OSCL-LXR linux-6.0.1/​Documentation/​kbuild/​modules.rst	Source navigation Diff markup Identifier search General search
	Version: linux-6.0.1 spark-3.0.0 hadoop-3.2.0-src hadoop-3.3.0-src spark-2.4.7 linux-4.15.13 hadoop-2.7.4-src Revert   Change

 =========================
 Building External Modules
 =========================
 
 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 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" 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::
 
                 $ make -C <path_to_kernel_src> 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
 
 2.2 Options
 ===========
 
         ($KDIR refers to the path of the kernel source directory.)
 
         make -C $KDIR M=$PWD
 
         -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.
 
         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.
 
 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.
 
         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>/extra/, but a prefix may
                 be added with INSTALL_MOD_PATH (discussed in section 5).
 
         clean
                 Remove all generated files in the module directory only.
 
         help
                 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)::
 
                 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 ./
 
 
 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::
 
         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 Documentation/kbuild/makefiles.rst.
 
 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
         8123_bin.o_shipped      <= Binary blob
 
 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.
 
         Example 1::
 
                 --> filename: Makefile
                 ifneq ($(KERNELRELEASE),)
                 # kbuild part of makefile
                 obj-m  := 8123.o
                 8123-y := 8123_if.o 8123_pci.o 8123_bin.o
 
                 else
                 # normal makefile
                 KDIR ?= /lib/modules/`uname -r`/build
 
                 default:
                         $(MAKE) -C $(KDIR) M=$$PWD
 
                 # Module specific targets
                 genbin:
                         echo "X" > 8123_bin.o_shipped
 
                 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.
 
 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:
 
         Example 2::
 
                 --> filename: Kbuild
                 obj-m  := 8123.o
                 8123-y := 8123_if.o 8123_pci.o 8123_bin.o
 
                 --> filename: Makefile
                 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.
 
         The next example shows a backward compatible version.
 
         Example 3::
 
                 --> filename: Kbuild
                 obj-m  := 8123.o
                 8123-y := 8123_if.o 8123_pci.o 8123_bin.o
 
                 --> filename: Makefile
                 ifneq ($(KERNELRELEASE),)
                 # kbuild part of makefile
                 include Kbuild
 
                 else
                 # normal makefile
                 KDIR ?= /lib/modules/`uname -r`/build
 
                 default:
                         $(MAKE) -C $(KDIR) M=$$PWD
 
                 # Module specific targets
                 genbin:
                         echo "X" > 8123_bin.o_shipped
 
                 endif
 
         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 <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::
 
                 8123-y := 8123_if.o 8123_pci.o 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::
 
                 obj-m := foo.o bar.o
                 foo-y := <foo_srcs>
                 bar-y := <bar_srcs>
 
         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/.
 
           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/.
 
 4.1 Kernel Includes
 -------------------
 
         To include a header file located under include/linux/, simply
         use::
 
                 #include <linux/module.h>
 
         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_<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 := -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 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::
 
                 .
                 |__ 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.
 
 
 5. 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)/extra/
 
 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::
 
                 $ 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.
 
 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."::
 
                 $ make INSTALL_MOD_DIR=gandalf -C $KDIR \
                        M=$PWD modules_install
                 => Install dir: /lib/modules/$(KERNELRELEASE)/gandalf/
 
 
 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)
 -----------------------------------------------
 
         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:
 
         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. 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::
 
                         ./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.
 
 
 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 should use
         kbuild for building and can therefore use the same methods as
         in-tree modules when testing for `CONFIG_` definitions.

This page was automatically generated by the 2.1.0 LXR engine. The LXR team