ptrace: __ptrace_detach: do __wake_up_parent() if we reap the tracee

The bug is old, it wasn't cause by recent changes.

Test case:

	static void *tfunc(void *arg)
	{
		int pid = (long)arg;

		assert(ptrace(PTRACE_ATTACH, pid, NULL, NULL) == 0);
		kill(pid, SIGKILL);

		sleep(1);
		return NULL;
	}

	int main(void)
	{
		pthread_t th;
		long pid = fork();

		if (!pid)
			pause();

		signal(SIGCHLD, SIG_IGN);
		assert(pthread_create(&th, NULL, tfunc, (void*)pid) == 0);

		int r = waitpid(-1, NULL, __WNOTHREAD);
		printf("waitpid: %d %m\n", r);

		return 0;
	}

Before the patch this program hangs, after this patch waitpid() correctly
fails with errno == -ECHILD.

The problem is, __ptrace_detach() reaps the EXIT_ZOMBIE tracee if its
->real_parent is our sub-thread and we ignore SIGCHLD.  But in this case
we should wake up other threads which can sleep in do_wait().

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: Roland McGrath <roland@redhat.com>
Cc: Vitaly Mayatskikh <vmayatsk@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 7 insertions, 4 deletions

diff --git a/kernel/ptrace.c b/kernel/ptrace.c
index 307c285af59e..23bd09cd042e 100644
--- a/kernel/ptrace.c
+++ b/kernel/ptrace.c
@@ -266,9 +266,10 @@ static int ignoring_children(struct sighand_struct *sigh)
  * or self-reaping.  Do notification now if it would have happened earlier.
  * If it should reap itself, return true.
  *
- * If it's our own child, there is no notification to do.
- * But if our normal children self-reap, then this child
- * was prevented by ptrace and we must reap it now.
+ * If it's our own child, there is no notification to do. But if our normal
+ * children self-reap, then this child was prevented by ptrace and we must
+ * reap it now, in that case we must also wake up sub-threads sleeping in
+ * do_wait().
  */
 static bool __ptrace_detach(struct task_struct *tracer, struct task_struct *p)
 {
@@ -278,8 +279,10 @@ static bool __ptrace_detach(struct task_struct *tracer, struct task_struct *p)
                 if (!task_detached(p) && thread_group_empty(p)) {
                         if (!same_thread_group(p->real_parent, tracer))
                                 do_notify_parent(p, p->exit_signal);
-                        else if (ignoring_children(tracer->sighand))
+                        else if (ignoring_children(tracer->sighand)) {
+                                __wake_up_parent(p, tracer);
                                 p->exit_signal = -1;
+                        }
                 }
                 if (task_detached(p)) {
                         /* Mark it as in the process of being reaped. */