Building External Modules
-This document describes how-to build an out-of-tree kernel module.
+This document describes how to build an out-of-tree kernel module.
=== Table of Contents
=== 1 Introduction
- === 2 How-to Build External Modules
+ === 2 How to Build External Modules
--- 2.1 Command Syntax
--- 2.2 Options
--- 2.3 Targets
--- 3.2 Separate Kbuild file and Makefile
--- 3.3 Binary Blobs
--- 3.4 Building Multiple Modules
- === 4. Include files
- --- 4.1 How to include files from the kernel include dir
- --- 4.2 External modules using an include/ dir
- --- 4.3 External modules using several directories
- === 5. Module installation
+ === 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 & Module.symvers
- --- 6.1 Symbols from the kernel (vmlinux + modules)
- --- 6.2 Symbols and external modules
- --- 6.3 Symbols from another external module
+ === 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
section 3.
-=== 2. How-to Build External Modules
+=== 2. How to Build External Modules
-To build external modules, you must have a pre-built kernel available
+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
The command to build an external module is:
- make -C <path_to_kernel_src> M=$PWD
+
$ 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
+ $ 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
+ $ make -C /lib/modules/`uname -r`/build M=$PWD modules_install
--- 2.2 Options
modules_install
Install the external module(s). The default location is
- /lib/modules/<kernel_release>/extra, but a prefix may
+ /lib/modules/<kernel_release>/extra/, but a prefix may
be added with INSTALL_MOD_PATH (discussed in section 5).
clean
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
+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
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
+ 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
+ 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:
--- 3.4 Building Multiple Modules
kbuild supports building multiple modules with a single build
- file. For example, if you want to build two modules, foo and
- bar, the kbuild lines would be:
+ 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>
It is that simple!
-=== 5. Include files
+=== 4. Include Files
-Include files are a necessity when a .c file uses something from other .c
-files (not strictly in the sense of C, but if good programming practice is
-used). Any module that consists of more than one .c file will have a .h file
-for one of the .c files.
+Within the kernel, header files are kept in standard locations
+according to the following rule:
-- If the .h file only describes a module internal interface, then the .h file
- shall be placed in the same directory as the .c files.
-- If the .h files describe an interface used by other parts of the kernel
- located in different directories, the .h files shall be located in
- include/linux/ or other include/ directories as appropriate.
+ * 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/.
-One exception for this rule is larger subsystems that have their own directory
-under include/ such as include/scsi. Another exception is arch-specific
-.h files which are located under include/asm-$(ARCH)/*.
+ 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/$(ARCH)/include/.
-External modules have a tendency to locate include files in a separate include/
-directory and therefore need to deal with this in their kbuild file.
+--- 4.1 Kernel Includes
---- 5.1 How to include files from the kernel include dir
+ To include a header file located under include/linux/, simply
+ use:
- When a module needs to include a file from include/linux/, then one
- just uses:
+ #include <linux/module.h>
- #include <linux/modules.h>
+ kbuild will add options to "gcc" so the relevant directories
+ are searched.
- kbuild will make sure to add options to gcc so the relevant
- directories are searched.
- Likewise for .h files placed in the same directory as the .c file.
+--- 4.2 Single Subdirectory
- #include "8123_if.h"
+ External modules tend to place header files in a separate
+ include/ directory where their source is located, although this
+ is not the usual kernel style. To inform kbuild of the
+ directory, use either ccflags-y or CFLAGS_<filename>.o.
- will do the job.
-
---- 5.2 External modules using an include/ dir
-
- External modules often locate their .h files in a separate include/
- directory although this is not usual kernel style. When an external
- module uses an include/ dir then kbuild needs to be told so.
- The trick here is to use either EXTRA_CFLAGS (take effect for all .c
- files) or CFLAGS_$F.o (take effect only for a single file).
-
- In our example, if we move 8123_if.h to a subdirectory named include/
- the resulting Kbuild file would look like:
+ 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
+ obj-m := 8123.o
- EXTRA_CFLAGS := -Iinclude
+ 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 kbuild limitation: there must be no space present.
-
---- 5.3 External modules using several directories
+ 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.
- If an external module does not follow the usual kernel style, but
- decides to spread files over several directories, then kbuild can
- handle this too.
+--- 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
- | +- hal/include/hardwareif.h
- +- include/complex.h
-
- To build a single module named complex.ko, we then need the following
+ .
+ |__ 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:
- Kbuild:
+ --> filename: Kbuild
obj-m := complex.o
complex-y := src/complex_main.o
complex-y += src/hal/hardwareif.o
- EXTRA_CFLAGS := -I$(src)/include
- EXTRA_CFLAGS += -I$(src)src/hal/include
+ 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.
- kbuild knows how to handle .o files located in another directory -
- although this is NOT recommended practice. The syntax is to specify
- the directory relative to the directory where the Kbuild file is
- located.
+ 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.
- To find the .h files, we have to explicitly tell kbuild where to look
- for the .h files. When kbuild executes, the current directory is always
- the root of the kernel tree (argument to -C) and therefore we have to
- tell kbuild how to find the .h files using absolute paths.
- $(src) will specify the absolute path to the directory where the
- Kbuild file are located when being build as an external module.
- Therefore -I$(src)/ is used to point out the directory of the Kbuild
- file and any additional path are just appended.
-=== 6. Module installation
+=== 5. Module Installation
-Modules which are included in the kernel are installed in the directory:
+Modules which are included in the kernel are installed in the
+directory:
- /lib/modules/$(KERNELRELEASE)/kernel
+ /lib/modules/$(KERNELRELEASE)/kernel/
-External modules are installed in the directory:
+And external modules are installed in:
- /lib/modules/$(KERNELRELEASE)/extra
+ /lib/modules/$(KERNELRELEASE)/extra/
---- 6.1 INSTALL_MOD_PATH
+--- 5.1 INSTALL_MOD_PATH
- Above are the default directories, but as always, some level of
- customization is possible. One can prefix the path using the variable
- 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/$(KERNELRELEASE)/kernel
-
- INSTALL_MOD_PATH may be set as an ordinary shell variable or as in the
- example above, can be specified on the command line when calling make.
- INSTALL_MOD_PATH has effect both when installing modules included in
- the kernel as well as when installing external modules.
+ => Install dir: /frodo/lib/modules/$(KERNELRELEASE)/kernel/
---- 6.2 INSTALL_MOD_DIR
+ 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.
- When installing external modules they are by default installed to a
- directory under /lib/modules/$(KERNELRELEASE)/extra, but one may wish
- to locate modules for a specific functionality in a separate
- directory. For this purpose, one can use INSTALL_MOD_DIR to specify an
- alternative name to 'extra'.
+--- 5.2 INSTALL_MOD_DIR
- $ make INSTALL_MOD_DIR=gandalf -C KERNELDIR \
- M=`pwd` modules_install
- => Install dir: /lib/modules/$(KERNELRELEASE)/gandalf
+ External modules are by default installed to a directory under
+ /lib/modules/$(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/
-=== 7. Module versioning & Module.symvers
-Module versioning is enabled by the CONFIG_MODVERSIONS tag.
+=== 6. Module Versioning
-Module versioning is used as a simple ABI consistency check. The Module
-versioning creates a CRC value of the full prototype for an exported symbol and
-when a module is loaded/used then the CRC values contained in the kernel are
-compared with similar values in the module. If they are not equal, then the
-kernel refuses to load the module.
+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.
+Module.symvers contains a list of all exported symbols from a kernel
+build.
---- 7.1 Symbols from the kernel (vmlinux + modules)
+--- 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 symbols, the corresponding CRC value
- is stored too.
+ 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>
- Sample:
+ <CRC> <Symbol> <module>
+
0x2d036834 scsi_remove_host drivers/scsi/scsi_mod
- For a kernel build without CONFIG_MODVERSIONS enabled, the crc
- would read: 0x00000000
+ For a kernel build without CONFIG_MODVERSIONS enabled, the CRC
+ would read 0x00000000.
Module.symvers serves two purposes:
- 1) It lists all exported symbols both from vmlinux and all modules
- 2) It lists the CRC if CONFIG_MODVERSIONS is enabled
-
---- 7.2 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 and to obtain all
- symbols, modpost reads Module.symvers from the kernel.
- 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.
-
---- 7.3 Symbols from another external module
-
- Sometimes, an external module uses exported symbols from another
- external module. Kbuild needs to have full knowledge on all symbols
- to avoid spitting out warnings about undefined symbols.
- Three solutions exist to let kbuild know all symbols of more than
- one external module.
- 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' and 'bar', and 'foo' needs
- symbols from 'bar', then one can use a common top-level kbuild
- file so both modules are compiled in same build.
-
- Consider following directory layout:
- ./foo/ <= contains the foo module
- ./bar/ <= contains the bar module
- The top-level Kbuild file would then look like:
-
- #./Kbuild: (this file may also be named Makefile)
+ 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/
- Executing:
- make -C $KDIR M=`pwd`
+ And executing
+
+ $ make -C $KDIR M=$PWD
- will then do the expected and compile both modules with full
- knowledge on symbols from both modules.
+ 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 module 'bar', one can copy the
- Module.symvers file from the compilation of the 'bar' module
- to the directory where the 'foo' module 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 in the Makefile
- 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 Makfile. These files will be
- loaded by modpost during the initialisation of its symbol
- tables.
-
-=== 8. Tips & Tricks
-
---- 8.1 Testing for CONFIG_FOO_BAR
-
- Modules often need to check for certain CONFIG_ options to decide if
- a specific feature shall be included in the module. When kbuild is used
- this is done by referencing the CONFIG_ variable directly.
+ 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 shall use kbuild when building
- and therefore can use the same methods as in-kernel modules when
- testing for CONFIG_ definitions.
+ 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.
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