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Re: [Xen-devel] [RFC PATCH 0/4] Add missing default labels to switch statements


On 22/02/2019 22:34, Andrew Cooper wrote:
> On 22/02/2019 22:11, Julien Grall wrote:
>> Hi Stefano,
>> On 22/02/2019 21:58, Stefano Stabellini wrote:
>>> On Fri, 22 Feb 2019, Andrew Cooper wrote:
>>>> On 22/02/2019 21:00, Stefano Stabellini wrote:
>>>>> On Fri, 22 Feb 2019, Julien Grall wrote:
>>>>>>>>> BTW, I checked the series with -Wswitch-default:
>>>>>>>>> -Wswitch-default
>>>>>>>>> Warn whenever a switch statement does not have a default case.
>>>>>>>>>> Furthermore, using BUG() is a pretty bad idea in switch.
>>>>>>>>> It is and not only in the switch. The reason I put BUG is that I tried
>>>>>>>>> to follow
>>>>>>>>> the existing "error handling" at those places.
>>>>>>>> It is not because BUG() is been used today in some places that we need 
>>>>>>>> to
>>>>>>>> continue to spread it.
>>>>>>>>> Use of BUG() itself is another topic which will also need to be
>>>>>>>>> addressed
>>>>>>>> So we should not add more of them...
>>>>>>> Again, I see this as a dedicated change. So, in the current series I 
>>>>>>> think
>>>>>>> it is
>>>>>>> acceptable to use the existing way of error handling if any at all.
>>>>>> That's not how it works in upstream. If you know some constructs are 
>>>>>> wrong, it
>>>>>> is best to try to address partially the problem directly then having so 
>>>>>> you
>>>>>> reduce the amounts of change afterwards.
>>>>>> So please try to not introduce more BUG() in the code base.
>>>>> Hi Oleksandr, Julien,
>>>>> Julien's right that we should not introduce any more BUG()s. In fact,
>>>>> each of them makes the code less safe, not more safe! The purpose of
>>>>> MISRAC 16.4 is "defensive programming": write the code in a way that is
>>>>> more (not less!) resilient to failure.
>>>>> So, I think it is a good idea to introduce a default label because it
>>>>> can help us spot unexpected issues. Instead of calling BUG() in the
>>>>> default handler, which is detrimental, we should return an error when
>>>>> possible, or just print a warning.
>>>> domain_crash() is almost always better than BUG().  It is very obvious
>>>> if it gets hit, and wont crash Xen.
>>> That's a good suggestion.
>>>>> As 16.4 clearly state, even a simple comment would be enough to address
>>>>> the rule. We just need to explain why a default label is not needed.
>>>>> Such as:
>>>>>     default:
>>>>>     /* unreachable because blah and blah */
>>>> What a simple comment doesn't do is avoid breaking -Wswitch.
>>> I don't know how to reconcile 16.4 with -Wswitch. One could argue that
>>> -Wswitch could be a good way to address 16.4, but then we introduce a
>>> compiler specific requirement. Typically gcc is not the compiler of
>>> choice for these environments, unfortunately forcing gcc is not an
>>> option.
>> Well, you could build with GCC and then build with your custom
>> compiler... But, GCC is pretty much the only choice for Xen on Arm today
>> as we don't build with clang and I pretty doubt we can build with compcert.
> So the suggestion I had was to have an overall CONFIG_MISRA which we can
> hide some of this nonsense behind, and then
> #define MISRA_BLE_DEFAULT default:
> #else
> #endif

This is pretty disgusting :). But then, it makes the code is bit more 
obscure. So how that rule is making Xen more safe? Furthermore, one 
default may not rule them all. So aren't we just adding code to make 
MISRA happy at the risk of introducing more bug?

> So when you disable CONFIG_MISRA, your compiler starts being able to
> help you again.
> TBH, it would also be nice to hide the SYMBOL nonsense behind, so we can
> continue doing it the efficient way for ~100% of the time.

My main concern is we only looked at 2 MISRA rules and we already need 
some nonsense code to spread everywhere. How much more are we going to 
get like that?


Julien Grall
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