Tags: openshift, cgroups, systemd

systemd containers on OpenShift with cgroups v2

systemd in a container is a practical reality of migrating nontrivial applications to container infrastructure. It is not the “cloud native” way, but many applications written in The Before Times cannot be broken up and rearchitected without a huge cost. And so, there is a demand to run containers that run systemd, which in turn manages application services.

FreeIPA is one example. Its traditional environment is a dedicated Linux server (ignoring replicas). There are many services which both interact among themselves, and process requests from external clients and other FreeIPA servers. The engineering effort to redesign FreeIPA as a suite of several containerised services is expected to be very high. Therefore our small team focused on bringing FreeIPA to OpenShift therefore decided to pursue the “monolithic container” approach.

Support for systemd containers in OpenShift, without hacks, is a prerequisite for this approach to viable. In this post I experiment with systemd containers in OpenShift and share my results.

Test application: HTTP server §

To test systemd containers on OpenShift, I created a Fedora-based container running the nginx HTTP server. I enable the nginx systemd and set the default command to /sbin/init, which is systemd. The server doesn’t host any interesting content, but if it responds to requests we know that systemd is working.

The Containerfile definition is:

FROM fedora:33-x86_64
RUN dnf -y install nginx && dnf clean all && systemctl enable nginx
EXPOSE 80
CMD [ "/sbin/init" ]

I built the container on my workstation and tagged it test-nginx. To check that the container works, I ran it locally and performed an HTTP request via curl:

% podman run --detach --publish 8080:80 test-nginx
2d8059e555c821d9ffcccd84bee88996207794957696c54e8d29787e8c33fab3

% curl --head localhost:8080
HTTP/1.1 200 OK
Server: nginx/1.18.0
Date: Thu, 25 Mar 2021 00:22:23 GMT
Content-Type: text/html
Content-Length: 5564
Last-Modified: Mon, 27 Jul 2020 22:20:49 GMT
Connection: keep-alive
ETag: "5f1f5341-15bc"
Accept-Ranges: bytes

% podman kill 2d8059e5
2d8059e555c821d9ffcccd84bee88996207794957696c54e8d29787e8c33fab3

The container works properly in podman. I proceed to testing it on OpenShift.

Running (privileged user) §

I performed my testing on an OpenShift 4.8 nightly cluster. The exact build is 4.8.0-0.nightly-2021-03-26-010831. As far as I’m aware, with respect to systemd and cgroups there are no major differences between OpenShift 4.7 (which is Generally Available) and the build I’m using. So results should be similar on OpenShift 4.7.

The Pod definition for my test service is:

apiVersion: v1
kind: Pod
metadata:
  name: nginx
  labels:
    app: nginx
spec:
  containers:
  - name: nginx
    image: quay.io/ftweedal/test-nginx:latest

I create the Pod, operating with the cluster admin credential. After a few seconds, the pod is running:

% oc create -f pod-nginx.yaml 
pod/nginx created

% oc get -o json pod/nginx | jq .status.phase
"Running"

Verifying that the service is working §

pod/nginx is running, but it is not exposed to other pods in the cluster, or to the outside world. To test that the server is working, I will expose it on the hostname nginx.apps.ft-48dev-5.idmocp.lab.eng.rdu2.redhat.com. First, observe that performing an HTTP request from my workstation fails because the service is not available:

% curl --head nginx.apps.ft-48dev-5.idmocp.lab.eng.rdu2.redhat.com
HTTP/1.0 503 Service Unavailable
pragma: no-cache
cache-control: private, max-age=0, no-cache, no-store
content-type: text/html

Now I create Service and Route objects to expose the nginx server. The Service definition is:

apiVersion: v1
kind: Service
metadata:
  name: nginx
spec:
  selector:
    app: nginx
  ports:
    - protocol: TCP
      port: 80

And the Route definition is:

apiVersion: v1
kind: Route
metadata:
  name: nginx
spec:
  host: nginx.apps.ft-48dev-5.idmocp.lab.eng.rdu2.redhat.com
  to:
    kind: Service
    name: nginx

I create the objects:

% oc create -f service-nginx.yaml 
service/nginx created

% oc create -f route-nginx.yaml
route.route.openshift.io/nginx created

After a few seconds I performed the HTTP request again, and it succeeded:

% curl --head nginx.apps.ft-48dev-5.idmocp.lab.eng.rdu2.redhat.com
HTTP/1.1 200 OK
server: nginx/1.18.0
date: Tue, 30 Mar 2021 08:16:23 GMT
content-type: text/html
content-length: 5564
last-modified: Mon, 27 Jul 2020 22:20:49 GMT
etag: "5f1f5341-15bc"
accept-ranges: bytes
set-cookie: 6cf5f3bc2fa4d24f45018c591d3617c3=6f2f093d36d535f1dde195e08a311bda; path=/; HttpOnly
cache-control: private

This confirms that the systemd container is running properly on OpenShift 4.8.

Low-level details §

Now I will inspect some low-level details of the container. I’ll do that in a debug shell on the worker node. So first, I query the pod’s worker node name and container ID:

% oc get -o json pod/nginx \
    | jq '.spec.nodeName,
          .status.containerStatuses[0].containerID'
"ft-48dev-5-f24l6-worker-0-q7lff"
"cri-o://d9d106cb65e4c965737ef66f15bd5b9e0988c386675e3404e24fd36e58284638"

Now I enter a debug shell on the worker node:

% oc debug node/ft-48dev-5-f24l6-worker-0-q7lff
Starting pod/ft-48dev-5-f24l6-worker-0-q7lff-debug ...
To use host binaries, run `chroot /host`
Pod IP: 10.8.1.64
If you don't see a command prompt, try pressing enter.
sh-4.2# chroot /host
sh-4.4# 

I use crictl to query the namespaces of the container:

sh-4.4# crictl inspect d9d106 \
        | jq .info.runtimeSpec.linux.namespaces[].type
"pid"
"network"
"ipc"
"uts"
"mount"

Observe that there are pid and mount namespaces (among others), but no cgroup namespace. The worker node and container are using cgroups v1.

The container_manage_cgroup SELinux boolean is off:

sh-4.4# getsebool container_manage_cgroup
container_manage_cgroup --> off

Now let’s see what processes are running in the container. We can query the PID of the initial container process via crictl inspect. Then I use pgrep(1) with the --ns option, which lists processes executing in the same namespace(s) as the specified PID:

sh-4.4# crictl inspect d9d106 | jq .info.pid
14591

sh-4.4# pgrep --ns 14591 | xargs ps -o user,pid,cmd --sort pid
USER         PID CMD
root       14591 /sbin/init
root       14625 /usr/lib/systemd/systemd-journald
systemd+   14636 /usr/lib/systemd/systemd-resolved
root       14642 /usr/lib/systemd/systemd-homed
root       14643 /usr/lib/systemd/systemd-logind
root       14646 /sbin/agetty -o -p -- \u --noclear --keep-baud console 115200,38400,9600 xterm
dbus       14647 /usr/bin/dbus-broker-launch --scope system --audit
dbus       14651 dbus-broker --log 4 --controller 9 --machine-id 2f2fcc4033c5428996568ca34219c72a --max-bytes 536870912 --max-fds 4096 --max-matches 16384 --audit
root       14654 nginx: master process /usr/sbin/nginx
polkitd    14655 nginx: worker process
polkitd    14656 nginx: worker process
polkitd    14657 nginx: worker process
polkitd    14658 nginx: worker process
polkitd    14659 nginx: worker process
polkitd    14660 nginx: worker process
polkitd    14661 nginx: worker process
polkitd    14662 nginx: worker process

The PID column shows the PIDs from the point of view of the host’s PID namespace. The first process (PID 1 inside the container) is systemd (/sbin/init). systemd has started other system services, and also nginx.

systemd is running as root on the host. The other processes run under various system accounts. The container does not have its own user namespace. This pod was created by a privileged account, which allows it to run as root on the host.

Running (unprivileged user) §

I created an unprivileged user called test, and granted it admin privileges (so it can create pods).

% oc create user test
user.user.openshift.io/test created 

% oc adm policy add-role-to-user admin test
clusterrole.rbac.authorization.k8s.io/admin added: "test"

I did not grant to the test account any Security Context Constraints (SCCs) that would allow it to run privileged containers or use host user accounts (including root).

Now I create the same nginx pod, as this user test. The pod fails to execute:

% oc --as test create -f pod-nginx.yaml
pod/nginx created

% oc get pod/nginx
NAME    READY   STATUS             RESTARTS   AGE
nginx   0/1     CrashLoopBackOff   1          23s

Let’s inspect the logs to see what went wrong:

% oc logs pod/nginx
%

There is no output. This baffled me, at first. Eventually I learned that Kubernetes, by default, does not allocate pseudo-terminal devices to containers. You can overcome this on a per-container basis by including tty: true in the Container object definition:

apiVersion: v1
kind: Pod
metadata:
  name: nginx
  labels:
    app: nginx
spec:
  containers:
  - name: nginx
    image: quay.io/ftweedal/test-nginx:latest
    tty: true

With the pseudo-terminal enabled, oc logs now shows the error output:

% oc logs pod/nginx
systemd v246.10-1.fc33 running in system mode. (+PAM +AUDIT +SELINUX +IMA -APPARMOR +SMACK +SYSVINIT +UTMP +LIBCRYPTSETUP +GCRYPT +GNUTLS +ACL +XZ +LZ4 +ZSTD +SECCOMP +BLKID +ELFUTILS +KMOD +IDN2 -IDN +PCRE2 default-hierarchy=unified)
Detected virtualization container-other.
Detected architecture x86-64.

Welcome to Fedora 33 (Container Image)!

Set hostname to <nginx>.
Failed to write /run/systemd/container, ignoring: Permission denied
Failed to create /kubepods.slice/kubepods-besteffort.slice/kubepods-besteffort-pod3bbed45f_634a_4f60_bb07_5f080c483f0f.slice/crio-90dead4cf549b844c4fb704765edfbba9e9e188b30299f484906f15d22b29fbd.scope/init.scope control group: Permission denied
Failed to allocate manager object: Permission denied
[!!!!!!] Failed to allocate manager object.
Exiting PID 1...

The user executing systemd does not have permissions to write the cgroup filesystem. Although cgroups are heirarchical, cgroups v1 does not support delegating management of part of the heirarchy to unprivileged users. But cgroups v2 does support this.

Set the SYSTEMD_LOG_LEVEL environment variable to info or debug to get more detail in the systemd log output.

Enabling cgroups v2 §

We can enable cgroups v2 (only) on worker nodes via the following MachineConfig object:

apiVersion: machineconfiguration.openshift.io/v1
kind: MachineConfig
metadata:
  name: enable-cgroupv2-workers
  labels:
    machineconfiguration.openshift.io/role: worker
spec:
  kernelArguments:
    - systemd.unified_cgroup_hierarchy=1
    - cgroup_no_v1="all"
    - psi=1

After creating the MachineConfig, the Machine Config Operator applies the configuration change and restarts each worker node, one by one. This occurs over several minutes.

Running (unprivileged; cgroups v2) §

After some time, all worker nodes have the updated kernel configuration to enable cgroups v2 and disable cgroups v1. I again created the pod as the unprivileged test user. And again, pod execution failed. But this time the error is different:

% oc --as test create -f pod-nginx.yaml
pod/nginx created

% oc get pod
NAME    READY   STATUS   RESTARTS   AGE
nginx   0/1     Error    1          12s

% oc logs pod/nginx
systemd v246.10-1.fc33 running in system mode. (+PAM +AUDIT +SELINUX +IMA -APPARMOR +SMACK +SYSVINIT +UTMP +LIBCRYPTSETUP +GCRYPT +GNUTLS +ACL +XZ +LZ4 +ZSTD +SECCOMP +BLKID +ELFUTILS +KMOD +IDN2 -IDN +PCRE2 default-hierarchy=unified)
Detected virtualization container-other.
Detected architecture x86-64.

Welcome to Fedora 33 (Container Image)!

Set hostname to <nginx>.
Failed to write /run/systemd/container, ignoring: Permission denied
Failed to create /init.scope control group: Permission denied
Failed to allocate manager object: Permission denied
[!!!!!!] Failed to allocate manager object.
Exiting PID 1...

The error suggests that the container now has its own cgroup namespace. I can confirm it by creating a pod debug container…

% oc debug pod/nginx
Starting pod/nginx-debug ...
Pod IP: 10.130.2.10
If you don't see a command prompt, try pressing enter.
sh-5.0$

…finding out the node and container ID…

% oc get -o json pod/nginx-debug \
    | jq '.spec.nodeName,
          .status.containerStatuses[0].containerID'
"ft-48dev-5-f24l6-worker-0-qv7kq"
"cri-o://e870d022d1c53adf94e36877312fcfef5ef0431ad9cf1fbe9c9d2ace02bee858"

…and analysing the container sandbox in a node debug shell:

sh-4.4# crictl inspect e870d02 \
        | jq .info.runtimeSpec.linux.namespaces[].type
"pid"
"network"
"ipc"
"uts"
"mount"
"cgroup"

The output confirms that the pod has a cgroup namespace. Despite this, the unprivileged user running systemd in the container does not have permission to manage the namespace. The oc logs output demonstrates this.

container_manage_cgroups SELinux boolean §

I have one more thing to try. The container_manage_cgroups SELinux boolean was disabled on the worker nodes (per default configuration). Perhaps it is still needed, even when using cgroups v2. I enabled it on the worker node (directly from the debug shell, for now):

sh-4.4# setsebool container_manage_cgroup on

I again created the nginx pod as the test user. It failed with the same error as the previous attempt, when container_manage_cgroup was off. So that was not the issue, or at least not the immediate issue.

Next steps §

At this point, I have successfully enabled cgroups v2 on worker nodes. Container sandboxes have their own cgroup namespace. But inside the container, systemd fails with permission errors when it attempts some cgroup management.

The next step is to test the systemd container in OpenShift with cgroups v2 enabled and user namespaces enabled. Both of these features are necessary for securely running a complex, systemd-based application in OpenShift. My hope is that enabling them together is the last step to getting systemd-based containers working properly in OpenShift. I will investigate and report the results in an upcoming post.

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