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