lfs-book/chapter06/kernfs.xml

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

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

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

    <para>Various file systems exported by the kernel are used to communicate to
    and from the kernel itself. These file systems are virtual in that no disk
    space is used for them. The content of the file systems resides in
    memory.</para>

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

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

  <sect2>
    <title>Creating Initial Device Nodes</title>

    <para>When the kernel boots the system, it requires the presence of a few
    device nodes, in particular the <filename
    class="devicefile">console</filename> and <filename
    class="devicefile">null</filename> devices. The device nodes must be created
    on the hard disk so that they are available before <command>udevd</command>
    has been started, and additionally when Linux is started with
    <parameter>init=/bin/bash</parameter>. Create the devices by running the
    following commands:</para>

<screen><userinput>mknod -m 600 $LFS/dev/console c 5 1
mknod -m 666 $LFS/dev/null c 1 3</userinput></screen>

  </sect2>

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

      <para>The recommended method of populating the <filename
      class="directory">/dev</filename> directory with devices is to mount a
      virtual filesystem (such as <systemitem
      class="filesystem">tmpfs</systemitem>) on the <filename
      class="directory">/dev</filename> directory, and allow the devices to be
      created dynamically on that virtual filesystem as they are detected or
      accessed. Device creation is generally done during the boot process
      by Udev. Since this new system does not yet have Udev and has not yet
      been booted, it is necessary to mount and populate <filename
      class="directory">/dev</filename> manually. This is accomplished by 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 to some other location. Use
      the following command to achieve this:</para>

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

  </sect2>

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

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

<screen><userinput>mount -vt devpts devpts $LFS/dev/pts -o gid=5,mode=620
mount -vt proc proc $LFS/proc
mount -vt sysfs sysfs $LFS/sys</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>.
      Inside a chroot environment, this temporary file system needs
      to be mounted separate from the host file system:</para>

<screen><userinput>if [ -h $LFS/dev/shm ]; then
  link=$(readlink $LFS/dev/shm)
  mkdir -p $LFS/$link
  mount -vt tmpfs shm $LFS/$link
  unset link
else
  mount -vt tmpfs shm $LFS/dev/shm
fi</userinput></screen>

  </sect2>

</sect1>