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

[Jan Beulich <jbeulich@xxxxxxxx>]

On 02.10.2024 08:54, Roberto Bagnara wrote:
> On 2024-10-02 08:09, Jan Beulich wrote:
>> 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.
> 
> You can do two things to obtain a second opinion:
> 
> 1) Use the MISRA forum (here is the link to the forum
>     section devoted to the side-effect rules of MISRA C:2012
>     and MISRA C:2023 (https://forum.misra.org.uk/forumdisplay.php?fid=168).
>     The MISRA C Working Group will, in due course, provide
>     you with an official answer to your questions about what,
>     for the interpretation of Rule 13.6, has to be considered
>     a side effect.
> 
> 2) Reach out to your ISO National Body and try to obtain
>     an official answer from ISO/IEC JTC1/SC22/WG14 (the
>     international standardization working group for the
>     programming language C) to your questions about what the
>     C Standard considers to be side effects.

I took the latter route, and to my (positive) surprise I got back an answer
the same day. There was a request for someone to confirm, but so far I didn't
see further replies. Since this is a German institution I raised the question
in German and got back an answer in German (attached); I've tried my best to
translate this without falsifying anything, but I've omitted non-technical
parts:

"Initialization of an object is never a side effect of the initialization
by itself. Of course expressions used for initialization can themselves have
side effects on other objects.

@Andreas: Do you agree? C after all has a far simpler object model than C++.
The (potential) change in object representation (i.e. the bytes underlying
the object) is not a side effect of object initialization, but its primary
purpose."

Further for Misra she added a reference to a Swiss person, but I think with
Bugseng we have sufficient expertise there.

Jan

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• To: Wellhöfer, Johannes <Johannes.Wellhoefer@xxxxxx>, "jbeulich@xxxxxxxx" <jbeulich@xxxxxxxx>
• From: Daniela Engert <dani@xxxxxxx>
• Date: Wed, 2 Oct 2024 15:31:12 +0200 (CEST)
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• Cc: Andreas Weis <der_ghulbus@xxxxxxxxxxxxxx>
• Delivered-to: jbeulich@xxxxxxxx
• Importance: Normal

Hallo zusammen,

bitte nicht wundern, aber sobald ich "Ingenieur" rieche, bin ich sofort beim 
"Du" (bin ja selber eine).
Mein Hauptfokus liegt zwar auf C++, und C ist für mich ein "Seiteneffekt", aber 
diese Frage kann ich vermutlich ebenfalls beantworten:

Die Initialisierung eines Objekts ist niemals ein Seiteneffekt der 
Initialisierung von sich selbst. In den Expressions, welche zur Initialisierung 
evaluiert werden, können jedoch sehr wohl Seiteneffekte in anderen Objekten 
auftreten.

@Andreas: kannst du dem zustimmen? C hat ja ein sehr viel einfacheres 
Objektmodell als C++. Die (potentielle) Änderung der Objektrepräsentation (d.h. 
die Bytes, welche das Objekt hinterlegen) ist ja kein Seiteneffekt der 
Objektinitialisierung, sondern sein primärer Zweck.

Wenn es um MISRA geht, wäre sicherlich P. Sommerlad der richtige 
Ansprechpartner (https://sommerlad.ch/). Ansonsten tagt das WG14 Komittee 
gerade, große Teile der WG14 Tafelrunde sind versammelt, Robert Seacord ist 
sein Chair.

lG Daniela



> Wellhöfer, Johannes <johannes.wellhoefer@xxxxxx> hat am 02.10.2024 15:02 CEST 
> geschrieben:
> 
> 
> Danke Manuela, fürs weiterleiten.
>  
>  Sehr geehrter Herr Beulich, 
>  
>  vielen Dank für Ihre Anfrage. Ich habe Frau Engert hier mit in den 
> Emailverlauf genommen. Sie arbeitet als deutsche Expertin in der 
> JTC1/SC22/WG14 mit.
> 
> @Daniela Engert können Sie Herrn Beulich mit seiner Frage (unten im Verlauf 
> gelb markiert) weiterhelfen?
>  
>  Mit freundlichen Grüßen
> 
> Johannes Wellhöfer
> Er/Sein
> Projektmanager
> DIN-Normenausschuss Informationstechnik und Anwendungen (NIA)
> DIN Deutsches Institut für Normung e. V. – Industrie und Informationstechnik 
> (DIN – NuS IIT IuA)
> 
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> 
> 
> 
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> -----Ursprüngliche Nachricht-----
>  Von: Tillack-Lübke, Manuela <Manuela.Tillack-luebke@xxxxxx> 
>  Gesendet: Mittwoch, 2. Oktober 2024 12:09
>  An: Wellhöfer, Johannes <Johannes.Wellhoefer@xxxxxx>
>  Betreff: WG: Klarstellung zu ISO/IEC JTC1/SC22/WG14
> 
> Lieber Johannes,
> 
> zuständigkeitshalber bitte ich Dich um Beantwortung der unten angefügten 
> Anfrage. Vielen Dank.
> 
> 
> Mit besten Grüßen
> 
> Manuela
> AP A. 634
> - 2419
> 
> 
> 
> -----Ursprüngliche Nachricht-----
> Von: info@xxxxxx <info@xxxxxx>
> Gesendet: Mittwoch, 2. Oktober 2024 10:17
> An: DIN Info <DIN-Info@xxxxxx>
> Betreff: Klarstellung zu ISO/IEC JTC1/SC22/WG14
> 
> Guten Tag,
> 
> folgende Nachricht wurde auf https://www.din.de/de an Sie geschickt.
> 
> Guten Tag,
> 
> im Zusammenhang mit Misra C:2012 ist eine Interpretationsfrage zum C99 
> Standard (ISO/IEC 9899:TC3) aufgetaucht. Zur Klärung wurde ich an den ISO 
> National Body verwiesen, der - soweit ich feststellen kann - Sie sind.
> 
> Die eigentliche Frage betrifft "side effects": Fällt die Initialisierung 
> eines Objekts auch darunter?
> 
> In der Hoffnung, dass Sie uns bei der Klärung behilflich sein können, 
> möglicherweise auch durch Verweis an eine andere geeignete Stelle, vielen 
> Dank im Voraus,
> Jan Beulich
> 
> Herr Jan Beulich
> jbeulich@xxxxxxxx

--- End Message ---