diff options
Diffstat (limited to 'Documentation/kbuild')
| -rw-r--r-- | Documentation/kbuild/kconfig-language.txt | 3 | ||||
| -rw-r--r-- | Documentation/kbuild/makefiles.txt | 7 | ||||
| -rw-r--r-- | Documentation/kbuild/modules.txt | 733 |
3 files changed, 370 insertions, 373 deletions
diff --git a/Documentation/kbuild/kconfig-language.txt b/Documentation/kbuild/kconfig-language.txt index b472e4e0ba67..2fe93ca7c77c 100644 --- a/Documentation/kbuild/kconfig-language.txt +++ b/Documentation/kbuild/kconfig-language.txt @@ -322,7 +322,8 @@ mainmenu: "mainmenu" <prompt> This sets the config program's title bar if the config program chooses -to use it. +to use it. It should be placed at the top of the configuration, before any +other statement. Kconfig hints diff --git a/Documentation/kbuild/makefiles.txt b/Documentation/kbuild/makefiles.txt index c787ae512120..0ef00bd6e54d 100644 --- a/Documentation/kbuild/makefiles.txt +++ b/Documentation/kbuild/makefiles.txt @@ -776,6 +776,13 @@ This will delete the directory debian, including all subdirectories. Kbuild will assume the directories to be in the same relative path as the Makefile if no absolute path is specified (path does not start with '/'). +To exclude certain files from make clean, use the $(no-clean-files) variable. +This is only a special case used in the top level Kbuild file: + + Example: + #Kbuild + no-clean-files := $(bounds-file) $(offsets-file) + Usually kbuild descends down in subdirectories due to "obj-* := dir/", but in the architecture makefiles where the kbuild infrastructure is not sufficient this sometimes needs to be explicit. diff --git a/Documentation/kbuild/modules.txt b/Documentation/kbuild/modules.txt index 0767cf69c69e..3fb39e0116b4 100644 --- a/Documentation/kbuild/modules.txt +++ b/Documentation/kbuild/modules.txt @@ -1,215 +1,185 @@ +Building External Modules -In this document you will find information about: -- how to build external modules -- how to make your module use the kbuild infrastructure -- how kbuild will install a kernel -- how to install modules in a non-standard location +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 Building external modules - --- 2.2 Available targets - --- 2.3 Available options - --- 2.4 Preparing the kernel tree for module build - --- 2.5 Building separate files for a module - === 3. Example commands - === 4. Creating a kbuild file for an external module - === 5. Include files - --- 5.1 How to include files from the kernel include dir - --- 5.2 External modules using an include/ dir - --- 5.3 External modules using several directories - === 6. Module installation - --- 6.1 INSTALL_MOD_PATH - --- 6.2 INSTALL_MOD_DIR - === 7. Module versioning & Module.symvers - --- 7.1 Symbols from the kernel (vmlinux + modules) - --- 7.2 Symbols and external modules - --- 7.3 Symbols from another external module - === 8. Tips & Tricks - --- 8.1 Testing for CONFIG_FOO_BAR + === 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 includes functionality for building modules both -within the kernel source tree and outside the kernel source tree. -The latter is usually referred to as external or "out-of-tree" -modules and is used both during development and for modules that -are not planned to be included in the kernel tree. +"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. -What is covered within this file is mainly information to authors -of 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. A complete example will be presented in -chapter 4, "Creating a kbuild file for an external module". +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 +=== 2. How to Build External Modules -kbuild offers functionality to build external modules, with the -prerequisite that there is a pre-built kernel available with full source. -A subset of the targets available when building the kernel is available -when building an external module. +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. ---- 2.1 Building external modules +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. - Use the following command to build an external module: +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. - make -C <path-to-kernel> M=`pwd` +--- 2.1 Command Syntax - For the running kernel use: + The command to build an external module is: - make -C /lib/modules/`uname -r`/build M=`pwd` + $ make -C <path_to_kernel_src> M=$PWD - For the above command to succeed, the kernel must have been - built with modules enabled. + The kbuild system knows that an external module is being built + due to the "M=<dir>" option given in the command. - To install the modules that were just built: + To build against the running kernel use: - make -C <path-to-kernel> M=`pwd` modules_install + $ make -C /lib/modules/`uname -r`/build M=$PWD - More complex examples will be shown later, the above should - be enough to get you started. + Then to install the module(s) just built, add the target + "modules_install" to the command: ---- 2.2 Available targets + $ make -C /lib/modules/`uname -r`/build M=$PWD modules_install - $KDIR refers to the path to the kernel source top-level directory +--- 2.2 Options - make -C $KDIR M=`pwd` - Will build the module(s) located in current directory. - All output files will be located in the same directory - as the module source. - No attempts are made to update the kernel source, and it is - a precondition that a successful make has been executed - for the kernel. + ($KDIR refers to the path of the kernel source directory.) - make -C $KDIR M=`pwd` modules - The modules target is implied when no target is given. - Same functionality as if no target was specified. - See description above. + make -C $KDIR M=$PWD - make -C $KDIR M=`pwd` modules_install - Install the external module(s). - Installation default is in /lib/modules/<kernel-version>/extra, - but may be prefixed with INSTALL_MOD_PATH - see separate - chapter. + -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. - make -C $KDIR M=`pwd` clean - Remove all generated files for the module - the kernel - source directory is not modified. + M=$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. - make -C $KDIR M=`pwd` help - help will list the available target when building external - modules. +--- 2.3 Targets ---- 2.3 Available options: + When building an external module, only a subset of the "make" + targets are available. - $KDIR refers to the path to the kernel source top-level directory + make -C $KDIR M=$PWD [target] - make -C $KDIR - Used to specify where to find the kernel source. - '$KDIR' represent the directory where the kernel source is. - Make will actually change directory to the specified directory - when executed but change back when finished. + 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. - make -C $KDIR M=`pwd` - M= is used to tell kbuild that an external module is - being built. - The option given to M= is the directory where the external - module (kbuild file) is located. - When an external module is being built only a subset of the - usual targets are available. + modules + The default target for external modules. It has the + same functionality as if no target was specified. See + description above. - make -C $KDIR SUBDIRS=`pwd` - Same as M=. The SUBDIRS= syntax is kept for backwards - compatibility. + 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). ---- 2.4 Preparing the kernel tree for module build + clean + Remove all generated files in the module directory only. - To make sure the kernel contains the information required to - build external modules the target 'modules_prepare' must be used. - 'modules_prepare' 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. + help + List the available targets for external modules. ---- 2.5 Building separate files for a module - It is possible to build single files which are part of a module. - This works equally well for the kernel, a module and even for - external modules. - Examples (module foo.ko, consist of bar.o, baz.o): - make -C $KDIR M=`pwd` bar.lst - make -C $KDIR M=`pwd` bar.o - make -C $KDIR M=`pwd` foo.ko - make -C $KDIR M=`pwd` / - - -=== 3. Example commands - -This example shows the actual commands to be executed when building -an external module for the currently running kernel. -In the example below, the distribution is supposed to use the -facility to locate output files for a kernel compile in a different -directory than the kernel source - but the examples will also work -when the source and the output files are mixed in the same directory. +--- 2.4 Building Separate Files -# Kernel source -/lib/modules/<kernel-version>/source -> /usr/src/linux-<version> - -# Output from kernel compile -/lib/modules/<kernel-version>/build -> /usr/src/linux-<version>-up - -Change to the directory where the kbuild file is located and execute -the following commands to build the module: + 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. - cd /home/user/src/module - make -C /usr/src/`uname -r`/source \ - O=/lib/modules/`uname-r`/build \ - M=`pwd` + 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 / -Then, to install the module use the following command: - make -C /usr/src/`uname -r`/source \ - O=/lib/modules/`uname-r`/build \ - M=`pwd` \ - modules_install +=== 3. Creating a Kbuild File for an External Module -If you look closely you will see that this is the same command as -listed before - with the directories spelled out. +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: -The above are rather long commands, and the following chapter -lists a few tricks to make it all easier. + 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: -=== 4. Creating a kbuild file for an external module + <module_name>-y := <src1>.o <src2>.o ... -kbuild is the build system for the kernel, and external modules -must use kbuild to stay compatible with changes in the build system -and to pick up the right flags to gcc etc. +NOTE: Further documentation describing the syntax used by kbuild is +located in Documentation/kbuild/makefiles.txt. -The kbuild file used as input shall follow the syntax described -in Documentation/kbuild/makefiles.txt. This chapter will introduce a few -more tricks to be used when dealing with external modules. +The examples below demonstrate how to create a build file for the +module 8123.ko, which is built from the following files: -In the following a Makefile will be created for a module with the -following files: 8123_if.c 8123_if.h 8123_pci.c 8123_bin.o_shipped <= Binary blob ---- 4.1 Shared Makefile for module and kernel +--- 3.1 Shared Makefile - An external module always includes a wrapper Makefile supporting - building the module using 'make' with no arguments. - The Makefile provided will most likely include additional - functionality such as test targets etc. and this part shall - be filtered away from kbuild since it may impact kbuild if - name clashes occurs. + 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 @@ -219,11 +189,11 @@ following files: 8123-y := 8123_if.o 8123_pci.o 8123_bin.o else - # Normal Makefile + # normal makefile + KDIR ?= /lib/modules/`uname -r`/build - KERNELDIR := /lib/modules/`uname -r`/build - all:: - $(MAKE) -C $(KERNELDIR) M=`pwd` $@ + default: + $(MAKE) -C $(KDIR) M=$$PWD # Module specific targets genbin: @@ -231,15 +201,20 @@ following files: endif - In example 1, the check for KERNELRELEASE is used to separate - the two parts of the Makefile. kbuild will only see the two - assignments whereas make will see everything except the two - kbuild assignments. + The check for KERNELRELEASE is used to separate the two parts + of the makefile. In the example, kbuild will only see the two + assignments, whereas "make" will see everything except these + two assignments. This is due to two passes made on the file: + the first pass is by the "make" instance run on the command + line; the second pass is by the kbuild system, which is + initiated by the parameterized "make" in the default target. + +--- 3.2 Separate Kbuild File and Makefile - In recent versions of the kernel, kbuild will look for a file named - Kbuild and as second option look for a file named Makefile. - Utilising the Kbuild file makes us split up the Makefile in example 1 - into two files as shown in example 2: + In newer versions of the kernel, kbuild will first look for a + file named "Kbuild," and only if that is not found, will it + then look for a makefile. Utilizing a "Kbuild" file allows us + to split up the makefile from example 1 into two files: Example 2: --> filename: Kbuild @@ -247,20 +222,21 @@ following files: 8123-y := 8123_if.o 8123_pci.o 8123_bin.o --> filename: Makefile - KERNELDIR := /lib/modules/`uname -r`/build - all:: - $(MAKE) -C $(KERNELDIR) M=`pwd` $@ + KDIR ?= /lib/modules/`uname -r`/build + + default: + $(MAKE) -C $(KDIR) M=$$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. - In example 2, we are down to two fairly simple files and for simple - files as used in this example the split is questionable. But some - external modules use Makefiles of several hundred lines and here it - really pays off to separate the kbuild part from the rest. - Example 3 shows a backward compatible version. + The next example shows a backward compatible version. Example 3: --> filename: Kbuild @@ -269,13 +245,15 @@ following files: --> filename: Makefile ifneq ($(KERNELRELEASE),) + # kbuild part of makefile include Kbuild + else - # Normal Makefile + # normal makefile + KDIR ?= /lib/modules/`uname -r`/build - KERNELDIR := /lib/modules/`uname -r`/build - all:: - $(MAKE) -C $(KERNELDIR) M=`pwd` $@ + default: + $(MAKE) -C $(KDIR) M=$$PWD # Module specific targets genbin: @@ -283,260 +261,271 @@ following files: endif - The trick here is to include the Kbuild file from Makefile, so - if an older version of kbuild picks up the Makefile, the Kbuild - file will be included. + 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. ---- 4.2 Binary blobs included in a module +--- 3.3 Binary Blobs - Some external modules needs to include a .o as a blob. kbuild - has support for this, but requires the blob file to be named - <filename>_shipped. In our example the blob is named - 8123_bin.o_shipped and when the kbuild rules kick in the file - 8123_bin.o is created as a simple copy off the 8213_bin.o_shipped file - with the _shipped part stripped of the filename. - This allows the 8123_bin.o filename to be used in the assignment to - the module. + 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. - Example 4: - obj-m := 8123.o 8123-y := 8123_if.o 8123_pci.o 8123_bin.o - In example 4, there is no distinction between the ordinary .c/.h files - and the binary file. But kbuild will pick up different rules to create - the .o file. + 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: -=== 5. Include files + obj-m := foo.o bar.o + foo-y := <foo_srcs> + bar-y := <bar_srcs> -Include files are a necessity when a .c file uses something from other .c -files (not strictly in the sense of C, but if good programming practice is -used). Any module that consists of more than one .c file will have a .h file -for one of the .c files. + It is that simple! -- If the .h file only describes a module internal interface, then the .h file - shall be placed in the same directory as the .c files. -- If the .h files describe an interface used by other parts of the kernel - located in different directories, the .h files shall be located in - include/linux/ or other include/ directories as appropriate. -One exception for this rule is larger subsystems that have their own directory -under include/ such as include/scsi. Another exception is arch-specific -.h files which are located under include/asm-$(ARCH)/*. +=== 4. Include Files -External modules have a tendency to locate include files in a separate include/ -directory and therefore need to deal with this in their kbuild file. +Within the kernel, header files are kept in standard locations +according to the following rule: ---- 5.1 How to include files from the kernel include dir + * If the header file only describes the internal interface of a + module, then the file is placed in the same directory as the + source files. + * If the header file describes an interface used by other parts + of the kernel that are located in different directories, then + the file is placed in include/linux/. - When a module needs to include a file from include/linux/, then one - just uses: + NOTE: There are two notable exceptions to this rule: larger + subsystems have their own directory under include/, such as + include/scsi; and architecture specific headers are located + under arch/$(ARCH)/include/. - #include <linux/modules.h> +--- 4.1 Kernel Includes - kbuild will make sure to add options to gcc so the relevant - directories are searched. - Likewise for .h files placed in the same directory as the .c file. + To include a header file located under include/linux/, simply + use: - #include "8123_if.h" + #include <linux/module.h> - will do the job. + kbuild will add options to "gcc" so the relevant directories + are searched. ---- 5.2 External modules using an include/ dir +--- 4.2 Single Subdirectory - External modules often locate their .h files in a separate include/ - directory although this is not usual kernel style. When an external - module uses an include/ dir then kbuild needs to be told so. - The trick here is to use either EXTRA_CFLAGS (take effect for all .c - files) or CFLAGS_$F.o (take effect only for a single file). + External modules tend to place header files in a separate + include/ directory where their source is located, although this + is not the usual kernel style. To inform kbuild of the + directory, use either ccflags-y or CFLAGS_<filename>.o. - In our example, if we move 8123_if.h to a subdirectory named include/ - the resulting Kbuild file would look like: + Using the example from section 3, if we moved 8123_if.h to a + subdirectory named include, the resulting kbuild file would + look like: --> filename: Kbuild - obj-m := 8123.o + obj-m := 8123.o - EXTRA_CFLAGS := -Iinclude + ccflags-y := -Iinclude 8123-y := 8123_if.o 8123_pci.o 8123_bin.o - Note that in the assignment there is no space between -I and the path. - This is a kbuild limitation: there must be no space present. + Note that in the assignment there is no space between -I and + the path. This is a limitation of kbuild: there must be no + space present. ---- 5.3 External modules using several directories - - If an external module does not follow the usual kernel style, but - decides to spread files over several directories, then kbuild can - handle this too. +--- 4.3 Several Subdirectories + kbuild can handle files that are spread over several directories. Consider the following example: - | - +- src/complex_main.c - | +- hal/hardwareif.c - | +- hal/include/hardwareif.h - +- include/complex.h - - To build a single module named complex.ko, we then need the following + . + |__ src + | |__ complex_main.c + | |__ hal + | |__ hardwareif.c + | |__ include + | |__ hardwareif.h + |__ include + |__ complex.h + + To build the module complex.ko, we then need the following kbuild file: - Kbuild: + --> filename: Kbuild obj-m := complex.o complex-y := src/complex_main.o complex-y += src/hal/hardwareif.o - EXTRA_CFLAGS := -I$(src)/include - EXTRA_CFLAGS += -I$(src)src/hal/include + ccflags-y := -I$(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. - kbuild knows how to handle .o files located in another directory - - although this is NOT recommended practice. The syntax is to specify - the directory relative to the directory where the Kbuild file is - located. + 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. - To find the .h files, we have to explicitly tell kbuild where to look - for the .h files. When kbuild executes, the current directory is always - the root of the kernel tree (argument to -C) and therefore we have to - tell kbuild how to find the .h files using absolute paths. - $(src) will specify the absolute path to the directory where the - Kbuild file are located when being build as an external module. - Therefore -I$(src)/ is used to point out the directory of the Kbuild - file and any additional path are just appended. -=== 6. Module installation +=== 5. Module Installation -Modules which are included in the kernel are installed in the directory: +Modules which are included in the kernel are installed in the +directory: - /lib/modules/$(KERNELRELEASE)/kernel + /lib/modules/$(KERNELRELEASE)/kernel/ -External modules are installed in the directory: +And external modules are installed in: - /lib/modules/$(KERNELRELEASE)/extra + /lib/modules/$(KERNELRELEASE)/extra/ ---- 6.1 INSTALL_MOD_PATH +--- 5.1 INSTALL_MOD_PATH - Above are the default directories, but as always, some level of - customization is possible. One can prefix the path using the variable - 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/$(KERNELRELEASE)/kernel - - INSTALL_MOD_PATH may be set as an ordinary shell variable or as in the - example above, can be specified on the command line when calling make. - INSTALL_MOD_PATH has effect both when installing modules included in - the kernel as well as when installing external modules. + => Install dir: /frodo/lib/modules/$(KERNELRELEASE)/kernel/ ---- 6.2 INSTALL_MOD_DIR + 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. - When installing external modules they are by default installed to a - directory under /lib/modules/$(KERNELRELEASE)/extra, but one may wish - to locate modules for a specific functionality in a separate - directory. For this purpose, one can use INSTALL_MOD_DIR to specify an - alternative name to 'extra'. +--- 5.2 INSTALL_MOD_DIR - $ make INSTALL_MOD_DIR=gandalf -C KERNELDIR \ - M=`pwd` modules_install - => Install dir: /lib/modules/$(KERNELRELEASE)/gandalf + External modules are by default installed to a directory under + /lib/modules/$(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/ -=== 7. Module versioning & Module.symvers -Module versioning is enabled by the CONFIG_MODVERSIONS tag. +=== 6. Module Versioning -Module versioning is used as a simple ABI consistency check. The Module -versioning creates a CRC value of the full prototype for an exported symbol and -when a module is loaded/used then the CRC values contained in the kernel are -compared with similar values in the module. If they are not equal, then the -kernel refuses to load the module. +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. +Module.symvers contains a list of all exported symbols from a kernel +build. ---- 7.1 Symbols from the kernel (vmlinux + modules) +--- 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 symbols, the corresponding CRC value - is stored too. + 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> - Sample: + <CRC> <Symbol> <module> + 0x2d036834 scsi_remove_host drivers/scsi/scsi_mod - For a kernel build without CONFIG_MODVERSIONS enabled, the crc - would read: 0x00000000 + For a kernel build without CONFIG_MODVERSIONS enabled, the CRC + would read 0x00000000. Module.symvers serves two purposes: - 1) It lists all exported symbols both from vmlinux and all modules - 2) It lists the CRC if CONFIG_MODVERSIONS is enabled - ---- 7.2 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 and to obtain all - symbols, modpost reads Module.symvers from the kernel. - 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. - ---- 7.3 Symbols from another external module - - Sometimes, an external module uses exported symbols from another - external module. Kbuild needs to have full knowledge on all symbols - to avoid spitting out warnings about undefined symbols. - Three solutions exist to let kbuild know all symbols of more than - one external module. - 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' and 'bar', and 'foo' needs - symbols from 'bar', then one can use a common top-level kbuild - file so both modules are compiled in same build. - - Consider following directory layout: - ./foo/ <= contains the foo module - ./bar/ <= contains the bar module - The top-level Kbuild file would then look like: - - #./Kbuild: (this file may also be named Makefile) + 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/ - Executing: - make -C $KDIR M=`pwd` + And executing + + $ make -C $KDIR M=$PWD - will then do the expected and compile both modules with full - knowledge on symbols from both modules. + 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 module 'bar', one can copy the - Module.symvers file from the compilation of the 'bar' module - to the directory where the 'foo' module 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 in the Makefile - 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 Makfile. These files will be - loaded by modpost during the initialisation of its symbol - tables. - -=== 8. Tips & Tricks - ---- 8.1 Testing for CONFIG_FOO_BAR - - 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. + 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-$(CONFIG_EXT2_FS) += ext2.o @@ -544,9 +533,9 @@ Module.symvers contains a list of all exported symbols from a kernel build. ext2-y := balloc.o bitmap.o dir.o ext2-$(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 shall use kbuild when building - and therefore can use the same methods as in-kernel modules when - testing for CONFIG_ definitions. + 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. |