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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>Enable some additional features if you are building a 64-bit
145 system. If you are using menuconfig, enable them in the order of
146 <parameter>CONFIG_PCI_MSI</parameter> first, then
147 <parameter>CONFIG_IRQ_REMAP</parameter>, at last
148 <parameter>CONFIG_X86_X2APIC</parameter> because an option only
149 shows up after its dependencies are selected.</para>
150
151 <xi:include xmlns:xi="http://www.w3.org/2001/XInclude"
152 href="kernel/x2apic.xml"/>
153
154 <para>If you are building a 32-bit system running on a hardware
155 with RAM more than 4GB, adjust the configuration so the kernel will
156 be able to use up to 64GB physical RAM:</para>
157
158 <xi:include xmlns:xi="http://www.w3.org/2001/XInclude"
159 href="kernel/highmem.xml"/>
160
161 <para>If the partition for the LFS system is in a NVME SSD (i. e. the
162 device node for the partition is <filename>/dev/nvme*</filename>
163 instead of <filename>/dev/sd*</filename>), enable NVME support or
164 the LFS system won't boot:</para>
165
166 <xi:include xmlns:xi="http://www.w3.org/2001/XInclude"
167 href="kernel/nvme.xml"/>
168 </note>
169
170 <note revision="systemd">
171 <para>While "The IPv6 Protocol" is not strictly
172 required, it is highly recommended by the systemd developers.</para>
173 </note>
174
175 <para>There are several other options that may be desired
176 depending on the requirements for the system. For a list of options needed
177 for BLFS packages, see the <ulink
178 url="&lfs-root;blfs/view/&short-version;/longindex.html#kernel-config-index">BLFS
179 Index of Kernel Settings</ulink>.</para>
180
181 <note>
182 <para>If your host hardware is using UEFI and you wish to boot the
183 LFS system with it, you should adjust some kernel configuration
184 following <ulink url="&blfs-book;postlfs/grub-setup.html#uefi-kernel">
185 the BLFS page</ulink> <emphasis role='bold'>even if you'll use the
186 UEFI bootloader from the host distro</emphasis>.</para>
187 </note>
188
189 <variablelist>
190 <title>The rationale for the above configuration items:</title>
191
192 <varlistentry>
193 <term><parameter>Randomize the address of the kernel image (KASLR)</parameter></term>
194 <listitem>
195 <para>Enable ASLR for kernel image, to mitigate some attacks based
196 on fixed addresses of sensitive data or code in the kernel.</para>
197 </listitem>
198 </varlistentry>
199
200 <varlistentry>
201 <term>
202 <parameter>
203 Compile the kernel with warnings as errors
204 </parameter>
205 </term>
206 <listitem>
207 <para>This may cause building failure if the compiler and/or
208 configuration are different from those of the kernel
209 developers.</para>
210 </listitem>
211 </varlistentry>
212
213 <varlistentry>
214 <term>
215 <parameter>
216 Enable kernel headers through /sys/kernel/kheaders.tar.xz
217 </parameter>
218 </term>
219 <listitem>
220 <para>This will require <command>cpio</command> building the kernel.
221 <command>cpio</command> is not installed by LFS.</para>
222 </listitem>
223 </varlistentry>
224
225 <varlistentry>
226 <term>
227 <parameter>
228 Configure standard kernel features (expert users)
229 </parameter>
230 </term>
231 <listitem>
232 <para>This will make some options show up in the configuration
233 interface but changing those options may be dangerous. Do not use
234 this unless you know what you are doing.</para>
235 </listitem>
236 </varlistentry>
237
238 <varlistentry>
239 <term><parameter>Strong Stack Protector</parameter></term>
240 <listitem>
241 <para>Enable SSP for the kernel. We've enabled it for the entire
242 userspace with <parameter>--enable-default-ssp</parameter>
243 configuring GCC, but the kernel does not use GCC default setting
244 for SSP. We enable it explicitly here.</para>
245 </listitem>
246 </varlistentry>
247
248 <varlistentry>
249 <term><parameter>Support for uevent helper</parameter></term>
250 <listitem>
251 <para>Having this option set may interfere with device
252 management when using Udev.</para>
253 </listitem>
254 </varlistentry>
255
256 <varlistentry>
257 <term><parameter>Maintain a devtmpfs</parameter></term>
258 <listitem>
259 <para>This will create automated device nodes which are populated by the
260 kernel, even without Udev running. Udev then runs on top of this,
261 managing permissions and adding symlinks. This configuration
262 item is required for all users of Udev.</para>
263 </listitem>
264 </varlistentry>
265
266 <varlistentry>
267 <term><parameter>Automount devtmpfs at /dev</parameter></term>
268 <listitem>
269 <para>This will mount the kernel view of the devices on /dev
270 upon switching to root filesystem just before starting
271 init.</para>
272 </listitem>
273 </varlistentry>
274
275 <varlistentry>
276 <term>
277 <parameter>
278 Enable legacy fbdev support for your modesetting driver
279 </parameter> and
280 <parameter>Framebuffer Console support</parameter>
281 </term>
282 <listitem>
283 <para>These are needed to display the Linux console on a
284 GPU driven by a DRI (Direct Rendering Infrastructure) driver.
285 If <option>CONFIG_DRM</option> (Direct Rendering Manager) is
286 enabled, you should enable these two options as well or you'll see
287 a blank screen once the DRI driver is loaded.</para>
288 </listitem>
289 </varlistentry>
290
291 <varlistentry>
292 <term><parameter>Support x2apic</parameter></term>
293 <listitem>
294 <para>Support running the interrupt controller of 64-bit x86
295 processors in x2APIC mode. x2APIC may be enabled by firmware on
296 64-bit x86 systems, and a kernel without this option enabled will
297 panic on boot if x2APIC is enabled by firmware. This option
298 has no effect, but also does no harm if x2APIC is disabled by the
299 firmware.</para>
300 </listitem>
301 </varlistentry>
302
303 </variablelist>
304
305 <para>Alternatively, <command>make oldconfig</command> may be more
306 appropriate in some situations. See the <filename>README</filename>
307 file for more information.</para>
308
309 <para>If desired, skip kernel configuration by copying the kernel
310 config file, <filename>.config</filename>, from the host system
311 (assuming it is available) to the unpacked <filename
312 class="directory">linux-&linux-version;</filename> directory. However,
313 we do not recommend this option. It is often better to explore all the
314 configuration menus and create the kernel configuration from
315 scratch.</para>
316
317 <para>Compile the kernel image and modules:</para>
318
319<screen><userinput remap="make">make</userinput></screen>
320
321 <para>If using kernel modules, module configuration in <filename
322 class="directory">/etc/modprobe.d</filename> may be required.
323 Information pertaining to modules and kernel configuration is
324 located in <xref linkend="ch-config-udev"/> and in the kernel
325 documentation in the <filename
326 class="directory">linux-&linux-version;/Documentation</filename> directory.
327 Also, <ulink role='man' url='&man;modprobe.d.5'>modprobe.d(5)</ulink>
328 may be of interest.</para>
329
330 <para>Unless module support has been disabled in the kernel configuration,
331 install the modules with:</para>
332
333<screen><userinput remap="install">make modules_install</userinput></screen>
334
335 <para>After kernel compilation is complete, additional steps are
336 required to complete the installation. Some files need to be copied to
337 the <filename class="directory">/boot</filename> directory.</para>
338
339 <caution>
340 <para>As the cross-built LFS system has a separate &boot-dir;
341 partition, the files copied below should go there. The easiest way to
342 do that is to create the entry for &boot-dir; in &fstab; first (read
343 the previous section for details), then issue the following command
344 as the &root; user:</para>
345
346<screen role="nodump"><userinput>mount /boot</userinput></screen>
347
348 <para>We don't need to specify the path to the device node, as it's
349 already given in <filename>/etc/fstab</filename>.</para>
350 </caution>
351
352 <para>The path to the kernel image may vary depending on the platform being
353 used. The filename below can be changed to suit your taste, but the stem of
354 the filename should be <emphasis>vmlinuz</emphasis> to be compatible with
355 the automatic setup of the boot process described in the next section. The
356 following command assumes an x86 architecture:</para>
357
358<screen><userinput remap="install">cp -iv arch/x86/boot/bzImage /boot/vmlinuz-&linux-version;-lfs-&version;</userinput></screen>
359
360 <para><filename>System.map</filename> is a symbol file for the kernel.
361 It maps the function entry points of every function in the kernel API,
362 as well as the addresses of the kernel data structures for the running
363 kernel. It is used as a resource when investigating kernel problems.
364 Issue the following command to install the map file:</para>
365
366<screen><userinput remap="install">cp -iv System.map /boot/System.map-&linux-version;</userinput></screen>
367
368 <para>The kernel configuration file <filename>.config</filename>
369 produced by the <command>make menuconfig</command> step
370 above contains all the configuration selections for the kernel
371 that was just compiled. It is a good idea to keep this file for future
372 reference:</para>
373
374<screen><userinput remap="install">cp -iv .config /boot/config-&linux-version;</userinput></screen>
375
376 <para>Install the documentation for the Linux kernel:</para>
377
378<screen><userinput remap="install">cp -r Documentation -T /usr/share/doc/linux-&linux-version;</userinput></screen>
379
380 <para>It is important to note that the files in the kernel source
381 directory are not owned by <emphasis>root</emphasis>. Whenever a
382 package is unpacked as user <emphasis>root</emphasis> (like we did
383 inside chroot), the files have the user and group IDs of whatever
384 they were on the packager's computer. This is usually not a problem
385 for any other package to be installed because the source tree is
386 removed after the installation. However, the Linux source tree is
387 often retained for a long time. Because of this, there is a chance
388 that whatever user ID the packager used will be assigned to somebody
389 on the machine. That person would then have write access to the kernel
390 source.</para>
391
392 <note>
393 <para>In many cases, the configuration of the kernel will need to be
394 updated for packages that will be installed later in BLFS. Unlike
395 other packages, it is not necessary to remove the kernel source tree
396 after the newly built kernel is installed.</para>
397
398 <para>If the kernel source tree is going to be retained, run
399 <command>chown -R 0:0</command> on the <filename
400 class="directory">linux-&linux-version;</filename> directory to ensure
401 all files are owned by user <emphasis>root</emphasis>.</para>
402 </note>
403
404 <warning>
405 <para>Some kernel documentation recommends creating a symlink from
406 <filename class="symlink">/usr/src/linux</filename> pointing to the kernel
407 source directory. This is specific to kernels prior to the 2.6 series and
408 <emphasis>must not</emphasis> be created on an LFS system as it can cause
409 problems for packages you may wish to build once your base LFS system is
410 complete.</para>
411 </warning>
412
413 <warning>
414 <para>The headers in the system's <filename
415 class="directory">include</filename> directory (<filename
416 class="directory">/usr/include</filename>) should
417 <emphasis>always</emphasis> be the ones against which Glibc was compiled,
418 that is, the sanitised headers installed in <xref
419 linkend="ch-tools-linux-headers"/>. Therefore, they should
420 <emphasis>never</emphasis> be replaced by either the raw kernel headers
421 or any other kernel sanitized headers.</para>
422 </warning>
423
424 </sect2>
425
426 <sect2 id="conf-modprobe" role="configuration">
427 <title>Configuring Linux Module Load Order</title>
428
429 <indexterm zone="conf-modprobe">
430 <primary sortas="e-/etc/modprobe.d/usb.conf">/etc/modprobe.d/usb.conf</primary>
431 </indexterm>
432
433 <para>Most of the time Linux modules are loaded automatically, but
434 sometimes it needs some specific direction. The program that loads
435 modules, <command>modprobe</command> or <command>insmod</command>, uses
436 <filename>/etc/modprobe.d/usb.conf</filename> for this purpose. This file
437 needs to be created so that if the USB drivers (ehci_hcd, ohci_hcd and
438 uhci_hcd) have been built as modules, they will be loaded in the correct
439 order; ehci_hcd needs to be loaded prior to ohci_hcd and uhci_hcd in order
440 to avoid a warning being output at boot time.</para>
441
442 <para>Create a new file <filename>/etc/modprobe.d/usb.conf</filename> by running
443 the following:</para>
444
445<screen><userinput>install -v -m755 -d /etc/modprobe.d
446cat &gt; /etc/modprobe.d/usb.conf &lt;&lt; "EOF"
447<literal># Begin /etc/modprobe.d/usb.conf
448
449install ohci_hcd /sbin/modprobe ehci_hcd ; /sbin/modprobe -i ohci_hcd ; true
450install uhci_hcd /sbin/modprobe ehci_hcd ; /sbin/modprobe -i uhci_hcd ; true
451
452# End /etc/modprobe.d/usb.conf</literal>
453EOF</userinput></screen>
454
455 </sect2>
456
457 <sect2 id="contents-kernel" role="content">
458 <title>Contents of Linux</title>
459
460 <segmentedlist>
461 <segtitle>Installed files</segtitle>
462 <segtitle>Installed directories</segtitle>
463
464 <seglistitem>
465 <seg>config-&linux-version;,
466 vmlinuz-&linux-version;-lfs-&version;,
467 and System.map-&linux-version;</seg>
468 <seg>/lib/modules, /usr/share/doc/linux-&linux-version;</seg>
469 </seglistitem>
470 </segmentedlist>
471
472 <variablelist>
473 <bridgehead renderas="sect3">Short Descriptions</bridgehead>
474 <?dbfo list-presentation="list"?>
475 <?dbhtml list-presentation="table"?>
476
477 <varlistentry id="config">
478 <term><filename>config-&linux-version;</filename></term>
479 <listitem>
480 <para>Contains all the configuration selections for the kernel</para>
481 <indexterm zone="ch-bootable-kernel config">
482 <primary sortas="e-/boot/config">/boot/config-&linux-version;</primary>
483 </indexterm>
484 </listitem>
485 </varlistentry>
486
487 <varlistentry id="lfskernel">
488 <term><filename>vmlinuz-&linux-version;-lfs-&version;</filename></term>
489 <listitem>
490 <para>The engine of the Linux system. When turning on the computer,
491 the kernel is the first part of the operating system that gets loaded.
492 It detects and initializes all components of the computer's hardware,
493 then makes these components available as a tree of files to the
494 software and turns a single CPU into a multitasking machine capable
495 of running scores of programs seemingly at the same time</para>
496 <indexterm zone="ch-bootable-kernel lfskernel">
497 <primary sortas="b-lfskernel">lfskernel-&linux-version;</primary>
498 </indexterm>
499 </listitem>
500 </varlistentry>
501
502 <varlistentry id="System.map">
503 <term><filename>System.map-&linux-version;</filename></term>
504 <listitem>
505 <para>A list of addresses and symbols; it maps the entry points and
506 addresses of all the functions and data structures in the
507 kernel</para>
508 <indexterm zone="ch-bootable-kernel System.map">
509 <primary sortas="e-/boot/System.map">/boot/System.map-&linux-version;</primary>
510 </indexterm>
511 </listitem>
512 </varlistentry>
513
514 </variablelist>
515
516 </sect2>
517
518</sect1>
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