@@ -15,17 +15,17 @@ This document describes how-to build an out-of-tree kernel module.
--- 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
@@ -298,236 +298,232 @@ module 8123.ko, which is built from the following files:
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
-
- When a module needs to include a file from include/linux/, then one
- just uses:
+ To include a header file located under include/linux/, simply
+ use:
#include <linux/modules.h>
- 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.
-
- #include "8123_if.h"
-
- will do the job.
+ kbuild will add options to "gcc" so the relevant directories
+ are searched.
---- 5.2 External modules using an include/ dir
+--- 4.2 Single Subdirectory
- 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_$F.o (take effect only for a single file).
+ External modules tend to place header files in a separate
+ include/ directory where their source is located, although this
+ is not the usual kernel style. To inform kbuild of the
+ directory use either ccflags-y or CFLAGS_<filename>.o.
- In our example, if we move 8123_if.h to a subdirectory named include/
- the resulting Kbuild file would look like:
+ Using the example from section 3, if we moved 8123_if.h to a
+ subdirectory named include, the resulting kbuild file would
+ look like:
--> filename: Kbuild
- obj-m := 8123.o
+ obj-m := 8123.o
- EXTRA_CFLAGS := -Iinclude
+ ccflags-y := -Iinclude
8123-y := 8123_if.o 8123_pci.o 8123_bin.o
- Note that in the assignment there is no space between -I and the path.
- This is a kbuild limitation: there must be no space present.
-
---- 5.3 External modules using several directories
+ Note that in the assignment there is no space between -I and
+ the path. This is a limitation of kbuild: there must be no
+ space present.
- If an external module does not follow the usual kernel style, but
- decides to spread files over several directories, then kbuild can
- handle this too.
+--- 4.3 Several Subdirectories
+ kbuild can handle files that are spread over several directories.
Consider the following example:
- |
- +- src/complex_main.c
- | +- hal/hardwareif.c
- | +- hal/include/hardwareif.h
- +- include/complex.h
-
- To build a single module named complex.ko, we then need the following
+ .
+ |__ src
+ | |__ complex_main.c
+ | |__ hal
+ | |__ hardwareif.c
+ | |__ include
+ | |__ hardwareif.h
+ |__ include
+ |__ complex.h
+
+ To build the module complex.ko, we then need the following
kbuild file:
- Kbuild:
+ --> filename: Kbuild
obj-m := complex.o
complex-y := src/complex_main.o
complex-y += src/hal/hardwareif.o
- EXTRA_CFLAGS := -I$(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/$(KERNELRELEASE)/kernel
-External modules are installed in the directory:
+And external modules are installed in:
/lib/modules/$(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/$(KERNELRELEASE)/kernel
- INSTALL_MOD_PATH may be set as an ordinary shell variable or as in the
- example above, can be specified on the command line when calling make.
- INSTALL_MOD_PATH has effect both when installing modules included in
- the kernel as well as when installing external modules.
+ INSTALL_MOD_PATH may be set as an ordinary shell variable or,
+ as shown above, can be specified on the command line when
+ calling "make." This has effect when installing both in-tree
+ and out-of-tree modules.
---- 6.2 INSTALL_MOD_DIR
+--- 5.2 INSTALL_MOD_DIR
- When installing external modules they are by default installed to a
- directory under /lib/modules/$(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'.
+ 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 KERNELDIR \
- M=`pwd` modules_install
+ $ make INSTALL_MOD_DIR=gandalf -C $KDIR \
+ M=$PWD modules_install
=> Install dir: /lib/modules/$(KERNELRELEASE)/gandalf
-=== 7. Module versioning & Module.symvers
+=== 6. Module Versioning
-Module versioning is enabled by the CONFIG_MODVERSIONS tag.
+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 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.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.
+--- 6.1 Symbols From the Kernel (vmlinux + modules)
---- 7.1 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, then you can use a
+ common top-level kbuild file so both modules are
+ compiled in the same build. Consider 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-$(CONFIG_EXT2_FS) += ext2.o
@@ -535,9 +531,9 @@ Module.symvers contains a list of all exported symbols from a kernel build.
ext2-y := balloc.o bitmap.o dir.o
ext2-$(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.