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![matt mooney](/assets/img/avatar_default.png)
Omit needless words and sentences; reorganize and tighten sentence structure; swap sections 2.2 and 2.3 for a more logical flow; remove section 3, therefore shifting 4->3; add to explanations; and add section on building multiple modules. Signed-off-by: matt mooney <mfm@muteddisk.com> Signed-off-by: Michal Marek <mmarek@suse.cz>
543 lines
18 KiB
Text
543 lines
18 KiB
Text
Building External Modules
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This document describes how-to build an out-of-tree kernel module.
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=== Table of Contents
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=== 1 Introduction
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=== 2 How-to Build External Modules
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--- 2.1 Command Syntax
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--- 2.2 Options
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--- 2.3 Targets
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--- 2.4 Building Separate Files
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=== 3. Creating a Kbuild File for an External Module
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--- 3.1 Shared Makefile
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--- 3.2 Separate Kbuild file and Makefile
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--- 3.3 Binary Blobs
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--- 3.4 Building Multiple Modules
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=== 4. Include files
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--- 4.1 How to include files from the kernel include dir
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--- 4.2 External modules using an include/ dir
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--- 4.3 External modules using several directories
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=== 5. Module installation
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--- 5.1 INSTALL_MOD_PATH
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--- 5.2 INSTALL_MOD_DIR
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=== 6. Module versioning & Module.symvers
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--- 6.1 Symbols from the kernel (vmlinux + modules)
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--- 6.2 Symbols and external modules
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--- 6.3 Symbols from another external module
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=== 7. Tips & Tricks
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--- 7.1 Testing for CONFIG_FOO_BAR
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=== 1. Introduction
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"kbuild" is the build system used by the Linux kernel. Modules must use
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kbuild to stay compatible with changes in the build infrastructure and
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to pick up the right flags to "gcc." Functionality for building modules
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both in-tree and out-of-tree is provided. The method for building
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either is similar, and all modules are initially developed and built
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out-of-tree.
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Covered in this document is information aimed at developers interested
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in building out-of-tree (or "external") modules. The author of an
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external module should supply a makefile that hides most of the
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complexity, so one only has to type "make" to build the module. This is
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easily accomplished, and a complete example will be presented in
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section 3.
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=== 2. How-to Build External Modules
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To build external modules, you must have a pre-built kernel available
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that contains the configuration and header files used in the build.
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Also, the kernel must have been built with modules enabled. If you are
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using a distribution kernel, there will be a package for the kernel you
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are running provided by your distribution.
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An alternative is to use the "make" target "modules_prepare." This will
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make sure the kernel contains the information required. The target
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exists solely as a simple way to prepare a kernel source tree for
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building external modules.
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NOTE: "modules_prepare" will not build Module.symvers even if
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CONFIG_MODVERSIONS is set; therefore, a full kernel build needs to be
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executed to make module versioning work.
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--- 2.1 Command Syntax
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The command to build an external module is:
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make -C <path_to_kernel_src> M=$PWD
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The kbuild system knows that an external module is being built
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due to the "M=<dir>" option given in the command.
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To build against the running kernel use:
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make -C /lib/modules/`uname -r`/build M=$PWD
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Then to install the module(s) just built, add the target
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"modules_install" to the command:
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make -C /lib/modules/`uname -r`/build M=$PWD modules_install
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--- 2.2 Options
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($KDIR refers to the path of the kernel source directory.)
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make -C $KDIR M=$PWD
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-C $KDIR
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The directory where the kernel source is located.
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"make" will actually change to the specified directory
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when executing and will change back when finished.
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M=$PWD
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Informs kbuild that an external module is being built.
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The value given to "M" is the absolute path of the
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directory where the external module (kbuild file) is
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located.
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--- 2.3 Targets
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When building an external module, only a subset of the "make"
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targets are available.
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make -C $KDIR M=$PWD [target]
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The default will build the module(s) located in the current
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directory, so a target does not need to be specified. All
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output files will also be generated in this directory. No
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attempts are made to update the kernel source, and it is a
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precondition that a successful "make" has been executed for the
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kernel.
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modules
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The default target for external modules. It has the
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same functionality as if no target was specified. See
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description above.
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modules_install
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Install the external module(s). The default location is
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/lib/modules/<kernel_release>/extra, but a prefix may
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be added with INSTALL_MOD_PATH (discussed in section 5).
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clean
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Remove all generated files in the module directory only.
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help
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List the available targets for external modules.
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--- 2.4 Building Separate Files
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It is possible to build single files that are part of a module.
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This works equally well for the kernel, a module, and even for
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external modules.
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Example (The module foo.ko, consist of bar.o and baz.o):
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make -C $KDIR M=$PWD bar.lst
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make -C $KDIR M=$PWD baz.o
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make -C $KDIR M=$PWD foo.ko
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make -C $KDIR M=$PWD /
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=== 3. Creating a Kbuild File for an External Module
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In the last section we saw the command to build a module for the
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running kernel. The module is not actually built, however, because a
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build file is required. Contained in this file will be the name of
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the module(s) being built, along with the list of requisite source
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files. The file may be as simple as a single line:
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obj-m := <module_name>.o
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The kbuild system will build <module_name>.o from <module_name>.c,
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and, after linking, will result in the kernel module <module_name>.ko.
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The above line can be put in either a "Kbuild" file or a "Makefile."
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When the module is built from multiple sources, an additional line is
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needed listing the files:
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<module_name>-y := <src1>.o <src2>.o ...
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NOTE: Further documentation describing the syntax used by kbuild is
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located in Documentation/kbuild/makefiles.txt.
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The examples below demonstrate how-to create a build file for the
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module 8123.ko, which is built from the following files:
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8123_if.c
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8123_if.h
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8123_pci.c
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8123_bin.o_shipped <= Binary blob
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--- 3.1 Shared Makefile
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An external module always includes a wrapper makefile that
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supports building the module using "make" with no arguments.
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This target is not used by kbuild; it is only for convenience.
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Additional functionality, such as test targets, can be included
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but should be filtered out from kbuild due to possible name
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clashes.
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Example 1:
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--> filename: Makefile
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ifneq ($(KERNELRELEASE),)
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# kbuild part of makefile
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obj-m := 8123.o
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8123-y := 8123_if.o 8123_pci.o 8123_bin.o
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else
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# normal makefile
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KDIR ?= /lib/modules/`uname -r`/build
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default:
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$(MAKE) -C $(KDIR) M=$$PWD
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# Module specific targets
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genbin:
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echo "X" > 8123_bin.o_shipped
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endif
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The check for KERNELRELEASE is used to separate the two parts
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of the makefile. In the example, kbuild will only see the two
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assignments, whereas "make" will see everything except these
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two assignments. This is due to two passes made on the file:
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the first pass is by the "make" instance run on the
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command line; the second pass is by the kbuild system, which is
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initiated by the parameterized "make" in the default target.
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--- 3.2 Separate Kbuild File and Makefile
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In newer versions of the kernel, kbuild will first look for a
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file named "Kbuild", and only if that is not found, will it
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then look for a makefile. Utilizing a "Kbuild" file allows us
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to split up the makefile from example 1 into two files:
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Example 2:
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--> filename: Kbuild
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obj-m := 8123.o
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8123-y := 8123_if.o 8123_pci.o 8123_bin.o
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--> filename: Makefile
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KDIR ?= /lib/modules/`uname -r`/build
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default:
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$(MAKE) -C $(KDIR) M=$$PWD
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# Module specific targets
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genbin:
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echo "X" > 8123_bin.o_shipped
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The split in example 2 is questionable due to the simplicity of
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each file; however, some external modules use makefiles
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consisting of several hundred lines, and here it really pays
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off to separate the kbuild part from the rest.
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The next example shows a backward compatible version.
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Example 3:
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--> filename: Kbuild
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obj-m := 8123.o
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8123-y := 8123_if.o 8123_pci.o 8123_bin.o
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--> filename: Makefile
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ifneq ($(KERNELRELEASE),)
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# kbuild part of makefile
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include Kbuild
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else
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# normal makefile
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KDIR ?= /lib/modules/`uname -r`/build
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default:
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$(MAKE) -C $(KDIR) M=$$PWD
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# Module specific targets
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genbin:
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echo "X" > 8123_bin.o_shipped
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endif
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Here the "Kbuild" file is included from the makefile. This
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allows an older version of kbuild, which only knows of
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makefiles, to be used when the "make" and kbuild parts are
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split into separate files.
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--- 3.3 Binary Blobs
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Some external modules need to include an object file as a blob.
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kbuild has support for this, but requires the blob file to be
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named <filename>_shipped. When the kbuild rules kick in, a copy
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of <filename>_shipped is created with _shipped stripped off,
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giving us <filename>. This shortened filename can be used in
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the assignment to the module.
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Throughout this section, 8123_bin.o_shipped has been used to
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build the kernel module 8123.ko; it has been included as
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8123_bin.o.
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8123-y := 8123_if.o 8123_pci.o 8123_bin.o
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Although there is no distinction between the ordinary source
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files and the binary file, kbuild will pick up different rules
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when creating the object file for the module.
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--- 3.4 Building Multiple Modules
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kbuild supports building multiple modules with a single build
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file. For example, if you want to build two modules, foo and
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bar, the kbuild lines would be:
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obj-m := foo.o bar.o
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foo-y := <foo_srcs>
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bar-y := <bar_srcs>
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It is that simple!
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=== 5. Include files
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Include files are a necessity when a .c file uses something from other .c
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files (not strictly in the sense of C, but if good programming practice is
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used). Any module that consists of more than one .c file will have a .h file
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for one of the .c files.
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- If the .h file only describes a module internal interface, then the .h file
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shall be placed in the same directory as the .c files.
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- If the .h files describe an interface used by other parts of the kernel
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located in different directories, the .h files shall be located in
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include/linux/ or other include/ directories as appropriate.
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One exception for this rule is larger subsystems that have their own directory
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under include/ such as include/scsi. Another exception is arch-specific
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.h files which are located under include/asm-$(ARCH)/*.
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External modules have a tendency to locate include files in a separate include/
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directory and therefore need to deal with this in their kbuild file.
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--- 5.1 How to include files from the kernel include dir
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When a module needs to include a file from include/linux/, then one
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just uses:
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#include <linux/modules.h>
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kbuild will make sure to add options to gcc so the relevant
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directories are searched.
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Likewise for .h files placed in the same directory as the .c file.
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#include "8123_if.h"
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will do the job.
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--- 5.2 External modules using an include/ dir
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External modules often locate their .h files in a separate include/
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directory although this is not usual kernel style. When an external
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module uses an include/ dir then kbuild needs to be told so.
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The trick here is to use either EXTRA_CFLAGS (take effect for all .c
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files) or CFLAGS_$F.o (take effect only for a single file).
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In our example, if we move 8123_if.h to a subdirectory named include/
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the resulting Kbuild file would look like:
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--> filename: Kbuild
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obj-m := 8123.o
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EXTRA_CFLAGS := -Iinclude
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8123-y := 8123_if.o 8123_pci.o 8123_bin.o
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Note that in the assignment there is no space between -I and the path.
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This is a kbuild limitation: there must be no space present.
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--- 5.3 External modules using several directories
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If an external module does not follow the usual kernel style, but
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decides to spread files over several directories, then kbuild can
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handle this too.
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Consider the following example:
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+- src/complex_main.c
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+- include/complex.h
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To build a single module named complex.ko, we then need the following
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kbuild file:
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Kbuild:
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obj-m := complex.o
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complex-y := src/complex_main.o
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complex-y += src/hal/hardwareif.o
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EXTRA_CFLAGS := -I$(src)/include
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EXTRA_CFLAGS += -I$(src)src/hal/include
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kbuild knows how to handle .o files located in another directory -
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although this is NOT recommended practice. The syntax is to specify
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the directory relative to the directory where the Kbuild file is
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located.
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To find the .h files, we have to explicitly tell kbuild where to look
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for the .h files. When kbuild executes, the current directory is always
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the root of the kernel tree (argument to -C) and therefore we have to
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tell kbuild how to find the .h files using absolute paths.
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$(src) will specify the absolute path to the directory where the
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Kbuild file are located when being build as an external module.
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Therefore -I$(src)/ is used to point out the directory of the Kbuild
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file and any additional path are just appended.
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=== 6. Module installation
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Modules which are included in the kernel are installed in the directory:
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/lib/modules/$(KERNELRELEASE)/kernel
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External modules are installed in the directory:
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/lib/modules/$(KERNELRELEASE)/extra
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--- 6.1 INSTALL_MOD_PATH
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Above are the default directories, but as always, some level of
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customization is possible. One can prefix the path using the variable
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INSTALL_MOD_PATH:
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$ make INSTALL_MOD_PATH=/frodo modules_install
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=> Install dir: /frodo/lib/modules/$(KERNELRELEASE)/kernel
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INSTALL_MOD_PATH may be set as an ordinary shell variable or as in the
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example above, can be specified on the command line when calling make.
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INSTALL_MOD_PATH has effect both when installing modules included in
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the kernel as well as when installing external modules.
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--- 6.2 INSTALL_MOD_DIR
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When installing external modules they are by default installed to a
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directory under /lib/modules/$(KERNELRELEASE)/extra, but one may wish
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to locate modules for a specific functionality in a separate
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directory. For this purpose, one can use INSTALL_MOD_DIR to specify an
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alternative name to 'extra'.
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$ make INSTALL_MOD_DIR=gandalf -C KERNELDIR \
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M=`pwd` modules_install
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=> Install dir: /lib/modules/$(KERNELRELEASE)/gandalf
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=== 7. Module versioning & Module.symvers
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Module versioning is enabled by the CONFIG_MODVERSIONS tag.
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Module versioning is used as a simple ABI consistency check. The Module
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versioning creates a CRC value of the full prototype for an exported symbol and
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when a module is loaded/used then the CRC values contained in the kernel are
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compared with similar values in the module. If they are not equal, then the
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kernel refuses to load the module.
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Module.symvers contains a list of all exported symbols from a kernel build.
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--- 7.1 Symbols from the kernel (vmlinux + modules)
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During a kernel build, a file named Module.symvers will be generated.
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Module.symvers contains all exported symbols from the kernel and
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compiled modules. For each symbols, the corresponding CRC value
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is stored too.
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The syntax of the Module.symvers file is:
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<CRC> <Symbol> <module>
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Sample:
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0x2d036834 scsi_remove_host drivers/scsi/scsi_mod
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For a kernel build without CONFIG_MODVERSIONS enabled, the crc
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would read: 0x00000000
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Module.symvers serves two purposes:
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1) It lists all exported symbols both from vmlinux and all modules
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2) It lists the CRC if CONFIG_MODVERSIONS is enabled
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--- 7.2 Symbols and external modules
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When building an external module, the build system needs access to
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the symbols from the kernel to check if all external symbols are
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defined. This is done in the MODPOST step and to obtain all
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symbols, modpost reads Module.symvers from the kernel.
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If a Module.symvers file is present in the directory where
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the external module is being built, this file will be read too.
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During the MODPOST step, a new Module.symvers file will be written
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containing all exported symbols that were not defined in the kernel.
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--- 7.3 Symbols from another external module
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Sometimes, an external module uses exported symbols from another
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external module. Kbuild needs to have full knowledge on all symbols
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to avoid spitting out warnings about undefined symbols.
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Three solutions exist to let kbuild know all symbols of more than
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one external module.
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The method with a top-level kbuild file is recommended but may be
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impractical in certain situations.
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Use a top-level Kbuild file
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If you have two modules: 'foo' and 'bar', and 'foo' needs
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symbols from 'bar', then one can use a common top-level kbuild
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file so both modules are compiled in same build.
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Consider following directory layout:
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./foo/ <= contains the foo module
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./bar/ <= contains the bar module
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The top-level Kbuild file would then look like:
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#./Kbuild: (this file may also be named Makefile)
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obj-y := foo/ bar/
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Executing:
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make -C $KDIR M=`pwd`
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will then do the expected and compile both modules with full
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knowledge on symbols from both modules.
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Use an extra Module.symvers file
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When an external module is built, a Module.symvers file is
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generated containing all exported symbols which are not
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defined in the kernel.
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To get access to symbols from module 'bar', one can copy the
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Module.symvers file from the compilation of the 'bar' module
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to the directory where the 'foo' module is built.
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During the module build, kbuild will read the Module.symvers
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file in the directory of the external module and when the
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build is finished, a new Module.symvers file is created
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containing the sum of all symbols defined and not part of the
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kernel.
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Use make variable KBUILD_EXTRA_SYMBOLS in the Makefile
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|
If it is impractical to copy Module.symvers from another
|
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module, you can assign a space separated list of files to
|
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KBUILD_EXTRA_SYMBOLS in your Makfile. These files will be
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|
loaded by modpost during the initialisation of its symbol
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|
tables.
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=== 8. Tips & Tricks
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--- 8.1 Testing for CONFIG_FOO_BAR
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Modules often need to check for certain CONFIG_ options to decide if
|
|
a specific feature shall be included in the module. When kbuild is used
|
|
this is done by referencing the CONFIG_ variable directly.
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|
|
|
#fs/ext2/Makefile
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|
obj-$(CONFIG_EXT2_FS) += ext2.o
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|
|
|
ext2-y := balloc.o bitmap.o dir.o
|
|
ext2-$(CONFIG_EXT2_FS_XATTR) += xattr.o
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|
|
|
External modules have traditionally used grep to check for specific
|
|
CONFIG_ settings directly in .config. This usage is broken.
|
|
As introduced before, external modules shall use kbuild when building
|
|
and therefore can use the same methods as in-kernel modules when
|
|
testing for CONFIG_ definitions.
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