selftests/pidfd: first test for multi-threaded exec polling

Add first test for premature thread-group leader exit.

Link: https://lore.kernel.org/r/20250320-work-pidfs-thread_group-v4-2-da678ce805bf@kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: brauner

tools/testing/selftests/pidfd/pidfd_info_test.c

@@ -236,7 +236,7 @@ static void *pidfd_info_pause_thread(void *arg)
 
 TEST_F(pidfd_info, thread_group)
 {
-	pid_t pid_leader, pid_thread;
+	pid_t pid_leader, pid_poller, pid_thread;
 	pthread_t thread;
 	int nevents, pidfd_leader, pidfd_thread, pidfd_leader_thread, ret;
 	int ipc_sockets[2];
@@ -262,6 +262,35 @@ TEST_F(pidfd_info, thread_group)
 		syscall(__NR_exit, EXIT_SUCCESS);
 	}
 
+	/*
+	 * Opening a PIDFD_THREAD aka thread-specific pidfd based on a
+	 * thread-group leader must succeed.
+	 */
+	pidfd_leader_thread = sys_pidfd_open(pid_leader, PIDFD_THREAD);
+	ASSERT_GE(pidfd_leader_thread, 0);
+
+	pid_poller = fork();
+	ASSERT_GE(pid_poller, 0);
+	if (pid_poller == 0) {
+		/*
+		 * We can't poll and wait for the old thread-group
+		 * leader to exit using a thread-specific pidfd. The
+		 * thread-group leader exited prematurely and
+		 * notification is delayed until all subthreads have
+		 * exited.
+		 */
+		fds.events = POLLIN;
+		fds.fd = pidfd_leader_thread;
+		nevents = poll(&fds, 1, 10000 /* wait 5 seconds */);
+		if (nevents != 0)
+			_exit(EXIT_FAILURE);
+		if (fds.revents & POLLIN)
+			_exit(EXIT_FAILURE);
+		if (fds.revents & POLLHUP)
+			_exit(EXIT_FAILURE);
+		_exit(EXIT_SUCCESS);
+	}
+
 	/* Retrieve the tid of the thread. */
 	EXPECT_EQ(close(ipc_sockets[1]), 0);
 	ASSERT_EQ(read_nointr(ipc_sockets[0], &pid_thread, sizeof(pid_thread)), sizeof(pid_thread));
@@ -275,12 +304,7 @@ TEST_F(pidfd_info, thread_group)
 	pidfd_thread = sys_pidfd_open(pid_thread, PIDFD_THREAD);
 	ASSERT_GE(pidfd_thread, 0);
 
-	/*
-	 * Opening a PIDFD_THREAD aka thread-specific pidfd based on a
-	 * thread-group leader must succeed.
-	 */
-	pidfd_leader_thread = sys_pidfd_open(pid_leader, PIDFD_THREAD);
-	ASSERT_GE(pidfd_leader_thread, 0);
+	ASSERT_EQ(wait_for_pid(pid_poller), 0);
 
 	/*
 	 * Note that pidfd_leader is a thread-group pidfd, so polling on it