[RFC PATCH 3/3] ptrace: ensure PTRACE_EVENT_EXIT won't stop if the tracee is killed by exec

[Oleg Nesterov <oleg@redhat.com>]

The previous patch fixed the deadlock when mt-exec waits for debugger
which should reap a zombie thread, but we can hit the same problem if
the killed sub-thread stops in ptrace_event(PTRACE_EVENT_EXIT). Change
ptrace_stop() to check signal->group_exit_task.

This is a user-visible change. But hopefully it can't break anything.
Note that the semantics of PTRACE_EVENT_EXIT was never really defined,
it depends on /dev/random. Just for example, currently a sub-thread
killed by exec will stop, but if it exits on its own and races with
exec it will not stop, so nobody can rely on PTRACE_EVENT_EXIT anyway.

We really need to finally define what PTRACE_EVENT_EXIT should actually
do, but this needs other changes.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
 kernel/signal.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/kernel/signal.c b/kernel/signal.c
index 334212044940..59f61e07905b 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -2376,6 +2376,10 @@ static int ptrace_stop(int exit_code, int why, unsigned long message,
 	if (!current->ptrace || __fatal_signal_pending(current))
 		return exit_code;
 
+	/* de_thread() -> wait_for_notify_count() waits for us */
+	if (current->signal->group_exec_task)
+		return exit_code;
+
 	set_special_state(TASK_TRACED);
 	current->jobctl |= JOBCTL_TRACED;
 
-- 
2.25.1.362.g51ebf55