[PATCH 2/2] 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
may_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 | 18 ++++++++++++++----
 1 file changed, 14 insertions(+), 4 deletions(-)

diff --git a/kernel/signal.c b/kernel/signal.c
index b78ce63..16e44d7 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -1738,6 +1738,20 @@ static inline int may_ptrace_stop(void)
 {
 	if (!likely(current->ptrace))
 		return 0;
+
+	/*
+	 * Other checks can only affect PTRACE_EVENT_EXIT, a task with the
+	 * pending SIGKILL can't block in TASK_TRACED. But PTRACE_EVENT_EXIT
+	 * can be reported after SIGKILL was already dequeued.
+	 */
+
+	/*
+	 * Coredump or exec. We must not stop in the latter case, debuger can
+	 * wait for cred_guard_mutex held by execing thread.
+	 */
+	if (current->signal->group_exit_task)
+		return 0;
+
 	/*
 	 * Are we in the middle of do_coredump?
 	 * If so and our tracer is also part of the coredump stopping
@@ -1746,10 +1760,6 @@ static inline int may_ptrace_stop(void)
 	 * If SIGKILL was already sent before the caller unlocked
 	 * ->siglock we must see ->core_state != NULL. Otherwise it
 	 * is safe to enter schedule().
-	 *
-	 * This is almost outdated, a task with the pending SIGKILL can't
-	 * block in TASK_TRACED. But PTRACE_EVENT_EXIT can be reported
-	 * after SIGKILL was already dequeued.
 	 */
 	if (unlikely(current->mm->core_state) &&
 	    unlikely(current->mm == current->parent->mm))
-- 
2.5.0