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<h1>Udev on Fedora</h1>
<h2>by Harald Hoyer</h2>
<p>
This document tries to reveal the secrets of udev and how it works on Fedora.
</p>
<p>
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.
</p>
<p>
The <a href="http://www.kernel.org/pub/linux/utils/kernel/hotplug/udev.html">udev homepage</a> and the
Linux-hotplug-devel mailing list <a href="https://lists.sourceforge.net/lists/listinfo/linux-hotplug-devel">https://lists.sourceforge.net/lists/listinfo/linux-hotplug-devel</a> are the main development sources.
</p><p>
</p>
<h2>What Does Udev Do?</h2>
<p>
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 <tt class="filename">/dev/</tt> directory, or it renames network interfaces.
</p><p>
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.
</p><p>
udev gets called by hotplug, if a module is loaded, and a device is added
or removed. udev looks in <tt class="filename">/sys/</tt>, 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 <tt class="filename">/etc/udev/rules.d/</tt> directory), which
specify the device node filename and symlinks, a device node is created
in <tt class="filename">/dev/</tt> with the permissions, which are specified in <tt class="filename">/etc/udev/permissions.d/</tt>.
</p>
After device node creation, removal, or network device renaming, udev
executes the programs in the directory tree under <tt class="filename">/etc/dev.d/</tt>. The
name of a program must end with the <tt class="filename">.dev</tt> 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:
<ul>
<li><tt class="filename">/etc/dev.d/$(DEVNAME)/*.dev</tt></li>
<li><tt class="filename">/etc/dev.d/$(SUBSYSTEM)/*.dev</tt></li>
<li><tt class="filename">/etc/dev.d/default/*.dev</tt></li>
</ul>
<h2>How is Udev Integrated on Fedora?</h2>
<h3><tt class="command">initrd</tt> / <tt class="command">initfs</tt></h3>
<p>
<tt class="command">mkinitrd</tt> copies <tt class="filename">/sbin/udev.static</tt>
to the <tt class="command">initrd</tt> <tt class="filename">/sbin/udev</tt> and symlinks it to
<tt class="filename">/sbin/udevstart</tt>.
</p><p>
After the kernel boots, it executes the nash script of the <tt class="command">initrd</tt>. This
mounts a tmpfs filesystem on <tt class="filename">/dev/</tt>. Instead of hotplug <tt class="command">/sbin/udev</tt> is
called in the <tt class="command">initrd</tt> phase. udevstart creates all device nodes for the
devices, which are compiled in the kernel and for the modules, which
are loaded by nash. </p>
<h4>Problems</h4>
The whole udev and hotplug infrastructure is
not available in <tt class="command">initrd</tt>. Thus no hotplug scripts, udev rules, and
permissions and no <tt class="filename">/etc/dev.d/</tt> scripts are executed for any hotplug
event, which is sent from the kernel.
<h3><tt class="filename">rc.sysinit</tt></h3>
<p> First, if SELinux is loaded and enabled,
the context of <tt class="filename">/dev/</tt> is set. <tt class="filename">rc.sysinit</tt> calls <tt class="filename">/sbin/start_udev</tt>.
<tt class="filename">start_udev</tt> mounts a tmpfs filesystem on <tt class="filename">/dev/</tt>, 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
<tt class="command">/sbin/udevstart</tt> is called again, which simulates the hotplug events in
the <tt class="command">initrd</tt> phase, to apply the whole udev rules and permissions. After
that <tt class="filename">rc.sysinit</tt> parses the ouput of <tt class="filename">/sbin/kmodule</tt> and loads every
module. This should provide device nodes for all hardware found on your
computer. </p>
<h3>Console User Permissions</h3>
<p>
<tt class="filename">/etc/dev.d/default/pam_console.dev</tt> is called whenever a device node is
created and calls <tt class="filename">/sbin/pam_console_setowner</tt> with the filename (and an
optional symlink) of the device node. This sets the permissions for
console users like specified in <tt class="filename">/etc/security/console.perms</tt>. </p>
<h2>Customizing Udev on Fedora</h2>
<p>
Read the manpage of udev and udevinfo.
Please try not to modify the files of RPM packages.
</p>
<h3>New Rules</h3>
<p>
New rules should be placed in a file, which ends in <tt class="filename">.rules</tt> in
<tt class="filename">/etc/udev/rules.d/</tt>. Please do not use <tt class="filename">50-udev.rules</tt>. The supported and
preferred way is to create rules without the "NAME" tag and only
create "SYMLINK"s. </p><p>
A nice document describing how to write rules can be found on <a href="http://www.reactivated.net/udevrules.php">http://www.reactivated.net/udevrules.php</a>.
</p>
<h3>Permissions</h3>
New permissions should be placed in a file, which ends in
<tt class="filename">.permissions</tt> in <tt class="filename">/etc/udev/permissions.d/</tt>. Please do not use
<tt class="filename">50-udev.permissions</tt>.
<h3>But I Really Want My Device Node!</h3>
<p>
Put them in <tt class="filename">/etc/udev/devices/</tt>, and they will get copied to <tt class="filename">/dev/</tt>. <a href="https://bugzilla.redhat.com/bugzilla">File a bugzilla entry</a>, if you think that should be done per default.
</p>
<h2>Updating to udev Without <tt class="filename">/dev/</tt></h2>
<p>
The steps to upgrade without Anaconda or a rescue CD are (NOT recommended):
</p>
<ul>
<li>start from a kernel-2.6
</li><li>Make sure <tt class="filename">/sys/</tt> is mounted
</li><li>Install the latest <tt class="filename">initscripts</tt> package
</li><li>Install the latest <tt class="filename">udev</tt> package
</li><li>Execute <tt class="command">/sbin/start_udev</tt>
</li><li>Install the latest <tt class="filename">mkinitrd</tt> package
</li><li>Install the latest <tt class="filename">kernel</tt> package
</li><li>Or execute <tt class="command">mkinitrd</tt> for your existing kernel(s)
</li></ul>
<h2>Udev without <tt class="command">initrd</tt></h2>
<p>Install Fedore Core as usual and reboot. Execute the following commands
</p>
<pre class="screen">
<tt class="command">
mkdir /tmp/dev
mount --move /dev /tmp/dev
sbin/MAKEDEV null console zero
mount --move /tmp/dev /dev
</tt></pre>
Install your kernel without an <tt class="command">initrd</tt>. Reboot.
<p>
You will get some SELinux errors, and syslogd will not work as expected.
</p>
<h2>Current Problems on Fedora</h2>
<p>
<a href="http://bugzilla.redhat.com/bugzilla/buglist.cgi?short_desc_type=allwordssubstr&component=udev&bug_status=ASSIGNED&bug_status=MODIFIED&bug_status=NEEDINFO&bug_status=NEW&bug_status=REOPENED&bug_status=VERIFIED&bug_severity=high&bug_severity=low&bug_severity=normal&bug_severity=security&bug_severity=translation&long_desc_type=allwordssubstr&bug_file_loc_type=allwordssubstr&status_whiteboard_type=allwordssubstr&fixed_in_type=allwordssubstr&devel_whiteboard_type=allwordssubstr&keywords_type=allwords&cust_facing=YES&emailassigned_to1=1&emailtype1=exact&emailreporter2=1&emailtype2=exact&bugidtype=include&chfieldto=Now&cmdtype=doit&remaction=run&namedcmd=blank&order=Reuse+same+sort+as+last+time&field0-0-0=noop&type0-0-0=noop">All open bugs for <code class="filename">udev</code></a>
</p>
<h3>Nvidia</h3>
<p>Quick solution: If you do not need rhgb, just load the nvidia module in <tt class="filename">/etc/rc.local</tt>
</p><p>
If you have udev >= 032-5, load the nvidia module:
</p>
<pre class="screen">
<tt class="command">
cp -a /dev/nvidia* /etc/udev/devices
chown root.root /etc/udev/devices/nvidia*
</tt></pre>
<p>The Bugzilla for this problem is <strike><a href="https://bugzilla.redhat.com/bugzilla/show_bug.cgi?id=133900">133900</a></strike>.</p>
<h3>Palm Pilot</h3>
<p>
If you have udev >= 032-5, execute the command:
</p>
<pre class="screen">
<tt class="command">
ln -s ttyUSB1 /etc/udev/devices/pilot
</tt></pre>
<h3>ISDN</h3>
<p>
If you have udev >= 032-5:
</p>
<pre class="screen">
<tt class="command">
/sbin/MAKEDEV -d /etc/udev/devices isdn
</tt></pre>
</pre>
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