1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
|
<html>
<head>
<link href="../lg.css" rel="stylesheet" type="text/css" media="screen, projection" />
<link rel="shortcut icon" href="../favicon.ico" />
<title>Compiling the Linux Kernel LG #111</title>
<style type="text/css" media="screen, projection">
<!--
-->
</style>
<link rel="alternate" type="application/rss+xml" title="LG RSS" href="lg.rss" />
<link rel="alternate" type="application/rdf+xml" title="LG RDF" href="lg.rdf" />
<link rel="alternate" type="application/atom+xml" title="LG Atom" href="lg.atom.xml" />
</head>
<body>
<img src="../gx/2003/newlogo-blank-200-gold2.jpg" id="logo" alt="Linux Gazette"/>
<p id="fun">...making Linux just a little more fun!</p>
<div class="content articlecontent">
<div id="previousnexttop">
<A HREF="engel.html" ><-- prev</A> | <A HREF="okopnik.html" >next --></A>
</div>
<h1>Compiling the Linux Kernel</h1>
<p id="by"><b>By <A HREF="../authors/krishnakumar.html">R. Krishnakumar</A></b></p>
<p>
<hr>
<em>This article will serve as a primer to people who are new to the
world of Linux hacking, and are attempting to compile the Linux kernel
from source. The various steps from downloading the kernel source to
booting from the new kernel image are explained. Also given are tips
on cleaning up the source code, doing verbose compilation etc. </em>
<hr>
<h3>1. Downloading the kernel source code</h3>
<p>In order to compile a new kernel we have to download the source code of
the Linux kernel. We can download the source from <a
href="http://www.kernel.org">www.kernel.org</a>. Here we
can find all versions of the Linux kernel source code. Let's take an
example. Suppose we want to compile the 2.6.9 version of the linux kernel.
We have to download the 2.6.9 source code from:
<p>
<a href="http://www.kernel.org/pub/linux/kernel/v2.6/linux-2.6.9.tar.bz2">http://www.kernel.org/pub/linux/kernel/v2.6/linux-2.6.9.tar.bz2</a>
<p> It's better to download the bzipped version, as that will be more
compressed than its gzipped counterpart; hence will take less
time to download. A <tt>wget</tt> from the command line will look like:
<blockquote>
<pre>
wget http://www.kernel.org/pub/linux/kernel/v2.6/linux-2.6.9.tar.bz2
</pre>
</blockquote>
<p> Once we download the required kernel version source, we need to bunzip
and untar it. We can do the following:
<p>
<blockquote>
<pre>
tar xvjf linux-2.6.9.tar.bz2
</pre>
</blockquote>
<p> The 'x' option is to denote the untarring (e'x'traction), 'v' for
verbose, 'j' for specifying that we need to bunzip the file before
untarring and 'f' for stating the name of the input file.
<p> The file will untar into the directory linux-2.6.9. Once it's
untarred '<tt>cd</tt>' to <tt>linux-2.6.9</tt>.
<h3>2. Configuring the kernel</h3>
We have to configure the kernel before we start compiling it.
During the configuration phase, we will select the components which
we want to be part of the kernel. For example: suppose we are using
the ext3 filesystem. Then we need to select the ext3 filesystem support
while configuring the kernel. Typically we have to run a
<blockquote>
<pre>
make menuconfig
</pre>
</blockquote>
This will bring up the ncurses interface for configuring the kernel.
There are other options such as 'make xconfig' and 'make config'.
The former will bring up the configuration menu in graphical mode and
the latter in text mode.
<p> Once we select the different components we want for our kernel, we
can exit the configuration interface. We should select the option
to save the configuration from the configuration menu, before exiting.
<p> After we have configured the kernel as mentioned above, we can find
a file named '<tt>.config</tt>' in the top level directory of the source.
This file is the configuration file. It contains various options and
their states (whether they are selected or not). For example, if we choose
to have the PCI support in our kernel we can find an entry of the form:
<blockquote>
<pre>
CONFIG_PCI=y
</pre>
</blockquote>
in the <tt>.config</tt> file. Similarly, options which are selected as not
required will appear as not set. Suppose we have not selected
the XFS filesystem support in our kernel we will find the following
in the <tt>.config</tt>
<blockquote>
<pre>
# CONFIG_XFS_FS is not set
</pre>
</blockquote>
<p> A great feature of 2.6 kernels is that if we are running
<tt>make menuconfig</tt> (or
xconfig or config) for the first time, then the configuration menu we are
presented with is based on our current kernel configuration. In my case,
I have a Fedora Core 1 system. The kernel which I run is '2.4.22-1.2115.nptl'.
Hence when I run a 'make menuconfig' for the first time on the source then the
configuration menu presented will contain the options as given in
'/boot/config-2.4.22-1.2115.nptl'.
<h3>3. Building Dependencies</h3>
This step is required in kernels prior to 2.6 series (here I am only
referring to the stable series kernels). For example if we are using a
2.4 kernel then we have to build the dependencies explicitly. We have
to run the following:
<blockquote>
<pre>
make dep
</pre>
</blockquote>
This will build the dependencies. But for a 2.6 kernel we can skip
this step. The dependencies are automatically created when making the
final image with a 2.6 kernel.
<h3>4. Creating the final image</h3>
We can build various types of kernel binary images.
We can build a plain kernel image, a gzipped compressed image
or a bzipped compressed image. Usual choice is the 'bzImage',
which is the linux kernel image in the bzipped compressed form.
We can create the bzImage by running
<blockquote>
<pre>
make bzImage
</pre>
</blockquote>
In 2.6 kernels this step will also resolve the dependencies and proceed
to create a bzImage image.
<p> After the compilation is over we can find the kernel image at the path
<tt>arch/i386/boot/bzImage</tt> in case of an image for a 386 based processor
(Pentium, AMD etc.).
<h3>5. Compiling and Installing the modules</h3>
In the configuring section if we have selected some components to be
built as kernel modules then we need to compile those modules.
To compile the modules we should run the command:
<blockquote>
<pre>
make modules
</pre>
</blockquote>
This command will compile the components (which are selected
for module compilation) to modules. In a 2.4 kernel the result
will be .o files of the corresponding components. But in a
2.6 kernel the output file will be a .ko module. For example
if we have given the option for the Network driver of Realtek
cards to be built as modules then after giving a 'make modules'
we can find in '<tt>driver/net/</tt>' a file named <tt>8139too.o</tt>
in the case of a 2.4 kernel and <tt>8139too.ko</tt> in the case of a
2.6 kernel.
<p> After we have compiled the modules, it's time now to install the modules.
To install the modules run:
<blockquote>
<pre>
make modules_install
</pre>
</blockquote>
as root. This will install the modules and other necessary files into the
<tt>/lib/modules/2.6.9</tt> directory.
<h3>6. Booting from the new kernel</h3>
Once we are done with the installation of modules, we
can go for an automatic installation procedure for the
kernel binary. We just have to run
<blockquote>
<pre>
make install
</pre>
</blockquote>
This will update the kernel image on to the <tt>/boot</tt> area, update
the configuration file of the bootloader (<tt>lilo.conf</tt> or
<tt>grub.conf</tt>) and then do the necessary actions to make the new
kernel bootable.
<p> After this we need to reboot the machine. When the machine boots
next time the boot menu will present us with the option to boot
from the new kernel we built. We choose that option and voila!!
boot into a kernel we built all by ourselves!
<h3>7. Manual installation of the kernel </h3>
In case '<tt>make install</tt>' does not work, or if we cannot perform an
automatic installation due to some other reason, we can go for a manual
installation of the kernel. For example, if we are using the grub boot
loader then we have to copy the bzImage into the boot partition and then
change the '<tt>/etc/grub.conf</tt>' to reflect the presence of the new
image. If we are having lilo boot loader then we have to copy the bzImage
to the boot location and then modify the <tt>lilo.conf</tt> and then run
the '<tt>lilo</tt>' command to make sure that next time we boot we will
have our new image as a choice to boot from. The following are the steps we
should perform as root user if we are using lilo boot loader: <blockquote>
<pre> cp -a arch/i386/boot/bzImage /boot/bzImage-2.6.9 </pre> </blockquote>
After this we add the following entry to /etc/lilo.conf
<blockquote>
<pre>
image=/boot/bzImage-2.6.9
label=2.6.9-kernel
root=your_root_disk
</pre>
</blockquote>
We should run lilo after this
<blockquote>
<pre>
lilo -v
</pre>
</blockquote>
We will reboot the machine after this. When we are prompted at
the lilo prompt enter '2.6.9-kernel' as the boot option and we
will be booting to the new custom built kernel.
<h3>8. Verbose compilation</h3>
We find that the compilation of the kernel is very quiet. Much less
information on what is getting compiled is shown on the screen while the
compilation proceeds.
<pre class="code">
#make bzImage
CHK include/linux/version.h
UPD include/linux/version.h
SPLIT include/linux/autoconf.h -> include/config/*
CC scripts/mod/empty.o
HOSTCC scripts/mod/mk_elfconfig
MKELF scripts/mod/elfconfig.h
HOSTCC scripts/mod/file2alias.o
HOSTCC scripts/mod/modpost.o
HOSTCC scripts/mod/sumversion.o
....
....
</pre>
If we need to know what commands are used for compilation,
then we need to give the verbose compilation option while compiling.
That is:
<blockquote>
<pre>
make bzImage V=1
</pre>
</blockquote>
This will output the commands which are executed while compiling.
Here is a snippet from the compilation output:
<pre class="code">
<..snip..>
make -f scripts/Makefile.build obj=init
gcc -Wp,-MD,init/.main.o.d -nostdinc -iwithprefix include -D__KERNEL__ -Iinclude -Wall
-Wstrict-prototypes -Wno-trigraphs -fno-strict-aliasing -fno-common -pipe -msoft-float
-mpreferred-stack-boundary=2 -march=i686 -Iinclude/asm-i386/mach-default -O2
-fomit-frame-pointer -DKBUILD_BASENAME=main -DKBUILD_MODNAME=main -c -o init/main.o
init/main.c
CHK include/linux/compile.h
UPD include/linux/compile.h
gcc -Wp,-MD,init/.version.o.d -nostdinc -iwithprefix include -D__KERNEL__ -Iinclude -Wall
-Wstrict-prototypes -Wno-trigraphs -fno-strict-aliasing -fno-common -pipe -msoft-float
-mpreferred-stack-boundary=2 -march=i686 -Iinclude/asm-i386/mach-default -O2
-fomit-frame-pointer -DKBUILD_BASENAME=version -DKBUILD_MODNAME=version -c -o
init/version.o init/version.c
<..snip..>
</pre>
<h3>9. Cleaning the kernel source</h3>
After we have initiated compilation once on the source if we want
to clean the object files and other temporary files then
we have to run the following:
<blockquote>
<pre>
make clean
</pre>
</blockquote>
This will remove most generated files but will keep the configuration
file.
<p> If we need an absolute cleaning, i.e. if we want to return the source to
the state in which it was before we started the compilation, then do a
<p>
<blockquote>
<pre>
make mrproper
</pre>
</blockquote>
This command will delete all generated files, the configuration file as
well as various backup files. This will in effect unwind all the changes
we made to the source. The source after this step will be as good as it
was just after the download and untar.
<h3>10. Conclusion</h3>
We have seen how to obtain the linux kernel source, how to configure it,
how to build the kernel image and modules, how to boot from the newly compiled
kernel and how to do a verbose compilation. Also we have seen how to clean up
the temporary files and configuration files which were created during the
compilation. The next step for a budding kernel hacker would be to modify the
kernel source and try experimenting with it.
</p>
<!-- *** BEGIN author bio *** -->
<P>
<P>
<!-- *** BEGIN bio *** -->
<hr>
<p>
<img align="left" alt="[BIO]" src="../gx/authors/krishnakumar.jpg" class="bio">
<em>
Krishnakumar loves to hack the Linux kernel. He works
for Hewlett-Packard and is a BTech from Govt. Engg. College
Thrissur.
</em>
<br clear="all">
<!-- *** END bio *** -->
<!-- *** END author bio *** -->
<div id="articlefooter">
<p>
Copyright © 2005, R. Krishnakumar. Released under the
<a href="http://linuxgazette.net/copying.html">Open Publication
license</a>. Linux Gazette is not produced, sponsored, or endorsed by
its prior host, SSC, Inc.
</p>
<p>
Published in Issue 111 of Linux Gazette, February 2005
</p>
</div>
<div id="previousnextbottom">
<A HREF="engel.html" ><-- prev</A> | <A HREF="okopnik.html" >next --></A>
</div>
</div>
<div id="navigation">
<a href="../index.html">Home</a>
<a href="../faq/index.html">FAQ</a>
<a href="../lg_index.html">Site Map</a>
<a href="../mirrors.html">Mirrors</a>
<a href="../mirrors.html">Translations</a>
<a href="../search.html">Search</a>
<a href="../archives.html">Archives</a>
<a href="../authors/index.html">Authors</a>
<a href="../contact.html">Contact Us</a>
</div>
<div id="breadcrumbs">
<a href="../index.html">Home</a> >
<a href="index.html">February 2005 (#111)</a> >
Article
</div>
<img src="../gx/2003/sit3-shine.7-2.gif" id="tux" alt="Tux"/>
</body>
</html>
|
