Udev

Last updated
udev
Developer(s) Greg Kroah-Hartman and Kay Sievers
Initial releaseNovember 2003;20 years ago (2003-11)
Stable release
255 [1] / 6 December 2023;5 months ago (6 December 2023)
Repository
Written in C
Operating system Linux
Type Device node
License GPLv2
Website Official website

udev (userspace /dev ) is a device manager for the Linux kernel. As the successor of devfsd and hotplug, udev primarily manages device nodes in the /dev directory. At the same time, udev also handles all user space events raised when hardware devices are added into the system or removed from it, including firmware loading as required by certain devices.

Contents

Rationale

It is an operating system's kernel that is responsible for providing an abstract interface of the hardware to the rest of the software. Being a monolithic kernel, the Linux kernel does exactly that: device drivers are part of the Linux kernel, and make up more than half of its source code. [2] Hardware can be accessed through system calls or over their device nodes.

To be able to deal with peripheral devices that are hotplug-capable in a user-friendly way, a part of handling all of these hotplug-capable hardware devices was handed over from the kernel to a daemon running in user-space. Running in user space serves security and stability purposes.

Design

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.

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 have 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 Unix systems, where the device nodes in the /dev directory have been a static set of files, the Linux udev device manager dynamically provides only the nodes for the devices actually present on a system. Although devfs used to provide similar functionality, Greg Kroah-Hartman cited a number of reasons [4] for preferring udev over devfs:

The udev, as a whole, is divided into three parts:

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 has been incorporated into systemd 183 Free and open-source-software display servers and UI toolkits.svg
udev has been incorporated into systemd 183

udev is a generic device manager running as a daemon on a Linux system and listening (via a netlink socket) to uevents the kernel sends out if a new device is initialized or a device is removed from the system. The udev package comes with an extensive set of rules that match against exported values of the event and properties of the discovered device. A matching rule will possibly name and create a device node and run configured programs to set up and configure the device.

udev rules can match on properties like the kernel subsystem, the kernel device name, the physical location of the device, or properties like the device's serial number. Rules can also request information from external programs to name a device or specify a custom name that will always be the same, regardless of the order devices are discovered by the system.

In the past a common way to use udev on Linux systems was to let it send events through a socket to HAL, which would perform further device-specific actions. For example, HAL would notify other software running on the system that the new hardware had arrived by issuing a broadcast message on the D-Bus IPC system to all interested processes. In this way, desktops such as GNOME or K Desktop Environment 3 could start the file browser to browse the file systems of newly attached USB flash drives and SD cards. [6]

By the middle of 2011 HAL had been deprecated by most Linux distributions as well as by the KDE, GNOME [7] and Xfce [8] desktop environments, among others. The functionality previously embodied in HAL has been integrated into udev itself, or moved to separate software such as udisks and upower.

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.

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 Gentoo Linux project created a fork of systemd's udev codebase in order to avoid dependency on the systemd architecture. The resulting fork is called eudev and it makes udev functionality available without systemd. A stated goal of the project is to keep eudev independent of any Linux distribution or init system. [13] The Gentoo project describes eudev as follows: [14]

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. [15] Two days later, Lennart Poettering suggested this patch be postponed until kdbus starts to be utilized by udev; at that point, the plan was to switch udev to use kdbus as the underlying messaging system, and to get rid of the userspace-to-userspace netlink-based transport. [16]

Authors

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

Related Research Articles

<span class="mw-page-title-main">Device driver</span> Software interface to attached devices

In computing, a device driver is a computer program that operates or controls a particular type of device that is attached to a computer or automaton. A driver provides a software interface to hardware devices, enabling operating systems and other computer programs to access hardware functions without needing to know precise details about the hardware being used.

D-Bus is a message-oriented middleware mechanism that allows communication between multiple processes running concurrently on the same machine. D-Bus was developed as part of the freedesktop.org project, initiated by GNOME developer Havoc Pennington to standardize services provided by Linux desktop environments such as GNOME and KDE.

<span class="mw-page-title-main">Linux kernel interfaces</span> An overview and comparison of the Linux kernel API and ABI.

The Linux kernel provides multiple interfaces to user-space and kernel-mode code that are used for varying purposes and that have varying properties by design. There are two types of application programming interface (API) in the Linux kernel:

  1. the "kernel–user space" API; and
  2. the "kernel internal" API.

The device mapper is a framework provided by the Linux kernel for mapping physical block devices onto higher-level virtual block devices. It forms the foundation of the logical volume manager (LVM), software RAIDs and dm-crypt disk encryption, and offers additional features such as file system snapshots.

<span class="mw-page-title-main">NetworkManager</span> Software

NetworkManager is a daemon that sits on top of libudev and other Linux kernel interfaces and provides a high-level interface for the configuration of the network interfaces.

In Linux systems, initrd is a scheme for loading a temporary root file system into memory, to be used as part of the Linux startup process. initrd and initramfs refer to two different methods of achieving this. Both are commonly used to make preparations before the real root file system can be mounted.

Netlink is a socket family used for inter-process communication (IPC) between both the kernel and userspace processes, and between different userspace processes, in a way similar to the Unix domain sockets available on certain Unix-like operating systems, including its original incarnation as a Linux kernel interface, as well as in the form of a later implementation on FreeBSD. Similarly to the Unix domain sockets, and unlike INET sockets, Netlink communication cannot traverse host boundaries. However, while the Unix domain sockets use the file system namespace, Netlink sockets are usually addressed by process identifiers (PIDs).

<span class="mw-page-title-main">EFI system partition</span> Partition used by Unified Extensible Firmware Interface

The EFIsystem partition or ESP is a partition on a data storage device that is used by computers that have the Unified Extensible Firmware Interface (UEFI). When a computer is booted, UEFI firmware loads files stored on the ESP to start operating systems and various utilities.

In the context of free and open-source software, proprietary software only available as a binary executable is referred to as a blob or binary blob. The term usually refers to a device driver module loaded into the kernel of an open-source operating system, and is sometimes also applied to code running outside the kernel, such as system firmware images, microcode updates, or userland programs. The term blob was first used in database management systems to describe a collection of binary data stored as a single entity.

<span class="mw-page-title-main">Greg Kroah-Hartman</span> American Linux kernel developer

Greg Kroah-Hartman is a major Linux kernel developer. As of April 2013, he is the Linux kernel maintainer for the -stable branch, the staging subsystem, USB, driver core, debugfs, kref, kobject, and the sysfs kernel subsystems, Userspace I/O, and TTY layer. He also created linux-hotplug, the udev project, and the Linux Driver Project. He worked for Novell in the SUSE Labs division and, as of 1 February 2012, works at the Linux Foundation.

HAL is a software subsystem for UNIX-like operating systems providing hardware abstraction.

mdadm is a Linux utility used to manage and monitor software RAID devices. It is used in modern Linux distributions in place of older software RAID utilities such as raidtools2 or raidtools.

The Linux booting process involves multiple stages and is in many ways similar to the BSD and other Unix-style boot processes, from which it derives. Although the Linux booting process depends very much on the computer architecture, those architectures share similar stages and software components, including system startup, bootloader execution, loading and startup of a Linux kernel image, and execution of various startup scripts and daemons. Those are grouped into 4 steps: system startup, bootloader stage, kernel stage, and init process. When a Linux system is powered up or reset, its processor will execute a specific firmware/program for system initialization, such as Power-on self-test, invoking the reset vector to start a program at a known address in flash/ROM, then load the bootloader into RAM for later execution. In personal computer (PC), not only limited to Linux-distro PC, this firmware/program is called BIOS, which is stored in the mainboard. In embedded Linux system, this firmware/program is called boot ROM. After being loaded into RAM, bootloader will execute to load the second-stage bootloader. The second-stage bootloader will load the kernel image into memory, decompress and initialize it then pass control to this kernel image. Second-stage bootloader also performs several operation on the system such as system hardware check, mounting the root device, loading the necessary kernel modules, etc. Finally, the first user-space process starts, and other high-level system initializations are performed.

GVfs is GNOME's userspace virtual filesystem designed to work with the I/O abstraction of GIO, a library available in GLib since version 2.15.1. It installs several modules that are automatically used by applications using the APIs of libgio. There is also FUSE support that allows applications not using GIO to access the GVfs filesystems.

In Unix-like operating systems, a device file, device node, or special file is an interface to a device driver that appears in a file system as if it were an ordinary file. There are also special files in DOS, OS/2, and Windows. These special files allow an application program to interact with a device by using its device driver via standard input/output system calls. Using standard system calls simplifies many programming tasks, and leads to consistent user-space I/O mechanisms regardless of device features and functions.

systemd Suite of system components for Linux

systemd is a software suite that provides an array of system components for Linux operating systems. The main aim is to unify service configuration and behavior across Linux distributions. Its primary component is a "system and service manager" – an init system used to bootstrap user space and manage user processes. It also provides replacements for various daemons and utilities, including device management, login management, network connection management, and event logging. The name systemd adheres to the Unix convention of naming daemons by appending the letter d. It also plays on the term "System D", which refers to a person's ability to adapt quickly and improvise to solve problems.

Consistent Network Device Naming is a convention for naming Ethernet adapters in Linux.

devfsd is a device manager for the Linux kernel. Primarily, it creates device nodes in the /dev directory when kernel drivers make the underlying hardware accessible. The nodes exist in a virtual device file system named devfs. In systems that support many different types of hardware, each of which has its own device nodes, this is more convenient than creating all possible device nodes beforehand and in a real filesystem.

<span class="mw-page-title-main">Kay Sievers</span>

Kay Sievers is a German computer programmer, best known for developing the udev device manager of Linux, systemd and the Gummiboot EFI bootloader. Kay Sievers made major contributions to Linux's hardware hotplug and device management subsystems. In 2012, together with Harald Hoyer, Sievers was the main driving force behind Fedora's merging of the /lib, /bin and /sbin file-system trees into /usr, a simplification which other distributions such as Arch Linux have since adopted.

<span class="mw-page-title-main">Harald Hoyer</span>

Harald Hoyer is a computer programmer and photographer, best known for developing the dracut initramfs generator and framework, the udev device manager of Linux, the systemd replacement for the System V init daemon and the Gummiboot EFI boot loader. Harald Hoyer also made various contributions to the Linux Kernel, starting 1997. In 2012, together with Kay Sievers, Hoyer was the main driving force behind merging the /lib, /bin and /sbin file system trees into /usr in the Fedora distribution

References

  1. ↑ "Release 255". 6 December 2023. Retrieved 19 December 2023.
  2. ↑ Marti, Don (2 July 2007). "Are top Linux developers losing the will to code?". ComputerworldUK. Archived from the original on 19 July 2016. Retrieved 2 June 2024.
  3. ↑ Pennington, Havoc (2003-07-10), Making Hardware Just Work
  4. ↑ Greg Kroah-Hartman. "udev and devfs - The final word". Archived from the original (Plain text) on 2011-07-09. Retrieved 2008-01-24.
  5. 1 2 3 "systemd/systemd". GitHub. Retrieved 2016-08-21.
  6. ↑ "Dynamic Device Management in Udev" (PDF). Linux Magazine. 2006-10-01. Retrieved 2008-07-14.[ permanent dead link ]
  7. ↑ "HALRemoval". 2011-06-28. Retrieved 2011-09-13.
  8. ↑ "Thunar-volman and the deprecation of HAL in Xfce". 2010-01-17. Retrieved 2017-12-25.
  9. ↑ Lennart Poettering (2010-04-25). "Relationship between udev, hal, Dbus and DeviceKit?".
  10. ↑ Sievers, Kay (2012-04-03). "The future of the udev source tree". linux-hotplug (Mailing list). Retrieved 2013-05-22.
  11. ↑ Sievers, Kay, "Commit importing udev into systemd", systemd, retrieved 2013-05-22
  12. ↑ Linus Torvalds (2012-10-03). "Re: udev breakages". linux-kernel (Mailing list). Retrieved 2014-10-28.
  13. ↑ "gentoo/eudev – README.md". GitHub . Retrieved 2017-12-25.
  14. ↑ "Gentoo Linux Projects – Gentoo eudev project" . Retrieved 2017-12-25.
  15. ↑ "[systemd-devel][PATCH] Drop the udev firmware loader". 2014-05-29.
  16. ↑ "[systemd-devel][PATCH] Drop the udev firmware loader". 2014-05-31.
This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.