KVM: selftests: Fix signedness issue with vCPU mmap size check

Check that the return value of KVM_GET_VCPU_MMAP_SIZE is non-negative
before comparing with sizeof(kvm_run). If KVM_GET_VCPU_MMAP_SIZE fails,
it will return -1, and `-1 > sizeof(kvm_run)` is true, so the ASSERT
passes.

There are no other locations in tools/testing/selftests/kvm that make
the same mistake.

Signed-off-by: James Houghton <jthoughton@google.com>
Link: https://lore.kernel.org/r/20250711001742.1965347-1-jthoughton@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: 48ca

tools/testing/selftests/kvm/lib/kvm_util.c

@@ -24,7 +24,7 @@ uint32_t guest_random_seed;
 struct guest_random_state guest_rng;
 static uint32_t last_guest_seed;
 
-static int vcpu_mmap_sz(void);
+static size_t vcpu_mmap_sz(void);
 
 int __open_path_or_exit(const char *path, int flags, const char *enoent_help)
 {
@@ -1321,14 +1321,14 @@ void vm_guest_mem_fallocate(struct kvm_vm *vm, uint64_t base, uint64_t size,
 }
 
 /* Returns the size of a vCPU's kvm_run structure. */
-static int vcpu_mmap_sz(void)
+static size_t vcpu_mmap_sz(void)
 {
 	int dev_fd, ret;
 
 	dev_fd = open_kvm_dev_path_or_exit();
 
 	ret = ioctl(dev_fd, KVM_GET_VCPU_MMAP_SIZE, NULL);
-	TEST_ASSERT(ret >= sizeof(struct kvm_run),
+	TEST_ASSERT(ret >= 0 && ret >= sizeof(struct kvm_run),
 		    KVM_IOCTL_ERROR(KVM_GET_VCPU_MMAP_SIZE, ret));
 
 	close(dev_fd);
@@ -1369,7 +1369,7 @@ struct kvm_vcpu *__vm_vcpu_add(struct kvm_vm *vm, uint32_t vcpu_id)
 	TEST_ASSERT_VM_VCPU_IOCTL(vcpu->fd >= 0, KVM_CREATE_VCPU, vcpu->fd, vm);
 
 	TEST_ASSERT(vcpu_mmap_sz() >= sizeof(*vcpu->run), "vcpu mmap size "
-		"smaller than expected, vcpu_mmap_sz: %i expected_min: %zi",
+		"smaller than expected, vcpu_mmap_sz: %zi expected_min: %zi",
 		vcpu_mmap_sz(), sizeof(*vcpu->run));
 	vcpu->run = (struct kvm_run *) mmap(NULL, vcpu_mmap_sz(),
 		PROT_READ | PROT_WRITE, MAP_SHARED, vcpu->fd, 0);