+Building External Modules
-In this document you will find information about:
-- how to build external modules
-- how to make your module use the kbuild infrastructure
-- how kbuild will install a kernel
-- how to install modules in a non-standard location
+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 Building external modules
- --- 2.2 Available targets
- --- 2.3 Available options
- --- 2.4 Preparing the kernel tree for module build
- --- 2.5 Building separate files for a module
- === 3. Example commands
- === 4. Creating a kbuild file for an external module
- === 5. Include files
- --- 5.1 How to include files from the kernel include dir
- --- 5.2 External modules using an include/ dir
- --- 5.3 External modules using several directories
- === 6. Module installation
- --- 6.1 INSTALL_MOD_PATH
- --- 6.2 INSTALL_MOD_DIR
- === 7. Module versioning & Module.symvers
- --- 7.1 Symbols from the kernel (vmlinux + modules)
- --- 7.2 Symbols and external modules
- --- 7.3 Symbols from another external module
- === 8. Tips & Tricks
- --- 8.1 Testing for CONFIG_FOO_BAR
+ === 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 includes functionality for building modules both
-within the kernel source tree and outside the kernel source tree.
-The latter is usually referred to as external or "out-of-tree"
-modules and is used both during development and for modules that
-are not planned to be included in the kernel tree.
+"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.
-What is covered within this file is mainly information to authors
-of 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. A complete example will be presented in
-chapter 4, "Creating a kbuild file for an external module".
+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
+=== 2. How to Build External Modules
-kbuild offers functionality to build external modules, with the
-prerequisite that there is a pre-built kernel available with full source.
-A subset of the targets available when building the kernel is available
-when building an external module.
+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.
---- 2.1 Building external modules
+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.
- Use the following command to build an external module:
+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.
- make -C <path-to-kernel> M=`pwd`
+--- 2.1 Command Syntax
- For the running kernel use:
+ The command to build an external module is:
- make -C /lib/modules/`uname -r`/build M=`pwd`
+ $ make -C <path_to_kernel_src> M=$PWD
- For the above command to succeed, the kernel must have been
- built with modules enabled.
+ The kbuild system knows that an external module is being built
+ due to the "M=<dir>" option given in the command.
- To install the modules that were just built:
+ To build against the running kernel use:
- make -C <path-to-kernel> M=`pwd` modules_install
+ $ make -C /lib/modules/`uname -r`/build M=$PWD
- More complex examples will be shown later, the above should
- be enough to get you started.
+ Then to install the module(s) just built, add the target
+ "modules_install" to the command:
---- 2.2 Available targets
+ $ make -C /lib/modules/`uname -r`/build M=$PWD modules_install
- $KDIR refers to the path to the kernel source top-level directory
+--- 2.2 Options
- make -C $KDIR M=`pwd`
- Will build the module(s) located in current directory.
- All output files will be located in the same directory
- as the module source.
- No attempts are made to update the kernel source, and it is
- a precondition that a successful make has been executed
- for the kernel.
+ ($KDIR refers to the path of the kernel source directory.)
- make -C $KDIR M=`pwd` modules
- The modules target is implied when no target is given.
- Same functionality as if no target was specified.
- See description above.
+ make -C $KDIR M=$PWD
- make -C $KDIR M=`pwd` modules_install
- Install the external module(s).
- Installation default is in /lib/modules/<kernel-version>/extra,
- but may be prefixed with INSTALL_MOD_PATH - see separate
- chapter.
+ -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.
- make -C $KDIR M=`pwd` clean
- Remove all generated files for the module - the kernel
- source directory is not modified.
+ M=$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.
- make -C $KDIR M=`pwd` help
- help will list the available target when building external
- modules.
+--- 2.3 Targets
---- 2.3 Available options:
+ When building an external module, only a subset of the "make"
+ targets are available.
- $KDIR refers to the path to the kernel source top-level directory
+ make -C $KDIR M=$PWD [target]
- make -C $KDIR
- Used to specify where to find the kernel source.
- '$KDIR' represent the directory where the kernel source is.
- Make will actually change directory to the specified directory
- when executed but change back when finished.
+ 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.
- make -C $KDIR M=`pwd`
- M= is used to tell kbuild that an external module is
- being built.
- The option given to M= is the directory where the external
- module (kbuild file) is located.
- When an external module is being built only a subset of the
- usual targets are available.
+ modules
+ The default target for external modules. It has the
+ same functionality as if no target was specified. See
+ description above.
- make -C $KDIR SUBDIRS=`pwd`
- Same as M=. The SUBDIRS= syntax is kept for backwards
- compatibility.
+ 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).
---- 2.4 Preparing the kernel tree for module build
+ clean
+ Remove all generated files in the module directory only.
- To make sure the kernel contains the information required to
- build external modules the target 'modules_prepare' must be used.
- 'modules_prepare' 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.
+ help
+ List the available targets for external modules.
---- 2.5 Building separate files for a module
- It is possible to build single files which are part of a module.
- This works equally well for the kernel, a module and even for
- external modules.
- Examples (module foo.ko, consist of bar.o, baz.o):
- make -C $KDIR M=`pwd` bar.lst
- make -C $KDIR M=`pwd` bar.o
- make -C $KDIR M=`pwd` foo.ko
- make -C $KDIR M=`pwd` /
-
-
-=== 3. Example commands
-
-This example shows the actual commands to be executed when building
-an external module for the currently running kernel.
-In the example below, the distribution is supposed to use the
-facility to locate output files for a kernel compile in a different
-directory than the kernel source - but the examples will also work
-when the source and the output files are mixed in the same directory.
+--- 2.4 Building Separate Files
-# Kernel source
-/lib/modules/<kernel-version>/source -> /usr/src/linux-<version>
-
-# Output from kernel compile
-/lib/modules/<kernel-version>/build -> /usr/src/linux-<version>-up
-
-Change to the directory where the kbuild file is located and execute
-the following commands to build the module:
+ 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.
- cd /home/user/src/module
- make -C /usr/src/`uname -r`/source \
- O=/lib/modules/`uname-r`/build \
- M=`pwd`
+ 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 /
-Then, to install the module use the following command:
- make -C /usr/src/`uname -r`/source \
- O=/lib/modules/`uname-r`/build \
- M=`pwd` \
- modules_install
+=== 3. Creating a Kbuild File for an External Module
-If you look closely you will see that this is the same command as
-listed before - with the directories spelled out.
+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:
-The above are rather long commands, and the following chapter
-lists a few tricks to make it all easier.
+ 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:
-=== 4. Creating a kbuild file for an external module
+ <module_name>-y := <src1>.o <src2>.o ...
-kbuild is the build system for the kernel, and external modules
-must use kbuild to stay compatible with changes in the build system
-and to pick up the right flags to gcc etc.
+NOTE: Further documentation describing the syntax used by kbuild is
+located in Documentation/kbuild/makefiles.txt.
-The kbuild file used as input shall follow the syntax described
-in Documentation/kbuild/makefiles.txt. This chapter will introduce a few
-more tricks to be used when dealing with external modules.
+The examples below demonstrate how to create a build file for the
+module 8123.ko, which is built from the following files:
-In the following a Makefile will be created for a module with the
-following files:
8123_if.c
8123_if.h
8123_pci.c
8123_bin.o_shipped <= Binary blob
---- 4.1 Shared Makefile for module and kernel
+--- 3.1 Shared Makefile
- An external module always includes a wrapper Makefile supporting
- building the module using 'make' with no arguments.
- The Makefile provided will most likely include additional
- functionality such as test targets etc. and this part shall
- be filtered away from kbuild since it may impact kbuild if
- name clashes occurs.
+ 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
8123-y := 8123_if.o 8123_pci.o 8123_bin.o
else
- # Normal Makefile
+ # normal makefile
+ KDIR ?= /lib/modules/`uname -r`/build
- KERNELDIR := /lib/modules/`uname -r`/build
- all::
- $(MAKE) -C $(KERNELDIR) M=`pwd` $@
+ default:
+ $(MAKE) -C $(KDIR) M=$$PWD
# Module specific targets
genbin:
endif
- In example 1, the check for KERNELRELEASE is used to separate
- the two parts of the Makefile. kbuild will only see the two
- assignments whereas make will see everything except the two
- kbuild assignments.
+ The check for KERNELRELEASE is used to separate the two parts
+ of the makefile. In the example, kbuild will only see the two
+ assignments, whereas "make" will see everything except these
+ two assignments. This is due to two passes made on the file:
+ the first pass is by the "make" instance run on the command
+ line; the second pass is by the kbuild system, which is
+ initiated by the parameterized "make" in the default target.
+
+--- 3.2 Separate Kbuild File and Makefile
- In recent versions of the kernel, kbuild will look for a file named
- Kbuild and as second option look for a file named Makefile.
- Utilising the Kbuild file makes us split up the Makefile in example 1
- into two files as shown in example 2:
+ In newer versions of the kernel, kbuild will first look for a
+ file named "Kbuild," and only if that is not found, will it
+ then look for a makefile. Utilizing a "Kbuild" file allows us
+ to split up the makefile from example 1 into two files:
Example 2:
--> filename: Kbuild
8123-y := 8123_if.o 8123_pci.o 8123_bin.o
--> filename: Makefile
- KERNELDIR := /lib/modules/`uname -r`/build
- all::
- $(MAKE) -C $(KERNELDIR) M=`pwd` $@
+ KDIR ?= /lib/modules/`uname -r`/build
+
+ default:
+ $(MAKE) -C $(KDIR) M=$$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.
- In example 2, we are down to two fairly simple files and for simple
- files as used in this example the split is questionable. But some
- external modules use Makefiles of several hundred lines and here it
- really pays off to separate the kbuild part from the rest.
- Example 3 shows a backward compatible version.
+ The next example shows a backward compatible version.
Example 3:
--> filename: Kbuild
--> filename: Makefile
ifneq ($(KERNELRELEASE),)
+ # kbuild part of makefile
include Kbuild
+
else
- # Normal Makefile
+ # normal makefile
+ KDIR ?= /lib/modules/`uname -r`/build
- KERNELDIR := /lib/modules/`uname -r`/build
- all::
- $(MAKE) -C $(KERNELDIR) M=`pwd` $@
+ default:
+ $(MAKE) -C $(KDIR) M=$$PWD
# Module specific targets
genbin:
endif
- The trick here is to include the Kbuild file from Makefile, so
- if an older version of kbuild picks up the Makefile, the Kbuild
- file will be included.
+ 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.
---- 4.2 Binary blobs included in a module
+--- 3.3 Binary Blobs
- Some external modules needs to include a .o as a blob. kbuild
- has support for this, but requires the blob file to be named
- <filename>_shipped. In our example the blob is named
- 8123_bin.o_shipped and when the kbuild rules kick in the file
- 8123_bin.o is created as a simple copy off the 8213_bin.o_shipped file
- with the _shipped part stripped of the filename.
- This allows the 8123_bin.o filename to be used in the assignment to
- the module.
+ 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.
- Example 4:
- obj-m := 8123.o
8123-y := 8123_if.o 8123_pci.o 8123_bin.o
- In example 4, there is no distinction between the ordinary .c/.h files
- and the binary file. But kbuild will pick up different rules to create
- the .o file.
+ 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:
-=== 5. Include files
+ obj-m := foo.o bar.o
+ foo-y := <foo_srcs>
+ bar-y := <bar_srcs>
-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.
+ It is that simple!
-- 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.
-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-$(ARCH)/*.
+=== 4. Include Files
-External modules have a tendency to locate include files in a separate include/
-directory and therefore need to deal with this in their kbuild file.
+Within the kernel, header files are kept in standard locations
+according to the following rule:
---- 5.1 How to include files from the kernel include dir
+ * If the header file only describes the internal interface of a
+ module, then the file is placed in the same directory as the
+ source files.
+ * If the header file describes an interface used by other parts
+ of the kernel that are located in different directories, then
+ the file is placed in include/linux/.
- When a module needs to include a file from include/linux/, then one
- just uses:
+ NOTE: There are two notable exceptions to this rule: larger
+ subsystems have their own directory under include/, such as
+ include/scsi; and architecture specific headers are located
+ under arch/$(ARCH)/include/.
- #include <linux/modules.h>
+--- 4.1 Kernel Includes
- 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.
+ To include a header file located under include/linux/, simply
+ use:
- #include "8123_if.h"
+ #include <linux/module.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_$F.o (take effect only for a single file).
+ External modules tend to place header files in a separate
+ include/ directory where their source is located, although this
+ is not the usual kernel style. To inform kbuild of the
+ directory, use either ccflags-y or CFLAGS_<filename>.o.
- In our example, if we move 8123_if.h to a subdirectory named include/
- the resulting Kbuild file would look like:
+ Using the example from section 3, if we moved 8123_if.h to a
+ subdirectory named include, the resulting kbuild file would
+ look like:
--> filename: Kbuild
- obj-m := 8123.o
+ obj-m := 8123.o
- EXTRA_CFLAGS := -Iinclude
+ ccflags-y := -Iinclude
8123-y := 8123_if.o 8123_pci.o 8123_bin.o
- Note that in the assignment there is no space between -I and the path.
- This is a kbuild limitation: there must be no space present.
+ Note that in the assignment there is no space between -I and
+ the path. This is a limitation of kbuild: there must be no
+ space present.
---- 5.3 External modules using several directories
-
- If an external module does not follow the usual kernel style, but
- decides to spread files over several directories, then kbuild can
- handle this too.
+--- 4.3 Several Subdirectories
+ kbuild can handle files that are spread over several directories.
Consider the following example:
- |
- +- src/complex_main.c
- | +- hal/hardwareif.c
- | +- hal/include/hardwareif.h
- +- include/complex.h
-
- To build a single module named complex.ko, we then need the following
+ .
+ |__ src
+ | |__ complex_main.c
+ | |__ hal
+ | |__ hardwareif.c
+ | |__ include
+ | |__ hardwareif.h
+ |__ include
+ |__ complex.h
+
+ To build the module complex.ko, we then need the following
kbuild file:
- Kbuild:
+ --> filename: Kbuild
obj-m := complex.o
complex-y := src/complex_main.o
complex-y += src/hal/hardwareif.o
- EXTRA_CFLAGS := -I$(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.
git.openpandora.org / pandora-kernel.git / blobdiff