__group_complete_signal(): fix coredump with group stop race

When __group_complete_signal() sees sig_kernel_coredump() signal, it starts
the group stop, but sets ->group_exit_task = t in a hope that "t" will
actually dequeue this signal and invoke do_coredump().  However, by the
time "t" enters get_signal_to_deliver() it is possible that the signal was
blocked/ignored or we have another pending !SIG_KERNEL_COREDUMP_MASK signal
which will be dequeued first.  This means the task could be stopped but not
killed.

Remove this code from __group_complete_signal().  Note also this patch
removes the bogus signal_wake_up(t, 1).  This thread can't be
STOPPED/TRACED, note the corresponding check in wants_signal().

Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Cc: Davide Libenzi <davidel@xmailserver.org>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Robin Holt <holt@sgi.com>
Cc: Roland McGrath <roland@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/signal.c

@@ -911,27 +911,6 @@ __group_complete_signal(int sig, struct task_struct *p)
 			} while_each_thread(p, t);
 			return;
 		}
-
-		/*
-		 * There will be a core dump.  We make all threads other
-		 * than the chosen one go into a group stop so that nothing
-		 * happens until it gets scheduled, takes the signal off
-		 * the shared queue, and does the core dump.  This is a
-		 * little more complicated than strictly necessary, but it
-		 * keeps the signal state that winds up in the core dump
-		 * unchanged from the death state, e.g. which thread had
-		 * the core-dump signal unblocked.
-		 */
-		rm_from_queue(SIG_KERNEL_STOP_MASK, &t->pending);
-		rm_from_queue(SIG_KERNEL_STOP_MASK, &p->signal->shared_pending);
-		p->signal->group_stop_count = 0;
-		p->signal->group_exit_task = t;
-		p = t;
-		do {
-			p->signal->group_stop_count++;
-			signal_wake_up(t, t == p);
-		} while_each_thread(p, t);
-		return;
 	}
 
 	/*
@@ -1762,15 +1741,6 @@ static int handle_group_stop(void)
 {
 	int stop_count;
 
-	if (current->signal->group_exit_task == current) {
-		/*
-		 * Group stop is so we can do a core dump,
-		 * We are the initiating thread, so get on with it.
-		 */
-		current->signal->group_exit_task = NULL;
-		return 0;
-	}
-
 	if (current->signal->flags & SIGNAL_GROUP_EXIT)
 		/*
 		 * Group stop is so another thread can do a core dump,