[PATCH 1/2] protect /sbin/init from unwanted signals more

[Oleg Nesterov <oleg@redhat.com>]

init ignores the SIG_DFL signals but we queue them anyway, including
SIGKILL. This is mostly OK, the signal will be dropped silently when
dequeued, but the pending SIGKILL has 2 bad implications:

	- it implies fatal_signal_pending(), so we confuse things
	  like wait_for_completion_killable/lock_page_killable.

	- for the sub-namespace inits, the pending SIGKILL can
	  mask (legacy_queue) the subsequent SIGKILL from the
	  parent namespace which must kill cinit reliably.
	  (preparation, cinits don't have SIGNAL_UNKILLABLE yet)

The patch can't help when init is ptraced, but ptracing of init is
not "safe" anyway.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>

--- K-IS/kernel/signal.c~1_INIT_IGN_KILL	2008-11-10 19:21:17.000000000 +0100
+++ K-IS/kernel/signal.c	2008-11-17 19:54:09.000000000 +0100
@@ -43,7 +43,13 @@ static struct kmem_cache *sigqueue_cache
 
 static void __user *sig_handler(struct task_struct *t, int sig)
 {
-	return t->sighand->action[sig - 1].sa.sa_handler;
+	void __user *h = t->sighand->action[sig - 1].sa.sa_handler;
+
+	/* drop SIGKILL early to not confuse wait_xxx_killable/etc */
+	if (unlikely(t->signal->flags & SIGNAL_UNKILLABLE) && h == SIG_DFL)
+		h = SIG_IGN;
+
+	return h;
 }
 
 static int sig_handler_ignored(void __user *handler, int sig)