Re: [PATCH 1/3] xen/keyhandler: add missing noreturn attribute

[Jan Beulich <jbeulich@xxxxxxxx>]

On 05.06.2025 14:26, Nicola Vetrini wrote:
> On 2025-06-05 14:22, Nicola Vetrini wrote:
>> On 2025-06-05 09:17, Jan Beulich wrote:
>>> On 05.06.2025 01:49, victorm.lira@xxxxxxx wrote:
>>>> From: Nicola Vetrini <nicola.vetrini@xxxxxxxxxxx>
>>>>
>>>> Function `reboot_machine' does not return, but lacks the `noreturn' 
>>>> attribute,
>>>> therefore causing a violation of MISRA C Rule 2.1: "A project shall 
>>>> not contain
>>>> unreachable code".
>>>
>>> Is this (uniformly) true? Looking at ...
>>>
>>>> --- a/xen/common/keyhandler.c
>>>> +++ b/xen/common/keyhandler.c
>>>> @@ -251,7 +251,7 @@ static void cf_check 
>>>> dump_hwdom_registers(unsigned char key)
>>>>      }
>>>>  }
>>>>
>>>> -static void cf_check reboot_machine(unsigned char key, bool unused)
>>>> +static void noreturn cf_check reboot_machine(unsigned char key, bool 
>>>> unused)
>>>>  {
>>>>      printk("'%c' pressed -> rebooting machine\n", key);
>>>>      machine_restart(0);
>>>
>>> ... generated code here, I can see that the compiler is perfectly able 
>>> to
>>> leverage the noreturn that machine_restart() has, resulting in no
>>> unreachable code to be generated. That is - neither in source nor in
>>> binary there is any unreachable code. Therefore I'm having a hard time
>>> seeing what the violation is here.
>>>
>>> That said, I certainly don't mind the addition of the (seemingly) 
>>> missing
>>> attribute. Otoh I wonder whether an attribute the removal of which has 
>>> no
>>> effect wouldn't count as "dead code" or alike, violating some other 
>>> rule.
>>>
>>
>> Inlining does not play a role in this case. Here reboot_machine() is 
>> marked as a violation because machine_restart() is noreturn and there 
>> is no other path upon which reboot_machine() may return, hence any 
>> function calling reboot_machine() could have portions that are 
>> inadvertently unreachable (as in never executed due to divergence) by 
>> not having the annotation.

Just that there's not going to be a 2nd caller, considering the purpose
of the function.

>> That said, in such trivial cases compilers 
>> are typically able to derive the property automatically, but they are 
>> not obliged to and, more importantly, the behavior may even differ with 
>> the same compiler using different optimization levels.
> 
> Just a note: in later revisions of MISRA C this has become a rule of its 
> own [1], which helps reduce confusion, but up to MISRA C:2012 Amendment 
> 2 (currently used by Xen), this is part of Rule 2.1.
> 
> [1] Rule 17.11: "A function that never returns should be declared with a 
> _Noreturn function specifier"

Oh, that's indeed quite a bit more explicit.

Jan