signals: protect init from unwanted signals more

(This is a modified version of the patch submitted by Oleg Nesterov
http://lkml.org/lkml/2008/11/18/249 and tries to address comments that
came up in that discussion)

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: NSukadev Bhattiprolu <sukadev@linux.vnet.ibm.com>
Acked-by: NRoland McGrath <roland@redhat.com>
Signed-off-by: NOleg Nesterov <oleg@tv-sign.ru>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Daniel Lezcano <daniel.lezcano@free.fr>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

kernel/signal.c

@@ -55,10 +55,21 @@ static int sig_handler_ignored(void __user *handler, int sig)
 		(handler == SIG_DFL && sig_kernel_ignore(sig));
 }
 
-static int sig_ignored(struct task_struct *t, int sig)
+static int sig_task_ignored(struct task_struct *t, int sig)
 {
 	void __user *handler;
 
+	handler = sig_handler(t, sig);
+
+	if (unlikely(t->signal->flags & SIGNAL_UNKILLABLE) &&
+			handler == SIG_DFL)
+		return 1;
+
+	return sig_handler_ignored(handler, sig);
+}
+
+static int sig_ignored(struct task_struct *t, int sig)
+{
 	/*
 	 * Blocked signals are never ignored, since the
 	 * signal handler may change by the time it is
@@ -67,8 +78,7 @@ static int sig_ignored(struct task_struct *t, int sig)
 	if (sigismember(&t->blocked, sig) || sigismember(&t->real_blocked, sig))
 		return 0;
 
-	handler = sig_handler(t, sig);
-	if (!sig_handler_ignored(handler, sig))
+	if (!sig_task_ignored(t, sig))
 		return 0;
 
 	/*