ptrace: ensure JOBCTL_STOP_SIGMASK is not zero after detach

This is the temporary simple fix for 3.2, we need more changes in this
area.

1. do_signal_stop() assumes that the running untraced thread in the
   stopped thread group is not possible. This was our goal but it is
   not yet achieved: a stopped-but-resumed tracee can clone the running
   thread which can initiate another group-stop.

   Remove WARN_ON_ONCE(!current->ptrace).

2. A new thread always starts with ->jobctl = 0. If it is auto-attached
   and this group is stopped, __ptrace_unlink() sets JOBCTL_STOP_PENDING
   but JOBCTL_STOP_SIGMASK part is zero, this triggers WANR_ON(!signr)
   in do_jobctl_trap() if another debugger attaches.

   Change __ptrace_unlink() to set the artificial SIGSTOP for report.

   Alternatively we could change ptrace_init_task() to copy signr from
   current, but this means we can copy it for no reason and hide the
   possible similar problems.
Acked-by: Tejun Heo <tj@kernel.org>
Cc: <stable@kernel.org>		[3.1]
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/ptrace.c

@@ -96,9 +96,20 @@ void __ptrace_unlink(struct task_struct *child)
 	 */
 	if (!(child->flags & PF_EXITING) &&
 	    (child->signal->flags & SIGNAL_STOP_STOPPED ||
-	     child->signal->group_stop_count))
+	     child->signal->group_stop_count)) {
 		child->jobctl |= JOBCTL_STOP_PENDING;
 
+		/*
+		 * This is only possible if this thread was cloned by the
+		 * traced task running in the stopped group, set the signal
+		 * for the future reports.
+		 * FIXME: we should change ptrace_init_task() to handle this
+		 * case.
+		 */
+		if (!(child->jobctl & JOBCTL_STOP_SIGMASK))
+			child->jobctl |= SIGSTOP;
+	}
+
 	/*
 	 * If transition to TASK_STOPPED is pending or in TASK_TRACED, kick
 	 * @child in the butt.  Note that @resume should be used iff @child

kernel/signal.c

@@ -1994,8 +1994,6 @@ static bool do_signal_stop(int signr)
 		 */
 		if (!(sig->flags & SIGNAL_STOP_STOPPED))
 			sig->group_exit_code = signr;
-		else
-			WARN_ON_ONCE(!current->ptrace);
 
 		sig->group_stop_count = 0;