udev - userspace device management
-For more information see the files in the docs/ directory.
+Integrating udev in the system has complex dependencies and differs from distro
+to distro. All major distros depend on udev these days and the system may not
+work without a properly installed version. The upstream udev project does not
+recommend to replace a distro's udev installation with the upstream version.
-Important Note:
- Integrating udev in the system is a whole lot of work, has complex dependencies
- and differs a lot from distro to distro. All reasonable distros depend on udev
- these days and the system will not work without it.
-
- The upstream udev project does not support or recomend to replace a distro's udev
- installation with the upstream version. The installation of a unmodified upstream
- version may render your system unusable! There is no "default" setup or a set
- of "default" rules provided by the upstream udev version.
+Tools and rules shipped by udev are not public API and may change at any time.
+Never call any private tool in /lib/udev from any external application, it might
+just go away in the next release. Access to udev information is only offered
+by udevadm and libudev. Tools and rules in /lib/udev, and the entire content of
+the /dev/.udev directory is private to udev and does change whenever needed.
Requirements:
- - 2.6 version of the Linux kernel.
-
- - The kernel must have sysfs, netlink, and hotplug enabled.
-
- - The proc filesystem must be mounted on /proc.
-
- - The sysfs filesystem must be mounted at /sys. No other location
- is supported.
-
+ - Version 2.6.25 of the Linux kernel with sysfs, procfs, signalfd, inotify,
+ unix domain sockets, networking and hotplug enabled:
+ CONFIG_HOTPLUG=y
+ CONFIG_UEVENT_HELPER_PATH=""
+ CONFIG_NET=y
+ CONFIG_UNIX=y
+ CONFIG_SYSFS=y
+ CONFIG_SYSFS_DEPRECATED*=n
+ CONFIG_PROC_FS=y
+ CONFIG_TMPFS=y
+ CONFIG_INOTIFY=y
+ CONFIG_SIGNALFD=y
+ CONFIG_TMPFS_POSIX_ACL=y (user ACLs for device nodes)
+ CONFIG_BLK_DEV_BSG=y (SCSI devices)
+
+ - For reliable operations, the kernel must not use the CONFIG_SYSFS_DEPRECATED*
+ option.
+
+ - Unix domain sockets (CONFIG_UNIX) as a loadable kernel module may work,
+ but it is not supported.
+
+ - The proc filesystem must be mounted on /proc, the sysfs filesystem must
+ be mounted at /sys. No other locations are supported by udev.
+
+ - The system must have the following group names resolvable at udev startup:
+ disk, cdrom, floppy, tape, audio, video, lp, tty, dialout, kmem.
+ Especially in LDAP setups, it is required, that getgrnam() is able to resolve
+ these group names with only the rootfs mounted, and while no network is
+ available.
+
+ - To build all udev extras, libacl, libglib2, libusb, usbutils, pciutils,
+ gperf are needed. These dependencies can be disabled with the
+ --disable-extras option.
Operation:
- - Udev creates and removes device nodes in /dev based on events
- the kernel sends out on device discovery or removal
-
- - Directly after mounting the root filesystem, the udevd daemon must be
- started by an init script.
-
- - From kernel version 2.6.15 on, the hotplug helper /sbin/hotplug should
- be disabled with an init script before kernel modules are loaded.
-
- - During bootup, /dev usually gets a tmpfs filesystem mounted which is
- populated from scratch by udev (created nodes don't survive a reboot,
- the /lib/udev/devices directory should be used for "static nodes").
-
- - Udev replaces the hotplug event management invoked from /sbin/hotplug
- by the udevd daemon, which receives the kernel events over netlink.
-
- - All kernel events are matched against a set of specified rules which
- make it possible to hook into the event processing.
-
- - Copies of the rules files for all major distros are in the etc/udev
- directory (you may look there how others distros are doing it).
-
-Compile Options:
- prefix
- Set this to the default root that you want to use only override
- this if you really know what you are doing even then, you probably
- don't do the right thing.
- DESTDIR
- Prefix for install target, used for package building.
- USE_LOG
- if set to 'true', udev is able to pass errors or debug information
- to syslog. This is very useful to see what udev is doing or not doing,
- it is enabled by default.
- DEBUG
- If set to 'true', very verbose debugging messages will be compiled
- into the udev binaries. The actual level of debugging is specified
- in the udev config file.
- STRIPCMD
- If udev is compiled for packaging an empty string can be passed
- to disable the stripping of the binaries.
- USE_SELINUX
- If set to 'true', udev will be built with SELinux support
- enabled. This is disabled by default.
- USE_KLIBC
- If set to 'true', udev is built and linked against klibc.
- Default value is 'false'. KLCC specifies the klibc compiler
- wrapper, usually located at /usr/bin/klcc.
- EXTRAS
- If set, will build the "extra" helper programs as specified
- as listed (see below for an example).
-
-If you want to build the udev helper program cdrom_id and scsi_id:
- make EXTRAS="extras/cdrom_id extras/scsi_id"
-
-Installation:
- - The install target intall the udev binaries in the default locations,
- all at boot time reqired binaries will be installed in /sbin.
-
- - The default location for scripts and binaries that are called from
- rules is /lib/udev.
-
- - It is recommended to use the /lib/udev/devices directory to place
- device nodes and symlinks in, which are copied to /dev at every boot.
- That way, nodes for broken subsystems or devices which can't be
- detected automatically by the kernel will always be available.
-
-Please direct any comment/question/concern to the linux-hotplug-devel mailing list at:
- linux-hotplug-devel@lists.sourceforge.net
+ Udev creates and removes device nodes in /dev, based on events the kernel
+ sends out on device discovery or removal.
+
+ - Early in the boot process, the /dev directory should get a 'tmpfs'
+ filesystem mounted, which is maintained by udev. Created nodes or changed
+ permissions will not survive a reboot, which is intentional.
+
+ - The content of /lib/udev/devices directory which contains the nodes,
+ symlinks and directories, which are always expected to be in /dev, should
+ be copied over to the tmpfs mounted /dev, to provide the required nodes
+ to initialize udev and continue booting.
+
+ - The deprecated hotplug helper /sbin/hotplug should be disabled in the
+ kernel configuration, it is not needed today, and may render the system
+ unusable because the kernel may create too many processes in parallel
+ so that the system runs out-of-memory.
+
+ - All kernel events are matched against a set of specified rules in
+ /lib/udev/rules.d which make it possible to hook into the event
+ processing to load required kernel modules and setup devices. For all
+ devices the kernel exports a major/minor number, udev will create a
+ device node with the default kernel name, or the one specified by a
+ matching udev rule.
+
+Please direct any comment/question to the linux-hotplug mailing list at:
+ linux-hotplug@vger.kernel.org
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