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: Christian Brauner <brauner@kernel.org>

diff --git a/tools/testing/selftests/pidfd/pidfd_info_test.c b/tools/testing/selftests/pidfd/pidfd_info_test.c
index 28a28ae4686a67474c4f62f4ab99d17f2db717df..4169780c9e55bbf563291c4575c35e329ff3c927 100644 (file)
--- a/tools/testing/selftests/pidfd/pidfd_info_test.c
+++ b/tools/testing/selftests/pidfd/pidfd_info_test.c
@@ -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];
@@ -439,6 +439,37 @@ TEST_F(pidfd_info, thread_group_exec)
                syscall(__NR_exit, EXIT_SUCCESS);
        }
 
+       /* 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'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.
+                *
+                * 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, 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));
@@ -447,33 +478,12 @@ TEST_F(pidfd_info, thread_group_exec)
        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);
-
-       /*
-        * We can poll and wait for the old thread-group leader to exit
-        * using a thread-specific pidfd.
-        *
-        * 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.).
-        */
-       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));
-
        /* 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. */