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1<?xml version="1.0" encoding="UTF-8"?>
2<!DOCTYPE sect1 PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
3 "http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd" [
4 <!ENTITY % general-entities SYSTEM "../general.ent">
5 %general-entities;
6]>
7
8<sect1 id="ch-bootable-kernel" role="wrap">
9 <?dbhtml filename="kernel.html"?>
10
11 <sect1info condition="script">
12 <productname>kernel</productname>
13 <productnumber>&linux-version;</productnumber>
14 <address>&linux-url;</address>
15 </sect1info>
16
17 <title>Linux-&linux-version;</title>
18
19 <indexterm zone="ch-bootable-kernel">
20 <primary sortas="a-Linux">Linux</primary>
21 </indexterm>
22
23 <sect2 role="package">
24 <title/>
25
26 <para>The Linux package contains the Linux kernel.</para>
27
28 <segmentedlist>
29 <segtitle>&buildtime;</segtitle>
30 <segtitle>&diskspace;</segtitle>
31
32 <seglistitem>
33 <seg>&linux-knl-sbu;</seg>
34 <seg>&linux-knl-du;</seg>
35 </seglistitem>
36 </segmentedlist>
37
38 </sect2>
39
40 <sect2 role="installation">
41 <title>Installation of the kernel</title>
42
43 <para>Building the kernel involves a few steps&mdash;configuration,
44 compilation, and installation. Read the <filename>README</filename> file
45 in the kernel source tree for alternative methods to the way this book
46 configures the kernel.</para>
47
48 <important>
49 <para>
50 Building the linux kernel for the first time is one of the most
51 challenging tasks in LFS. Getting it right depends on the specific
52 hardware for the target system and your specific needs. There are
53 almost 12,000 configuration items that are available for the kernel
54 although only about a third of them are needed for most computers. The
55 LFS editors recommend that users not familiar with this process follow
56 the procedures below fairly closely. The objective is to get an
57 initial system to a point where you can log in at the command line when
58 you reboot later in <xref linkend="ch-finish-reboot" role='.'/>
59 At this point optimization and customization is not a goal.
60 </para>
61
62
63 <para>
64 For general information on kernel configuration see <ulink
65 url="&hints-root;kernel-configuration.txt"/>. Additional information
66 about configuring and building the kernel can be found at <ulink
67 url="&anduin-sources;/kernel-nutshell/"/>.
68 These references are a bit
69 dated, but still give a reasonable overview of the process.
70 </para>
71
72 <para>
73 If all else fails, you can ask for help on the <ulink
74 url="https://www.linuxfromscratch.org/mail.html">lfs-support</ulink>
75 mailing list. Note that subscribing is required in order for the list
76 to avoid spam.
77 </para>
78 </important>
79
80 <para>Prepare for compilation by running the following command:</para>
81
82<screen><userinput remap="pre">make mrproper</userinput></screen>
83
84 <para>This ensures that the kernel tree is absolutely clean. The
85 kernel team recommends that this command be issued prior to each
86 kernel compilation. Do not rely on the source tree being clean after
87 un-tarring.</para>
88
89 <para>There are several ways to configure the kernel options. Usually,
90 this is done through a menu-driven interface, for example:</para>
91
92<screen role="nodump"><userinput>make menuconfig</userinput></screen>
93
94 <variablelist>
95 <title>The meaning of optional make environment variables:</title>
96
97 <varlistentry>
98 <term><parameter>LANG=&lt;host_LANG_value&gt; LC_ALL=</parameter></term>
99 <listitem>
100 <para>This establishes the locale setting to the one used on the
101 host. This may be needed for a proper menuconfig ncurses interface
102 line drawing on a UTF-8 linux text console.</para>
103
104 <para>If used, be sure to replace
105 <replaceable>&lt;host_LANG_value&gt;</replaceable> by the value of
106 the <envar>$LANG</envar> variable from your host. You can
107 alternatively use instead the host's value of <envar>$LC_ALL</envar>
108 or <envar>$LC_CTYPE</envar>.</para>
109 </listitem>
110 </varlistentry>
111
112 <varlistentry>
113 <term><command>make menuconfig</command></term>
114 <listitem>
115 <para>This launches an ncurses menu-driven interface. For other
116 (graphical) interfaces, type <command>make help</command>.</para>
117 </listitem>
118 </varlistentry>
119 </variablelist>
120
121 <!-- Support for compiling a keymap into the kernel is deliberately removed -->
122
123
124 <note>
125 <?dbfo keep-together="auto"?>
126 <para>A good starting place for setting up the kernel configuration is to
127 run <command>make defconfig</command>. This will set the base
128 configuration to a good state that takes your current system architecture
129 into account.</para>
130
131 <para>Be sure to enable/disable/set the following features or the system might
132 not work correctly or boot at all:</para>
133
134 <!-- To editors: for updating kernel configuration, edit
135 kernel/*.toml and regenerate kernel/*.xml with
136 "make -C kernel KERNEL_TREE=</usr/src/linux-&linux-version> -->
137
138 <xi:include xmlns:xi="http://www.w3.org/2001/XInclude"
139 href="kernel/sysv.xml"/>
140
141 <xi:include xmlns:xi="http://www.w3.org/2001/XInclude"
142 href="kernel/systemd.xml"/>
143
144 <para>If the partition for the LFS system is in a NVME SSD (i. e. the
145 device node for the partition is <filename>/dev/nvme*</filename>
146 instead of <filename>/dev/sd*</filename>), enable NVME support or
147 the LFS system won't boot:</para>
148
149 <xi:include xmlns:xi="http://www.w3.org/2001/XInclude"
150 href="kernel/nvme.xml"/>
151 </note>
152
153 <note revision="systemd">
154 <para>While "The IPv6 Protocol" is not strictly
155 required, it is highly recommended by the systemd developers.</para>
156 </note>
157
158 <para>There are several other options that may be desired
159 depending on the requirements for the system. For a list of options needed
160 for BLFS packages, see the <ulink
161 url="&lfs-root;blfs/view/&short-version;/longindex.html#kernel-config-index">BLFS
162 Index of Kernel Settings</ulink>.</para>
163
164 <note>
165 <para>If your host hardware is using UEFI and you wish to boot the
166 LFS system with it, you should adjust some kernel configuration
167 following <ulink url="&blfs-book;postlfs/grub-setup.html#uefi-kernel">
168 the BLFS page</ulink> <emphasis role='bold'>even if you'll use the
169 UEFI bootloader from the host distro</emphasis>.</para>
170 </note>
171
172 <variablelist>
173 <title>The rationale for the above configuration items:</title>
174
175 <varlistentry>
176 <term><parameter>Randomize the address of the kernel image (KASLR)</parameter></term>
177 <listitem>
178 <para>Enable ASLR for kernel image, to mitigate some attacks based
179 on fixed addresses of sensitive data or code in the kernel.</para>
180 </listitem>
181 </varlistentry>
182
183 <varlistentry>
184 <term>
185 <parameter>
186 Compile the kernel with warnings as errors
187 </parameter>
188 </term>
189 <listitem>
190 <para>This may cause building failure if the compiler and/or
191 configuration are different from those of the kernel
192 developers.</para>
193 </listitem>
194 </varlistentry>
195
196 <varlistentry>
197 <term>
198 <parameter>
199 Enable kernel headers through /sys/kernel/kheaders.tar.xz
200 </parameter>
201 </term>
202 <listitem>
203 <para>This will require <command>cpio</command> building the kernel.
204 <command>cpio</command> is not installed by LFS.</para>
205 </listitem>
206 </varlistentry>
207
208 <varlistentry>
209 <term>
210 <parameter>
211 Configure standard kernel features (expert users)
212 </parameter>
213 </term>
214 <listitem>
215 <para>This will make some options show up in the configuration
216 interface but changing those options may be dangerous. Do not use
217 this unless you know what you are doing.</para>
218 </listitem>
219 </varlistentry>
220
221 <varlistentry>
222 <term><parameter>Strong Stack Protector</parameter></term>
223 <listitem>
224 <para>Enable SSP for the kernel. We've enabled it for the entire
225 userspace with <parameter>--enable-default-ssp</parameter>
226 configuring GCC, but the kernel does not use GCC default setting
227 for SSP. We enable it explicitly here.</para>
228 </listitem>
229 </varlistentry>
230
231 <varlistentry>
232 <term><parameter>Support for uevent helper</parameter></term>
233 <listitem>
234 <para>Having this option set may interfere with device
235 management when using Udev.</para>
236 </listitem>
237 </varlistentry>
238
239 <varlistentry>
240 <term><parameter>Maintain a devtmpfs</parameter></term>
241 <listitem>
242 <para>This will create automated device nodes which are populated by the
243 kernel, even without Udev running. Udev then runs on top of this,
244 managing permissions and adding symlinks. This configuration
245 item is required for all users of Udev.</para>
246 </listitem>
247 </varlistentry>
248
249 <varlistentry>
250 <term><parameter>Automount devtmpfs at /dev</parameter></term>
251 <listitem>
252 <para>This will mount the kernel view of the devices on /dev
253 upon switching to root filesystem just before starting
254 init.</para>
255 </listitem>
256 </varlistentry>
257
258 <varlistentry>
259 <term><parameter>Enable the generic EFI decompressor</parameter></term>
260 <listitem>
261 <para>Create the bootable image as an EFI application that carries
262 the actual kernel image in compressed form. It can make the
263 bootable image 50% smaller.</para>
264 </listitem>
265 </varlistentry>
266
267 <varlistentry>
268 <term>
269 <parameter>
270 Enable legacy fbdev support for your modesetting driver
271 </parameter> and
272 <parameter>Framebuffer Console support</parameter>
273 </term>
274 <listitem>
275 <para>These are needed to display the Linux console on a
276 GPU driven by a DRI (Direct Rendering Infrastructure) driver.
277 If <option>CONFIG_DRM</option> (Direct Rendering Manager) is
278 enabled, you should enable these two options as well or you'll see
279 a blank screen once the DRI driver is loaded.</para>
280 </listitem>
281 </varlistentry>
282
283 </variablelist>
284
285 <para>Alternatively, <command>make oldconfig</command> may be more
286 appropriate in some situations. See the <filename>README</filename>
287 file for more information.</para>
288
289 <para>If desired, skip kernel configuration by copying the kernel
290 config file, <filename>.config</filename>, from the host system
291 (assuming it is available) to the unpacked <filename
292 class="directory">linux-&linux-version;</filename> directory. However,
293 we do not recommend this option. It is often better to explore all the
294 configuration menus and create the kernel configuration from
295 scratch.</para>
296
297 <para>Compile the kernel image and modules:</para>
298
299<screen><userinput remap="make">make</userinput></screen>
300
301 <para>If using kernel modules, module configuration in <filename
302 class="directory">/etc/modprobe.d</filename> may be required.
303 Information pertaining to modules and kernel configuration is
304 located in <xref linkend="ch-config-udev"/> and in the kernel
305 documentation in the <filename
306 class="directory">linux-&linux-version;/Documentation</filename> directory.
307 Also, <ulink role='man' url='&man;modprobe.d.5'>modprobe.d(5)</ulink>
308 may be of interest.</para>
309
310 <para>Unless module support has been disabled in the kernel configuration,
311 install the modules with:</para>
312
313<screen><userinput remap="install">make modules_install</userinput></screen>
314
315 <para>After kernel compilation is complete, additional steps are
316 required to complete the installation. Some files need to be copied to
317 the <filename class="directory">/boot</filename> directory.</para>
318
319 <caution>
320 <para>If you've decided to use a separate &boot-dir; partition for the
321 LFS system (maybe sharing a &boot-dir; partition with the host
322 distro) , the files copied below should go there. The easiest way to
323 do that is to create the entry for &boot-dir; in &fstab; first (read
324 the previous section for details), then issue the following command
325 as the &root; user in the
326 <emphasis>chroot environment</emphasis>:</para>
327
328<screen role="nodump"><userinput>mount /boot</userinput></screen>
329
330 <para>The path to the device node is omitted in the command because
331 <command>mount</command> can read it from &fstab;.</para>
332 </caution>
333
334 <para>The path to the kernel image may vary depending on the platform being
335 used. The filename below can be changed to suit your taste, but the stem of
336 the filename should be <emphasis>vmlinuz</emphasis> to be compatible with
337 the automatic setup of the boot process described in the next section. The
338 following command assumes an ARM64 architecture with an EFI boot loader
339 (for example, GRUB built in &ch-final;):</para>
340
341<screen><userinput remap="install">cp -iv arch/arm64/boot/vmlinuz.efi /boot/vmlinuz-&linux-version;-lfs-&version;</userinput></screen>
342
343 <para><filename>System.map</filename> is a symbol file for the kernel.
344 It maps the function entry points of every function in the kernel API,
345 as well as the addresses of the kernel data structures for the running
346 kernel. It is used as a resource when investigating kernel problems.
347 Issue the following command to install the map file:</para>
348
349<screen><userinput remap="install">cp -iv System.map /boot/System.map-&linux-version;</userinput></screen>
350
351 <para>The kernel configuration file <filename>.config</filename>
352 produced by the <command>make menuconfig</command> step
353 above contains all the configuration selections for the kernel
354 that was just compiled. It is a good idea to keep this file for future
355 reference:</para>
356
357<screen><userinput remap="install">cp -iv .config /boot/config-&linux-version;</userinput></screen>
358
359 <para>Install the documentation for the Linux kernel:</para>
360
361<screen><userinput remap="install">cp -r Documentation -T /usr/share/doc/linux-&linux-version;</userinput></screen>
362
363 <para>It is important to note that the files in the kernel source
364 directory are not owned by <emphasis>root</emphasis>. Whenever a
365 package is unpacked as user <emphasis>root</emphasis> (like we did
366 inside chroot), the files have the user and group IDs of whatever
367 they were on the packager's computer. This is usually not a problem
368 for any other package to be installed because the source tree is
369 removed after the installation. However, the Linux source tree is
370 often retained for a long time. Because of this, there is a chance
371 that whatever user ID the packager used will be assigned to somebody
372 on the machine. That person would then have write access to the kernel
373 source.</para>
374
375 <note>
376 <para>In many cases, the configuration of the kernel will need to be
377 updated for packages that will be installed later in BLFS. Unlike
378 other packages, it is not necessary to remove the kernel source tree
379 after the newly built kernel is installed.</para>
380
381 <para>If the kernel source tree is going to be retained, run
382 <command>chown -R 0:0</command> on the <filename
383 class="directory">linux-&linux-version;</filename> directory to ensure
384 all files are owned by user <emphasis>root</emphasis>.</para>
385 </note>
386
387 <warning>
388 <para>Some kernel documentation recommends creating a symlink from
389 <filename class="symlink">/usr/src/linux</filename> pointing to the kernel
390 source directory. This is specific to kernels prior to the 2.6 series and
391 <emphasis>must not</emphasis> be created on an LFS system as it can cause
392 problems for packages you may wish to build once your base LFS system is
393 complete.</para>
394 </warning>
395
396 <warning>
397 <para>The headers in the system's <filename
398 class="directory">include</filename> directory (<filename
399 class="directory">/usr/include</filename>) should
400 <emphasis>always</emphasis> be the ones against which Glibc was compiled,
401 that is, the sanitised headers installed in <xref
402 linkend="ch-tools-linux-headers"/>. Therefore, they should
403 <emphasis>never</emphasis> be replaced by either the raw kernel headers
404 or any other kernel sanitized headers.</para>
405 </warning>
406
407 </sect2>
408
409 <sect2 id="conf-modprobe" role="configuration">
410 <title>Configuring Linux Module Load Order</title>
411
412 <indexterm zone="conf-modprobe">
413 <primary sortas="e-/etc/modprobe.d/usb.conf">/etc/modprobe.d/usb.conf</primary>
414 </indexterm>
415
416 <para>Most of the time Linux modules are loaded automatically, but
417 sometimes it needs some specific direction. The program that loads
418 modules, <command>modprobe</command> or <command>insmod</command>, uses
419 <filename>/etc/modprobe.d/usb.conf</filename> for this purpose. This file
420 needs to be created so that if the USB drivers (ehci_hcd, ohci_hcd and
421 uhci_hcd) have been built as modules, they will be loaded in the correct
422 order; ehci_hcd needs to be loaded prior to ohci_hcd and uhci_hcd in order
423 to avoid a warning being output at boot time.</para>
424
425 <para>Create a new file <filename>/etc/modprobe.d/usb.conf</filename> by running
426 the following:</para>
427
428<screen><userinput>install -v -m755 -d /etc/modprobe.d
429cat &gt; /etc/modprobe.d/usb.conf &lt;&lt; "EOF"
430<literal># Begin /etc/modprobe.d/usb.conf
431
432install ohci_hcd /sbin/modprobe ehci_hcd ; /sbin/modprobe -i ohci_hcd ; true
433install uhci_hcd /sbin/modprobe ehci_hcd ; /sbin/modprobe -i uhci_hcd ; true
434
435# End /etc/modprobe.d/usb.conf</literal>
436EOF</userinput></screen>
437
438 </sect2>
439
440 <sect2 id="contents-kernel" role="content">
441 <title>Contents of Linux</title>
442
443 <segmentedlist>
444 <segtitle>Installed files</segtitle>
445 <segtitle>Installed directories</segtitle>
446
447 <seglistitem>
448 <seg>config-&linux-version;,
449 vmlinuz-&linux-version;-lfs-&version;,
450 and System.map-&linux-version;</seg>
451 <seg>/lib/modules, /usr/share/doc/linux-&linux-version;</seg>
452 </seglistitem>
453 </segmentedlist>
454
455 <variablelist>
456 <bridgehead renderas="sect3">Short Descriptions</bridgehead>
457 <?dbfo list-presentation="list"?>
458 <?dbhtml list-presentation="table"?>
459
460 <varlistentry id="config">
461 <term><filename>config-&linux-version;</filename></term>
462 <listitem>
463 <para>Contains all the configuration selections for the kernel</para>
464 <indexterm zone="ch-bootable-kernel config">
465 <primary sortas="e-/boot/config">/boot/config-&linux-version;</primary>
466 </indexterm>
467 </listitem>
468 </varlistentry>
469
470 <varlistentry id="lfskernel">
471 <term><filename>vmlinuz-&linux-version;-lfs-&version;</filename></term>
472 <listitem>
473 <para>The engine of the Linux system. When turning on the computer,
474 the kernel is the first part of the operating system that gets loaded.
475 It detects and initializes all components of the computer's hardware,
476 then makes these components available as a tree of files to the
477 software and turns a single CPU into a multitasking machine capable
478 of running scores of programs seemingly at the same time</para>
479 <indexterm zone="ch-bootable-kernel lfskernel">
480 <primary sortas="b-lfskernel">lfskernel-&linux-version;</primary>
481 </indexterm>
482 </listitem>
483 </varlistentry>
484
485 <varlistentry id="System.map">
486 <term><filename>System.map-&linux-version;</filename></term>
487 <listitem>
488 <para>A list of addresses and symbols; it maps the entry points and
489 addresses of all the functions and data structures in the
490 kernel</para>
491 <indexterm zone="ch-bootable-kernel System.map">
492 <primary sortas="e-/boot/System.map">/boot/System.map-&linux-version;</primary>
493 </indexterm>
494 </listitem>
495 </varlistentry>
496
497 </variablelist>
498
499 </sect2>
500
501</sect1>
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