source: BOOK/bootable/x86/kernel.xml@ b2bbca4

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<sect1 id="ch-bootable-kernel" role="wrap">
  <?dbhtml filename="kernel.html"?>

  <title>Linux-&linux-version;</title>

  <indexterm zone="ch-bootable-kernel">
    <primary sortas="a-Linux">Linux</primary>
  </indexterm>

  <sect2 role="package"><title/>
    <para>The Linux package contains the Linux kernel.</para>

  </sect2>

  <sect2 role="installation">
    <title>Installation of the kernel</title>

    <para os="a">Building the kernel involves a few steps&mdash;configuration,
    compilation, and installation. Read the <filename>README</filename>
    file in the kernel source tree for alternative methods to the way this
    book configures the kernel.</para>

    <para os="b">Prepare for compilation by running the following command:</para>

<screen os="c"><userinput>make mrproper</userinput></screen>

    <para os="d">This ensures that the kernel tree is absolutely clean. The
    kernel team recommends that this command be issued prior to each
    kernel compilation. Do not rely on the source tree being clean after
    un-tarring.</para>

    <note os="kc"><para>A good starting place for setting up the kernel configuration
    is to run <command>make defconfig</command>. This will set the base
    configuration to a good state that takes your current system architecture
    into account.</para>

    <para>Be sure to enable or disable following features or the system might not
    work correctly or boot at all:</para>

    <screen role="nodump">General setup  ---&gt;
  [*] Control Group support
Processor type and features  ---&gt;
  [*] Enable seccomp to safely compute untrusted bytecode
Networking support  ---&gt;
  Networking options  ---&gt;
    &lt;*&gt; The IPv6 protocol
Device Drivers  ---&gt;
  Generic Driver Options  ---&gt;
    ()  path to uevent helper
    [*] Maintain a devtmpfs filesystem to mount at /dev
    [ ] Fallback user-helper invocation for firmware loading
File systems  ---&gt;
  [*] Inotify support for userspace
  &lt;*&gt; Kernel automounter version 4 support (also supports v3)
  Pseudo filesystems  ---&gt;
    [*] Tmpfs POSIX Access Control Lists
    [*] Tmpfs extended attributes</screen></note>

    <note><para>While "The IPv6 Protocol" is not strictly required, it is
    highly recommended by the Systemd developers.</para></note>

    <para os="h">Configure the kernel via a menu-driven interface.
    Please note that the udev bootscript requires "rtc", "tmpfs" and "devtmpfs" to be
    enabled and built into the kernel, not as modules. CBLFS has
    some information regarding particular kernel configuration requirements of
    packages outside of CLFS at <ulink
    url="&cblfs-root;"/>:</para>

<screen os="i"><userinput>make menuconfig</userinput></screen>

    <para os="j">Alternatively, <command>make oldconfig</command> may be more
    appropriate in some situations. See the <filename>README</filename>
    file for more information.</para>

    <para os="k">If desired, skip kernel configuration by copying the kernel
    config file, <filename>.config</filename>, from the host system
    (assuming it is available) to the root directory of the unpacked kernel
    sources. However, we do not recommend this option. It is often better
    to explore all the configuration menus and create the kernel configuration
    from scratch.</para>

    <para os="m">Compile the kernel image and modules:</para>

<screen os="n"><userinput>make</userinput></screen>

    <para os="o">If using kernel modules, an
    <filename>/etc/modprobe.conf</filename> file may be needed.
    Information pertaining to modules and kernel configuration is
    located in the kernel documentation in the <filename
    class="directory">Documentation</filename> directory of the kernel
    sources tree. Also, <filename>modprobe.conf(5)</filename> may
    be of interest.</para>

    <para os="p">Be very careful when reading other documentation relating to
    kernel modules because it usually applies to 2.4.x kernels only. As
    far as we know, kernel configuration issues specific to Hotplug and
    Eudev are not documented. The problem is that Eudev will create a device
    node only if Hotplug or a user-written script inserts the corresponding
    module into the kernel, and not all modules are detectable by Hotplug.
    Note that statements like the one below in the
    <filename>/etc/modprobe.conf</filename> file do not work with Eudev:</para>

<screen os="q"><literal>alias char-major-XXX some-module</literal></screen>

    <para os="r">Because of the complications with Eudev and modules,
    we strongly recommend starting with a completely non-modular kernel
    configuration, especially if this is the first time using Eudev.</para>

    <para os="s">Install the modules, if the kernel configuration uses
    them:</para>

<screen os="t"><userinput>make modules_install</userinput></screen>

    <para os="s2">Install the firmware, if the kernel configuration uses
    them:</para>

<screen os="t2"><userinput>make firmware_install</userinput></screen>

    <para os="u">After kernel compilation is complete, additional steps are
    required to complete the installation. Some files need to be copied to
    the <filename class="directory">/boot</filename> directory.</para>

    <para os="v">Issue the following command to install the kernel:</para>

<screen><userinput>cp -v arch/i386/boot/bzImage /boot/vmlinuz-clfs-&linux-version;</userinput></screen>

    <para os="w"><filename>System.map</filename> is a symbol file for the kernel.
    It maps the function entry points of every function in the kernel API,
    as well as the addresses of the kernel data structures for the running
    kernel. Issue the following command to install the map file:</para>

<screen os="w1"><userinput>cp -v System.map /boot/System.map-&linux-version;</userinput></screen>

    <para os="x">The kernel configuration file <filename>.config</filename>
    produced by the <command>make menuconfig</command> step above contains
    all the configuration selections for the kernel that was just compiled.
    It is a good idea to keep this file for future reference:</para>

<screen os="x1"><userinput>cp -v .config /boot/config-&linux-version;</userinput></screen>

    <para os="y">It is important to note that the files in the kernel source
    directory are not owned by <systemitem class="username">root</systemitem>.
    Whenever a package is unpacked as user <systemitem
    class="username">root</systemitem> (like we do inside the final-system
    build environment), the files have the user and group IDs of whatever
    they were on the packager's computer. This is usually not a problem
    for any other package to be installed because the source tree is
    removed after the installation. However, the Linux source tree is
    often retained for a long time. Because of this, there is a chance
    that whatever user ID the packager used will be assigned to somebody
    on the machine. That person would then have write access to the kernel
    source.</para>

    <para os="y1">If the kernel source tree is going to retained, run
    <command>chown -R 0:0</command> on the <filename
    class="directory">linux-&linux-version;</filename> directory to
    ensure all files are owned by user <systemitem
    class="username">root</systemitem>.</para>

    <warning os="z">
      <para>Some kernel documentation recommends creating a symlink from
      <filename class="symlink">/usr/src/linux</filename> pointing to the
      kernel source directory. This is specific to kernels prior to the
      2.6 series and <emphasis>must not</emphasis> be created on a CLFS
      system as it can cause problems for packages you may wish to build
      once your base CLFS system is complete.</para>

      <para>Also, the headers in the system's <filename
      class="directory">include</filename> directory should
      <emphasis>always</emphasis> be the ones against which Glibc was
      compiled and should <emphasis>never</emphasis> be replaced by headers
      from a different kernel version.</para>
    </warning>

  </sect2>

  <sect2 id="contents-kernel" role="content">
    <title>Contents of Linux</title>

    <segmentedlist>
      <segtitle>Installed files</segtitle>
      <segtitle>Installed directory</segtitle>

      <seglistitem>
        <seg>config-[linux-version], clfskernel-[linux-version],
        and System.map-[linux-version]</seg>
        <seg>/lib/modules</seg>
      </seglistitem>
    </segmentedlist>

    <variablelist>
      <bridgehead renderas="sect3">Short Descriptions</bridgehead>
      <?dbfo list-presentation="list"?>
      <?dbhtml list-presentation="table"?>

      <varlistentry id="config">
        <term><filename>config-[linux-version]</filename></term>
        <listitem>
          <para>Contains all the configuration selections for the kernel</para>
          <indexterm zone="ch-bootable-kernel config">
            <primary sortas="e-/boot/config">/boot/config-[linux-version]</primary>
          </indexterm>
        </listitem>
      </varlistentry>

      <varlistentry id="clfskernel">
        <term><filename>clfskernel-[linux-version]</filename></term>
        <listitem>
          <para>The engine of the Linux system. When turning on the
          computer, the kernel is the first part of the operating system
          that gets loaded. It detects and initializes all components of
          the computer's hardware, then makes these components available
          as a tree of files to the software and turns a single CPU into
          a multitasking machine capable of running scores of programs
          seemingly at the same time.</para>
          <indexterm zone="ch-bootable-kernel clfskernel">
            <primary sortas="b-clfskernel">clfskernel-[linux-version]</primary>
          </indexterm>
        </listitem>
      </varlistentry>

      <varlistentry id="System.map">
        <term><filename>System.map-[linux-version]</filename></term>
        <listitem>
          <para>A list of addresses and symbols; it maps the entry points
          and addresses of all the functions and data structures in the
          kernel</para>
          <indexterm zone="ch-bootable-kernel System.map">
            <primary sortas="e-/boot/System.map">/boot/System.map-[linux-version]</primary>
          </indexterm>
        </listitem>
      </varlistentry>

    </variablelist>

  </sect2>

</sect1>