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

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Thu, 5 Jun 2025, Jan Beulich wrote:
> 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.

Does it mean you would ack the patch? :-)