udev

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udev (userspace

Device drivers are part of the Linux kernel, in which their primary functions include device discovery, detecting device state changes, and similar low-level hardware functions. After loading a device driver into memory from the kernel, detected events are sent out to the userspace daemon udevd. It is the device manager, udevd, that catches all of these events and then decides what shall happen next. For this, udevd has a very comprehensive set of configuration files, which can all be adjusted by the computer administrator, according to their needs.

• In case a new storage device is connected over USB, udevd is notified by the kernel and itself notifies the udisksd-daemon. That daemon could then mount the file systems.
• In case a new Ethernet cable is plugged into the Ethernet NIC, udevd is notified by the kernel and itself notifies the NetworkManager-daemon. The NetworkManager-daemon could start dhclient for that NIC, or configure according to some manual configuration.

The complexity of doing so forced application authors to re-implement hardware support logic. Some hardware devices also required privileged helper programs to prepare them for use. These often has to be invoked in ways that could be awkward to express with the Unix permissions model (for example, allowing users to join wireless networks only if they are logged into the video console). Application authors resorted to using setuid binaries or run service daemons to provide their own access control and privilege separation, potentially introducing security holes each time.^[3]

HAL was created to deal with these challenges, but is now deprecated in most Linux distributions, its functionality being replaced by udevd.

Overview

Unlike traditional

• Library libudev that allows access to device information; it was incorporated into the systemd 183 software bundle.^[5]
• User space daemon udevd that manages the virtual /dev.
• Administrative
  command-line utility
  udevadm for diagnostics.

The system gets calls from the kernel via netlink socket. Earlier versions used hotplug, adding a link to themselves in /etc/hotplug.d/default with this purpose.

Operation

udev is a generic device manager running as a

• udev provides low-level access to the linux device tree. Allows programs to enumerate devices and their properties and get notifications when devices come and go.
• dbus is a framework to allow programs to communicate with each other, securely, reliably, and with a high-level object-oriented programming interface.
• udisks (formerly known as DeviceKit-disks) is a daemon that sits on top of libudev and other kernel interfaces and provides a high-level interface to storage devices and is accessible via dbus to applications.
• upower (formerly known as DeviceKit-power) is a daemon that sits on top of libudev and other kernel interfaces and provides a high-level interface to power management and is accessible via dbus to applications.
• NetworkManager is a daemon that sits on top of libudev and other kernel interfaces (and a couple of other daemons) and provides a high-level interface to network configuration and setup and is accessible via dbus to apps.^[9]

udev receives messages from the kernel, and passes them on to subsystem daemons such as Network Manager. Applications talk to Network Manager over D-Bus.

${\displaystyle {\ce {Kernel -> udev -> Network Manager <=> D-Bus <=> Firefox}}}$

HAL is obsolete and only used by legacy code. Ubuntu 10.04 shipped without HAL. Initially a new daemon DeviceKit was planned to replace certain aspects of HAL, but in March 2009, DeviceKit was deprecated in favor of adding the same code to udev as a package: udev-extras, and some functions have now moved to udev proper.

History

udev was introduced in Linux 2.5. The Linux kernel version 2.6.13 introduced or updated a new version of the uevent interface. A system using a new version of udev will not boot with kernels older than 2.6.13 unless udev is disabled and a traditional /dev directory is used for device access.

In April 2012, udev's codebase was merged into the systemd source tree, making systemd 183 the first version to include udev.^[5][10][11] In October 2012, Linus Torvalds criticized Kay Sievers's approach to udev maintenance and bug fixing related to firmware loading, stating:^[12]

Yes, doing it in the kernel is "more robust". But don't play games, and stop the lying. It's more robust because we have maintainers that care, and because we know that regressions are not something we can play fast and loose with. If something breaks, and we don't know what the right fix for that breakage is, we revert the thing that broke. So yes, we're clearly better off doing it in the kernel. Not because firmware loading cannot be done in user space. But simply because udev maintenance since Greg gave it up has gone downhill.

In 2012, the

eudev is a fork of systemd-udev with the goal of obtaining better compatibility with existing software such as OpenRC and Upstart, older kernels, various toolchains and anything else required by users and various distributions.

On May 29, 2014, support for firmware loading through udev was dropped from systemd, as it has been decided that it is the kernel's task to load firmware.

udev was developed by Greg Kroah-Hartman and Kay Sievers, with much help from Dan Stekloff, among others.

References

External links

• Official website