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Based on kernel version 3.13. Page generated on 2014-01-20 22:03 EST.

1	Building External Modules
2	
3	This document describes how to build an out-of-tree kernel module.
4	
5	=== Table of Contents
6	
7		=== 1 Introduction
8		=== 2 How to Build External Modules
9		   --- 2.1 Command Syntax
10		   --- 2.2 Options
11		   --- 2.3 Targets
12		   --- 2.4 Building Separate Files
13		=== 3. Creating a Kbuild File for an External Module
14		   --- 3.1 Shared Makefile
15		   --- 3.2 Separate Kbuild file and Makefile
16		   --- 3.3 Binary Blobs
17		   --- 3.4 Building Multiple Modules
18		=== 4. Include Files
19		   --- 4.1 Kernel Includes
20		   --- 4.2 Single Subdirectory
21		   --- 4.3 Several Subdirectories
22		=== 5. Module Installation
23		   --- 5.1 INSTALL_MOD_PATH
24		   --- 5.2 INSTALL_MOD_DIR
25		=== 6. Module Versioning
26		   --- 6.1 Symbols From the Kernel (vmlinux + modules)
27		   --- 6.2 Symbols and External Modules
28		   --- 6.3 Symbols From Another External Module
29		=== 7. Tips & Tricks
30		   --- 7.1 Testing for CONFIG_FOO_BAR
31	
32	
33	
34	=== 1. Introduction
35	
36	"kbuild" is the build system used by the Linux kernel. Modules must use
37	kbuild to stay compatible with changes in the build infrastructure and
38	to pick up the right flags to "gcc." Functionality for building modules
39	both in-tree and out-of-tree is provided. The method for building
40	either is similar, and all modules are initially developed and built
41	out-of-tree.
42	
43	Covered in this document is information aimed at developers interested
44	in building out-of-tree (or "external") modules. The author of an
45	external module should supply a makefile that hides most of the
46	complexity, so one only has to type "make" to build the module. This is
47	easily accomplished, and a complete example will be presented in
48	section 3.
49	
50	
51	=== 2. How to Build External Modules
52	
53	To build external modules, you must have a prebuilt kernel available
54	that contains the configuration and header files used in the build.
55	Also, the kernel must have been built with modules enabled. If you are
56	using a distribution kernel, there will be a package for the kernel you
57	are running provided by your distribution.
58	
59	An alternative is to use the "make" target "modules_prepare." This will
60	make sure the kernel contains the information required. The target
61	exists solely as a simple way to prepare a kernel source tree for
62	building external modules.
63	
64	NOTE: "modules_prepare" will not build Module.symvers even if
65	CONFIG_MODVERSIONS is set; therefore, a full kernel build needs to be
66	executed to make module versioning work.
67	
68	--- 2.1 Command Syntax
69	
70		The command to build an external module is:
71	
72			$ make -C <path_to_kernel_src> M=$PWD
73	
74		The kbuild system knows that an external module is being built
75		due to the "M=<dir>" option given in the command.
76	
77		To build against the running kernel use:
78	
79			$ make -C /lib/modules/`uname -r`/build M=$PWD
80	
81		Then to install the module(s) just built, add the target
82		"modules_install" to the command:
83	
84			$ make -C /lib/modules/`uname -r`/build M=$PWD modules_install
85	
86	--- 2.2 Options
87	
88		($KDIR refers to the path of the kernel source directory.)
89	
90		make -C $KDIR M=$PWD
91	
92		-C $KDIR
93			The directory where the kernel source is located.
94			"make" will actually change to the specified directory
95			when executing and will change back when finished.
96	
97		M=$PWD
98			Informs kbuild that an external module is being built.
99			The value given to "M" is the absolute path of the
100			directory where the external module (kbuild file) is
101			located.
102	
103	--- 2.3 Targets
104	
105		When building an external module, only a subset of the "make"
106		targets are available.
107	
108		make -C $KDIR M=$PWD [target]
109	
110		The default will build the module(s) located in the current
111		directory, so a target does not need to be specified. All
112		output files will also be generated in this directory. No
113		attempts are made to update the kernel source, and it is a
114		precondition that a successful "make" has been executed for the
115		kernel.
116	
117		modules
118			The default target for external modules. It has the
119			same functionality as if no target was specified. See
120			description above.
121	
122		modules_install
123			Install the external module(s). The default location is
124			/lib/modules/<kernel_release>/extra/, but a prefix may
125			be added with INSTALL_MOD_PATH (discussed in section 5).
126	
127		clean
128			Remove all generated files in the module directory only.
129	
130		help
131			List the available targets for external modules.
132	
133	--- 2.4 Building Separate Files
134	
135		It is possible to build single files that are part of a module.
136		This works equally well for the kernel, a module, and even for
137		external modules.
138	
139		Example (The module foo.ko, consist of bar.o and baz.o):
140			make -C $KDIR M=$PWD bar.lst
141			make -C $KDIR M=$PWD baz.o
142			make -C $KDIR M=$PWD foo.ko
143			make -C $KDIR M=$PWD /
144	
145	
146	=== 3. Creating a Kbuild File for an External Module
147	
148	In the last section we saw the command to build a module for the
149	running kernel. The module is not actually built, however, because a
150	build file is required. Contained in this file will be the name of
151	the module(s) being built, along with the list of requisite source
152	files. The file may be as simple as a single line:
153	
154		obj-m := <module_name>.o
155	
156	The kbuild system will build <module_name>.o from <module_name>.c,
157	and, after linking, will result in the kernel module <module_name>.ko.
158	The above line can be put in either a "Kbuild" file or a "Makefile."
159	When the module is built from multiple sources, an additional line is
160	needed listing the files:
161	
162		<module_name>-y := <src1>.o <src2>.o ...
163	
164	NOTE: Further documentation describing the syntax used by kbuild is
165	located in Documentation/kbuild/makefiles.txt.
166	
167	The examples below demonstrate how to create a build file for the
168	module 8123.ko, which is built from the following files:
169	
170		8123_if.c
171		8123_if.h
172		8123_pci.c
173		8123_bin.o_shipped	<= Binary blob
174	
175	--- 3.1 Shared Makefile
176	
177		An external module always includes a wrapper makefile that
178		supports building the module using "make" with no arguments.
179		This target is not used by kbuild; it is only for convenience.
180		Additional functionality, such as test targets, can be included
181		but should be filtered out from kbuild due to possible name
182		clashes.
183	
184		Example 1:
185			--> filename: Makefile
186			ifneq ($(KERNELRELEASE),)
187			# kbuild part of makefile
188			obj-m  := 8123.o
189			8123-y := 8123_if.o 8123_pci.o 8123_bin.o
190	
191			else
192			# normal makefile
193			KDIR ?= /lib/modules/`uname -r`/build
194	
195			default:
196				$(MAKE) -C $(KDIR) M=$$PWD
197	
198			# Module specific targets
199			genbin:
200				echo "X" > 8123_bin.o_shipped
201	
202			endif
203	
204		The check for KERNELRELEASE is used to separate the two parts
205		of the makefile. In the example, kbuild will only see the two
206		assignments, whereas "make" will see everything except these
207		two assignments. This is due to two passes made on the file:
208		the first pass is by the "make" instance run on the command
209		line; the second pass is by the kbuild system, which is
210		initiated by the parameterized "make" in the default target.
211	
212	--- 3.2 Separate Kbuild File and Makefile
213	
214		In newer versions of the kernel, kbuild will first look for a
215		file named "Kbuild," and only if that is not found, will it
216		then look for a makefile. Utilizing a "Kbuild" file allows us
217		to split up the makefile from example 1 into two files:
218	
219		Example 2:
220			--> filename: Kbuild
221			obj-m  := 8123.o
222			8123-y := 8123_if.o 8123_pci.o 8123_bin.o
223	
224			--> filename: Makefile
225			KDIR ?= /lib/modules/`uname -r`/build
226	
227			default:
228				$(MAKE) -C $(KDIR) M=$$PWD
229	
230			# Module specific targets
231			genbin:
232				echo "X" > 8123_bin.o_shipped
233	
234		The split in example 2 is questionable due to the simplicity of
235		each file; however, some external modules use makefiles
236		consisting of several hundred lines, and here it really pays
237		off to separate the kbuild part from the rest.
238	
239		The next example shows a backward compatible version.
240	
241		Example 3:
242			--> filename: Kbuild
243			obj-m  := 8123.o
244			8123-y := 8123_if.o 8123_pci.o 8123_bin.o
245	
246			--> filename: Makefile
247			ifneq ($(KERNELRELEASE),)
248			# kbuild part of makefile
249			include Kbuild
250	
251			else
252			# normal makefile
253			KDIR ?= /lib/modules/`uname -r`/build
254	
255			default:
256				$(MAKE) -C $(KDIR) M=$$PWD
257	
258			# Module specific targets
259			genbin:
260				echo "X" > 8123_bin.o_shipped
261	
262			endif
263	
264		Here the "Kbuild" file is included from the makefile. This
265		allows an older version of kbuild, which only knows of
266		makefiles, to be used when the "make" and kbuild parts are
267		split into separate files.
268	
269	--- 3.3 Binary Blobs
270	
271		Some external modules need to include an object file as a blob.
272		kbuild has support for this, but requires the blob file to be
273		named <filename>_shipped. When the kbuild rules kick in, a copy
274		of <filename>_shipped is created with _shipped stripped off,
275		giving us <filename>. This shortened filename can be used in
276		the assignment to the module.
277	
278		Throughout this section, 8123_bin.o_shipped has been used to
279		build the kernel module 8123.ko; it has been included as
280		8123_bin.o.
281	
282			8123-y := 8123_if.o 8123_pci.o 8123_bin.o
283	
284		Although there is no distinction between the ordinary source
285		files and the binary file, kbuild will pick up different rules
286		when creating the object file for the module.
287	
288	--- 3.4 Building Multiple Modules
289	
290		kbuild supports building multiple modules with a single build
291		file. For example, if you wanted to build two modules, foo.ko
292		and bar.ko, the kbuild lines would be:
293	
294			obj-m := foo.o bar.o
295			foo-y := <foo_srcs>
296			bar-y := <bar_srcs>
297	
298		It is that simple!
299	
300	
301	=== 4. Include Files
302	
303	Within the kernel, header files are kept in standard locations
304	according to the following rule:
305	
306		* If the header file only describes the internal interface of a
307		  module, then the file is placed in the same directory as the
308		  source files.
309		* If the header file describes an interface used by other parts
310		  of the kernel that are located in different directories, then
311		  the file is placed in include/linux/.
312	
313		  NOTE: There are two notable exceptions to this rule: larger
314		  subsystems have their own directory under include/, such as
315		  include/scsi; and architecture specific headers are located
316		  under arch/$(ARCH)/include/.
317	
318	--- 4.1 Kernel Includes
319	
320		To include a header file located under include/linux/, simply
321		use:
322	
323			#include <linux/module.h>
324	
325		kbuild will add options to "gcc" so the relevant directories
326		are searched.
327	
328	--- 4.2 Single Subdirectory
329	
330		External modules tend to place header files in a separate
331		include/ directory where their source is located, although this
332		is not the usual kernel style. To inform kbuild of the
333		directory, use either ccflags-y or CFLAGS_<filename>.o.
334	
335		Using the example from section 3, if we moved 8123_if.h to a
336		subdirectory named include, the resulting kbuild file would
337		look like:
338	
339			--> filename: Kbuild
340			obj-m := 8123.o
341	
342			ccflags-y := -Iinclude
343			8123-y := 8123_if.o 8123_pci.o 8123_bin.o
344	
345		Note that in the assignment there is no space between -I and
346		the path. This is a limitation of kbuild: there must be no
347		space present.
348	
349	--- 4.3 Several Subdirectories
350	
351		kbuild can handle files that are spread over several directories.
352		Consider the following example:
353	
354		.
355		|__ src
356		|   |__ complex_main.c
357		|   |__ hal
358		|	|__ hardwareif.c
359		|	|__ include
360		|	    |__ hardwareif.h
361		|__ include
362		    |__ complex.h
363	
364		To build the module complex.ko, we then need the following
365		kbuild file:
366	
367			--> filename: Kbuild
368			obj-m := complex.o
369			complex-y := src/complex_main.o
370			complex-y += src/hal/hardwareif.o
371	
372			ccflags-y := -I$(src)/include
373			ccflags-y += -I$(src)/src/hal/include
374	
375		As you can see, kbuild knows how to handle object files located
376		in other directories. The trick is to specify the directory
377		relative to the kbuild file's location. That being said, this
378		is NOT recommended practice.
379	
380		For the header files, kbuild must be explicitly told where to
381		look. When kbuild executes, the current directory is always the
382		root of the kernel tree (the argument to "-C") and therefore an
383		absolute path is needed. $(src) provides the absolute path by
384		pointing to the directory where the currently executing kbuild
385		file is located.
386	
387	
388	=== 5. Module Installation
389	
390	Modules which are included in the kernel are installed in the
391	directory:
392	
393		/lib/modules/$(KERNELRELEASE)/kernel/
394	
395	And external modules are installed in:
396	
397		/lib/modules/$(KERNELRELEASE)/extra/
398	
399	--- 5.1 INSTALL_MOD_PATH
400	
401		Above are the default directories but as always some level of
402		customization is possible. A prefix can be added to the
403		installation path using the variable INSTALL_MOD_PATH:
404	
405			$ make INSTALL_MOD_PATH=/frodo modules_install
406			=> Install dir: /frodo/lib/modules/$(KERNELRELEASE)/kernel/
407	
408		INSTALL_MOD_PATH may be set as an ordinary shell variable or,
409		as shown above, can be specified on the command line when
410		calling "make." This has effect when installing both in-tree
411		and out-of-tree modules.
412	
413	--- 5.2 INSTALL_MOD_DIR
414	
415		External modules are by default installed to a directory under
416		/lib/modules/$(KERNELRELEASE)/extra/, but you may wish to
417		locate modules for a specific functionality in a separate
418		directory. For this purpose, use INSTALL_MOD_DIR to specify an
419		alternative name to "extra."
420	
421			$ make INSTALL_MOD_DIR=gandalf -C $KDIR \
422			       M=$PWD modules_install
423			=> Install dir: /lib/modules/$(KERNELRELEASE)/gandalf/
424	
425	
426	=== 6. Module Versioning
427	
428	Module versioning is enabled by the CONFIG_MODVERSIONS tag, and is used
429	as a simple ABI consistency check. A CRC value of the full prototype
430	for an exported symbol is created. When a module is loaded/used, the
431	CRC values contained in the kernel are compared with similar values in
432	the module; if they are not equal, the kernel refuses to load the
433	module.
434	
435	Module.symvers contains a list of all exported symbols from a kernel
436	build.
437	
438	--- 6.1 Symbols From the Kernel (vmlinux + modules)
439	
440		During a kernel build, a file named Module.symvers will be
441		generated. Module.symvers contains all exported symbols from
442		the kernel and compiled modules. For each symbol, the
443		corresponding CRC value is also stored.
444	
445		The syntax of the Module.symvers file is:
446			<CRC>	    <Symbol>	       <module>
447	
448			0x2d036834  scsi_remove_host   drivers/scsi/scsi_mod
449	
450		For a kernel build without CONFIG_MODVERSIONS enabled, the CRC
451		would read 0x00000000.
452	
453		Module.symvers serves two purposes:
454		1) It lists all exported symbols from vmlinux and all modules.
455		2) It lists the CRC if CONFIG_MODVERSIONS is enabled.
456	
457	--- 6.2 Symbols and External Modules
458	
459		When building an external module, the build system needs access
460		to the symbols from the kernel to check if all external symbols
461		are defined. This is done in the MODPOST step. modpost obtains
462		the symbols by reading Module.symvers from the kernel source
463		tree. If a Module.symvers file is present in the directory
464		where the external module is being built, this file will be
465		read too. During the MODPOST step, a new Module.symvers file
466		will be written containing all exported symbols that were not
467		defined in the kernel.
468	
469	--- 6.3 Symbols From Another External Module
470	
471		Sometimes, an external module uses exported symbols from
472		another external module. kbuild needs to have full knowledge of
473		all symbols to avoid spliitting out warnings about undefined
474		symbols. Three solutions exist for this situation.
475	
476		NOTE: The method with a top-level kbuild file is recommended
477		but may be impractical in certain situations.
478	
479		Use a top-level kbuild file
480			If you have two modules, foo.ko and bar.ko, where
481			foo.ko needs symbols from bar.ko, you can use a
482			common top-level kbuild file so both modules are
483			compiled in the same build. Consider the following
484			directory layout:
485	
486			./foo/ <= contains foo.ko
487			./bar/ <= contains bar.ko
488	
489			The top-level kbuild file would then look like:
490	
491			#./Kbuild (or ./Makefile):
492				obj-y := foo/ bar/
493	
494			And executing
495	
496				$ make -C $KDIR M=$PWD
497	
498			will then do the expected and compile both modules with
499			full knowledge of symbols from either module.
500	
501		Use an extra Module.symvers file
502			When an external module is built, a Module.symvers file
503			is generated containing all exported symbols which are
504			not defined in the kernel. To get access to symbols
505			from bar.ko, copy the Module.symvers file from the
506			compilation of bar.ko to the directory where foo.ko is
507			built. During the module build, kbuild will read the
508			Module.symvers file in the directory of the external
509			module, and when the build is finished, a new
510			Module.symvers file is created containing the sum of
511			all symbols defined and not part of the kernel.
512	
513		Use "make" variable KBUILD_EXTRA_SYMBOLS
514			If it is impractical to copy Module.symvers from
515			another module, you can assign a space separated list
516			of files to KBUILD_EXTRA_SYMBOLS in your build file.
517			These files will be loaded by modpost during the
518			initialization of its symbol tables.
519	
520	
521	=== 7. Tips & Tricks
522	
523	--- 7.1 Testing for CONFIG_FOO_BAR
524	
525		Modules often need to check for certain CONFIG_ options to
526		decide if a specific feature is included in the module. In
527		kbuild this is done by referencing the CONFIG_ variable
528		directly.
529	
530			#fs/ext2/Makefile
531			obj-$(CONFIG_EXT2_FS) += ext2.o
532	
533			ext2-y := balloc.o bitmap.o dir.o
534			ext2-$(CONFIG_EXT2_FS_XATTR) += xattr.o
535	
536		External modules have traditionally used "grep" to check for
537		specific CONFIG_ settings directly in .config. This usage is
538		broken. As introduced before, external modules should use
539		kbuild for building and can therefore use the same methods as
540		in-tree modules when testing for CONFIG_ definitions.
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