Building External Modules

  This document describes how to build an out-of-tree kernel module.

  
  === Table of Contents
  
  	=== 1 Introduction

  	=== 2 How to Build External Modules

  	   --- 2.1 Command Syntax
  	   --- 2.2 Options
  	   --- 2.3 Targets
  	   --- 2.4 Building Separate Files
  	=== 3. Creating a Kbuild File for an External Module
  	   --- 3.1 Shared Makefile
  	   --- 3.2 Separate Kbuild file and Makefile
  	   --- 3.3 Binary Blobs
  	   --- 3.4 Building Multiple Modules

  	=== 4. Include Files
  	   --- 4.1 Kernel Includes
  	   --- 4.2 Single Subdirectory
  	   --- 4.3 Several Subdirectories
  	=== 5. Module Installation

  	   --- 5.1 INSTALL_MOD_PATH
  	   --- 5.2 INSTALL_MOD_DIR

  	=== 6. Module Versioning
  	   --- 6.1 Symbols From the Kernel (vmlinux + modules)
  	   --- 6.2 Symbols and External Modules
  	   --- 6.3 Symbols From Another External Module

  	=== 7. Tips & Tricks
  	   --- 7.1 Testing for CONFIG_FOO_BAR

  
  
  
  === 1. Introduction

  "kbuild" is the build system used by the Linux kernel. Modules must use
  kbuild to stay compatible with changes in the build infrastructure and
  to pick up the right flags to "gcc." Functionality for building modules
  both in-tree and out-of-tree is provided. The method for building
  either is similar, and all modules are initially developed and built
  out-of-tree.

  Covered in this document is information aimed at developers interested
  in building out-of-tree (or "external") modules. The author of an
  external module should supply a makefile that hides most of the
  complexity, so one only has to type "make" to build the module. This is
  easily accomplished, and a complete example will be presented in
  section 3.

  === 2. How to Build External Modules

  To build external modules, you must have a prebuilt kernel available

  that contains the configuration and header files used in the build.
  Also, the kernel must have been built with modules enabled. If you are
  using a distribution kernel, there will be a package for the kernel you
  are running provided by your distribution.

  An alternative is to use the "make" target "modules_prepare." This will
  make sure the kernel contains the information required. The target
  exists solely as a simple way to prepare a kernel source tree for
  building external modules.

  NOTE: "modules_prepare" will not build Module.symvers even if
  CONFIG_MODVERSIONS is set; therefore, a full kernel build needs to be
  executed to make module versioning work.

  --- 2.1 Command Syntax

  	The command to build an external module is:

  		$ make -C <path_to_kernel_src> M=$PWD

  	The kbuild system knows that an external module is being built
  	due to the "M=<dir>" option given in the command.

  	To build against the running kernel use:

  		$ make -C /lib/modules/`uname -r`/build M=$PWD

  	Then to install the module(s) just built, add the target
  	"modules_install" to the command:

  		$ make -C /lib/modules/`uname -r`/build M=$PWD modules_install

  --- 2.2 Options

  	($KDIR refers to the path of the kernel source directory.)

  	make -C $KDIR M=$PWD

  	-C $KDIR
  		The directory where the kernel source is located.
  		"make" will actually change to the specified directory
  		when executing and will change back when finished.

  	M=$PWD
  		Informs kbuild that an external module is being built.
  		The value given to "M" is the absolute path of the
  		directory where the external module (kbuild file) is
  		located.

  --- 2.3 Targets

  	When building an external module, only a subset of the "make"
  	targets are available.

  	make -C $KDIR M=$PWD [target]

  	The default will build the module(s) located in the current
  	directory, so a target does not need to be specified. All
  	output files will also be generated in this directory. No
  	attempts are made to update the kernel source, and it is a
  	precondition that a successful "make" has been executed for the
  	kernel.

  	modules
  		The default target for external modules. It has the
  		same functionality as if no target was specified. See
  		description above.

  	modules_install
  		Install the external module(s). The default location is

  		/lib/modules/<kernel_release>/extra/, but a prefix may

  		be added with INSTALL_MOD_PATH (discussed in section 5).

  	clean
  		Remove all generated files in the module directory only.

  	help
  		List the available targets for external modules.

  --- 2.4 Building Separate Files

  	It is possible to build single files that are part of a module.
  	This works equally well for the kernel, a module, and even for
  	external modules.

  	Example (The module foo.ko, consist of bar.o and baz.o):
  		make -C $KDIR M=$PWD bar.lst
  		make -C $KDIR M=$PWD baz.o
  		make -C $KDIR M=$PWD foo.ko
  		make -C $KDIR M=$PWD /

  === 3. Creating a Kbuild File for an External Module

  In the last section we saw the command to build a module for the
  running kernel. The module is not actually built, however, because a
  build file is required. Contained in this file will be the name of
  the module(s) being built, along with the list of requisite source
  files. The file may be as simple as a single line:

  	obj-m := <module_name>.o

  The kbuild system will build <module_name>.o from <module_name>.c,
  and, after linking, will result in the kernel module <module_name>.ko.
  The above line can be put in either a "Kbuild" file or a "Makefile."
  When the module is built from multiple sources, an additional line is
  needed listing the files:

  	<module_name>-y := <src1>.o <src2>.o ...

  NOTE: Further documentation describing the syntax used by kbuild is
  located in Documentation/kbuild/makefiles.txt.

  The examples below demonstrate how to create a build file for the

  module 8123.ko, which is built from the following files:

  	8123_if.c
  	8123_if.h
  	8123_pci.c
  	8123_bin.o_shipped	<= Binary blob

  --- 3.1 Shared Makefile

  	An external module always includes a wrapper makefile that
  	supports building the module using "make" with no arguments.
  	This target is not used by kbuild; it is only for convenience.
  	Additional functionality, such as test targets, can be included
  	but should be filtered out from kbuild due to possible name
  	clashes.

  
  	Example 1:
  		--> filename: Makefile
  		ifneq ($(KERNELRELEASE),)
  		# kbuild part of makefile
  		obj-m  := 8123.o
  		8123-y := 8123_if.o 8123_pci.o 8123_bin.o
  
  		else

  		# normal makefile
  		KDIR ?= /lib/modules/`uname -r`/build

  		default:
  			$(MAKE) -C $(KDIR) M=$$PWD

  
  		# Module specific targets
  		genbin:

  			echo "X" > 8123_bin.o_shipped

  
  		endif

  	The check for KERNELRELEASE is used to separate the two parts
  	of the makefile. In the example, kbuild will only see the two
  	assignments, whereas "make" will see everything except these
  	two assignments. This is due to two passes made on the file:

  	the first pass is by the "make" instance run on the command
  	line; the second pass is by the kbuild system, which is

  	initiated by the parameterized "make" in the default target.

  --- 3.2 Separate Kbuild File and Makefile
  
  	In newer versions of the kernel, kbuild will first look for a

  	file named "Kbuild," and only if that is not found, will it

  	then look for a makefile. Utilizing a "Kbuild" file allows us
  	to split up the makefile from example 1 into two files:

  
  	Example 2:
  		--> filename: Kbuild
  		obj-m  := 8123.o
  		8123-y := 8123_if.o 8123_pci.o 8123_bin.o
  
  		--> filename: Makefile

  		KDIR ?= /lib/modules/`uname -r`/build
  
  		default:
  			$(MAKE) -C $(KDIR) M=$$PWD

  
  		# Module specific targets
  		genbin:

  			echo "X" > 8123_bin.o_shipped

  	The split in example 2 is questionable due to the simplicity of
  	each file; however, some external modules use makefiles
  	consisting of several hundred lines, and here it really pays
  	off to separate the kbuild part from the rest.

  	The next example shows a backward compatible version.

  
  	Example 3:
  		--> filename: Kbuild
  		obj-m  := 8123.o
  		8123-y := 8123_if.o 8123_pci.o 8123_bin.o
  
  		--> filename: Makefile
  		ifneq ($(KERNELRELEASE),)

  		# kbuild part of makefile

  		include Kbuild

  		else

  		# normal makefile
  		KDIR ?= /lib/modules/`uname -r`/build

  		default:
  			$(MAKE) -C $(KDIR) M=$$PWD

  
  		# Module specific targets
  		genbin:

  			echo "X" > 8123_bin.o_shipped

  
  		endif

  	Here the "Kbuild" file is included from the makefile. This
  	allows an older version of kbuild, which only knows of
  	makefiles, to be used when the "make" and kbuild parts are
  	split into separate files.

  --- 3.3 Binary Blobs

  	Some external modules need to include an object file as a blob.
  	kbuild has support for this, but requires the blob file to be
  	named <filename>_shipped. When the kbuild rules kick in, a copy
  	of <filename>_shipped is created with _shipped stripped off,
  	giving us <filename>. This shortened filename can be used in
  	the assignment to the module.
  
  	Throughout this section, 8123_bin.o_shipped has been used to
  	build the kernel module 8123.ko; it has been included as
  	8123_bin.o.

  		8123-y := 8123_if.o 8123_pci.o 8123_bin.o

  	Although there is no distinction between the ordinary source
  	files and the binary file, kbuild will pick up different rules
  	when creating the object file for the module.
  
  --- 3.4 Building Multiple Modules
  
  	kbuild supports building multiple modules with a single build

  	file. For example, if you wanted to build two modules, foo.ko
  	and bar.ko, the kbuild lines would be:

  
  		obj-m := foo.o bar.o
  		foo-y := <foo_srcs>
  		bar-y := <bar_srcs>
  
  	It is that simple!

  === 4. Include Files

  Within the kernel, header files are kept in standard locations
  according to the following rule:

  	* If the header file only describes the internal interface of a
  	  module, then the file is placed in the same directory as the
  	  source files.
  	* If the header file describes an interface used by other parts
  	  of the kernel that are located in different directories, then
  	  the file is placed in include/linux/.

  	  NOTE: There are two notable exceptions to this rule: larger
  	  subsystems have their own directory under include/, such as
  	  include/scsi; and architecture specific headers are located
  	  under arch/$(ARCH)/include/.

  --- 4.1 Kernel Includes

  	To include a header file located under include/linux/, simply
  	use:

  		#include <linux/module.h>

  	kbuild will add options to "gcc" so the relevant directories
  	are searched.

  --- 4.2 Single Subdirectory

  	External modules tend to place header files in a separate
  	include/ directory where their source is located, although this
  	is not the usual kernel style. To inform kbuild of the

  	directory, use either ccflags-y or CFLAGS_<filename>.o.

  	Using the example from section 3, if we moved 8123_if.h to a
  	subdirectory named include, the resulting kbuild file would
  	look like:

  
  		--> filename: Kbuild

  		obj-m := 8123.o

  		ccflags-y := -Iinclude

  		8123-y := 8123_if.o 8123_pci.o 8123_bin.o

  	Note that in the assignment there is no space between -I and
  	the path. This is a limitation of kbuild: there must be no
  	space present.

  --- 4.3 Several Subdirectories

  	kbuild can handle files that are spread over several directories.

  	Consider the following example:

  	.
  	|__ src
  	|   |__ complex_main.c
  	|   |__ hal
  	|	|__ hardwareif.c
  	|	|__ include
  	|	    |__ hardwareif.h
  	|__ include
  	    |__ complex.h
  
  	To build the module complex.ko, we then need the following

  	kbuild file:

  		--> filename: Kbuild

  		obj-m := complex.o
  		complex-y := src/complex_main.o
  		complex-y += src/hal/hardwareif.o

  		ccflags-y := -I$(src)/include
  		ccflags-y += -I$(src)/src/hal/include

  	As you can see, kbuild knows how to handle object files located
  	in other directories. The trick is to specify the directory
  	relative to the kbuild file's location. That being said, this
  	is NOT recommended practice.

  	For the header files, kbuild must be explicitly told where to
  	look. When kbuild executes, the current directory is always the
  	root of the kernel tree (the argument to "-C") and therefore an
  	absolute path is needed. $(src) provides the absolute path by
  	pointing to the directory where the currently executing kbuild
  	file is located.

  === 5. Module Installation

  Modules which are included in the kernel are installed in the
  directory:

  	/lib/modules/$(KERNELRELEASE)/kernel/

  And external modules are installed in:

  	/lib/modules/$(KERNELRELEASE)/extra/

  --- 5.1 INSTALL_MOD_PATH

  	Above are the default directories but as always some level of
  	customization is possible. A prefix can be added to the
  	installation path using the variable INSTALL_MOD_PATH:

  
  		$ make INSTALL_MOD_PATH=/frodo modules_install

  		=> Install dir: /frodo/lib/modules/$(KERNELRELEASE)/kernel/

  	INSTALL_MOD_PATH may be set as an ordinary shell variable or,
  	as shown above, can be specified on the command line when
  	calling "make." This has effect when installing both in-tree
  	and out-of-tree modules.

  --- 5.2 INSTALL_MOD_DIR

  	External modules are by default installed to a directory under

  	/lib/modules/$(KERNELRELEASE)/extra/, but you may wish to
  	locate modules for a specific functionality in a separate
  	directory. For this purpose, use INSTALL_MOD_DIR to specify an
  	alternative name to "extra."

  		$ make INSTALL_MOD_DIR=gandalf -C $KDIR \
  		       M=$PWD modules_install

  		=> Install dir: /lib/modules/$(KERNELRELEASE)/gandalf/

  === 6. Module Versioning

  Module versioning is enabled by the CONFIG_MODVERSIONS tag, and is used
  as a simple ABI consistency check. A CRC value of the full prototype
  for an exported symbol is created. When a module is loaded/used, the
  CRC values contained in the kernel are compared with similar values in
  the module; if they are not equal, the kernel refuses to load the
  module.

  Module.symvers contains a list of all exported symbols from a kernel
  build.

  --- 6.1 Symbols From the Kernel (vmlinux + modules)

  	During a kernel build, a file named Module.symvers will be
  	generated. Module.symvers contains all exported symbols from
  	the kernel and compiled modules. For each symbol, the
  	corresponding CRC value is also stored.

  
  	The syntax of the Module.symvers file is:

  		<CRC>	    <Symbol>	       <module>

  		0x2d036834  scsi_remove_host   drivers/scsi/scsi_mod

  	For a kernel build without CONFIG_MODVERSIONS enabled, the CRC
  	would read 0x00000000.

  	Module.symvers serves two purposes:

  	1) It lists all exported symbols from vmlinux and all modules.
  	2) It lists the CRC if CONFIG_MODVERSIONS is enabled.
  
  --- 6.2 Symbols and External Modules
  
  	When building an external module, the build system needs access
  	to the symbols from the kernel to check if all external symbols
  	are defined. This is done in the MODPOST step. modpost obtains
  	the symbols by reading Module.symvers from the kernel source
  	tree. If a Module.symvers file is present in the directory
  	where the external module is being built, this file will be
  	read too. During the MODPOST step, a new Module.symvers file
  	will be written containing all exported symbols that were not
  	defined in the kernel.
  
  --- 6.3 Symbols From Another External Module
  
  	Sometimes, an external module uses exported symbols from
  	another external module. kbuild needs to have full knowledge of
  	all symbols to avoid spitting out warnings about undefined
  	symbols. Three solutions exist for this situation.
  
  	NOTE: The method with a top-level kbuild file is recommended
  	but may be impractical in certain situations.
  
  	Use a top-level kbuild file
  		If you have two modules, foo.ko and bar.ko, where

  		foo.ko needs symbols from bar.ko, you can use a

  		common top-level kbuild file so both modules are

  		compiled in the same build. Consider the following

  		directory layout:
  
  		./foo/ <= contains foo.ko
  		./bar/ <= contains bar.ko
  
  		The top-level kbuild file would then look like:
  
  		#./Kbuild (or ./Makefile):

  			obj-y := foo/ bar/

  		And executing

  			$ make -C $KDIR M=$PWD

  		will then do the expected and compile both modules with

  		full knowledge of symbols from either module.

  
  	Use an extra Module.symvers file

  		When an external module is built, a Module.symvers file
  		is generated containing all exported symbols which are
  		not defined in the kernel. To get access to symbols
  		from bar.ko, copy the Module.symvers file from the
  		compilation of bar.ko to the directory where foo.ko is
  		built. During the module build, kbuild will read the
  		Module.symvers file in the directory of the external
  		module, and when the build is finished, a new
  		Module.symvers file is created containing the sum of
  		all symbols defined and not part of the kernel.
  
  	Use "make" variable KBUILD_EXTRA_SYMBOLS
  		If it is impractical to copy Module.symvers from
  		another module, you can assign a space separated list

  		of files to KBUILD_EXTRA_SYMBOLS in your build file.
  		These files will be loaded by modpost during the

  		initialization of its symbol tables.

  === 7. Tips & Tricks
  
  --- 7.1 Testing for CONFIG_FOO_BAR
  
  	Modules often need to check for certain CONFIG_ options to
  	decide if a specific feature is included in the module. In
  	kbuild this is done by referencing the CONFIG_ variable
  	directly.

  
  		#fs/ext2/Makefile
  		obj-$(CONFIG_EXT2_FS) += ext2.o
  
  		ext2-y := balloc.o bitmap.o dir.o
  		ext2-$(CONFIG_EXT2_FS_XATTR) += xattr.o

  	External modules have traditionally used "grep" to check for
  	specific CONFIG_ settings directly in .config. This usage is
  	broken. As introduced before, external modules should use
  	kbuild for building and can therefore use the same methods as
  	in-tree modules when testing for CONFIG_ definitions.