source: chapter10/kernel.xml@ ed431efc

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1<?xml version="1.0" encoding="ISO-8859-1"?>
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"/>. At this point
59 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><parameter>Framebuffer Console support</parameter></term>
269 <listitem>
270 <para>This is needed to display the Linux console on a frame
271 buffer device. To allow the kernel to print debug messages at an
272 early boot stage, it shouldn't be built as a kernel module
273 unless an initramfs will be used. And, if
274 <option>CONFIG_DRM</option> (Direct Rendering Manager) is enabled,
275 it's likely <option>CONFIG_DRM_FBDEV_EMULATION</option> (Enable
276 legacy fbdev support for your modesetting driver) should be
277 enabled as well.</para>
278 </listitem>
279 </varlistentry>
280
281 </variablelist>
282
283 <para>Alternatively, <command>make oldconfig</command> may be more
284 appropriate in some situations. See the <filename>README</filename>
285 file for more information.</para>
286
287 <para>If desired, skip kernel configuration by copying the kernel
288 config file, <filename>.config</filename>, from the host system
289 (assuming it is available) to the unpacked <filename
290 class="directory">linux-&linux-version;</filename> directory. However,
291 we do not recommend this option. It is often better to explore all the
292 configuration menus and create the kernel configuration from
293 scratch.</para>
294
295 <para>Fix an issue causing all kernel modules built with Binutils-2.41
296 or later not loadable:</para>
297
298<screen><userinput remap="pre">cat &gt;&gt; arch/loongarch/Makefile &lt;&lt; EOF
299<literal>KBUILD_AFLAGS_MODULE += -Wa,-mno-relax
300KBUILD_CFLAGS_MODULE += -Wa,-mno-relax</literal>
301EOF</userinput></screen>
302
303 <para>Compile the kernel image and modules:</para>
304
305<screen><userinput remap="make">make</userinput></screen>
306
307 <para>If using kernel modules, module configuration in <filename
308 class="directory">/etc/modprobe.d</filename> may be required.
309 Information pertaining to modules and kernel configuration is
310 located in <xref linkend="ch-config-udev"/> and in the kernel
311 documentation in the <filename
312 class="directory">linux-&linux-version;/Documentation</filename> directory.
313 Also, <filename>modprobe.d(5)</filename> may be of interest.</para>
314
315 <para>Unless module support has been disabled in the kernel configuration,
316 install the modules with:</para>
317
318<screen><userinput remap="install">make modules_install</userinput></screen>
319
320 <para>After kernel compilation is complete, additional steps are
321 required to complete the installation. Some files need to be copied to
322 the <filename class="directory">/boot</filename> directory.</para>
323
324 <caution>
325 <para>If you've decided to use a separate &boot-dir; partition for the
326 LFS system (maybe sharing a &boot-dir; partition with the host
327 distro) , the files copied below should go there. The easiest way to
328 do that is to create the entry for &boot-dir; in &fstab; first (read
329 the previous section for details), then issue the following command
330 as the &root; user in the
331 <emphasis>chroot environment</emphasis>:</para>
332
333<screen role="nodump"><userinput>mount /boot</userinput></screen>
334
335 <para>The path to the device node is omitted in the command because
336 <command>mount</command> can read it from &fstab;.</para>
337 </caution>
338
339 <para>The path to the kernel image may vary depending on the platform being
340 used. The filename below can be changed to suit your taste, but the stem of
341 the filename should be <emphasis>vmlinuz</emphasis> to be compatible with
342 the automatic setup of the boot process described in the next section. The
343 following command assumes a LoongArch platform with an EFI boot loader
344 (for example, GRUB built in &ch-final;):</para>
345
346<screen><userinput remap="install">cp -iv arch/loongarch/boot/vmlinuz.efi /boot/vmlinuz-&linux-version;-lfs-&version;</userinput></screen>
347
348 <para><filename>System.map</filename> is a symbol file for the kernel.
349 It maps the function entry points of every function in the kernel API,
350 as well as the addresses of the kernel data structures for the running
351 kernel. It is used as a resource when investigating kernel problems.
352 Issue the following command to install the map file:</para>
353
354<screen><userinput remap="install">cp -iv System.map /boot/System.map-&linux-version;</userinput></screen>
355
356 <para>The kernel configuration file <filename>.config</filename>
357 produced by the <command>make menuconfig</command> step
358 above contains all the configuration selections for the kernel
359 that was just compiled. It is a good idea to keep this file for future
360 reference:</para>
361
362<screen><userinput remap="install">cp -iv .config /boot/config-&linux-version;</userinput></screen>
363
364 <para>Install the documentation for the Linux kernel:</para>
365
366<screen><userinput remap="install">cp -r Documentation -T /usr/share/doc/linux-&linux-version;</userinput></screen>
367
368 <para>It is important to note that the files in the kernel source
369 directory are not owned by <emphasis>root</emphasis>. Whenever a
370 package is unpacked as user <emphasis>root</emphasis> (like we did
371 inside chroot), the files have the user and group IDs of whatever
372 they were on the packager's computer. This is usually not a problem
373 for any other package to be installed because the source tree is
374 removed after the installation. However, the Linux source tree is
375 often retained for a long time. Because of this, there is a chance
376 that whatever user ID the packager used will be assigned to somebody
377 on the machine. That person would then have write access to the kernel
378 source.</para>
379
380 <note>
381 <para>In many cases, the configuration of the kernel will need to be
382 updated for packages that will be installed later in BLFS. Unlike
383 other packages, it is not necessary to remove the kernel source tree
384 after the newly built kernel is installed.</para>
385
386 <para>If the kernel source tree is going to be retained, run
387 <command>chown -R 0:0</command> on the <filename
388 class="directory">linux-&linux-version;</filename> directory to ensure
389 all files are owned by user <emphasis>root</emphasis>.</para>
390 </note>
391
392 <warning>
393 <para>Some kernel documentation recommends creating a symlink from
394 <filename class="symlink">/usr/src/linux</filename> pointing to the kernel
395 source directory. This is specific to kernels prior to the 2.6 series and
396 <emphasis>must not</emphasis> be created on an LFS system as it can cause
397 problems for packages you may wish to build once your base LFS system is
398 complete.</para>
399 </warning>
400
401 <warning>
402 <para>The headers in the system's <filename
403 class="directory">include</filename> directory (<filename
404 class="directory">/usr/include</filename>) should
405 <emphasis>always</emphasis> be the ones against which Glibc was compiled,
406 that is, the sanitised headers installed in <xref
407 linkend="ch-tools-linux-headers"/>. Therefore, they should
408 <emphasis>never</emphasis> be replaced by either the raw kernel headers
409 or any other kernel sanitized headers.</para>
410 </warning>
411
412 </sect2>
413
414 <sect2 id="conf-modprobe" role="configuration">
415 <title>Configuring Linux Module Load Order</title>
416
417 <indexterm zone="conf-modprobe">
418 <primary sortas="e-/etc/modprobe.d/usb.conf">/etc/modprobe.d/usb.conf</primary>
419 </indexterm>
420
421 <para>Most of the time Linux modules are loaded automatically, but
422 sometimes it needs some specific direction. The program that loads
423 modules, <command>modprobe</command> or <command>insmod</command>, uses
424 <filename>/etc/modprobe.d/usb.conf</filename> for this purpose. This file
425 needs to be created so that if the USB drivers (ehci_hcd, ohci_hcd and
426 uhci_hcd) have been built as modules, they will be loaded in the correct
427 order; ehci_hcd needs to be loaded prior to ohci_hcd and uhci_hcd in order
428 to avoid a warning being output at boot time.</para>
429
430 <para>Create a new file <filename>/etc/modprobe.d/usb.conf</filename> by running
431 the following:</para>
432
433<screen><userinput>install -v -m755 -d /etc/modprobe.d
434cat &gt; /etc/modprobe.d/usb.conf &lt;&lt; "EOF"
435<literal># Begin /etc/modprobe.d/usb.conf
436
437install ohci_hcd /sbin/modprobe ehci_hcd ; /sbin/modprobe -i ohci_hcd ; true
438install uhci_hcd /sbin/modprobe ehci_hcd ; /sbin/modprobe -i uhci_hcd ; true
439
440# End /etc/modprobe.d/usb.conf</literal>
441EOF</userinput></screen>
442
443 </sect2>
444
445 <sect2 id="contents-kernel" role="content">
446 <title>Contents of Linux</title>
447
448 <segmentedlist>
449 <segtitle>Installed files</segtitle>
450 <segtitle>Installed directories</segtitle>
451
452 <seglistitem>
453 <seg>config-&linux-version;,
454 vmlinuz-&linux-version;-lfs-&version;,
455 and System.map-&linux-version;</seg>
456 <seg>/lib/modules, /usr/share/doc/linux-&linux-version;</seg>
457 </seglistitem>
458 </segmentedlist>
459
460 <variablelist>
461 <bridgehead renderas="sect3">Short Descriptions</bridgehead>
462 <?dbfo list-presentation="list"?>
463 <?dbhtml list-presentation="table"?>
464
465 <varlistentry id="config">
466 <term><filename>config-&linux-version;</filename></term>
467 <listitem>
468 <para>Contains all the configuration selections for the kernel</para>
469 <indexterm zone="ch-bootable-kernel config">
470 <primary sortas="e-/boot/config">/boot/config-&linux-version;</primary>
471 </indexterm>
472 </listitem>
473 </varlistentry>
474
475 <varlistentry id="lfskernel">
476 <term><filename>vmlinuz-&linux-version;-lfs-&version;</filename></term>
477 <listitem>
478 <para>The engine of the Linux system. When turning on the computer,
479 the kernel is the first part of the operating system that gets loaded.
480 It detects and initializes all components of the computer's hardware,
481 then makes these components available as a tree of files to the
482 software and turns a single CPU into a multitasking machine capable
483 of running scores of programs seemingly at the same time</para>
484 <indexterm zone="ch-bootable-kernel lfskernel">
485 <primary sortas="b-lfskernel">lfskernel-&linux-version;</primary>
486 </indexterm>
487 </listitem>
488 </varlistentry>
489
490 <varlistentry id="System.map">
491 <term><filename>System.map-&linux-version;</filename></term>
492 <listitem>
493 <para>A list of addresses and symbols; it maps the entry points and
494 addresses of all the functions and data structures in the
495 kernel</para>
496 <indexterm zone="ch-bootable-kernel System.map">
497 <primary sortas="e-/boot/System.map">/boot/System.map-&linux-version;</primary>
498 </indexterm>
499 </listitem>
500 </varlistentry>
501
502 </variablelist>
503
504 </sect2>
505
506</sect1>
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