[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [PATCH 1/3] xen/keyhandler: add missing noreturn attribute
On 06.06.2025 02:09, Stefano Stabellini wrote: > 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? :-) With an improved description I may at least no longer object to it. Jan
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