Linux build module makefile

Building External Modules¶

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

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:

The kbuild system knows that an external module is being built due to the “M= ” option given in the command.

To build against the running kernel use:

Then to install the module(s) just built, add the target “modules_install” to the command:

2.2 Options¶

($KDIR refers to the path of the kernel source directory.)

make -C $KDIR M=$PWD

The directory where the kernel source is located. “make” will actually change to the specified directory when executing and will change back when finished.

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.

The default target for external modules. It has the same functionality as if no target was specified. See description above.

Install the external module(s). The default location is /lib/modules/ /extra/, but a prefix may be added with INSTALL_MOD_PATH (discussed in section 5).

Remove all generated files in the module directory only.

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):

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:

The kbuild system will build .o from .c, and, after linking, will result in the kernel module .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:

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:

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.

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:

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.

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 _shipped. When the kbuild rules kick in, a copy of _shipped is created with _shipped stripped off, giving us . 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:

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:

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/.

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/.

4.1 Kernel Includes¶

To include a header file located under include/linux/, simply use:

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_ .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:

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:

To build the module complex.ko, we then need the following kbuild file:

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:

And external modules are installed in:

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:

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.”:

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)В¶

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:

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.

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

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. 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:

The top-level kbuild file would then look like:

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.

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:

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.

© Copyright The kernel development community.

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Makefile для самых маленьких

Не очень строгий перевод материала mrbook.org/tutorials/make Мне в свое время очень не хватило подобной методички для понимания базовых вещей о make. Думаю, будет хоть кому-нибудь интересно. Хотя эта технология и отмирает, но все равно используется в очень многих проектах. Кармы на хаб «Переводы» не хватило, как только появится возможность — добавлю и туда. Добавил в Переводы. Если есть ошибки в оформлении, то прошу указать на них. Буду исправлять.

Статья будет интересная прежде всего изучающим программирование на C/C++ в UNIX-подобных системах от самых корней, без использования IDE.

Компилировать проект ручками — занятие весьма утомительное, особенно когда исходных файлов становится больше одного, и для каждого из них надо каждый раз набивать команды компиляции и линковки. Но не все так плохо. Сейчас мы будем учиться создавать и использовать Мейкфайлы. Makefile — это набор инструкций для программы make, которая помогает собирать программный проект буквально в одно касание.

Для практики понадобится создать микроскопический проект а-ля Hello World из четырех файлов в одном каталоге:

Все скопом можно скачать отсюда
Автор использовал язык C++, знать который совсем не обязательно, и компилятор g++ из gcc. Любой другой компилятор скорее всего тоже подойдет. Файлы слегка подправлены, чтобы собирались gcc 4.7.1

Программа make

Если запустить
make
то программа попытается найти файл с именем по умолчание Makefile в текущем каталоге и выполнить инструкции из него. Если в текущем каталоге есть несколько мейкфайлов, то можно указать на нужный вот таким образом:
make -f MyMakefile
Есть еще множество других параметров, нам пока не нужных. О них можно узнать в ман-странице.

Процесс сборки

Компилятор берет файлы с исходным кодом и получает из них объектные файлы. Затем линковщик берет объектные файлы и получает из них исполняемый файл. Сборка = компиляция + линковка.

Компиляция руками

Самый простой способ собрать программу:
g++ main.cpp hello.cpp factorial.cpp -o hello
Каждый раз набирать такое неудобно, поэтому будем автоматизировать.

Самый простой Мейкфайл

В нем должны быть такие части:

Для нашего примера мейкфайл будет выглядеть так:

Обратите внимание, что строка с командой должна начинаться с табуляции! Сохраните это под именем Makefile-1 в каталоге с проектом и запустите сборку командой make -f Makefile-1
В первом примере цель называется all . Это цель по умолчанию для мейкфайла, которая будет выполняться, если никакая другая цель не указана явно. Также у этой цели в этом примере нет никаких зависимостей, так что make сразу приступает к выполнению нужной команды. А команда в свою очередь запускает компилятор.

Использование зависимостей

Использовать несколько целей в одном мейкфайле полезно для больших проектов. Это связано с тем, что при изменении одного файла не понадобится пересобирать весь проект, а можно будет обойтись пересборкой только измененной части. Пример:

Это надо сохранить под именем Makefile-2 все в том же каталоге

Теперь у цели all есть только зависимость, но нет команды. В этом случае make при вызове последовательно выполнит все указанные в файле зависимости этой цели.
Еще добавилась новая цель clean . Она традиционно используется для быстрой очистки всех результатов сборки проекта. Очистка запускается так: make -f Makefile-2 clean

Использование переменных и комментариев

Переменные широко используются в мейкфайлах. Например, это удобный способ учесть возможность того, что проект будут собирать другим компилятором или с другими опциями.

Это Makefile-3
Переменные — очень удобная штука. Для их использования надо просто присвоить им значение до момента их использования. После этого можно подставлять их значение в нужное место вот таким способом: $(VAR)

Что делать дальше

После этого краткого инструктажа уже можно пробовать создавать простые мейкфайлы самостоятельно. Дальше надо читать серьезные учебники и руководства. Как финальный аккорд можно попробовать самостоятельно разобрать и осознать такой универсальный мейкфайл, который можно в два касания адаптировать под практически любой проект:

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