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
