[PATCH 3/3] signals: allow the kernel to actually kill /sbin/init

[Oleg Nesterov <oleg@tv-sign.ru>]

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>

--- 25/kernel/signal.c~3_INIT_SIGSEGV	2008-03-16 15:45:56.000000000 +0300
+++ 25/kernel/signal.c	2008-03-16 16:14:56.000000000 +0300
@@ -877,7 +877,8 @@ specific_send_sig_info(int sig, struct s
  * 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)
@@ -897,6 +898,8 @@ force_sig_info(int sig, struct siginfo *
 			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);