Re: [RFC PATCH 3/8] kmod - teach call_usermodehelper() to use a namespace

[Oleg Nesterov <oleg@redhat.com>]

On 02/08, Ian Kent wrote:
>
> On Fri, 2015-02-06 at 07:08 -0500, Jeff Layton wrote:
> > On Thu, 05 Feb 2015 10:34:11 +0800
> > Ian Kent <ikent@redhat.com> wrote:
> >
> > > +{
> > > +	struct task_struct *tsk;
> > > +
> > > +	rcu_read_lock();
> > > +	tsk = find_task_by_vpid(1);
> > > +	if (tsk)
> > > +		get_task_struct(tsk);
> > > +	rcu_read_unlock();
> >
> > I'm not terribly familiar with the task_struct lifetime rules...
> >
> > I assume that you can be assured that tsk won't go away while you hold
> > the rcu_read_lock, but is doing a get_task_struct while holding it
> > sufficient to pin it after you drop the lock?
> >
> > IOW, could the refcount on the task_struct do a 0->1 transition here and
> > end up being freed anyway after you've grabbed a reference?
>
> Good point, I thought getting a reference under he read lock would be
> enough but maybe I need more checks as I do with dentrys. I'll check
> that.

This is fine. If find_task_by_vpid() succeeds then delayed_put_task_struct()
can't be called until rcu_read_unlock() at least, so this task_struct has
a reference.


But I can't understand why do you need this helper... I guess I need to read
the whole series first. find_task_by_vpid(1) can never fail, but it can be
zombie... At seems we only need this task_struct for task_pid_nr(tsk) in
umh_enter_ns(tsk) ? Confused, but please ignore.

Oleg.