Developing hotplug interfaces

This page describes how developers can implement an interface that supports USB hotplugging,via snapd’s built-in hotplug support.

For a general user overview, including enabling this functionality and hotplug interface management, see Hotplug support.

Hotplug support is currently under active development, and will become widely available with the release of snapd 2.39.

Adding hotplug interfaces

The code for a snapd interface has be modified to support hotplug, and subsequently create slots on the system snap, by implementing the following new function:

HotplugDeviceDetected(deviceInfo *hotplug.HotplugDeviceInfo) (*hotplug.ProposedSlot, error)

When defined, HotplugDeviceDetected is executed whenever a any hotplug device is connected to the system. The purpose of the function is to decide whether a freshly connected device is relevant for any given interface by returning one of the following:

  • if the connected device is not relevant, the function should return nil, nil
  • if the connected device is relevant, a definition of the proposed hotplug slot that needs to be created should be returned (see below)
  • if the connected device is relevant, but it’s impossible to create a slot definition for it, an error should be returned

It is important to note that this function may be called multiple times as the device connected because, in many cases, the kernel creates various pseudo- and virtual- devices for given physical device. The function should filter out irrelevant calls and only create a single slot definition for the actual device.

The returned slot is a proposed slot because snapd’s hotplug subsystem mediates the creation of the final slot. It may simply update the slot if:

  • the device is being reconnected
  • the slot previously existed and had connections

Attribute access methods

The above function receives the hotplug.HotplugDeviceInfo structure which contains all the attributes provided by the udev event for the device, and offers the following methods to access them:

  • Subsystem() string

    Returns the name of the kernel subsystem of the device, for example tty or sound.

  • DeviceName() string

    Corresponds to udev’s DEVNAME attribute and provides the actual device path, e.g. /dev/ttyUSB0. This is the path that the interface is generally interested in, as far as confinement rules are concerned.

  • DevicePath() string

    Corresponds to udev’s DEVPATH attribute and provides device path under the sysfs filesystem, e.g. /sys/devices/pci0000:00/0000:00:14.0/usb1/1-2.

  • DeviceType() string

    Corresponds to udev’s DEVTYPE attribute, e.g. disk.

  • Major() string and Minor() string

    Correspond to udev’s MAJOR and MINOR attributes which provide major/minor device numbers (if applicable).

  • Attribute(name string) (string, bool)

    A generic method to query any udev attribute. It returns false if the attribute is not present in udev event data.

The hotplug.ProposedSlot structure is created by HotplugDeviceDetected in response to a udev event, and may define a name and label for the slot.

However, they are both optional and, in most cases, should be empty. Snapd’s hotplug subsystem will automatically generate them by probing and sanitising some of the well-defined udev attributes, such as model and vendor names. This ensures the resulting name, whether provided by interface code or left empty for snapd to figure out, is unique.

However, the hotplug.ProposedSlot must have some attributes set, and in typical cases, should at least include an attribute that carries the path of the device.

The path attribute can then be read by the following methods of the interface (and others, as applicable), to create respective security profiles for the snap and the given device:

  • AppArmorConnectedSlot
  • SecCompConnectedPlug
  • SecCompConnectedSlot

Example implementation

The following is one potentially complete implementation of HotplugDeviceDetected:

func (iface *myInterface) HotplugDeviceDetected(di *hotplug.HotplugDeviceInfo) (*hotplug.ProposedSlot, error) {
	bus, _ := di.Attribute("ID_BUS”)
        // some arbitrary criteria to filter irrelevant devices out
	if di.Subsystem() != "tty" || bus != "usb” || di.Major() != „123” )  {
		return nil, nil

	slot := hotplug.ProposedSlot{
		Attrs: map[string]interface{}{
			"path": di.DeviceName(),
        return slot, nil

When creating filtering logic for the above function, it is useful to inspect udev attributes of the respective device. One way of doing this with the is with the udevadm command. This can be used to report all currently existing devices (eg. udevadm info -e) or run in monitor mode (udevadm monitor -p) to continuously report all udev event, along with their attributed, as devices connected and unplugged.

As an example, the following is an example of output from udevadm for a USB serial port adapter:

P: /devices/pci0000:00/0000:00:11.0/0000:02:00.0/usb2/2-2/2-2.1/2-2.1:1.0/ttyUSB0/tty/ttyUSB0
N: ttyUSB0
S: serial/by-id/usb-FTDI_FT232R_USB_UART_AH06W0EQ-if00-port0
S: serial/by-path/pci-0000:02:00.0-usb-0:2.1:1.0-port0
E: DEVLINKS=/dev/serial/by-id/usb-FTDI_FT232R_USB_UART_AH06W0EQ-if00-port0 /dev/serial/by-path/pci-0000:02:00.0-usb-0:2.1:1.0-port0
E: DEVNAME=/dev/ttyUSB0
E: DEVPATH=/devices/pci0000:00/0000:00:11.0/0000:02:00.0/usb2/2-2/2-2.1/2-2.1:1.0/ttyUSB0/tty/ttyUSB0
E: ID_BUS=usb
E: ID_PATH=pci-0000:02:00.0-usb-0:2.1:1.0
E: ID_PATH_TAG=pci-0000_02_00_0-usb-0_2_1_1_0
E: ID_PCI_CLASS_FROM_DATABASE=Serial bus controller
E: ID_TYPE=generic
E: ID_USB_DRIVER=ftdi_sio
E: ID_VENDOR_FROM_DATABASE=Future Technology Devices International, Ltd
E: MAJOR=188
E: TAGS=:systemd:
E: USEC_INITIALIZED=415122796440

Snapd hardware interfaces, whose slots may appear as part of their gadget.yaml definition, for example, allowing gadget in their allow-installation section of the base declaration, must additionally implement the following function:

HandledByGadget(deviceInfo *HotplugDeviceInfo, slot *snap.SlotInfo) bool

The above function acts as a predicate that should return true if the device described by deviceInfo is the same as the one represented by the given slot. It’s called by the hotplug subsystem whenever a device is connected, and it receives slot(s) of the given interface defined statically in gadget.yaml.

In the typical cases where gadget slots are defined by means of device paths, the implementation of this method becomes a simple comparison of device path and the path attribute of the slot, for example:

func (iface *myInterface) HandledByGadget(di *hotplug.HotplugDeviceInfo, slot *snap.SlotInfo) bool {
  var path string
  if err := slot.Attr("path", &path); err != nil {
    return false
  return di.DeviceName() == path

Hotplug and interface hooks

When a supported device is connected to the system, snapd creates a hotplug slot for its respective interface. If the slot is then connected to a plug, either manually by the user, or via the auto-connect mechanism, the following interface hooks are executed if they exist:

  • prepare-plug-<plugname>
  • connect-plug-<plugnname> (for the snap on the plug side)

Similarly, when the device is disconnected and its slot had been connected to a plug, its disconnect interface hooks get executed, eg:

  • disconnect-plug-<plugname>

This mechanism can be used by snaps to react to devices appearing/disappearing from their connected plugs. Please refer to the documentation on Interface Hooks for more details on this functionality.

Last updated 5 years ago.