linux-kernel - Re: [PATCHv2 7/7] cgroup: mount cgroupns-root when inside non-init cgroupns

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Date:	Mon, 3 Nov 2014 16:17:11 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	Aditya Kali <adityakali@...gle.com>
Cc:	Tejun Heo <tj@...nel.org>, Li Zefan <lizefan@...wei.com>,
	Serge Hallyn <serge.hallyn@...ntu.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	cgroups@...r.kernel.org,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Linux API <linux-api@...r.kernel.org>,
	Ingo Molnar <mingo@...hat.com>,
	Linux Containers <containers@...ts.linux-foundation.org>,
	Rohit Jnagal <jnagal@...gle.com>
Subject: Re: [PATCHv2 7/7] cgroup: mount cgroupns-root when inside non-init cgroupns

On Mon, Nov 3, 2014 at 4:12 PM, Aditya Kali <adityakali@...gle.com> wrote:
> On Mon, Nov 3, 2014 at 3:48 PM, Andy Lutomirski <luto@...capital.net> wrote:
>> On Mon, Nov 3, 2014 at 3:23 PM, Aditya Kali <adityakali@...gle.com> wrote:
>>> On Mon, Nov 3, 2014 at 3:15 PM, Andy Lutomirski <luto@...capital.net> wrote:
>>>> On Mon, Nov 3, 2014 at 3:12 PM, Aditya Kali <adityakali@...gle.com> wrote:
>>>>> On Fri, Oct 31, 2014 at 5:07 PM, Andy Lutomirski <luto@...capital.net> wrote:
>>>>>> On Fri, Oct 31, 2014 at 12:19 PM, Aditya Kali <adityakali@...gle.com> wrote:
>>>>>>>         if (opts->flags & CGRP_ROOT_SANE_BEHAVIOR) {
>>>>>>>                 pr_warn("sane_behavior: this is still under development and its behaviors will change, proceed at your own risk\n");
>>>>>>> -               if (nr_opts != 1) {
>>>>>>> +               if (nr_opts > 1) {
>>>>>>>                         pr_err("sane_behavior: no other mount options allowed\n");
>>>>>>>                         return -EINVAL;
>>>>>>
>>>>>> This looks wrong.  But, if you make the change above, then it'll be right.
>>>>>>
>>>>>
>>>>> It would have been nice if simple 'mount -t cgroup cgroup <mnt>' from
>>>>> cgroupns does the right thing automatically.
>>>>>
>>>>
>>>> This is a debatable point, but it's not what I meant.  Won't your code
>>>> let 'mount -t cgroup -o one_evil_flag cgroup mountpoint' through?
>>>>
>>>
>>> I don't think so. This check "if (nr_opts > 1)" is nested under "if
>>> (opts->flags & CGRP_ROOT_SANE_BEHAVIOR)". So we know that there is
>>> atleast 1 option ('__DEVEL__sane_behavior') present (implicit or not).
>>> Addition of 'one_evil_flag' will make nr_opts = 2 and result in EINVAL
>>> here.
>>
>> But the implicit __DEVEL__sane_behavior doesn't increment nr_opts, right?
>>
>
> Yes. Hence this change makes sure that we don't return EINVAL when
> nr_opts == 0 or nr_opts == 1 :)
> That way, both of the following are equivalent when inside non-init cgroupns:
>
> (1) $ mount -t cgroup -o __DEVEL__sane_behavior cgroup mountpoint
> (2) $ mount -t cgroup cgroup mountpoint
>
> Any other mount option will trigger the error here.

I still don't get it.  Can you walk me through why mount -o
some_other_option -t cgroup cgroup mountpoint causes -EINVAL?

--Andy
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