USB hotplug for USB serial devices

Product support balenaOS
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1

What is the current recommended way to handle USB serial devices being plugged in?

I am working on porting an open source IoT gateway to balenaOS for the commercial pilot of an IoT hub product. The gateway has an add-ons system which adds support for a wide range of hardware including Zigbee and Z-Wave USB dongles (e.g. the Aeotec Z-Stick Z-Wave dongle or the Digi XStick or Conbee II Zigbee dongle).

On Raspbian the add-ons can automatically detect USB serial devices that are plugged into the device at runtime and use them.

Running inside a Docker container on balenaOS I can statically map a device in docker-compose.yml if I know what path it will be mounted at, e.g.

    devices:
      - /dev/ttyACM0:/dev/ttyACM0

However, if that device is not plugged in when the Docker image starts up it will simply refuse to start and the (headless) gateway becomes inoperable.

Ideally I would like to be able to dynamically detect USB dongles when they are plugged in using USB hotplug and automatically map them into the Docker container.

If I could create a bind mount to just bind the whole of the host’s /dev directory then I could configure device_cgroup_rules to allow the kernel to access certain classes of device, and perhaps use cap_add to add SYS_RAWIO for low level access (I do not want the container to be running privileged mode since that negates most of the security benefits of using a container), e.g.

    volumes:
      - '/dev:/dev' # Mount the host's dev directory
    device_cgroup_rules:
      - 'c 188:* rmw' # Allow USB serial devices (XBee, Conbee III, Sonoff)
      - 'c 166:* rmw' # Allow ACM devices (Conbee II)
    cap_add:
        - SYS_RAWIO

However, since bind mounts are not allowed this is not possible.

I can imagine some elaborate sidecar container that runs in privileged mode and pipes serial interfaces into unix sockets, but that would require re-architecting my whole gateway application and its 100 or so adapter add-ons in order for it to run on balenaOS.

Is there a recommended way to detect USB devices being plugged in via USB hotplug and automatically map them into a container?

2

Hello @benfrancis,

Well we do have a way to let the container use the whole /dev, however I believe it does need to be privileged:

Ideally I would like to be able to dynamically detect USB dongles when they are plugged in using USB hotplug and automatically map them into the Docker container.

I think that could be doable with a set of carefully crafted udev rules with proper access rights and clever volume management.

If you start this work and report here, we can try to guide you to the best possible outcome.

Regards,

Yann