Kill some obsolete sub-thread-ptrace stuff

There is a couple of subtle checks which were needed to handle ptracing from
the same thread group. This was deprecated a long ago, imho this code just
complicates the understanding.

And, the "->parent->signal->flags & SIGNAL_GROUP_EXIT" check in exit_notify()
is not right. SIGNAL_GROUP_EXIT can mean exec(), not exit_group(). This means
ptracer can lose a ptraced zombie on exec(). Minor problem, but still the bug.

Signed-off-by: Oleg Nesterov <[e-mail]>
Acked-by: Roland McGrath <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

kernel/exit.c

···
 		__kill_pgrp_info(SIGCONT, SEND_SIG_PRIV, pgrp);
 	}
 
-	/* Let father know we died 
+	/* Let father know we died
 	 *
 	 * Thread signals are configurable, but you aren't going to use
 	 * that to send signals to arbitary processes. 
···
 	 * If our self_exec id doesn't match our parent_exec_id then
 	 * we have changed execution domain as these two values started
 	 * the same after a fork.
-	 *	
 	 */
-	
 	if (tsk->exit_signal != SIGCHLD && tsk->exit_signal != -1 &&
 	    ( tsk->parent_exec_id != t->self_exec_id  ||
 	      tsk->self_exec_id != tsk->parent_exec_id)
···
 	}
 
 	state = EXIT_ZOMBIE;
-	if (tsk->exit_signal == -1 &&
-	    (likely(tsk->ptrace == 0) ||
-	     unlikely(tsk->parent->signal->flags & SIGNAL_GROUP_EXIT)))
+	if (tsk->exit_signal == -1 && likely(!tsk->ptrace))
 		state = EXIT_DEAD;
 	tsk->exit_state = state;
 

kernel/signal.c

···
 		    (current->ptrace & PT_ATTACHED)))
 		return 0;
 
-	if (unlikely(current->signal == current->parent->signal) &&
-	    unlikely(current->signal->flags & SIGNAL_GROUP_EXIT))
-		return 0;
-
 	/*
 	 * Are we in the middle of do_coredump?
 	 * If so and our tracer is also part of the coredump stopping