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+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 spliitting 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.
+