Dynamic Kernel Module Support (DKMS) / Loadable Kernel Modules (LKM)

To compile any Kernel Modules, you first need to deploy the Kernel Headers.

DKMS

DKMS is supported by running the DKMS scripts inside a privileged container.

To deploy containers that compiles DKMS modules, you will need to ensure that you bind-mount /usr/src and /lib/modules.

To deploy containers that run any DKMS operations (i.e., modprobe), you will need to ensure that you bind-mount /lib/modules.

By default, the /lib/modules folder is already available in the console deployed via RancherOS System Services, but not /usr/src. You will likely need to deploy your own container for compilation purposes.

To learn more about Docker’s privileged mode, or to limit capabilities, please review the Docker Runtime privilege and Linux capabilities documentation.

cloud-config Example

myservice:
  image: ...
  privileged: true
  volumes:
  - /lib/modules:/lib/modules
  - /usr/src:/usr/src

Docker Example

For one-off operations, it’s useful to use --rm to clean up containers when operations complete.

$ sudo system-docker run -it --rm --name dkms-install -v /usr/src:/usr/src -v /lib/modules:/lib/modules ubuntu sh -c 'apt-get update && apt-get install -y sysdig-dkms'

The same approach can be utilized with the User Docker Daemon, just replace sudo system-docker with docker.

LKM Dependencies

In some situations, another Kernel Module might need loading prior to any module you’re trying to add.

In this example, we’ll reference the v4l2loopback DKMS module, which requires probing videodev into the Kernel space and is not on any filesystem by default.

First, you must enable kernel-extras, then modprobe your dependencies and subsequent modules:

sudo ros service enable kernel-extras
sudo ros service up -d kernel-extras

This will overlay all the compiled modules into /lib/modules/$(uname -r) that are configured in the default RancherOS Kernel config.

Now you are ready to add your Modules into the Kernel space:

sudo modprobe videodev
sudo modprobe v4l2loopback

To see which modules are pre-built, you can either do a listing of all .ko (kernel object) files, or review the Kernel config:

find /lib*/modules/$(uname -r) -name *.ko | less
#or
zcat /proc/config.gz | less

For more information regarding modifying the Kernel, please review the Custom Kernels documentation.

Auto-Loading Modules

Kernel Modules can be automatically loaded with the rancher.modules cloud-config field.

#cloud-config
rancher:
  modules: [btrfs]

This functionality is also available via a kernel parameter. For example, the btrfs module could be automatically loaded with rancher.modules=[btrfs] as a kernel parameter.

Ubuntu-based Kernel Manipulation

For images that are or derive from Ubuntu, you will need some small packages for depmod(kmod) and modprobe(module-init-tools):

sudo apt-get install kmod module-init-tools

Most packages should already list these as dependencies in Aptitude, as well as gcc and related libs for packages that require compilation (which is most).

Troubleshooting

Messing around with the Kernel can be tricky, so here’s some common issues:

kernel source for this kernel does not seem to be installed.

Simply put, the Kernel Headers (or Source) cannot be found; enable them via the Kernel Headers System Service.

Operation not Permitted

When inside a container, you might see similar to the following:
modprobe: ERROR: could not insert 'videodev': Operation not permitted

This is in reference to your container’s privileges, not your user (i.e., sudo will not fix this).

Instead, ensure you started the container with --privileged or the cloud-config setting described above.

modprobe: ERROR: could not insert ‘v4l2loopback’: Unknown symbol in module, or unknown parameter (see dmesg)

Again, using v4l2loopback as an example, but this can happen for any module.

As stated, check out dmesg to see what the issue is. Chances are you’ll see something like the following:

[  322.734052] v4l2loopback: module verification failed: signature and/or required key missing - tainting kernel
[  322.734141] v4l2loopback: Unknown symbol video_ioctl2 (err 0)
[  322.734454] v4l2loopback: Unknown symbol v4l2_ctrl_handler_init_class (err 0)
[  322.734526] v4l2loopback: Unknown symbol video_devdata (err 0)
[  322.734563] v4l2loopback: Unknown symbol v4l2_ctrl_new_custom (err 0)
[  322.734599] v4l2loopback: Unknown symbol video_unregister_device (err 0)
[  322.734635] v4l2loopback: Unknown symbol video_device_alloc (err 0)
[  322.734696] v4l2loopback: Unknown symbol v4l2_device_register (err 0)
[  322.734732] v4l2loopback: Unknown symbol __video_register_device (err 0)
[  322.734765] v4l2loopback: Unknown symbol v4l2_ctrl_handler_free (err 0)
[  322.734796] v4l2loopback: Unknown symbol v4l2_device_unregister (err 0)
[  322.734828] v4l2loopback: Unknown symbol video_device_release (err 0)

This one can be trickier to evaluate, so start searching Google for symbol names to figure out which modules they derive from.

In this example, video_ioctl2 comes from videodev and can be simply inserted via the kernel-extras overlay described above.

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