Linux kbuild что это

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.

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Kbuild¶

Output files¶

modules.order¶

This file records the order in which modules appear in Makefiles. This is used by modprobe to deterministically resolve aliases that match multiple modules.

modules.builtin¶

This file lists all modules that are built into the kernel. This is used by modprobe to not fail when trying to load something builtin.

modules.builtin.modinfo¶

This file contains modinfo from all modules that are built into the kernel. Unlike modinfo of a separate module, all fields are prefixed with module name.

Environment variables¶

KCPPFLAGS¶

Additional options to pass when preprocessing. The preprocessing options will be used in all cases where kbuild does preprocessing including building C files and assembler files.

KAFLAGS¶

Additional options to the assembler (for built-in and modules).

AFLAGS_MODULE¶

Additional assembler options for modules.

AFLAGS_KERNEL¶

Additional assembler options for built-in.

KCFLAGS¶

Additional options to the C compiler (for built-in and modules).

CFLAGS_KERNEL¶

Additional options for $(CC) when used to compile code that is compiled as built-in.

CFLAGS_MODULE¶

Additional module specific options to use for $(CC).

LDFLAGS_MODULE¶

Additional options used for $(LD) when linking modules.

HOSTCFLAGS¶

Additional flags to be passed to $(HOSTCC) when building host programs.

HOSTCXXFLAGS¶

Additional flags to be passed to $(HOSTCXX) when building host programs.

HOSTLDFLAGS¶

Additional flags to be passed when linking host programs.

HOSTLDLIBS¶

Additional libraries to link against when building host programs.

KBUILD_KCONFIG¶

Set the top-level Kconfig file to the value of this environment variable. The default name is “Kconfig”.

KBUILD_VERBOSE¶

Set the kbuild verbosity. Can be assigned same values as “V=…”.

See make help for the full list.

Setting “V=…” takes precedence over KBUILD_VERBOSE.

KBUILD_EXTMOD¶

Set the directory to look for the kernel source when building external modules.

Setting “M=…” takes precedence over KBUILD_EXTMOD.

KBUILD_OUTPUT¶

Specify the output directory when building the kernel.

The output directory can also be specified using “O=…”.

Setting “O=…” takes precedence over KBUILD_OUTPUT.

KBUILD_EXTRA_WARN¶

Specify the extra build checks. The same value can be assigned by passing W=… from the command line.

See make help for the list of the supported values.

Setting “W=…” takes precedence over KBUILD_EXTRA_WARN.

KBUILD_DEBARCH¶

For the deb-pkg target, allows overriding the normal heuristics deployed by deb-pkg. Normally deb-pkg attempts to guess the right architecture based on the UTS_MACHINE variable, and on some architectures also the kernel config. The value of KBUILD_DEBARCH is assumed (not checked) to be a valid Debian architecture.

ARCH¶

Set ARCH to the architecture to be built.

In most cases the name of the architecture is the same as the directory name found in the arch/ directory.

But some architectures such as x86 and sparc have aliases.

x86: i386 for 32 bit, x86_64 for 64 bit

sh: sh for 32 bit, sh64 for 64 bit

sparc: sparc32 for 32 bit, sparc64 for 64 bit

CROSS_COMPILE¶

Specify an optional fixed part of the binutils filename. CROSS_COMPILE can be a part of the filename or the full path.

CROSS_COMPILE is also used for ccache in some setups.

Additional options for sparse.

CF is often used on the command-line like this:

INSTALL_PATH¶

INSTALL_PATH specifies where to place the updated kernel and system map images. Default is /boot, but you can set it to other values.

INSTALLKERNEL¶

Install script called when using “make install”. The default name is “installkernel”.

The script will be called with the following arguments:

$1 — kernel version

$2 — kernel image file

$3 — kernel map file

$4 — default install path (use root directory if blank)

The implementation of “make install” is architecture specific and it may differ from the above.

INSTALLKERNEL is provided to enable the possibility to specify a custom installer when cross compiling a kernel.

MODLIB¶

Specify where to install modules. The default value is:

The value can be overridden in which case the default value is ignored.

INSTALL_MOD_PATH¶

INSTALL_MOD_PATH specifies a prefix to MODLIB for module directory relocations required by build roots. This is not defined in the makefile but the argument can be passed to make if needed.

INSTALL_MOD_STRIP¶

INSTALL_MOD_STRIP, if defined, will cause modules to be stripped after they are installed. If INSTALL_MOD_STRIP is ‘1’, then the default option –strip-debug will be used. Otherwise, INSTALL_MOD_STRIP value will be used as the options to the strip command.

INSTALL_HDR_PATH¶

INSTALL_HDR_PATH specifies where to install user space headers when executing “make headers_*”.

The default value is:

$(objtree) is the directory where output files are saved. The output directory is often set using “O=…” on the commandline.

The value can be overridden in which case the default value is ignored.

KBUILD_ABS_SRCTREE¶

Kbuild uses a relative path to point to the tree when possible. For instance, when building in the source tree, the source tree path is ‘.’

Setting this flag requests Kbuild to use absolute path to the source tree. There are some useful cases to do so, like when generating tag files with absolute path entries etc.

KBUILD_SIGN_PIN¶

This variable allows a passphrase or PIN to be passed to the sign-file utility when signing kernel modules, if the private key requires such.

KBUILD_MODPOST_WARN¶

KBUILD_MODPOST_WARN can be set to avoid errors in case of undefined symbols in the final module linking stage. It changes such errors into warnings.

KBUILD_MODPOST_NOFINAL¶

KBUILD_MODPOST_NOFINAL can be set to skip the final link of modules. This is solely useful to speed up test compiles.

KBUILD_EXTRA_SYMBOLS¶

For modules that use symbols from other modules. See more details in Building External Modules .

ALLSOURCE_ARCHS¶

For tags/TAGS/cscope targets, you can specify more than one arch to be included in the databases, separated by blank space. E.g.:

To get all available archs you can also specify all. E.g.:

KBUILD_BUILD_TIMESTAMP¶

Setting this to a date string overrides the timestamp used in the UTS_VERSION definition (uname -v in the running kernel). The value has to be a string that can be passed to date -d. The default value is the output of the date command at one point during build.

KBUILD_BUILD_USER, KBUILD_BUILD_HOST¶

These two variables allow to override the user @ host string displayed during boot and in /proc/version. The default value is the output of the commands whoami and host, respectively.

LLVM¶

If this variable is set to 1, Kbuild will use Clang and LLVM utilities instead of GCC and GNU binutils to build the kernel.

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