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

[Thomas Gleixner]

Andrei,

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

> > > +
> > > +	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