Re: [PATCHv6 01/36] ns: Introduce Time Namespace

Andrei,

On Thu, 15 Aug 2019, Andrei Vagin wrote:

On Thu, Aug 15, 2019 at 07:19:12PM +0200, Thomas Gleixner wrote:

quoted

quoted

+static int timens_install(struct nsproxy *nsproxy, struct ns_common *new)
+{
+	struct time_namespace *ns = to_time_ns(new);
+
+	if (!current_is_single_threaded())
+		return -EUSERS;
+
+	if (!ns_capable(ns->user_ns, CAP_SYS_ADMIN) ||
+	    !ns_capable(current_user_ns(), CAP_SYS_ADMIN))
+		return -EPERM;
+
+	get_time_ns(ns);
+	get_time_ns(ns);

Why is this a double get?

Because we change nsproxy->time_ns and nsproxy->time_ns_for_children.

Probably, I need to reorgonize the code this way:

	get_time_ns(ns);
	put_time_ns(nsproxy->time_ns);
	nsproxy->time_ns = ns;

	get_time_ns(ns);
	put_time_ns(nsproxy->time_ns_for_children);
	nsproxy->time_ns_for_children = ns;

That's better. A few comments about refcounting might be useful as well.
 

quoted

quoted

+	put_time_ns(nsproxy->time_ns);
+	put_time_ns(nsproxy->time_ns_for_children);
+	nsproxy->time_ns = ns;
+	nsproxy->time_ns_for_children = ns;
+	ns->initialized = true;
+	return 0;
+}
+
+int timens_on_fork(struct nsproxy *nsproxy, struct task_struct *tsk)
+{
+	struct ns_common *nsc = &nsproxy->time_ns_for_children->ns;
+	struct time_namespace *ns = to_time_ns(nsc);
+
+	if (nsproxy->time_ns == nsproxy->time_ns_for_children)
+		return 0;

Doesn't this need to take a refcount on fork? Maybe not, but see below.

quoted

quoted

+
+	get_time_ns(ns);
+	put_time_ns(nsproxy->time_ns);
+	nsproxy->time_ns = ns;
+	ns->initialized = true;

Isn't that one initialized already?

When we discussed time namespaces last year, we decided that clock
offsets can be set only before any task enters a namespace.

When a namespace is created, ns->initialized is set to false. When a
task enters the namespace, ns->initialized is set to true.

Right. I'm probably just hopelessly confused by this nsproxy->time_ns
vs. nsproxy->time_ns_for_children->ns thing.
 

Namespace clock offsets can be changed only if ns->initialized is false.

A new task can be forked to a specified time namespace or it can call
setns, so we set ns->initialized to true in timens_on_fork and
timens_install.

Some comments explaining that logic above would be really helpful.

Thanks,

	tglx