[PATCH v3 1/2] KVM: x86: Update number of entries for KVM_GET_CPUID2 on success, not failure

[Sean Christopherson]

Update cpuid->nent if and only if kvm_vcpu_ioctl_get_cpuid2() succeeds.
The sole caller copies @cpuid to userspace only on success, i.e. the
existing code effectively does nothing.

Arguably, KVM should report the number of entries when returning -E2BIG so
that userspace doesn't have to guess the size, but all other similar KVM
ioctls() don't report the size either, i.e. userspace is conditioned to
guess.

Suggested-by: Takahiro Itazuri <itazur@xxxxxxxxxx>
Link: https://lore.kernel.org/all/20230410141820.57328-1-itazur@xxxxxxxxxx
Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
---
 arch/x86/kvm/cpuid.c | 13 ++++---------
 1 file changed, 4 insertions(+), 9 deletions(-)

diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index 0c9660a07b23..241f554f1764 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -501,20 +501,15 @@ int kvm_vcpu_ioctl_get_cpuid2(struct kvm_vcpu *vcpu,
 			      struct kvm_cpuid2 *cpuid,
 			      struct kvm_cpuid_entry2 __user *entries)
 {
-	int r;
-
-	r = -E2BIG;
 	if (cpuid->nent < vcpu->arch.cpuid_nent)
-		goto out;
-	r = -EFAULT;
+		return -E2BIG;
+
 	if (copy_to_user(entries, vcpu->arch.cpuid_entries,
 			 vcpu->arch.cpuid_nent * sizeof(struct kvm_cpuid_entry2)))
-		goto out;
-	return 0;
+		return -EFAULT;
 
-out:
 	cpuid->nent = vcpu->arch.cpuid_nent;
-	return r;
+	return 0;
 }
 
 /* Mask kvm_cpu_caps for @leaf with the raw CPUID capabilities of this CPU. */
-- 
2.41.0.rc0.172.g3f132b7071-goog