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Udev on Fedora

by Harald Hoyer

- This document tries to reveal the secrets of udev and how it works on Fedora. -

- udev was developed by Greg Kroah-Hartman <greg@kroah.com> with much - help from Dan Stekloff <dsteklof@us.ibm.com>, Kay Sievers <kay.sievers@vrfy.org>, - and many others. -

- The udev homepage and the - Linux-hotplug-devel mailing list https://lists.sourceforge.net/lists/listinfo/linux-hotplug-devel are the main development sources. -

- -

What Does Udev Do?

- udev provides a dynamic device directory containing only the files for - actually present devices. It creates or removes device node files usually - located in the /dev/ directory, or it renames network interfaces. -

- As part of the hotplug subsystem, udev is executed if a kernel device - is added or removed from the system. On device creation, udev reads - the sysfs directory of the given device to collect device attributes - like label, serial number or bus device number. These attributes may - be used as keys to determine a unique name for the device. udev maintains - a database for devices present on the system. - On device removal, udev queries its database for the name of the device - file to be deleted. -

- udev gets called by hotplug, if a module is loaded, and a device is added - or removed. udev looks in /sys/, if the driver provides a "dev" file, which - contains the major and minor number for a device node to communicate with - the driver. After looking in the udev rules (in the /etc/udev/rules.d/ directory), which - specify the device node filename and symlinks, a device node is created - in /dev/ with the permissions, which are specified in /etc/udev/permissions.d/. -

- After device node creation, removal, or network device renaming, udev - executes the programs in the directory tree under /etc/dev.d/. The - name of a program must end with the .dev suffix, to be recognized. - In addition to the hotplug environment variables, DEVNAME is exported - to make the name of the created node or the name the network device is - renamed to, available to the executed program. The programs in every - directory are sorted in lexical order, while the directories are - searched in the following order: -

/etc/dev.d/$(DEVNAME)/*.dev
/etc/dev.d/$(SUBSYSTEM)/*.dev
/etc/dev.d/default/*.dev

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How is Udev Integrated on Fedora?

`initrd` / `initfs`

- mkinitrd copies /sbin/udev.static - to the initrd /sbin/udev and symlinks it to - /sbin/udevstart. -

-After the kernel boots, it executes the nash script of the initrd. This -mounts a tmpfs filesystem on /dev/. Instead of hotplug /sbin/udev is -called in the initrd phase. udevstart creates all device nodes for the -devices, which are compiled in the kernel and for the modules, which -are loaded by nash.

- -

Problems

- -The whole udev and hotplug infrastructure is -not available in initrd. Thus no hotplug scripts, udev rules, and -permissions and no /etc/dev.d/ scripts are executed for any hotplug -event, which is sent from the kernel. - -

`rc.sysinit`

First, if SELinux is loaded and enabled, -the context of /dev/ is set. rc.sysinit calls /sbin/start_udev. -start_udev mounts a tmpfs filesystem on /dev/, if there is none already -mounted. Then it creates some device nodes, which need module -autoloading, or where there is no kernel module. After that -/sbin/udevstart is called again, which simulates the hotplug events in -the initrd phase, to apply the whole udev rules and permissions. After -that rc.sysinit parses the ouput of /sbin/kmodule and loads every -module. This should provide device nodes for all hardware found on your -computer.

Console User Permissions

-/etc/dev.d/default/pam_console.dev is called whenever a device node is -created and calls /sbin/pam_console_setowner with the filename (and an -optional symlink) of the device node. This sets the permissions for -console users like specified in /etc/security/console.perms.

Customizing Udev on Fedora

- Read the manpage of udev and udevinfo. - Please try not to modify the files of RPM packages. -

New Rules

-New rules should be placed in a file, which ends in .rules in -/etc/udev/rules.d/. Please do not use 50-udev.rules. The supported and -preferred way is to create rules without the "NAME" tag and only -create "SYMLINK"s.

- A nice document describing how to write rules can be found on http://www.reactivated.net/udevrules.php. -

- -

Permissions

-New permissions should be placed in a file, which ends in -.permissions in /etc/udev/permissions.d/. Please do not use -50-udev.permissions. - -

But I Really Want My Device Node!

- Put them in /etc/udev/devices/, and they will get copied to /dev/. File a bugzilla entry, if you think that should be done per default. -

- -

Updating to udev Without `/dev/`

- The steps to upgrade without Anaconda or a rescue CD are (NOT recommended): -

- -

start from a kernel-2.6 -
Make sure /sys/ is mounted -
Install the latest initscripts package -
Install the latest udev package -
Execute /sbin/start_udev -
Install the latest mkinitrd package -
Install the latest kernel package -
Or execute mkinitrd for your existing kernel(s) -

- -

Udev without `initrd`

- -

Install Fedore Core as usual and reboot. Execute the following commands -

- -

-
-mkdir /tmp/dev
-mount --move /dev /tmp/dev
-sbin/MAKEDEV null console zero
-mount --move /tmp/dev /dev
-

- Install your kernel without an initrd. Reboot. -

- You will get some SELinux errors, and syslogd will not work as expected. -

Current Problems on Fedora

- All open bugs for udev -

Nvidia

Quick solution: If you do not need rhgb, just load the nvidia module in /etc/rc.local -

- If you have udev >= 032-5, load the nvidia module: -

-
-cp -a /dev/nvidia* /etc/udev/devices
-chown root.root /etc/udev/devices/nvidia*
-

- -

The Bugzilla for this problem is ~~133900~~.

Palm Pilot

- If you have udev >= 032-5, execute the command: -

-
-ln -s ttyUSB1 /etc/udev/devices/pilot
-

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ISDN

- If you have udev >= 032-5: -

-
-/sbin/MAKEDEV -d /etc/udev/devices isdn
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