tests: add waitid() tests for non-blocking pidfds

Verify that the PIDFD_NONBLOCK flag works with pidfd_open() and that
waitid() with a non-blocking pidfd returns EAGAIN:

	TAP version 13
	1..3
	# Starting 3 tests from 1 test cases.
	#  RUN           global.wait_simple ...
	#            OK  global.wait_simple
	ok 1 global.wait_simple
	#  RUN           global.wait_states ...
	#            OK  global.wait_states
	ok 2 global.wait_states
	#  RUN           global.wait_nonblock ...
	#            OK  global.wait_nonblock
	ok 3 global.wait_nonblock
	# PASSED: 3 / 3 tests passed.
	# Totals: pass:3 fail:0 xfail:0 xpass:0 skip:0 error:0
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Cc: Shuah Khan <shuah@kernel.org>
Cc: linux-kselftest@vger.kernel.org
Link: https://lore.kernel.org/r/20200902102130.147672-5-christian.brauner@ubuntu.com

tools/testing/selftests/pidfd/pidfd.h

@@ -46,6 +46,10 @@
 #define __NR_pidfd_getfd -1
 #endif
 
+#ifndef PIDFD_NONBLOCK
+#define PIDFD_NONBLOCK O_NONBLOCK
+#endif
+
 /*
  * The kernel reserves 300 pids via RESERVED_PIDS in kernel/pid.c
  * That means, when it wraps around any pid < 300 will be skipped.

tools/testing/selftests/pidfd/pidfd_wait.c

@@ -21,6 +21,11 @@
 
 #define ptr_to_u64(ptr) ((__u64)((uintptr_t)(ptr)))
 
+/* Attempt to de-conflict with the selftests tree. */
+#ifndef SKIP
+#define SKIP(s, ...)	XFAIL(s, ##__VA_ARGS__)
+#endif
+
 static pid_t sys_clone3(struct clone_args *args)
 {
 	return syscall(__NR_clone3, args, sizeof(struct clone_args));
@@ -65,7 +70,7 @@ TEST(wait_simple)
 	pidfd = -1;
 
 	pid = sys_clone3(&args);
-	ASSERT_GE(pid, 1);
+	ASSERT_GE(pid, 0);
 
 	if (pid == 0)
 		exit(EXIT_SUCCESS);
@@ -133,4 +138,88 @@ TEST(wait_states)
 	EXPECT_EQ(close(pidfd), 0);
 }
 
+TEST(wait_nonblock)
+{
+	int pidfd, status = 0;
+	unsigned int flags = 0;
+	pid_t parent_tid = -1;
+	struct clone_args args = {
+		.parent_tid = ptr_to_u64(&parent_tid),
+		.flags = CLONE_PARENT_SETTID,
+		.exit_signal = SIGCHLD,
+	};
+	int ret;
+	pid_t pid;
+	siginfo_t info = {
+		.si_signo = 0,
+	};
+
+	/*
+	 * Callers need to see ECHILD with non-blocking pidfds when no child
+	 * processes exists.
+	 */
+	pidfd = sys_pidfd_open(getpid(), PIDFD_NONBLOCK);
+	EXPECT_GE(pidfd, 0) {
+		/* pidfd_open() doesn't support PIDFD_NONBLOCK. */
+		ASSERT_EQ(errno, EINVAL);
+		SKIP(return, "Skipping PIDFD_NONBLOCK test");
+	}
+
+	ret = sys_waitid(P_PIDFD, pidfd, &info, WEXITED, NULL);
+	ASSERT_LT(ret, 0);
+	ASSERT_EQ(errno, ECHILD);
+	EXPECT_EQ(close(pidfd), 0);
+
+	pid = sys_clone3(&args);
+	ASSERT_GE(pid, 0);
+
+	if (pid == 0) {
+		kill(getpid(), SIGSTOP);
+		exit(EXIT_SUCCESS);
+	}
+
+	pidfd = sys_pidfd_open(pid, PIDFD_NONBLOCK);
+	EXPECT_GE(pidfd, 0) {
+		/* pidfd_open() doesn't support PIDFD_NONBLOCK. */
+		ASSERT_EQ(errno, EINVAL);
+		SKIP(return, "Skipping PIDFD_NONBLOCK test");
+	}
+
+	flags = fcntl(pidfd, F_GETFL, 0);
+	ASSERT_GT(flags, 0);
+	ASSERT_GT((flags & O_NONBLOCK), 0);
+
+	/*
+	 * Callers need to see EAGAIN/EWOULDBLOCK with non-blocking pidfd when
+	 * child processes exist but none have exited.
+	 */
+	ret = sys_waitid(P_PIDFD, pidfd, &info, WEXITED, NULL);
+	ASSERT_LT(ret, 0);
+	ASSERT_EQ(errno, EAGAIN);
+
+	/*
+	 * Callers need to continue seeing 0 with non-blocking pidfd and
+	 * WNOHANG raised explicitly when child processes exist but none have
+	 * exited.
+	 */
+	ret = sys_waitid(P_PIDFD, pidfd, &info, WEXITED | WNOHANG, NULL);
+	ASSERT_EQ(ret, 0);
+
+	ASSERT_EQ(sys_waitid(P_PIDFD, pidfd, &info, WSTOPPED, NULL), 0);
+	ASSERT_EQ(info.si_signo, SIGCHLD);
+	ASSERT_EQ(info.si_code, CLD_STOPPED);
+	ASSERT_EQ(info.si_pid, parent_tid);
+
+	ASSERT_EQ(sys_pidfd_send_signal(pidfd, SIGCONT, NULL, 0), 0);
+
+	ASSERT_EQ(fcntl(pidfd, F_SETFL, (flags & ~O_NONBLOCK)), 0);
+
+	ASSERT_EQ(sys_waitid(P_PIDFD, pidfd, &info, WEXITED, NULL), 0);
+	ASSERT_EQ(info.si_signo, SIGCHLD);
+	ASSERT_EQ(info.si_code, CLD_EXITED);
+	ASSERT_EQ(info.si_pid, parent_tid);
+
+	EXPECT_EQ(close(pidfd), 0);
+}
+
 TEST_HARNESS_MAIN