[Devel] Re: [RFC,PATCH] fix /sbin/init signal handling

[Serge E. Hallyn]

Quoting Oleg Nesterov (oleg at tv-sign.ru):
> (Not for inclusion yet, against 2.6.23-rc2, untested)
> 
> Currently, /sbin/init is protected from unhandled signals by the
> "current == child_reaper(current)" check in get_signal_to_deliver().
> This is not enough, we have multiple problems:
> 
> 	- this doesn't work for multi-threaded inits, and we can't
> 	  fix this by simply making this check group-wide.
> 
> 	- /sbin/init and kernel threads are not protected from
> 	  handle_stop_signal(). Minor problem, but not good and
> 	  allows to "steal" SIGCONT or change ->signal->flags.
> 
> 	- /sbin/init is not protected from __group_complete_signal(),
> 	  sig_fatal() can set SIGNAL_GROUP_EXIT and block exec(), kill
> 	  sub-threads, set ->group_stop_count, etc.
> 
> Also, with support for multiple pid namespaces, we need an ability to
> actually kill the sub-namespace's init from the parent namespace. In
> this case it is not possible (without painful and intrusive changes)
> to make the "should we honor this signal" decision on the receiver's
> side.
> 
> Hopefully this patch (adds 43 bytes to kernel/signal.o) can solve
> these problems.
> 
> Notes:
> 
> 	- Blocked signals are never ignored, so init still can receive
> 	  a pending blocked signal after sigprocmask(SIG_UNBLOCK).
> 	  Easy to fix, but probably we can ignore this issue.
> 
> 	- this patch allows us to simplify de_thread() playing games
> 	  with pid_ns->child_reaper.
> 
> (Side note: the current behaviour of things like force_sig_info_fault()
>  is not very good, init should not ignore these signals and go to the
>  endless loop. Exit + panic is imho better, easy to chamge)
> 
> Oleg.
> 
> --- t/kernel/signal.c~INITSIGS	2007-08-19 14:39:35.000000000 +0400
> +++ t/kernel/signal.c	2007-08-19 19:00:27.000000000 +0400
> @@ -39,11 +39,35 @@
> 
>  static struct kmem_cache *sigqueue_cachep;
> 
> +static int sig_init_ignore(struct task_struct *tsk)
> +{
> +	// Currently this check is a bit racy with exec(),
> +	// we can _simplify_ de_thread and close the race.
> +	if (likely(!is_init(tsk->group_leader)))
> +		return 0;
> +
> +	// ---------------- Multiple pid namespaces ----------------
> +	// if (current is from tsk's parent pid_ns && !in_interrupt())
> +	//	return 0;
> +
> +	return 1;
> +}
> +
> +static int sig_task_ignore(struct task_struct *tsk, int sig)
> +{
> +	void __user * handler = tsk->sighand->action[sig-1].sa.sa_handler;
> +
> +	if (handler == SIG_IGN)
> +		return 1;
> +
> +	if (handler != SIG_DFL)
> +		return 0;
> +
> +	return sig_kernel_ignore(sig) || sig_init_ignore(tsk);
> +}

Looks good.  AFAICS init gets exactly those signals for which it
installed a signal handler.

> 
>  static int sig_ignored(struct task_struct *t, int sig)
>  {
> -	void __user * handler;
> -
>  	/*
>  	 * Tracers always want to know about signals..
>  	 */
> @@ -58,10 +82,7 @@ static int sig_ignored(struct task_struc
>  	if (sigismember(&t->blocked, sig))
>  		return 0;
> 
> -	/* Is it explicitly or implicitly ignored? */
> -	handler = t->sighand->action[sig-1].sa.sa_handler;
> -	return   handler == SIG_IGN ||
> -		(handler == SIG_DFL && sig_kernel_ignore(sig));
> +	return sig_task_ignore(t, sig);
>  }

Looks good.

> 
>  /*
> @@ -569,6 +590,9 @@ static void handle_stop_signal(int sig, 
>  		 */
>  		return;
> 
> +	if (sig_init_ignore(p))
> +		return;
> +
>  	if (sig_kernel_stop(sig)) {
>  		/*
>  		 * This is a stop signal.  Remove SIGCONT from all queues.
> @@ -1841,14 +1865,6 @@ relock:
>  		if (sig_kernel_ignore(signr)) /* Default is nothing. */
>  			continue;
> 
> -		/*
> -		 * Init of a pid space gets no signals it doesn't want from
> -		 * within that pid space. It can of course get signals from
> -		 * its parent pid space.
> -		 */
> -		if (current == child_reaper(current))
> -			continue;
> -

Ok, so the idea is that this will now be caught when the signal is sent,
using sig_ignored(), (i.e at send_sigqueue, send_group_sigqueue,
specific_send_sig_info, and __group_send_sig_info) and so doesn't need
to be checked here?

I was hoping that meant that sig_init_ignore() would always be called
with current as the sending process, but I guess that's not the case?
At least in get_signal_to_deliver() we might resend a signal, though
I guess we assume the signal comes from current->parent, so maybe we
can pass that as an argument...

>  		if (sig_kernel_stop(signr)) {
>  			/*
>  			 * The default action is to stop all threads in
> @@ -2300,13 +2316,10 @@ int do_sigaction(int sig, struct k_sigac
>  	k = &current->sighand->action[sig-1];
> 
>  	spin_lock_irq(&current->sighand->siglock);
> -	if (signal_pending(current)) {
> -		/*
> -		 * If there might be a fatal signal pending on multiple
> -		 * threads, make sure we take it before changing the action.
> -		 */
> +	if (current->signal->flags & SIGNAL_GROUP_EXIT) {
>  		spin_unlock_irq(&current->sighand->siglock);
> -		return -ERESTARTNOINTR;
> +		/* The return value doesn't matter, SIGKILL is pending */
> +		return -EINTR;
>  	}

Looks right, based on the original comment.

> 
>  	if (oact)
> @@ -2327,8 +2340,7 @@ int do_sigaction(int sig, struct k_sigac
>  		 *   (for example, SIGCHLD), shall cause the pending signal to
>  		 *   be discarded, whether or not it is blocked"
>  		 */
> -		if (act->sa.sa_handler == SIG_IGN ||
> -		   (act->sa.sa_handler == SIG_DFL && sig_kernel_ignore(sig))) {
> +		if (sig_task_ignore(current, sig)) {
>  			struct task_struct *t = current;
>  			sigemptyset(&mask);
>  			sigaddset(&mask, sig);

Haven't tested, but the patch reads good.

thanks,
-serge
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