Re: [PATCHv1 7/8] cgroup: cgroup namespace setns support

[Aditya Kali]

On Tue, Oct 21, 2014 at 12:02 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
> On Tue, Oct 21, 2014 at 11:49 AM, Aditya Kali <adityakali@xxxxxxxxxx> wrote:
>> On Mon, Oct 20, 2014 at 10:49 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
>>> On Mon, Oct 20, 2014 at 10:42 PM, Eric W. Biederman
>>> <ebiederm@xxxxxxxxxxxx> wrote:
>>>>
>>>> I do wonder if we think of this as chcgrouproot if there is a simpler
>>>> implementation.
>>>
>>> Could be.  I'll defer to Aditya for that one.
>>>
>>
>> More than chcgrouproot, its probably closer to pivot_cgroup_root. In
>> addition to restricting the process to a cgroup-root, new processes
>> entering the container should also be implicitly contained within the
>> cgroup-root of that container.
>
> Why?  Concretely, why should this be in the kernel namespace code
> instead of in userspace?
>

Userspace can do it too. Though then there will be possibility of
having processes in the same mount namespace with different
cgroup-roots. Deriving contents of /proc/<pid>/cgroup becomes even
more complex. Thats another reason why it might not be good idea to
tie cgroups with mount namespace.

>> Implementing pivot_cgroup_root would
>> probably involve overloading mount-namespace to now understand cgroup
>> filesystem too. I did attempt combining cgroupns-root with mntns
>> earlier (not via a new syscall though), but came to the conclusion
>> that its just simpler to have a separate cgroup namespace and get
>> clear semantics. One of the issues was that implicitly changing cgroup
>> on setns to mntns seemed like a huge undesirable side-effect.
>>
>> About pinning: I really feel that it should be OK to pin processes
>> within cgroupns-root. I think thats one of the most important feature
>> of cgroup-namespace since its most common usecase is to containerize
>> un-trusted processes - processes that, for their entire lifetime, need
>> to remain inside their container.
>
> So don't let them out.  None of the other namespaces have this kind of
> constraint:
>
>  - If you're in a mntns, you can still use fds from outside.
>  - If you're in a netns, you can still use sockets from outside the namespace.
>  - If you're in an ipcns, you can still use ipc handles from outside.

But none of the namespaces allow you to allocate new fds/sockets/ipc
handles in the outside namespace. I think moving a process outside of
cgroupns-root is like allocating a resource outside of your namespace.

>
> etc.

>
>> And with explicit permission from
>> cgroup subsystem (something like cgroup.may_unshare as you had
>> suggested previously), we can make sure that unprivileged processes
>> cannot pin themselves. Also, maintaining this invariant (your current
>> cgroup is always under your cgroupns-root) keeps the code and the
>> semantics simple.
>
> I actually think it makes the semantics more complex.  The less policy
> you stick in the kernel, the easier it is to understand the impact of
> that policy.
>

My inclination is towards keeping things simpler - both in code as
well as in configuration. I agree that cgroupns might seem
"less-flexible", but in its current form, it encourages consistent
container configuration. If you have a process that needs to move
around between cgroups belonging to different containers, then that
process should probably not be inside any container's cgroup
namespace. Allowing that will just make the cgroup namespace
pretty-much meaningless.

>>
>> If we ditch the pinning requirement and allow the containarized
>> process to move outside of its cgroupns-root, we will have to address
>> atleast the following:
>> * what does its /proc/self/cgroup  (and /proc/<pid>/cgroup in general)
>> look like? We might need to just not show anything in
>> /proc/<pid>/cgroup in such case (for default hierarchy).
>
> The process should see the cgroup path relative to its cgroup ns.
> Whether this requires a new /proc mount or happens automatically is an
> open question.  (I *hate* procfs for reasons like this.)
>
>> * how should future setns() and unshare() by such process behave?
>
> Open question.
>
>> * 'mount -t cgroup cgroup <mnt>' by such a process will yield unexpected result
>
> You could disallow that and instead require 'mount -t cgroup -o
> cgrouproot=. cgroup mnt' where '.' will be resolved at mount time
> relative to the caller's cgroupns.
>
>> * container will not remain migratable
>
> Why not?
>

Well, the processes running outside of cgroupns root will be exposed
to information outside of the container (i.e., its /proc/self/cgroup
will show paths involving other containers and potentially system
level information). So unless you even restore them, it will be
difficult to restore these processes. The whole point of virtualizing
the /proc/self/cgroup view was so that the processes don't see outside
cgroups.

>> * added code complexity to handle above scenarios
>>
>> I understand that having process pinned to a cgroup hierarchy might
>> seem inconvenient. But even today (without cgroup namespaces), moving
>> a task from one cgroup to another can fail for reasons outside of
>> control of the task attempting the move (even if its privileged). So
>> the userspace should already handle this scenario. I feel its not
>> worth to add complexity in the kernel for this.
>
> --Andy



-- 
Aditya
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