linux-kernel - Re: [PATCH 2/2] pid_ns: Introduce ioctl to set vector of ns_last

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Date:   Thu, 27 Apr 2017 18:12:57 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Kirill Tkhai <ktkhai@...tuozzo.com>
Cc:     serge@...lyn.com, ebiederm@...ssion.com, agruenba@...hat.com,
        linux-api@...r.kernel.org, linux-kernel@...r.kernel.org,
        paul@...l-moore.com, viro@...iv.linux.org.uk, avagin@...nvz.org,
        linux-fsdevel@...r.kernel.org, mtk.manpages@...il.com,
        akpm@...ux-foundation.org, luto@...capital.net,
        gorcunov@...nvz.org, mingo@...nel.org, keescook@...omium.org
Subject: Re: [PATCH 2/2] pid_ns: Introduce ioctl to set vector of
 ns_last_pid's on ns hierarhy

On 04/26, Kirill Tkhai wrote:
>
> On 26.04.2017 18:53, Oleg Nesterov wrote:
> >
> >> +static long set_last_pid_vec(struct pid_namespace *pid_ns,
> >> +			     struct pidns_ioc_req *req)
> >> +{
> >> +	char *str, *p;
> >> +	int ret = 0;
> >> +	pid_t pid;
> >> +
> >> +	read_lock(&tasklist_lock);
> >> +	if (!pid_ns->child_reaper)
> >> +		ret = -EINVAL;
> >> +	read_unlock(&tasklist_lock);
> >> +	if (ret)
> >> +		return ret;
> >
> > why do you need to check ->child_reaper under tasklist_lock? this looks pointless.
> >
> > In fact I do not understand how it is possible to hit pid_ns->child_reaper == NULL,
> > there must be at least one task in this namespace, otherwise you can't open a file
> > which has f_op == ns_file_operations, no?
>
> Sure, it's impossible to pick a pid_ns, if there is no the pid_ns's tasks. I added
> it under impression of
> https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/commit/?id=dfda351c729733a401981e8738ce497eaffcaa00
> but here it's completely wrong. It will be removed in v2.

Hmm. But if I read this commit correctly then we really need to check
pid_ns->child_reaper != NULL ?

Currently we can't pick an "empty" pid_ns. But after the commit above a task
can do sys_unshare(CLONE_NEWPID), another (or the same) task can open its
/proc/$pid/ns/pid_for_children and call ns_ioctl() before the 1st alloc_pid() ?

Or I am totally confused?

Oleg.