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

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Date:	Mon, 3 Nov 2014 16:12:53 -0800
From:	Aditya Kali <adityakali@...gle.com>
To:	Andy Lutomirski <luto@...capital.net>
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 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.


> --Andy

-- 
Aditya
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