Explicitly free the array of CPUID entries at the end of the Hyper-V CPUID
test, mainly in anticipation of moving management of the array into the
main test helper.

Cc: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 tools/testing/selftests/kvm/x86_64/hyperv_cpuid.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/tools/testing/selftests/kvm/x86_64/hyperv_cpuid.c b/tools/testing/selftests/kvm/x86_64/hyperv_cpuid.c
index 9a0fcc713350..09f9874d7705 100644
--- a/tools/testing/selftests/kvm/x86_64/hyperv_cpuid.c
+++ b/tools/testing/selftests/kvm/x86_64/hyperv_cpuid.c
@@ -164,6 +164,7 @@ int main(int argc, char *argv[])
 
 	hv_cpuid_entries = kvm_get_supported_hv_cpuid();
 	test_hv_cpuid(hv_cpuid_entries, kvm_cpu_has(X86_FEATURE_VMX));
+	free((void *)hv_cpuid_entries);
 
 out:
 	kvm_vm_free(vm);
-- 
2.47.1.688.g23fc6f90ad-goog

Sean Christopherson <seanjc@google.com> writes:

> Explicitly free the array of CPUID entries at the end of the Hyper-V CPUID
> test, mainly in anticipation of moving management of the array into the
> main test helper.
>
> Cc: Vitaly Kuznetsov <vkuznets@redhat.com>
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> ---
>  tools/testing/selftests/kvm/x86_64/hyperv_cpuid.c | 1 +
>  1 file changed, 1 insertion(+)
>
> diff --git a/tools/testing/selftests/kvm/x86_64/hyperv_cpuid.c b/tools/testing/selftests/kvm/x86_64/hyperv_cpuid.c
> index 9a0fcc713350..09f9874d7705 100644
> --- a/tools/testing/selftests/kvm/x86_64/hyperv_cpuid.c
> +++ b/tools/testing/selftests/kvm/x86_64/hyperv_cpuid.c
> @@ -164,6 +164,7 @@ int main(int argc, char *argv[])
>  
>  	hv_cpuid_entries = kvm_get_supported_hv_cpuid();
>  	test_hv_cpuid(hv_cpuid_entries, kvm_cpu_has(X86_FEATURE_VMX));
> +	free((void *)hv_cpuid_entries);

vcpu_get_supported_hv_cpuid() allocates memory for the resulting array
each time, however, kvm_get_supported_hv_cpuid() was designed after
what's now kvm_get_supported_cpuid() (afair) so it has an optimization
to ask KVM just once:

        static struct kvm_cpuid2 *cpuid;
        int kvm_fd;

        if (cpuid)
                return cpuid;

        cpuid = allocate_kvm_cpuid2(MAX_NR_CPUID_ENTRIES);
        kvm_fd = open_kvm_dev_path_or_exit();
	...

and it seems that if we free hv_cpuid_entries here, next time we call
kvm_get_supported_hv_cpuid() an already freed memory will be returned.
This doesn't matter in in this patch as we're about to quit anyway but
with the next one in the series it becomes problematic.

>  
>  out:
>  	kvm_vm_free(vm);

-- 
Vitaly

On Fri, Jan 17, 2025, Vitaly Kuznetsov wrote:
> Sean Christopherson <seanjc@google.com> writes:
> 
> > Explicitly free the array of CPUID entries at the end of the Hyper-V CPUID
> > test, mainly in anticipation of moving management of the array into the
> > main test helper.
> >
> > Cc: Vitaly Kuznetsov <vkuznets@redhat.com>
> > Signed-off-by: Sean Christopherson <seanjc@google.com>
> > ---
> >  tools/testing/selftests/kvm/x86_64/hyperv_cpuid.c | 1 +
> >  1 file changed, 1 insertion(+)
> >
> > diff --git a/tools/testing/selftests/kvm/x86_64/hyperv_cpuid.c b/tools/testing/selftests/kvm/x86_64/hyperv_cpuid.c
> > index 9a0fcc713350..09f9874d7705 100644
> > --- a/tools/testing/selftests/kvm/x86_64/hyperv_cpuid.c
> > +++ b/tools/testing/selftests/kvm/x86_64/hyperv_cpuid.c
> > @@ -164,6 +164,7 @@ int main(int argc, char *argv[])
> >  
> >  	hv_cpuid_entries = kvm_get_supported_hv_cpuid();
> >  	test_hv_cpuid(hv_cpuid_entries, kvm_cpu_has(X86_FEATURE_VMX));
> > +	free((void *)hv_cpuid_entries);
> 
> vcpu_get_supported_hv_cpuid() allocates memory for the resulting array
> each time, however, kvm_get_supported_hv_cpuid() was designed after
> what's now kvm_get_supported_cpuid() (afair) so it has an optimization
> to ask KVM just once:
> 
>         static struct kvm_cpuid2 *cpuid;
>         int kvm_fd;
> 
>         if (cpuid)
>                 return cpuid;
> 
>         cpuid = allocate_kvm_cpuid2(MAX_NR_CPUID_ENTRIES);
>         kvm_fd = open_kvm_dev_path_or_exit();
> 	...
> 
> and it seems that if we free hv_cpuid_entries here, next time we call
> kvm_get_supported_hv_cpuid() an already freed memory will be returned.
> This doesn't matter in in this patch as we're about to quit anyway but
> with the next one in the series it becomes problematic.

Ow.  I totally missed that.  I'll drop this patch, and then adjust the next one
to do:

	/*
	 * Note, the CPUID array returned by the system-scoped helper is a one-
	 * time allocation, i.e. must not be freed.
	 */
	if (vcpu)
		free((void *)hv_cpuid_entries);


I'll post a v2 once I've actually tested.

Thanks!