Building External Modules¶

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

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 the compiler. 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 Creating a Kbuild File for an External Module.

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.

Command Syntax¶

The command to build an external module is:
$ make -C <path_to_kernel_dir> 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
Starting from Linux 6.13, you can use the -f option instead of -C. This will avoid unnecessary change of the working directory. The external module will be output to the directory where you invoke make.

$ make -f /lib/modules/uname -r/build/Makefile M=$PWD

Options¶

($KDIR refers to the path of the kernel source directory, or the path of the kernel output directory if the kernel was built in a separate build directory.)

You can optionally pass MO= option if you want to build the modules in a separate directory.

make -C $KDIR M=$PWD [MO=$BUILD_DIR]

-C $KDIR
The directory that contains the kernel and relevant build artifacts used for building an external module. “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.

MO=$BUILD_DIR
Specifies a separate output directory for the external module.

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>/updates/, but a prefix may be added with INSTALL_MOD_PATH (discussed in section Module Installation).

clean
Remove all generated files in the module directory only.

help
List the available targets for external modules.

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 ./

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 Linux Kernel Makefiles.

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

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

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

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

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.

Separate Kbuild File and Makefile¶

Kbuild will first look for a file named “Kbuild”, and if it is not found, it will then look for “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

--> filename: Makefile
KDIR ?= /lib/modules/`uname -r`/build

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

Linux 6.13 and later support another way. The external module Makefile can include the kernel Makefile directly, rather than invoking sub Make.

Example 3:
--> filename: Kbuild
obj-m  := 8123.o
8123-y := 8123_if.o 8123_pci.o

--> filename: Makefile
KDIR ?= /lib/modules/$(shell uname -r)/build
export KBUILD_EXTMOD := $(realpath $(dir $(lastword $(MAKEFILE_LIST))))
include $(KDIR)/Makefile

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!

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/$(SRCARCH)/include/.

Kernel Includes¶

To include a header file located under include/linux/, simply use:
#include <linux/module.h>
kbuild will add options to the compiler so the relevant directories are searched.

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 := -I $(src)/include
8123-y := 8123_if.o 8123_pci.o

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.

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)/updates/

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.

INSTALL_MOD_DIR¶

External modules are by default installed to a directory under /lib/modules/$(KERNELRELEASE)/updates/, 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 “updates.”:
$ make INSTALL_MOD_DIR=gandalf -C $KDIR \
       M=$PWD modules_install
=> Install dir: /lib/modules/$(KERNELRELEASE)/gandalf/

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.

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>                         <Export Type>     <Namespace>

0xe1cc2a05  usb_stor_suspend drivers/usb/storage/usb-storage  EXPORT_SYMBOL_GPL USB_STORAGE
The fields are separated by tabs and values may be empty (e.g. if no namespace is defined for an exported symbol).

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

Module.symvers serves two purposes:

It lists all exported symbols from vmlinux and all modules.

It lists the CRC if CONFIG_MODVERSIONS is enabled.

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. During the MODPOST step, a new Module.symvers file will be written containing all exported symbols from that external module.

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. Two 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-m := 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 “make” variable KBUILD_EXTRA_SYMBOLS
If it is impractical to add a top-level kbuild file, 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.

Tips & Tricks¶

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