KVM: selftests: Verify stats fd can be dup()'d and read

Expand the binary stats test to verify that a stats fd can be dup()'d
and read, to (very) roughly simulate userspace passing around the file.
Adding the dup() test is primarily an intermediate step towards verifying
that userspace can read VM/vCPU stats before _and_ after userspace closes
its copy of the VM fd; the dup() test itself is only mildly interesting.

Link: https://lore.kernel.org/r/20230711230131.648752-7-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

tools/testing/selftests/kvm/kvm_binary_stats_test.c

@@ -232,17 +232,23 @@ int main(int argc, char *argv[])
 	}
 
 	/*
-	 * Check stats read for every VM and vCPU, with a variety of testcases.
+	 * Check stats read for every VM and vCPU, with a variety of flavors.
 	 * Note, stats_test() closes the passed in stats fd.
 	 */
 	for (i = 0; i < max_vm; ++i) {
+		/*
+		 * Verify that creating multiple userspace references to a
+		 * single stats file works and doesn't cause explosions.
+		 */
 		vm_stats_fds = vm_get_stats_fd(vms[i]);
+		stats_test(dup(vm_stats_fds));
 
 		/* Verify userspace can instantiate multiple stats files. */
 		stats_test(vm_get_stats_fd(vms[i]));
 
 		for (j = 0; j < max_vcpu; ++j) {
 			vcpu_stats_fds[j] = vcpu_get_stats_fd(vcpus[i * max_vcpu + j]);
+			stats_test(dup(vcpu_stats_fds[j]));
 			stats_test(vcpu_get_stats_fd(vcpus[i * max_vcpu + j]));
 		}