signals: allow the kernel to actually kill /sbin/init

Currently the buggy /sbin/init hangs if SIGSEGV/etc happens.  The kernel sends
the signal, init dequeues it and ignores, returns from the exception, repeats
the faulting instruction, and so on forever.

Imho, such a behaviour is not good.  I think that the explicit loud death of
the buggy /sbin/init is better than the silent hang.

Change force_sig_info() to clear SIGNAL_UNKILLABLE when the task should be
really killed.

Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
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

@@ -892,7 +892,8 @@ specific_send_sig_info(int sig, struct siginfo *info, struct task_struct *t)
  * since we do not want to have a signal handler that was blocked
  * be invoked when user space had explicitly blocked it.
  *
- * We don't want to have recursive SIGSEGV's etc, for example.
+ * We don't want to have recursive SIGSEGV's etc, for example,
+ * that is why we also clear SIGNAL_UNKILLABLE.
  */
 int
 force_sig_info(int sig, struct siginfo *info, struct task_struct *t)
@@ -912,6 +913,8 @@ force_sig_info(int sig, struct siginfo *info, struct task_struct *t)
 			recalc_sigpending_and_wake(t);
 		}
 	}
+	if (action->sa.sa_handler == SIG_DFL)
+		t->signal->flags &= ~SIGNAL_UNKILLABLE;
 	ret = specific_send_sig_info(sig, info, t);
 	spin_unlock_irqrestore(&t->sighand->siglock, flags);