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

[Jan Beulich <jbeulich@xxxxxxxx>]

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?

Jan