the rule and the justification are the same, you can declare only 
>>> once and use the same tag on top of the offending lines, so /* SAF-2-safe 
>>> MC3R1.R9.1 */,
>>
>> +1
>>
>> I'm puzzled by the wording vs comment placement though: The comments
>> are inserted ahead of the macro invocations, so there are no "following
>> local variables". Plus does this imply the comment would suppress the
>> checking on _all_ of them, rather than just the one that was confirmed
>> to be safe? What if another new one was added, that actually introduces
>> a problem?
>>
>>> also, I remember some maintainers not happy about the misra rule being put 
>>> after the tag, now I don’t recall who
>>
>> Me, at least. The annotations should be tool-agnostic imo, or else the
>> more tools we use, the longer these comments might get.
> 
> No problem for both: Given the earlier remarks by Julien on patch 1/4, I 
> think we can devise something along the lines of
> 
> /* SAF-x-safe MISRA:C 2012 Rule 9.1 <Justification> */
> int var;
> 
> and then write a generic justification in docs/misra/safe.json, while
> <Justification> contains a specific one (e.g., this loop does so and so 
> to ensure that no access to unset variables happens).

But that still mentions the rule. What if a new MISRA:C 2025 appears,
with different rule numbers? I don't mind the comment mentioning, in
a sufficiently terse way, what problem is circumvented. But I don't
think tools or specifications should be referenced here. That's the
purpose of the referenced "database" entry.

Jan