source: chapter07/kernfs.xml@ 2bf32ff

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<sect1 id="ch-tools-kernfs">
  <?dbhtml filename="kernfs.html"?>

  <title>Preparing Virtual Kernel File Systems</title>

  <indexterm zone="ch-tools-kernfs">
    <primary sortas="e-/dev/">/dev/*</primary>
  </indexterm>

    <para>Applications running in user space utilize various file 
    systems exported by the kernel to communicate 
    with the kernel itself. These file systems are virtual: no disk
    space is used for them. The content of the file systems resides in
    memory. These file systems must be mounted in the $LFS directory tree
    so the applications can find them in the chroot environment.</para>

    <para>Begin by creating directories on which the file systems will be
    mounted:</para>

<screen><userinput>mkdir -pv $LFS/{dev,proc,sys,run}</userinput></screen>

  <sect2 id="ch-tools-bindmount">
    <title>Mounting and Populating /dev</title>

      <para>During a normal boot of the LFS system, the kernel automatically
      mounts the <systemitem class="filesystem">devtmpfs</systemitem>
      filesystem on the
      <filename class="directory">/dev</filename> directory; the kernel
      creates device nodes on that virtual filesystem during the boot process
      or when a device is first detected or accessed. The udev daemon may
      change the owner or permission of the device nodes created by the
      kernel, or create new device nodes or symlinks to ease the work of
      distro maintainers or system administrators.  (See
      <xref linkend='ch-config-udev-device-node-creation'/> for details.)
      If the host kernel supports &devtmpfs;, we can simply mount a
      &devtmpfs; at <filename class='directory'>$LFS/dev</filename> and rely
      on the kernel to populate it (the LFS building process does not need
      the additional work onto &devtmpfs; by udev daemon).</para>

      <para>But, some host kernels may lack &devtmpfs; support and these
      host distros maintain the content of
      <filename class="directory">/dev</filename> with different methods.
      So the only host-agnostic way for populating
      <filename class="directory">$LFS/dev</filename> is
      bind mounting the host system's
      <filename class="directory">/dev</filename> directory. A bind mount is
      a special type of mount that allows you to create a mirror of a
      directory or mount point at some other location. Use the following
      command to do this:</para>

<screen><userinput>mount -v --bind /dev $LFS/dev</userinput></screen>

  </sect2>

  <sect2 id="ch-tools-kernfsmount">
    <title>Mounting Virtual Kernel File Systems</title>

      <para>Now mount the remaining virtual kernel filesystems:</para>

<screen><userinput>mount -v --bind /dev/pts $LFS/dev/pts
mount -vt proc proc $LFS/proc
mount -vt sysfs sysfs $LFS/sys
mount -vt tmpfs tmpfs $LFS/run</userinput></screen>
<!--
    <variablelist>
      <title>The meaning of the mount options for devpts:</title>

      <varlistentry>
        <term><parameter>gid=5</parameter></term>
        <listitem>
          <para>This ensures that all devpts-created device nodes are owned by
          group ID 5.  This is the ID we will use later on for the <systemitem
          class="groupname">tty</systemitem> group.  We use the group ID instead
          of a name, since the host system might use a different ID for its
          <systemitem class="groupname">tty</systemitem> group.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><parameter>mode=0620</parameter></term>
        <listitem>
          <para>This ensures that all devpts-created device nodes have mode 0620
          (user readable and writable, group writable).  Together with the
          option above, this ensures that devpts will create device nodes that
          meet the requirements of grantpt(), meaning the Glibc
          <command>pt_chown</command> helper binary (which is not installed by
          default) is not necessary.</para>
        </listitem>
      </varlistentry>

    </variablelist>
-->
      <para>In some host systems, <filename>/dev/shm</filename> is a
      symbolic link to <filename class="directory">/run/shm</filename>.
      The /run tmpfs was mounted above so in this case only a
      directory needs to be created.</para>

      <para>In other host systems <filename>/dev/shm</filename> is a mount point 
      for a tmpfs. In that case the mount of /dev above will only create
      /dev/shm as a directory in the chroot environment. In this situation
      we must explicitly mount a tmpfs:</para>

<screen><userinput>if [ -h $LFS/dev/shm ]; then
  mkdir -pv $LFS/$(readlink $LFS/dev/shm)
else
  mount -t tmpfs -o nosuid,nodev tmpfs $LFS/dev/shm
fi</userinput></screen>

  </sect2>

</sect1>