Unverified Commit 9b6f723d authored by Christian Brauner's avatar Christian Brauner
Browse files

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

Ensure that during a multi-threaded exec and premature thread-group
leader exit no exit notification is generated.

Link: https://lore.kernel.org/r/20250320-work-pidfs-thread_group-v4-3-da678ce805bf@kernel.org


Signed-off-by: default avatarChristian Brauner <brauner@kernel.org>
parent db7ce91e

tools/testing/selftests/pidfd/pidfd_info_test.c

+48 −24
Original line number Diff line number Diff line
@@ -413,7 +413,7 @@ static void *pidfd_info_thread_exec(void *arg) 


TEST_F(pidfd_info, thread_group_exec)

{

	pid_t pid_leader, pid_thread;

	pid_t pid_leader, pid_poller, pid_thread;

	pthread_t thread;

	int nevents, pidfd_leader, pidfd_leader_thread, pidfd_thread, ret;

	int ipc_sockets[2];
@@ -439,41 +439,51 @@ TEST_F(pidfd_info, thread_group_exec) 
		syscall(__NR_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));



	/* Opening a thread as a PIDFD_THREAD must succeed. */

	pidfd_thread = sys_pidfd_open(pid_thread, PIDFD_THREAD);

	ASSERT_GE(pidfd_thread, 0);



	/* Open a thread-specific pidfd for the thread-group leader. */

	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 poll and wait for the old thread-group leader to exit

	 * using a thread-specific pidfd.

		 * 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.

		 *

	 * This only works until the thread has execed. When the thread

	 * has execed it will have taken over the old thread-group

	 * leaders struct pid. Calling poll after the thread execed will

	 * thus block again because a new thread-group has started (Yes,

	 * it's fscked.).

		 * When the thread has execed it will taken over the old

		 * thread-group leaders struct pid. Calling poll after

		 * the thread execed will thus block again because a new

		 * thread-group has started.

		 */

		fds.events = POLLIN;

		fds.fd = pidfd_leader_thread;

	nevents = poll(&fds, 1, -1);

	ASSERT_EQ(nevents, 1);

	/* The thread-group leader has exited. */

	ASSERT_TRUE(!!(fds.revents & POLLIN));

	/* The thread-group leader hasn't been reaped. */

	ASSERT_FALSE(!!(fds.revents & POLLHUP));

		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));



	/* Opening a thread as a PIDFD_THREAD must succeed. */

	pidfd_thread = sys_pidfd_open(pid_thread, PIDFD_THREAD);

	ASSERT_GE(pidfd_thread, 0);



	/* Now that we've opened a thread-specific pidfd the thread can exec. */

	ASSERT_EQ(write_nointr(ipc_sockets[0], &pid_thread, sizeof(pid_thread)), sizeof(pid_thread));

	EXPECT_EQ(close(ipc_sockets[0]), 0);



	ASSERT_EQ(wait_for_pid(pid_poller), 0);



	/* Wait until the kernel has SIGKILLed the thread. */

	fds.events = POLLHUP;

	fds.fd = pidfd_thread;
@@ -506,6 +516,20 @@ TEST_F(pidfd_info, thread_group_exec) 


	/* Take down the thread-group leader. */

	EXPECT_EQ(sys_pidfd_send_signal(pidfd_leader, SIGKILL, NULL, 0), 0);



	/*

	 * Afte the exec we're dealing with an empty thread-group so now

	 * we must see an exit notification on the thread-specific pidfd

	 * for the thread-group leader as there's no subthread that can

	 * revive the struct pid.

	 */

	fds.events = POLLIN;

	fds.fd = pidfd_leader_thread;

	nevents = poll(&fds, 1, -1);

	ASSERT_EQ(nevents, 1);

	ASSERT_TRUE(!!(fds.revents & POLLIN));

	ASSERT_FALSE(!!(fds.revents & POLLHUP));



	EXPECT_EQ(sys_waitid(P_PIDFD, pidfd_leader, NULL, WEXITED), 0);



	/* Retrieve exit information for the thread-group leader. */