构建Linux内核扩展模块

╰+哭是因爲堅強的太久メ 2022-04-10 04:24 610阅读 0赞

官方文档  
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.txt.

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/$(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 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>
    
    	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.