[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [PATCH 1/3] xen/keyhandler: add missing noreturn attribute
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? :-)
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