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4b6769f612
We must never access random devices in /dev which do not belong to the event we are handling. Hard-coding /dev/hidrawX, and looping over all devices is absolutely not acceptable --> hook into hidraw events. We can not relay on (rather random) properties merged into the parent device by earlier rules --> use libudev to find the sibling device with a matching interface. Libusb does not fit into udev's use case. We never want want to scan and open() all usb devices in the system, just to find the device we are already handling the event for --> put all the stupid scanning into a single function and prepare for a fixed libusb or drop it later.
udev - userspace device management
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
/dev/.udev/ is private to udev.
Requirements:
- 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 tape devices)
- For reliable operation, the kernel must not use the CONFIG_SYSFS_DEPRECATED*
option.
- Unix domain sockets (CONFIG_UNIX) as a loadable kernel module 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.
- 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 old hotplug helper /sbin/hotplug should be disabled in the kernel
configuration, it is not needed, and may render the system unusable
because of a fork-bombing behavior.
- 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/concern to the linux-hotplug mailing list at:
linux-hotplug@vger.kernel.org