Kay Sievers
19b66dc57c
Udev does no longer automatically create udev rules in /etc from the device hotplug path. No device name reservation will happen anymore; this model creates too many problems for setups with many device changes or media which is booted on different hardware. Enumerated device names which are based on device discovery order or on persistent on-disk name reservation will in general not be supported by udev in the future. It is a problem that can not be solved properly, and it always creates new problems at the same time it tries to solve the original one. Udev will no longer pretend it can solve these issues, and people should switch to available alternatives which provide the far better compromise. From now on, udev will only create /dev/cdrom for the first optical drive, and if the drive is capable /dev/dvd. No other devices will get any compatibility symlinks or enumerated device names like cdrom1, cdrom2, and so on. The /dev/cdrom and /dev/dvd links have by default a negative link priority, which will cause them to be overwritten by any other device which clains the same names with already existing udev rules. If stable device names are needed, the /dev/disk/by-id/ links, which uniquely identify a specific piece of hardware should be used. The links usually contain a device serial number and the link names will not depend on device discovery order. If completely identical devices with identical or no serial number need to be handled at the same time, the /dev/disk/by-path/ links can be used. These links depend on the physical port which is used to connect the device. It will change when the same device is moved to a different port or host adapter. If custom names are needed, custom udev rules which match on specific device properties need to be added by the administrator.
udev - Linux userspace device management
Integrating udev in the system has complex dependencies and may differ from
distribution to distribution. A system may not be able to boot up or work
reliably without a properly installed udev version. The upstream udev project
does not recommend replacing a distro's udev installation with the upstream
version.
The upstream udev project's set of default rules may require a most recent
kernel release to work properly.
Tools and rules shipped by udev are not public API and may change at any time.
Never call any private tool in /usr/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 /usr/lib/udev and the entire contents
of the /run/udev directory are private to udev and do change whenever needed.
Requirements:
- Version 2.6.34 of the Linux kernel with sysfs, procfs, signalfd, inotify,
unix domain sockets, networking and hotplug enabled
- Some architectures might need a later kernel, that supports accept4(),
or need to backport the accept4() syscall wiring in the kernel.
- These options are required:
CONFIG_DEVTMPFS=y
CONFIG_HOTPLUG=y
CONFIG_INOTIFY_USER=y
CONFIG_NET=y
CONFIG_PROC_FS=y
CONFIG_SIGNALFD=y
CONFIG_SYSFS=y
CONFIG_SYSFS_DEPRECATED*=n
CONFIG_UEVENT_HELPER_PATH=""
- These options might be needed:
CONFIG_BLK_DEV_BSG=y (SCSI devices)
CONFIG_TMPFS_POSIX_ACL=y (user ACLs for device nodes)
- The /dev directory needs the 'devtmpfs' filesystem mounted.
Udev only manages the permissions and ownership of the
kernel-provided device nodes, and possibly creates additional symlinks.
- Udev requires /run to be writable, which is usually done by mounting a
'tmpfs' filesystem.
- This version of udev does not work properly with the CONFIG_SYSFS_DEPRECATED*
option enabled.
- 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.
- The proc filesystem must be mounted on /proc, and the sysfs filesystem must
be mounted at /sys. No other locations are supported by a standard
udev installation.
- The default rule sset requires the following group names resolvable at udev startup:
disk, cdrom, floppy, tape, audio, video, lp, tty, dialout, and kmem.
Especially in LDAP setups, it is required that getgrnam() be able to resolve
these group names with only the rootfs mounted and while no network is
available.
- Some udev extras have external dependencies like:
libglib2, usbutils, pciutils, and gperf.
All these extras can be disabled with configure options.
Setup:
- The udev daemon should be started to handle device events sent by the kernel.
During bootup, the events for already existing devices can be replayed, so
that they are configured by udev. The systemd service files contain the
needed commands to start the udev daemon and the coldplug sequence.
- Restarting the daemon never applies any rules to existing devices.
- New/changed rule files are picked up automatically; there is usually no
daemon restart or signal needed.
Operation:
- Based on events the kernel sends out on device creation/removal, udev
creates/removes device nodes and symlinks in the /dev directory.
- All kernel events are matched against a set of specified rules, which
possibly hook into the event processing and load required kernel
modules to set up devices. For all devices, the kernel exports a major/minor
number; if needed, udev creates a device node with the default kernel
device name. If specified, udev applies permissions/ownership to the device
node, creates additional symlinks pointing to the node, and executes
programs to handle the device.
- The events udev handles, and the information udev merges into its device
database, can be accessed with libudev:
http://www.kernel.org/pub/linux/utils/kernel/hotplug/libudev/
http://www.kernel.org/pub/linux/utils/kernel/hotplug/gudev/
For more details about udev and udev rules, see the udev man pages:
http://www.kernel.org/pub/linux/utils/kernel/hotplug/udev/
Please direct any comment/question to the linux-hotplug mailing list at:
linux-hotplug@vger.kernel.org