KVM: selftests: Fix hang in hardware_disable_test

If /dev/kvm is not available then hardware_disable_test will hang
indefinitely because the child process exits before posting to the
semaphore for which the parent is waiting.

Fix this by making the parent periodically check if the child has
exited. We have to be careful to forward the child's exit status to
preserve a KSFT_SKIP status.

I considered just checking for /dev/kvm before creating the child
process, but there are so many other reasons why the child could exit
early that it seemed better to handle that as general case.

Tested:

$ ./hardware_disable_test
/dev/kvm not available, skipping test
$ echo $?
4
$ modprobe kvm_intel
$ ./hardware_disable_test
$ echo $?
0
Signed-off-by: David Matlack <dmatlack@google.com>
Message-Id: <20210514230521.2608768-1-dmatlack@google.com>
Reviewed-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

tools/testing/selftests/kvm/hardware_disable_test.c

@@ -132,6 +132,36 @@ static void run_test(uint32_t run)
 	TEST_ASSERT(false, "%s: [%d] child escaped the ninja\n", __func__, run);
 }
 
+void wait_for_child_setup(pid_t pid)
+{
+	/*
+	 * Wait for the child to post to the semaphore, but wake up periodically
+	 * to check if the child exited prematurely.
+	 */
+	for (;;) {
+		const struct timespec wait_period = { .tv_sec = 1 };
+		int status;
+
+		if (!sem_timedwait(sem, &wait_period))
+			return;
+
+		/* Child is still running, keep waiting. */
+		if (pid != waitpid(pid, &status, WNOHANG))
+			continue;
+
+		/*
+		 * Child is no longer running, which is not expected.
+		 *
+		 * If it exited with a non-zero status, we explicitly forward
+		 * the child's status in case it exited with KSFT_SKIP.
+		 */
+		if (WIFEXITED(status))
+			exit(WEXITSTATUS(status));
+		else
+			TEST_ASSERT(false, "Child exited unexpectedly");
+	}
+}
+
 int main(int argc, char **argv)
 {
 	uint32_t i;
@@ -148,7 +178,7 @@ int main(int argc, char **argv)
 			run_test(i); /* This function always exits */
 
 		pr_debug("%s: [%d] waiting semaphore\n", __func__, i);
-		sem_wait(sem);
+		wait_for_child_setup(pid);
 		r = (rand() % DELAY_US_MAX) + 1;
 		pr_debug("%s: [%d] waiting %dus\n", __func__, i, r);
 		usleep(r);