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 19:17:51 +0300
From:   Kirill Tkhai <ktkhai@...tuozzo.com>
To:     Oleg Nesterov <oleg@...hat.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 27.04.2017 19:12, Oleg Nesterov wrote:
> 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() ?

Another task can't open /proc/$pid/ns/pid_for_children before the 1st alloc_pid(),
because pid_for_children is available to open only after the 1st alloc_pid().
So, it's impossible to call ioctl() on it.
 
> Or I am totally confused?