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

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Date:   Fri, 16 Aug 2019 08:34:35 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Andrei Vagin <avagin@...il.com>
cc:     Dmitry Safonov <dima@...sta.com>, linux-kernel@...r.kernel.org,
        Dmitry Safonov <0x7f454c46@...il.com>,
        Andrei Vagin <avagin@...nvz.org>,
        Adrian Reber <adrian@...as.de>,
        Andy Lutomirski <luto@...nel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Cyrill Gorcunov <gorcunov@...nvz.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
        Jann Horn <jannh@...gle.com>, Jeff Dike <jdike@...toit.com>,
        Oleg Nesterov <oleg@...hat.com>,
        Pavel Emelyanov <xemul@...tuozzo.com>,
        Shuah Khan <shuah@...nel.org>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        containers@...ts.linux-foundation.org, criu@...nvz.org,
        linux-api@...r.kernel.org, x86@...nel.org
Subject: 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:
> > > +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