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

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Message-ID: <CAGr1F2Ecd3vCkDHVpFevZDLt+J49MimYh0va9u7JEJVTLHq8uQ@mail.gmail.com>
Date:	Mon, 3 Nov 2014 16:49:41 -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 4:17 PM, Andy Lutomirski <luto@...capital.net> wrote:
> 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?
>

Argh! You are right. I was totally convinced that this works. But it
clearly doesn't if you specify 1 legit mount option. I wanted to make
it work for both cases (1) and (2) above. But then this check will
have to be changed :(
Sorry about the back and forth. I am just going to make it return
EINVAL if __DEVEL_sane_behavior is not specified as suggested in the
beginning.

> --Andy

-- 
Aditya
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