Linux Kernel Hacking 之 Buildinge External Modules

Buildinge External Modules 2.6

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.

2. How to Build External Modules

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

Options

$KDIR refers to the path of the kernel source directory

make -C $KDIR M=$PWD

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.

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 specified. 
No attemptes 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][1]

clean
    Remove all genenrated files in the module directory only.

help
    List the available targets for external modules.

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 .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:

<module_name>-y := <src1>.o <src2>.o ...

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

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.

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

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.

Binary Blobs

注：模块使用已经编译好的obj文件

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.

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/.

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.

Single Subdirectory

注：可以通过ccflags-y指定添加的include目录

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.

Several Subdirectories

注：可以通过‘+=’添加一个模块的多个obj和头文件

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

Note: $(src) is a relative path which points to the directory
where the Makefile is located. Always use $(src) when
referring to files located in the src tree.

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/$k(KERNELRELEASE)/ernel/

And external modules are installed in:

/lib/modules/$(KERNELRELEASE)/extra/

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.

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.

Symbols From the Kernel (vmlinux + modules)

注：编译外部模块时，会在外部模块目录生成Module.symvers

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.

Symbols and Extern Modules(MODPOST Check Symbol in Module.symvers)

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.

Symbols From Another External Module(How to Build Dependent Modules)

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

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.

8. Conclusions

• Command

  $(MAKE) -C $(KDIR) M=$PWD [Target]

• Makefile

  --> 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
  
  
  
  --> 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
  

• Dependence

  • Use an extra Module.symvers file
  • Use “make” variable KBUILD_EXTRA_SYMBOLS