#include <linux/init.h>#include <linux/kernel.h>#include <linux/module.h>static int __init hello_init(void) {    printk(KERN_INFO "hello_init\n");    return 0;}static void __exit hello_exit(void) { printk(KERN_INFO "hello_exit\n"); }module_init(hello_init);module_exit(hello_exit);MODULE_LICENSE("GPL");MODULE_AUTHOR("Hyeonki Hong <hhk7734@gmail.com>");MODULE_DESCRIPTION("Print init and exit");
ifneq ($(KERNELRELEASE),)obj-m := hello.oelseKERNELDIR ?= /lib/modules/$(shell uname -r)/buildPWD       := $(shell pwd)modules:$(MAKE) -C $(KERNELDIR) M=$(PWD) modulesmodules_install:    $(MAKE) -C$(KERNELDIR) M=$(PWD) modules_installclean:$(MAKE) -C $(KERNELDIR) M=$(PWD) cleanendif
ls /lib/modules/$(uname -r)/kernel/drivers sudo depmod sudo modprobe hello sudo modprobe -r hello dmesg Building External ModulesThis 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 usekbuild to stay compatible with changes in the build infrastructure andto pick up the right flags to "gcc." Functionality for building modulesboth in-tree and out-of-tree is provided. The method for buildingeither is similar, and all modules are initially developed and builtout-of-tree.Covered in this document is information aimed at developers interestedin building out-of-tree (or "external") modules. The author of anexternal module should supply a makefile that hides most of thecomplexity, so one only has to type "make" to build the module. This iseasily accomplished, and a complete example will be presented insection 3.=== 2. How to Build External ModulesTo build external modules, you must have a prebuilt kernel availablethat contains the configuration and header files used in the build.Also, the kernel must have been built with modules enabled. If you areusing a distribution kernel, there will be a package for the kernel youare running provided by your distribution.An alternative is to use the "make" target "modules_prepare." This willmake sure the kernel contains the information required. The targetexists solely as a simple way to prepare a kernel source tree forbuilding external modules.NOTE: "modules_prepare" will not build Module.symvers even ifCONFIG_MODVERSIONS is set; therefore, a full kernel build needs to beexecuted 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 ModuleIn the last section we saw the command to build a module for therunning kernel. The module is not actually built, however, because abuild file is required. Contained in this file will be the name ofthe module(s) being built, along with the list of requisite sourcefiles. The file may be as simple as a single line: obj-m := <module_name>.oThe 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 isneeded listing the files: <module_name>-y := <src1>.o <src2>.o ...NOTE: Further documentation describing the syntax used by kbuild islocated in Documentation/kbuild/makefiles.txt.The examples below demonstrate how to create a build file for themodule 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 FilesWithin the kernel, header files are kept in standard locationsaccording 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/$(ARCH)/include/.--- 4.1 Kernel Includes To include a header file located under include/linux/, simply use: #include <linux/module.h> kbuild will add options to "gcc" so the relevant directories are searched.--- 4.2 Single Subdirectory External modules tend to place header files in a separate include/ directory where their source is located, although this is not the usual kernel style. To inform kbuild of the directory, use either ccflags-y or CFLAGS_<filename>.o. Using the example from section 3, if we moved 8123_if.h to a subdirectory named include, the resulting kbuild file would look like: --> filename: Kbuild obj-m := 8123.o ccflags-y := -Iinclude 8123-y := 8123_if.o 8123_pci.o 8123_bin.o Note that in the assignment there is no space between -I and the path. This is a limitation of kbuild: there must be no space present.--- 4.3 Several Subdirectories kbuild can handle files that are spread over several directories. Consider the following example: . |__ src | |__ complex_main.c | |__ hal | |__ hardwareif.c | |__ include | |__ hardwareif.h |__ include |__ complex.h To build the module complex.ko, we then need the following kbuild file: --> filename: Kbuild obj-m := complex.o complex-y := src/complex_main.o complex-y += src/hal/hardwareif.o ccflags-y := -I$(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 InstallationModules which are included in the kernel are installed in thedirectory: /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 VersioningModule versioning is enabled by the CONFIG_MODVERSIONS tag, and is usedas a simple ABI consistency check. A CRC value of the full prototypefor an exported symbol is created. When a module is loaded/used, theCRC values contained in the kernel are compared with similar values inthe module; if they are not equal, the kernel refuses to load themodule.Module.symvers contains a list of all exported symbols from a kernelbuild.--- 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> 0x2d036834 scsi_remove_host drivers/scsi/scsi_mod 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. If a Module.symvers file is present in the directory where the external module is being built, this file will be read too. During the MODPOST step, a new Module.symvers file will be written containing all exported symbols that were not defined in the kernel.--- 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. Three 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-y := 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 an extra Module.symvers file  When an external module is built, a Module.symvers file  is generated containing all exported symbols which are  not defined in the kernel. To get access to symbols  from bar.ko, copy the Module.symvers file from the  compilation of bar.ko to the directory where foo.ko is  built. During the module build, kbuild will read the  Module.symvers file in the directory of the external  module, and when the build is finished, a new  Module.symvers file is created containing the sum of  all symbols defined and not part of the kernel. Use "make" variable KBUILD_EXTRA_SYMBOLS  If it is impractical to copy Module.symvers from  another module, 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.