Linux kernel module information

Kernel module (Русский)

Модули ядра — это отдельные кусочки кода, которые могут быть загружены и выгружены из ядра по мере необходимости. Они расширяют функциональность ядра без необходимости перезагрузки системы.

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Обзор

Чтобы создать модуль ядра, вы можете прочитать The Linux Kernel Module Programming Guide. Модуль можно сконфигурировать как вкомпилированный, а можно как загружаемый. Чтобы иметь возможность динамически загружать или выгружать модуль, его необходимо сконфигурировать как загружаемый модуль в настройке ядра (в этом случае строка, относящаяся к модулю должна быть отмечена буквой M ).

Модули хранятся в /usr/lib/modules/kernel_release . Чтобы узнать текущую версию вашего ядра, используйте команду uname -r .

Получение информации

Чтобы узнать, какие модули ядра загружены в настоящий момент:

Чтобы показать информацию о модуле:

Чтобы вывести список опций, с которыми загружен модуль:

Чтобы отобразить настройки для всех модулей:

Чтобы отобразить настройки для отдельного модуля:

Чтобы узнать зависимости модуля (или его псевдонима), включая сам модуль:

Автоматическое управление модулями

Сегодня все необходимые загрузки модулей делаются автоматически с помощью udev, поэтому если вам не нужно загружать какие-либо модули, не входящие в стандартное ядро, вам не придётся прописывать модули, требующиеся для загрузки в каком-либо конфигурационном файле. Однако, бывают случаи, когда вам необходимо загружать свой модуль в процессе загрузки или наоборот не загружать какой-то стандартный модуль, чтобы ваш компьютер правильно функционировал.

Чтобы дополнительные модули ядра загружались автоматически в процессе загрузки, создаются статические списки в конфигурационных файлах в директории /etc/modules-load.d/ . Каждый конфигурационный файл называется по схеме /etc/modules-load.d/

.conf . Эти конфигурационные файлы содержат список названий модулей ядра, которые необходимо грузить, разделённых переносом строки. Пустые строки и строки, в которых первым непробельным символом является # или ; , игнорируются.

Смотрите modules-load.d(5) для дополнительной информации.

Управление модулями вручную

Управление модулями ядра производится с помощью утилит, предоставляемых пакетом kmod . Вы можете использовать эти утилиты вручную.

Загрузка модуля из другого места (для тех модулей, которых нет в /lib/modules/$(uname -r)/ ):

Альтернативный вариант выгрузки модуля:

Настройка параметров модуля

Чтобы передать параметр модулю ядра, вы можете воспользоваться конфигурационным файлом в modprobe или использовать командную строку ядра.

С помощью файлов в /etc/modprobe.d/

Файлы в директории /etc/modprobe.d/ можно использовать для передачи настроек модуля в udev, который через modprobe управляет загрузкой модулей во время загрузки системы. Конфигурационные файлы в этой директории могут иметь любое имя, оканчивающееся расширением .conf . Синтаксис следующий:

С помощью командной строки ядра

Если модуль вкомпилирован в ядро, вы также можете передать параметры модулю с помощью командной строки ядра. Для всех стандартных загрузчиков, подойдёт следующий синтаксис:

Просто добавьте это в загрузчике в строку с ядром, как описано в параметрах ядра.

Создание псевдонимов

Псевдонимы (алиасы) — это альтернативные названия для модуля. Например: alias my-mod really_long_modulename означает, что вы можете использовать modprobe my-mod вместо modprobe really_long_modulename . Вы также можете использовать звёздочки в стиле shell, то есть alias my-mod* really_long_modulename будет иметь тот же эффект, что и modprobe my-mod-something . Создайте алиас:

У некоторых модулей есть алиасы, которые используются для их автоматической загрузки, когда они потребуются определённой программе. Отключение этих алиасов может предотвратить их автоматическую загрузку, при этом остаётся возможность из загрузки вручную.

Запрет загрузки

В терминах модулей ядра blacklisting означает механизм, предотвращающий загрузку какого-то модуля. Это может понадобиться, например если вам не нужна работа какого-то оборудования или если загрузка данного модуля вызывает проблемы: например, могут быть два модуля ядра, которые пытаются управлять одним и тем же оборудованием, и их совместная загрузка приводит к конфликту.

Некоторые модули загружаются как часть initramfs. Команда mkinitcpio -M напечатает все автоматически обнаруженные модули: для предотвращения initramfs от загрузки каких-то из этих модулей, занесите их в чёрный список в /etc/modprobe.d/modprobe.conf . Команда mkinitcpio -v отобразит все модули, которые необходимы некоторым хукам (например, filesystems хук, block хук и т.д.). Не забудьте добавить этот .conf файл в секцию FILES в /etc/mkinitcpio.conf , если вы этого ещё не сделали, пересоберите initramfs после того, как вы запретили загрузку модулей, а затем перезагрузитесь.

С помощью файлов в /etc/modprobe.d/

Создайте .conf файл в /etc/modprobe.d/ и добавьте строку для каждого модуля, который вы хотите запретить, используя ключевое слово blacklist . Например, если вы хотите запретить загружать модуль pcspkr :

Можно изменить такое поведение. Команда install заставляет modprobe запускать вашу собственную команду вместо вставки модуля в ядро как обычно. Поэтому вы можете насильно сделать так, чтобы модуль никогда не загружался:

Это запретит данный модуль и все модули, зависящие от него.

С помощью командной строки ядра

Вы также можете запрещать модули из загрузчика.

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Kernel module

Kernel modules are pieces of code that can be loaded and unloaded into the kernel upon demand. They extend the functionality of the kernel without the need to reboot the system.

To create a kernel module, you can read The Linux Kernel Module Programming Guide. A module can be configured as built-in or loadable. To dynamically load or remove a module, it has to be configured as a loadable module in the kernel configuration (the line related to the module will therefore display the letter M ).

Contents

Obtaining information

Modules are stored in /usr/lib/modules/kernel_release . You can use the command uname -r to get your current kernel release version.

To show what kernel modules are currently loaded:

To show information about a module:

To list the options that are set for a loaded module:

To display the comprehensive configuration of all the modules:

To display the configuration of a particular module:

List the dependencies of a module (or alias), including the module itself:

Automatic module loading with systemd

Today, all necessary modules loading is handled automatically by udev, so if you do not need to use any out-of-tree kernel modules, there is no need to put modules that should be loaded at boot in any configuration file. However, there are cases where you might want to load an extra module during the boot process, or blacklist another one for your computer to function properly.

Kernel modules can be explicitly listed in files under /etc/modules-load.d/ for systemd to load them during boot. Each configuration file is named in the style of /etc/modules-load.d/program.conf . Configuration files simply contain a list of kernel modules names to load, separated by newlines. Empty lines and lines whose first non-whitespace character is # or ; are ignored.

See modules-load.d(5) for more details.

Manual module handling

Kernel modules are handled by tools provided by kmod package. You can use these tools manually.

To load a module:

To load a module by filename (i.e. one that is not installed in /usr/lib/modules/$(uname -r)/ ):

To unload a module:

Setting module options

To pass a parameter to a kernel module, you can pass them manually with modprobe or assure certain parameters are always applied using a modprobe configuration file or by using the kernel command line.

Manually at load time using modprobe

The basic way to pass parameters to a module is using the modprobe command. Parameters are specified on command line using simple key=value assignments:

Using files in /etc/modprobe.d/

Files in /etc/modprobe.d/ directory can be used to pass module settings to udev, which will use modprobe to manage the loading of the modules during system boot. Configuration files in this directory can have any name, given that they end with the .conf extension. The syntax is:

Using kernel command line

If the module is built into the kernel, you can also pass options to the module using the kernel command line. For all common bootloaders, the following syntax is correct:

Simply add this to your bootloader’s kernel-line, as described in Kernel Parameters.

Aliasing

Aliases are alternate names for a module. For example: alias my-mod really_long_modulename means you can use modprobe my-mod instead of modprobe really_long_modulename . You can also use shell-style wildcards, so alias my-mod* really_long_modulename means that modprobe my-mod-something has the same effect. Create an alias:

Some modules have aliases which are used to automatically load them when they are needed by an application. Disabling these aliases can prevent automatic loading but will still allow the modules to be manually loaded.

Blacklisting

Blacklisting, in the context of kernel modules, is a mechanism to prevent the kernel module from loading. This could be useful if, for example, the associated hardware is not needed, or if loading that module causes problems: for instance there may be two kernel modules that try to control the same piece of hardware, and loading them together would result in a conflict.

Some modules are loaded as part of the initramfs. mkinitcpio -M will print out all automatically detected modules: to prevent the initramfs from loading some of those modules, blacklist them in a .conf file under /etc/modprobe.d and it shall be added in by the modconf hook during image generation. Running mkinitcpio -v will list all modules pulled in by the various hooks (e.g. filesystems hook, block hook, etc.). Remember to add that .conf file to the FILES array in /etc/mkinitcpio.conf if you do not have the modconf hook in your HOOKS array (e.g. you have deviated from the default configuration), and once you have blacklisted the modules regenerate the initramfs, and reboot afterwards.

Using files in /etc/modprobe.d/

Create a .conf file inside /etc/modprobe.d/ and append a line for each module you want to blacklist, using the blacklist keyword. If for example you want to prevent the pcspkr module from loading:

However, there is a workaround for this behaviour; the install command instructs modprobe to run a custom command instead of inserting the module in the kernel as normal, so you can force the module to always fail loading with:

This will effectively blacklist that module and any other that depends on it.

Using kernel command line

You can also blacklist modules from the bootloader.

Simply add module_blacklist=modname1,modname2,modname3 to your bootloader’s kernel line, as described in Kernel parameters.

Troubleshooting

Modules do not load

In case a specific module does not load and the boot log (accessible by running journalctl -b as root) says that the module is blacklisted, but the directory /etc/modprobe.d/ does not show a corresponding entry, check another modprobe source folder at /usr/lib/modprobe.d/ for blacklisting entries.

A module will not be loaded if the «vermagic» string contained within the kernel module does not match the value of the currently running kernel. If it is known that the module is compatible with the current running kernel the «vermagic» check can be ignored with modprobe —force-vermagic .

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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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