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  57                 <h1>Udev on Fedora</h1>
  58                 <h2>by Harald Hoyer</h2>
  59                 <p>
  60                 This document tries to reveal the secrets of udev and how it works on Fedora.
  61                 </p>
  62                 <p>
  63                 udev was developed by Greg  Kroah-Hartman  &lt;greg@kroah.com&gt;  with  much
  64                 help  from  Dan  Stekloff &lt;dsteklof@us.ibm.com&gt;, Kay Sievers &lt;kay.sievers@vrfy.org&gt;,
  65                  and many others.
  66                 </p>
  67                 <p>
  68                 The <a href="http://www.kernel.org/pub/linux/utils/kernel/hotplug/udev.html">udev homepage</a> and the
  69                 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.
  70                 </p><p>
  71                 </p>
  72
  73                 <h2>What Does Udev Do?</h2>
  74                 <p>
  75                 udev  provides a dynamic device directory containing only the files for
  76                 actually present devices. It creates or removes device node files  usually
  77                 located in the <tt class="filename">/dev/</tt> directory, or it renames network interfaces.
  78                 </p><p>
  79                 As  part  of the hotplug subsystem, udev is executed if a kernel device
  80                 is added or removed from the system.  On device  creation,  udev  reads
  81                 the  sysfs  directory  of the given device to collect device attributes
  82                 like label, serial number or bus device number.  These  attributes  may
  83                 be  used as keys to determine a unique name for the device.  udev maintains
  84                 a database for devices present on the system.
  85                 On device removal, udev queries its database for the name of the device
  86                 file to be deleted.
  87                 </p><p>
  88                 udev gets called by hotplug, if a module is loaded, and a device is added
  89                 or removed. udev looks in <tt class="filename">/sys/</tt>, if the driver provides a "dev" file, which
  90                 contains the major and minor number for a device node to communicate with
  91                 the driver. After looking in the udev rules (in the <tt class="filename">/etc/udev/rules.d/</tt> directory), which
  92                 specify the device node filename and symlinks, a device node is created
  93                 in <tt class="filename">/dev/</tt> with the permissions, which are specified in <tt class="filename">/etc/udev/permissions.d/</tt>.
  94                 </p>
  95                 After device node creation, removal, or network device  renaming,  udev
  96                 executes  the  programs  in  the directory tree under <tt class="filename">/etc/dev.d/</tt>.  The
  97                 name of a program must end with the <tt class="filename">.dev</tt> suffix, to be recognized.
  98                 In addition to the hotplug environment variables, DEVNAME  is  exported
  99                 to make the name of the created node or the name the network device is
 100                 renamed to, available to the executed program. The  programs  in  every
 101                 directory  are  sorted  in  lexical  order,  while  the directories are
 102                 searched in the following order:
 103                 <ul>
 104                 <li><tt class="filename">/etc/dev.d/$(DEVNAME)/*.dev</tt></li>
 105                 <li><tt class="filename">/etc/dev.d/$(SUBSYSTEM)/*.dev</tt></li>
 106                 <li><tt class="filename">/etc/dev.d/default/*.dev</tt></li>
 107                 </ul>
 108
 109                 <h2>How is Udev Integrated on Fedora?</h2>
 110                 <h3><tt class="command">initrd</tt> / <tt class="command">initfs</tt></h3>
 111                 <p>
 112                 <tt class="command">mkinitrd</tt> copies <tt class="filename">/sbin/udev.static</tt>
 113                 to the <tt class="command">initrd</tt> <tt class="filename">/sbin/udev</tt> and symlinks it to
 114                 <tt class="filename">/sbin/udevstart</tt>.
 115                 </p><p>
 116 After the kernel boots, it executes the nash script of the <tt class="command">initrd</tt>. This
 117 mounts a tmpfs filesystem on <tt class="filename">/dev/</tt>. Instead of hotplug <tt class="command">/sbin/udev</tt> is
 118 called in the <tt class="command">initrd</tt> phase. udevstart creates all device nodes for the
 119 devices, which are compiled in the kernel and for the modules, which
 120 are loaded by nash. </p>
 121
 122 <h4>Problems</h4>
 123
 124 The whole udev and hotplug infrastructure is
 125 not available in <tt class="command">initrd</tt>. Thus no hotplug scripts, udev rules, and
 126 permissions and no <tt class="filename">/etc/dev.d/</tt> scripts are executed for any hotplug
 127 event, which is sent from the kernel.
 128
 129 <h3><tt class="filename">rc.sysinit</tt></h3>
 130                 <p> First, if SELinux is loaded and enabled,
 131 the context of <tt class="filename">/dev/</tt> is set. <tt class="filename">rc.sysinit</tt> calls <tt class="filename">/sbin/start_udev</tt>.
 132 <tt class="filename">start_udev</tt> mounts a tmpfs filesystem on <tt class="filename">/dev/</tt>, if there is none already
 133 mounted. Then it creates some device nodes, which need module
 134 autoloading, or where there is no kernel module. After that
 135 <tt class="command">/sbin/udevstart</tt> is called again, which simulates the hotplug events in
 136 the <tt class="command">initrd</tt> phase, to apply the whole udev rules and permissions. After
 137 that <tt class="filename">rc.sysinit</tt> parses the ouput of <tt class="filename">/sbin/kmodule</tt> and loads every
 138 module. This should provide device nodes for all hardware found on your
 139 computer. </p>
 140                 <h3>Console User Permissions</h3>
 141                 <p>
 142 <tt class="filename">/etc/dev.d/default/pam_console.dev</tt> is called whenever a device node is
 143 created and calls <tt class="filename">/sbin/pam_console_setowner</tt> with the filename (and an
 144 optional symlink) of the device node. This sets the permissions for
 145 console users like specified in <tt class="filename">/etc/security/console.perms</tt>. </p>
 146                 <h2>Customizing Udev on Fedora</h2>
 147                 <p>
 148                 Read the manpage of udev and udevinfo.
 149                 Please try not to modify the files of RPM packages.
 150                 </p>
 151                 <h3>New Rules</h3>
 152                 <p>
 153 New rules should be placed in a file, which ends in <tt class="filename">.rules</tt> in
 154 <tt class="filename">/etc/udev/rules.d/</tt>. Please do not use <tt class="filename">50-udev.rules</tt>. The supported and
 155 preferred way is to create rules without the "NAME" tag and only
 156 create "SYMLINK"s. </p><p>
 157                 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>.
 158                 </p>
 159
 160 <h3>Permissions</h3>
 161 New permissions should be placed in a file, which ends in
 162 <tt class="filename">.permissions</tt> in <tt class="filename">/etc/udev/permissions.d/</tt>. Please do not use
 163 <tt class="filename">50-udev.permissions</tt>.
 164
 165 <h3>But I Really Want My Device Node!</h3>
 166                 <p>
 167                 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.
 168                 </p>
 169
 170                 <h2>Updating to udev Without <tt class="filename">/dev/</tt></h2>
 171                 <p>
 172                 The steps to upgrade without Anaconda or a rescue CD are (NOT recommended):
 173                 </p>
 174
 175                 <ul>
 176                 <li>start from a kernel-2.6
 177                 </li><li>Make sure <tt class="filename">/sys/</tt> is mounted
 178                 </li><li>Install the latest <tt class="filename">initscripts</tt> package
 179                 </li><li>Install the latest <tt class="filename">udev</tt> package
 180                 </li><li>Execute <tt class="command">/sbin/start_udev</tt>
 181                 </li><li>Install the latest <tt class="filename">mkinitrd</tt> package
 182                 </li><li>Install the latest <tt class="filename">kernel</tt> package
 183                 </li><li>Or execute <tt class="command">mkinitrd</tt> for your existing kernel(s)
 184                 </li></ul>
 185
 186                 <h2>Udev without <tt class="command">initrd</tt></h2>
 187
 188                 <p>Install Fedore Core as usual and reboot. Execute the following commands
 189                 </p>
 190
 191 <pre class="screen">
 192 <tt class="command">
 193 mkdir /tmp/dev
 194 mount --move /dev /tmp/dev
 195 sbin/MAKEDEV null console zero
 196 mount --move /tmp/dev /dev
 197 </tt></pre>
 198                 Install your kernel without an <tt class="command">initrd</tt>. Reboot.
 199                 <p>
 200                 You will get some SELinux errors, and syslogd will not work as expected.
 201                 </p>
 202                 <h2>Current Problems on Fedora</h2>
 203                 <p>
 204                 <a href="http://bugzilla.redhat.com/bugzilla/buglist.cgi?short_desc_type=allwordssubstr&amp;component=udev&amp;bug_status=ASSIGNED&amp;bug_status=MODIFIED&amp;bug_status=NEEDINFO&amp;bug_status=NEW&amp;bug_status=REOPENED&amp;bug_status=VERIFIED&amp;bug_severity=high&amp;bug_severity=low&amp;bug_severity=normal&amp;bug_severity=security&amp;bug_severity=translation&amp;long_desc_type=allwordssubstr&amp;bug_file_loc_type=allwordssubstr&amp;status_whiteboard_type=allwordssubstr&amp;fixed_in_type=allwordssubstr&amp;devel_whiteboard_type=allwordssubstr&amp;keywords_type=allwords&amp;cust_facing=YES&amp;emailassigned_to1=1&amp;emailtype1=exact&amp;emailreporter2=1&amp;emailtype2=exact&amp;bugidtype=include&amp;chfieldto=Now&amp;cmdtype=doit&amp;remaction=run&amp;namedcmd=blank&amp;order=Reuse+same+sort+as+last+time&amp;field0-0-0=noop&amp;type0-0-0=noop">All open bugs for <code class="filename">udev</code></a>
 205                 </p>
 206                 <h3>Nvidia</h3>
 207                 <p>Quick solution: If you do not need rhgb, just load the nvidia module in <tt class="filename">/etc/rc.local</tt>
 208                 </p><p>
 209                 If you have udev &gt;= 032-5, load the nvidia module:
 210                 </p>
 211 <pre class="screen">
 212 <tt class="command">
 213 cp -a /dev/nvidia* /etc/udev/devices
 214 chown root.root /etc/udev/devices/nvidia*
 215 </tt></pre>
 216
 217                 <p>The Bugzilla for this problem is <strike><a href="https://bugzilla.redhat.com/bugzilla/show_bug.cgi?id=133900">133900</a></strike>.</p>
 218                 <h3>Palm Pilot</h3>
 219                 <p>
 220                 If you have udev &gt;= 032-5, execute the command:
 221                 </p>
 222 <pre class="screen">
 223 <tt class="command">
 224 ln -s ttyUSB1 /etc/udev/devices/pilot
 225 </tt></pre>
 226
 227                 <h3>ISDN</h3>
 228                 <p>
 229                 If you have udev &gt;= 032-5:
 230                 </p>
 231 <pre class="screen">
 232 <tt class="command">
 233 /sbin/MAKEDEV -d /etc/udev/devices isdn
 234 </tt></pre>
 235                 </pre>
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