linux-kernel - Re: [PATCH 2/3] pidns: Guarantee that the pidns init will be the last pidns process reaped.

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Date:	Thu, 17 May 2012 19:00:15 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Pavel Emelyanov <xemul@...allels.com>,
	Cyrill Gorcunov <gorcunov@...nvz.org>,
	Louis Rilling <louis.rilling@...labs.com>,
	Mike Galbraith <efault@....de>
Subject: Re: [PATCH 2/3] pidns: Guarantee that the pidns init will be the
	last pidns process reaped.

On 05/16, Eric W. Biederman wrote:
>
> Oleg Nesterov <oleg@...hat.com> writes:
>
> > Hmm. I don't think the patch is 100% correct. Afaics, this needs more
> > delay_pidns_leader() checks.
> >
> > For example. Suppose we have a CLONE_NEWPID zombie I, it has an
> > EXIT_DEAD child D so delay_pidns_leader(I) == T.
> >
> > Now suppose that I->real_parent exits, lets denote this task as P.
> >
> > Suppose that P->real_parent ignores SIGCHLD.
> >
> > In this case P will do release_task(I) prematurely. And worse, when
> > D finally does realease_task(D) it will do realease_task(I) again.
>
> Good point.  I will fix that and post a patch shortly.  It doesn't
> need a full delay_pidns_leader test just a test for children.

This will add more complications. And even this is not enough, I guess.
For example __ptrace_detach()...

I agree, the idea to "hack" release_task() so that it switches to
init is clever, but imho this is too clever ;)

Seriously, what do you think about the patch below? Or something
like this. It is still based on your suggestion to check ->children,
but it is much, much more simple and understandable.

Just in case... Even with the PF_EXITING check __wake_up_parent()
can be wrong, but this is very unlikely and harmless.

What do you think?

> In looking for any other weird corner case bugs I am noticing that
> I don't think I handled the case of a ptraced init quite right.
> I don't understand the change signaling semantics when the
> ptracer is our parent.

Do you mean the "if (tsk->ptrace)" code in exit_notify() ? Nobody
understand it ;) Last time this code was modified by me (iirc), but
I simply tried to preserve the previous behaviour.

Oleg.

--- x/kernel/exit.c
+++ x/kernel/exit.c
@@ -63,6 +63,13 @@ static void exit_mm(struct task_struct *
 
 static void __unhash_process(struct task_struct *p, bool group_dead)
 {
+	struct task_struct *parent = p->parent;
+	bool parent_is_init = false;
+
+#ifdef CONFIG_PID_NS
+	parent_is_init = (task_active_pid_ns(p)->child_reaper == parent);
+#endif
+
 	nr_threads--;
 	detach_pid(p, PIDTYPE_PID);
 	if (group_dead) {
@@ -72,6 +79,11 @@ static void __unhash_process(struct task
 		list_del_rcu(&p->tasks);
 		list_del_init(&p->sibling);
 		__this_cpu_dec(process_counts);
+
+		if (parent_is_init && (parent->flags & PF_EXITING)) {
+			if (list_empty(&parent->children))
+				__wake_up_parent(p, parent);
+		}
 	}
 	list_del_rcu(&p->thread_group);
 }
--- x/kernel/pid_namespace.c
+++ x/kernel/pid_namespace.c
@@ -184,6 +184,9 @@ void zap_pid_ns_processes(struct pid_nam
 		rc = sys_wait4(-1, NULL, __WALL, NULL);
 	} while (rc != -ECHILD);
 
+	wait_event(&current->signal->wait_chldexit,
+			list_empty(&current->children));
+
 	if (pid_ns->reboot)
 		current->signal->group_exit_code = pid_ns->reboot;
 

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