Re: [PATCH 1/2] x86/cpu-policy: enable build of fuzzing harness by default

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 06/01/2026 9:49 am, Jan Beulich wrote:
> On 06.01.2026 10:36, Jan Beulich wrote:
>> On 22.12.2025 17:53, Jan Beulich wrote:
>>> ... on x86, to make sure its bit-rotting can be limited at least a little.
>>>
>>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>>>
>>> --- a/tools/fuzz/Makefile
>>> +++ b/tools/fuzz/Makefile
>>> @@ -4,6 +4,7 @@ include $(XEN_ROOT)/tools/Rules.mk
>>>  SUBDIRS-y :=
>>>  SUBDIRS-y += libelf
>>>  SUBDIRS-y += x86_instruction_emulator
>>> +SUBDIRS-$(CONFIG_X86_64) += cpu-policy
>>>  
>>>  .PHONY: all clean distclean install uninstall
>>>  all clean distclean install uninstall: %: subdirs-%
>>>
>> As it turns out this causes build failures on Ubuntu (and only there, and 
>> only
>> with gcc, which I don't understand):
>>
>> afl-policy-fuzzer.c: In function 'main':
>> afl-policy-fuzzer.c:153:9: error: ignoring return value of 'fread', declared 
>> with attribute warn_unused_result [-Werror=unused-result]
>>          fread(cp, sizeof(*cp), 1, fp);
>>          ^
>> cc1: all warnings being treated as errors
>>
>> Given how the code uses calloc() up front I don't really see why evaluating
>> the return value would actually be meaningful here, so I'm inclined to add a
>> cast to void (provided that would make a difference, which I have yet to
>> check). Opinions?
> Simply casting doesn't work. Hence what I'm intending to do is
>
> --- a/tools/fuzz/cpu-policy/afl-policy-fuzzer.c
> +++ b/tools/fuzz/cpu-policy/afl-policy-fuzzer.c
> @@ -133,6 +133,7 @@ int main(int argc, char **argv)
>  #endif
>      {
>          struct cpu_policy *cp = NULL;
> +        size_t size;
>  
>          if ( fp != stdin )
>          {
> @@ -150,9 +151,9 @@ int main(int argc, char **argv)
>          if ( !cp )
>              goto skip;
>  
> -        fread(cp, sizeof(*cp), 1, fp);
> +        size = fread(cp, sizeof(*cp), 1, fp);
>  
> -        if ( !feof(fp) )
> +        if ( !size || !feof(fp) )
>              goto skip;
>  
>          check_policy(cp);
>
> along with amending the description:
>
> "Since on Ubuntu fread()'s return value needs evaluating, adjust the code
>  there to also skip the test when there's no data at all."
>
> May I keep your ack with that adjustment?

Yes.  This is just harness code, so whatever compiles.

~Andrew