change kernel threads to ignore signals instead of blocking them

Currently kernel threads use sigprocmask(SIG_BLOCK) to protect against
signals.  This doesn't prevent the signal delivery, this only blocks
signal_wake_up().  Every "killall -33 kthreadd" means a "struct siginfo"
leak.

Change kthreadd_setup() to set all handlers to SIG_IGN instead of blocking
them (make a new helper ignore_signals() for that).  If the kernel thread
needs some signal, it should use allow_signal() anyway, and in that case it
should not use CLONE_SIGHAND.

Note that we can't change daemonize() (should die!) in the same way,
because it can be used along with CLONE_SIGHAND.  This means that
allow_signal() still should unblock the signal to work correctly with
daemonize()ed threads.

However, disallow_signal() doesn't block the signal any longer but ignores
it.

NOTE: with or without this patch the kernel threads are not protected from
handle_stop_signal(), this seems harmless, but not good.
Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Acked-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/sched.h

@@ -1317,6 +1317,7 @@ extern int in_egroup_p(gid_t);
 
 extern void proc_caches_init(void);
 extern void flush_signals(struct task_struct *);
+extern void ignore_signals(struct task_struct *);
 extern void flush_signal_handlers(struct task_struct *, int force_default);
 extern int dequeue_signal(struct task_struct *tsk, sigset_t *mask, siginfo_t *info);
 

kernel/exit.c

@@ -347,7 +347,7 @@ int disallow_signal(int sig)
 		return -EINVAL;
 
 	spin_lock_irq(&current->sighand->siglock);
-	sigaddset(&current->blocked, sig);
+	current->sighand->action[(sig)-1].sa.sa_handler = SIG_IGN;
 	recalc_sigpending();
 	spin_unlock_irq(&current->sighand->siglock);
 	return 0;

kernel/kthread.c

@@ -215,24 +215,13 @@ EXPORT_SYMBOL(kthread_stop);
 static __init void kthreadd_setup(void)
 {
 	struct task_struct *tsk = current;
-	struct k_sigaction sa;
-	sigset_t blocked;
 
 	set_task_comm(tsk, "kthreadd");
 
-	/* Block and flush all signals */
-	sigfillset(&blocked);
-	sigprocmask(SIG_BLOCK, &blocked, NULL);
-	flush_signals(tsk);
+	ignore_signals(tsk);
 
-	/* SIG_IGN makes children autoreap: see do_notify_parent(). */
-	sa.sa.sa_handler = SIG_IGN;
-	sa.sa.sa_flags = 0;
-	siginitset(&sa.sa.sa_mask, sigmask(SIGCHLD));
-	do_sigaction(SIGCHLD, &sa, (struct k_sigaction *)0);
-
-	set_user_nice(current, -5);
-	set_cpus_allowed(current, CPU_MASK_ALL);
+	set_user_nice(tsk, -5);
+	set_cpus_allowed(tsk, CPU_MASK_ALL);
 }
 
 int kthreadd(void *unused)

kernel/signal.c

@@ -209,6 +209,16 @@ void flush_signals(struct task_struct *t)
 	spin_unlock_irqrestore(&t->sighand->siglock, flags);
 }
 
+void ignore_signals(struct task_struct *t)
+{
+	int i;
+
+	for (i = 0; i < _NSIG; ++i)
+		t->sighand->action[i].sa.sa_handler = SIG_IGN;
+
+	flush_signals(t);
+}
+
 /*
  * Flush all handlers for a task.
  */