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 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.
+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:
- - Version 2.6.19 of the Linux kernel for reliable operation of this release of
- udev. The kernel may have a requirement on udev too, see Documentation/Changes
- in the kernel source tree for the actual dependency.
+ - Version 2.6.27 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_USER=y
+ CONFIG_SIGNALFD=y
+ CONFIG_TMPFS_POSIX_ACL=y (user ACLs for device nodes)
+ CONFIG_BLK_DEV_BSG=y (SCSI devices)
- - The kernel must have sysfs, unix domain sockets and networking enabled.
- (unix domain sockets (CONFIG_UNIX) as a loadable kernel module may work,
- but it does not make any sense - don't complain if anything goes wrong.)
+ - For reliable operations, the kernel must not use the CONFIG_SYSFS_DEPRECATED*
+ option.
- - The proc filesystem must be mounted on /proc/, the sysfs filesystem must
- be mounted at /sys/. No other locations are supported by udev.
+ - 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
+ Udev creates and removes device nodes in /dev, based on events the kernel
sends out on device discovery or removal.
- - Very early in the boot process, the /dev/ directory should get a 'tmpfs'
- filesystem mounted, which is populated from scratch by udev. Created nodes
- or changed permissions will not survive a reboot, which is intentional.
+ - 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,
+ - 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 old hotplug helper /sbin/hotplug should be disabled on bootup, before
- actions like loading kernel modules are taken, which may cause a lot of
- events.
-
- - The udevd daemon must be started on bootup to receive netlink uevents
- from the kernel driver core.
+ - 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
+ /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/concern to the linux-hotplug mailing list at:
+Please direct any comment/question to the linux-hotplug mailing list at:
linux-hotplug@vger.kernel.org
~ianmdlvl / elogind.git / blobdiff