Re: [PATCH] xen: address violation of MISRA C Rule 11.1

[Jan Beulich <jbeulich@xxxxxxxx>]

On 13.12.2024 15:02, Alessandro Zucchelli wrote:
> On 2024-12-13 11:08, Jan Beulich wrote:
>> On 13.12.2024 01:53, Stefano Stabellini wrote:
>>> On Thu, 12 Dec 2024, Jan Beulich wrote:
>>>> On 12.12.2024 03:27, Stefano Stabellini wrote:
>>>>> On Wed, 11 Dec 2024, Jan Beulich wrote:
>>>>>> On 11.12.2024 12:02, Alessandro Zucchelli wrote:
>>>>>>> Rule 11.1 states as following: "Conversions shall not be performed
>>>>>>> between a pointer to a function and any other type".
>>>>>>>
>>>>>>> Functions "__machine_restart" and "__machine_halt" in 
>>>>>>> "x86/shutdown.c"
>>>>>>> and "halt_this_cpu" in "arm/shutdown.c" are defined as noreturn
>>>>>>> functions and subsequently passed as parameters to function calls.
>>>>>>> This violates the rule in Clang, where the "noreturn" attribute is
>>>>>>> considered part of the function"s type.
>>>>>>
>>>>>> I'm unaware of build issues with Clang, hence can you clarify how 
>>>>>> Clang's
>>>>>> view comes into play here? In principle various attributes ought to 
>>>>>> be
>>>>>> part of a function's type; iirc that's also the case for gcc. Yet 
>>>>>> how
>>>>>> that matters to Eclair is still entirely unclear to me.
>>>>>>
>>>>>>> By removing the "noreturn"
>>>>>>> attribbute and replacing it with uses of the ASSERT_UNREACHABLE 
>>>>>>> macro,
>>>>>>> these violations are addressed.
>>>>>>
>>>>>> Papered over, I'd say. What about release builds, for example?
>>>>>>
>>>>>> Deleting the attribute also has a clear downside 
>>>>>> documentation-wise. If
>>>>>> we really mean to remove them from what the compiler gets to see, I 
>>>>>> think
>>>>>> we ought to still retain them in commented-out shape.
>>>>>
>>>>> Another option would be to #define noreturn to nothing for ECLAIR 
>>>>> builds ?
>>>>
>>>> That again would feel like papering over things. Plus I don't know if 
>>>> that's
>>>> an option at all.
>>>
>>> What is "papering over" and what is a "nice solution" is often up to 
>>> the
>>> personal opinions.
>>>
>>> From my point of view, Alessandro's patch doesn't make the code worse.
>>> The ASSERT_UNREACHABLE solution is OK. I do agree with you that it
>>> should not be required for us to remove "noreturn", but I don't think 
>>> we
>>> have used it consistently anyway across the Xen codebase.
>>> ASSERT_UNREACHABLE is also a form of documentation that the function
>>> does not return.
>>>
>>> In conclusion, I think all three options are acceptable:
>>> 1) this patch as is
>>> 2) this patch plus /* noreturn */ as a comment
>>> 3) #define noreturn to nothing just for ECLAIR builds
>>>
>>> I don't mind either way, maybe option 2) is the best compromise.
>>
>> The variant with least impact on what we currently have (generated code
>> wise) is 3), though, which hence would be my preference (well, not 
>> exactly
>> a preference, but the least bad one).
> 
> Another option could be to encapsulate these function pointer casts as 
> follows:
> #define REMOVE_NORETURN(x) (void(*)(void*))(x)
> This approach allows us to retain the noreturn attribute and the 
> associated optimizations;
> note that the encapsulating macro will need to be deviated then.

And then have one such macro for every attribute that may need zapping?
What if there are multiple? Any macro may do, yet which one to use would
be unclear. What if only some attributes need zapping, and some need
retaining?

Jan