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97 lines
4.4 KiB
Plaintext
97 lines
4.4 KiB
Plaintext
udev - Linux userspace device management
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Integrating udev in the system has complex dependencies and may differ from
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distribution to distribution. A system may not be able to boot up or work
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reliably without a properly installed udev version. The upstream udev project
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does not recommend replacing a distro's udev installation with the upstream
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version.
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The upstream udev project's set of default rules may require a most recent
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kernel release to work properly. This is currently version 2.6.31.
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Tools and rules shipped by udev are not public API and may change at any time.
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Never call any private tool in /lib/udev from any external application; it might
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just go away in the next release. Access to udev information is only offered
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by udevadm and libudev. Tools and rules in /lib/udev and the entire contents of
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the /dev/.udev directory are private to udev and do change whenever needed.
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Requirements:
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- Version 2.6.27 of the Linux kernel with sysfs, procfs, signalfd, inotify,
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unix domain sockets, networking and hotplug enabled:
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CONFIG_HOTPLUG=y
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CONFIG_UEVENT_HELPER_PATH=""
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CONFIG_NET=y
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CONFIG_UNIX=y
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CONFIG_SYSFS=y
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CONFIG_SYSFS_DEPRECATED*=n
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CONFIG_PROC_FS=y
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CONFIG_TMPFS=y
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CONFIG_INOTIFY_USER=y
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CONFIG_SIGNALFD=y
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CONFIG_TMPFS_POSIX_ACL=y (user ACLs for device nodes)
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CONFIG_BLK_DEV_BSG=y (SCSI devices)
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- Udev does not work with the CONFIG_SYSFS_DEPRECATED* option.
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- Unix domain sockets (CONFIG_UNIX) as a loadable kernel module may work,
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but it is not supported.
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- The deprecated hotplug helper /sbin/hotplug should be disabled in the
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kernel configuration, it is not needed today, and may render the system
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unusable because the kernel may create too many processes in parallel
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so that the system runs out-of-memory.
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- The proc filesystem must be mounted on /proc, and the sysfs filesystem must
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be mounted at /sys. No other locations are supported by a standard
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udev installation.
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- The system must have the following group names resolvable at udev startup:
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disk, cdrom, floppy, tape, audio, video, lp, tty, dialout, and kmem.
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Especially in LDAP setups, it is required that getgrnam() be able to resolve
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these group names with only the rootfs mounted and while no network is
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available.
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- The 'udev extras' has the following dependencies:
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libacl, libglib2, libusb, usbutils, pciutils, and gperf.
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These dependencies can be disabled with the --disable-extras configure option.
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Setup:
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- At bootup, the /dev directory should get the 'devtmpfs' filesystem
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mounted. Udev manages the permissions and ownership of the kernel-created
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device nodes, and udev possibly creates additional symlinks. If needed, udev also
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works on an empty 'tmpfs' filesystem, but some static device nodes like
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/dev/null, /dev/console, /dev/kmsg are needed to be able to start udev itself.
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- The udev daemon should be started to handle device events sent by the kernel.
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During bootup, the kernel can be asked to send events for all already existing
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devices so that they too can be configured by udev. This is usually done by:
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/sbin/udevadm trigger --type=subsystems
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/sbin/udevadm trigger --type=devices
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- Restarting the daemon never applies any rules to existing devices.
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- New/changed rule files are picked up automatically; there is no daemon
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restart or signal needed.
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Operation:
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- Udev creates/removes device nodes in /dev, based on events the kernel
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sends out on device creation/removal.
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- All kernel events are matched against a set of specified rules, which
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possibly hook into the event processing and load required kernel
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modules to setup devices. For all devices the kernel exports a major/minor
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number; if needed, udev creates a device node with the default kernel
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name. If specified, udev applies permissions/ownership to the device
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node, creates additional symlinks pointing to the node, and executes
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programs to handle the device.
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- The events udev handles, and the information udev merges into its device
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database, can be accessed with libudev:
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http://www.kernel.org/pub/linux/utils/kernel/hotplug/libudev/
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http://www.kernel.org/pub/linux/utils/kernel/hotplug/gudev/
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For more details about udev and udev rules see the udev(7) man page.
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Please direct any comment/question to the linux-hotplug mailing list at:
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linux-hotplug@vger.kernel.org
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