Re: [PATCHv1 0/8] CGroup Namespaces

From: Andy Lutomirski
Date: Tue Oct 14 2014 - 18:43:39 EST


On Mon, Oct 13, 2014 at 2:23 PM, Aditya Kali <adityakali@xxxxxxxxxx> wrote:
> Second take at the Cgroup Namespace patch-set.
>
> Major changes form RFC (V0):
> 1. setns support for cgroupns
> 2. 'mount -t cgroup cgroup <mntpt>' from inside a cgroupns now
> mounts the cgroup hierarcy with cgroupns-root as the filesystem root.
> 3. writes to cgroup files outside of cgroupns-root are not allowed
> 4. visibility of /proc/<pid>/cgroup is further restricted by not showing
> anything if the <pid> is in a sibling cgroupns and its cgroup falls outside
> your cgroupns-root.
>
> More details in the writeup below.
>
> Background
> Cgroups and Namespaces are used together to create âvirtualâ
> containers that isolates the host environment from the processes
> running in container. But since cgroups themselves are not
> âvirtualizedâ, the task is always able to see global cgroups view
> through cgroupfs mount and via /proc/self/cgroup file.
>
> $ cat /proc/self/cgroup
> 0:cpuset,cpu,cpuacct,memory,devices,freezer,hugetlb:/batchjobs/c_job_id1
>
> This exposure of cgroup names to the processes running inside a
> container results in some problems:
> (1) The container names are typically host-container-management-agent
> (systemd, docker/libcontainer, etc.) data and leaking its name (or
> leaking the hierarchy) reveals too much information about the host
> system.
> (2) It makes the container migration across machines (CRIU) more
> difficult as the container names need to be unique across the
> machines in the migration domain.
> (3) It makes it difficult to run container management tools (like
> docker/libcontainer, lmctfy, etc.) within virtual containers
> without adding dependency on some state/agent present outside the
> container.
>
> Note that the feature proposed here is completely different than the
> âns cgroupâ feature which existed in the linux kernel until recently.
> The ns cgroup also attempted to connect cgroups and namespaces by
> creating a new cgroup every time a new namespace was created. It did
> not solve any of the above mentioned problems and was later dropped
> from the kernel. Incidentally though, it used the same config option
> name CONFIG_CGROUP_NS as used in my prototype!
>
> Introducing CGroup Namespaces
> With unified cgroup hierarchy
> (Documentation/cgroups/unified-hierarchy.txt), the containers can now
> have a much more coherent cgroup view and its easy to associate a
> container with a single cgroup. This also allows us to virtualize the
> cgroup view for tasks inside the container.
>
> The new CGroup Namespace allows a process to âunshareâ its cgroup
> hierarchy starting from the cgroup its currently in.
> For Ex:
> $ cat /proc/self/cgroup
> 0:cpuset,cpu,cpuacct,memory,devices,freezer,hugetlb:/batchjobs/c_job_id1
> $ ls -l /proc/self/ns/cgroup
> lrwxrwxrwx 1 root root 0 2014-07-15 10:37 /proc/self/ns/cgroup -> cgroup:[4026531835]
> $ ~/unshare -c # calls unshare(CLONE_NEWCGROUP) and execâs /bin/bash
> [ns]$ ls -l /proc/self/ns/cgroup
> lrwxrwxrwx 1 root root 0 2014-07-15 10:35 /proc/self/ns/cgroup ->
> cgroup:[4026532183]
> # From within new cgroupns, process sees that its in the root cgroup
> [ns]$ cat /proc/self/cgroup
> 0:cpuset,cpu,cpuacct,memory,devices,freezer,hugetlb:/
>
> # From global cgroupns:
> $ cat /proc/<pid>/cgroup
> 0:cpuset,cpu,cpuacct,memory,devices,freezer,hugetlb:/batchjobs/c_job_id1
>
> # Unshare cgroupns along with userns and mountns
> # Following calls unshare(CLONE_NEWCGROUP|CLONE_NEWUSER|CLONE_NEWNS), then
> # sets up uid/gid map and execâs /bin/bash
> $ ~/unshare -c -u -m
>
> # Originally, we were in /batchjobs/c_job_id1 cgroup. Mount our own cgroup
> # hierarchy.
> [ns]$ mount -t cgroup cgroup /tmp/cgroup
> [ns]$ ls -l /tmp/cgroup
> total 0
> -r--r--r-- 1 root root 0 2014-10-13 09:32 cgroup.controllers
> -r--r--r-- 1 root root 0 2014-10-13 09:32 cgroup.populated
> -rw-r--r-- 1 root root 0 2014-10-13 09:25 cgroup.procs
> -rw-r--r-- 1 root root 0 2014-10-13 09:32 cgroup.subtree_control
>
> The cgroupns-root (/batchjobs/c_job_id1 in above example) becomes the
> filesystem root for the namespace specific cgroupfs mount.
>
> The virtualization of /proc/self/cgroup file combined with restricting
> the view of cgroup hierarchy by namespace-private cgroupfs mount
> should provide a completely isolated cgroup view inside the container.
>
> In its current form, the cgroup namespaces patcheset provides following
> behavior:
>
> (1) The ârootâ cgroup for a cgroup namespace is the cgroup in which
> the process calling unshare is running.
> For ex. if a process in /batchjobs/c_job_id1 cgroup calls unshare,
> cgroup /batchjobs/c_job_id1 becomes the cgroupns-root.
> For the init_cgroup_ns, this is the real root (â/â) cgroup
> (identified in code as cgrp_dfl_root.cgrp).
>
> (2) The cgroupns-root cgroup does not change even if the namespace
> creator process later moves to a different cgroup.
> $ ~/unshare -c # unshare cgroupns in some cgroup
> [ns]$ cat /proc/self/cgroup
> 0:cpuset,cpu,cpuacct,memory,devices,freezer,hugetlb:/
> [ns]$ mkdir sub_cgrp_1
> [ns]$ echo 0 > sub_cgrp_1/cgroup.procs
> [ns]$ cat /proc/self/cgroup
> 0:cpuset,cpu,cpuacct,memory,devices,freezer,hugetlb:/sub_cgrp_1
>
> (3) Each process gets its CGROUPNS specific view of
> /proc/<pid>/cgroup.
> (a) Processes running inside the cgroup namespace will be able to see
> cgroup paths (in /proc/self/cgroup) only inside their root cgroup
> [ns]$ sleep 100000 & # From within unshared cgroupns
> [1] 7353
> [ns]$ echo 7353 > sub_cgrp_1/cgroup.procs
> [ns]$ cat /proc/7353/cgroup
> 0:cpuset,cpu,cpuacct,memory,devices,freezer,hugetlb:/sub_cgrp_1
>
> (b) From global cgroupns, the real cgroup path will be visible:
> $ cat /proc/7353/cgroup
> 0:cpuset,cpu,cpuacct,memory,devices,freezer,hugetlb:/batchjobs/c_job_id1/sub_cgrp_1

This is a little weird. Not sure it's a problem.

>
> (c) From a sibling cgroupns (cgroupns root-ed at a sibling cgroup), no cgroup
> path will be visible:
> # ns2's cgroupns-root is at '/batchjobs/c_job_id2'
> [ns2]$ cat /proc/7353/cgroup
> [ns2]$
> This is same as when cgroup hierarchy is not mounted at all.
> (In correct container setup though, it should not be possible to
> access PIDs in another container in the first place.)
>
> (4) Processes inside a cgroupns are not allowed to move out of the
> cgroupns-root. This is true even if a privileged process in global
> cgroupns tries to move the process out of its cgroupns-root.
>
> # From global cgroupns
> $ cat /proc/7353/cgroup
> 0:cpuset,cpu,cpuacct,memory,devices,freezer,hugetlb:/batchjobs/c_job_id1/sub_cgrp_1
> # cgroupns-root for 7353 is /batchjobs/c_job_id1
> $ echo 7353 > batchjobs/c_job_id2/cgroup.procs
> -bash: echo: write error: Operation not permitted
>

>
> (6) When some thread from a multi-threaded process unshares its
> cgroup-namespace, the new cgroupns gets applied to the entire
> process (all the threads). This should be OK since
> unified-hierarchy only allows process-level containerization. So
> all the threads in the process will have the same cgroup. And both
> - changing cgroups and unsharing namespaces - are protected under
> threadgroup_lock(task).

This seems odd to me. Does unsharing the cgroupns unshare for all
tasks in the process? If not, then I think that it shouldn't change
the cgroup either.

What did you end up doing to grant permission to unshare the cgroup ns?

--Andy
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