diff options
Diffstat (limited to 'Documentation/kbuild')
| -rw-r--r-- | Documentation/kbuild/Kconfig.recursion-issue-01 | 57 | ||||
| -rw-r--r-- | Documentation/kbuild/Kconfig.recursion-issue-02 | 63 | ||||
| -rw-r--r-- | Documentation/kbuild/Kconfig.select-break | 33 | ||||
| -rw-r--r-- | Documentation/kbuild/gcc-plugins.rst | 133 | ||||
| -rw-r--r-- | Documentation/kbuild/headers_install.rst | 44 | ||||
| -rw-r--r-- | Documentation/kbuild/index.rst | 30 | ||||
| -rw-r--r-- | Documentation/kbuild/issues.rst | 15 | ||||
| -rw-r--r-- | Documentation/kbuild/kbuild.rst | 310 | ||||
| -rw-r--r-- | Documentation/kbuild/kconfig-language.rst | 772 | ||||
| -rw-r--r-- | Documentation/kbuild/kconfig-macro-language.rst | 247 | ||||
| -rw-r--r-- | Documentation/kbuild/kconfig.rst | 326 | ||||
| -rw-r--r-- | Documentation/kbuild/llvm.rst | 222 | ||||
| -rw-r--r-- | Documentation/kbuild/makefiles.rst | 1659 | ||||
| -rw-r--r-- | Documentation/kbuild/modules.rst | 561 | ||||
| -rw-r--r-- | Documentation/kbuild/reproducible-builds.rst | 137 |
15 files changed, 4609 insertions, 0 deletions
diff --git a/Documentation/kbuild/Kconfig.recursion-issue-01 b/Documentation/kbuild/Kconfig.recursion-issue-01 new file mode 100644 index 0000000000..e8877db046 --- /dev/null +++ b/Documentation/kbuild/Kconfig.recursion-issue-01 @@ -0,0 +1,57 @@ +# Simple Kconfig recursive issue +# ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +# +# Test with: +# +# make KBUILD_KCONFIG=Documentation/kbuild/Kconfig.recursion-issue-01 allnoconfig +# +# This Kconfig file has a simple recursive dependency issue. In order to +# understand why this recursive dependency issue occurs lets consider what +# Kconfig needs to address. We iterate over what Kconfig needs to address +# by stepping through the questions it needs to address sequentially. +# +# * What values are possible for CORE? +# +# CORE_BELL_A_ADVANCED selects CORE, which means that it influences the values +# that are possible for CORE. So for example if CORE_BELL_A_ADVANCED is 'y', +# CORE must be 'y' too. +# +# * What influences CORE_BELL_A_ADVANCED ? +# +# As the name implies CORE_BELL_A_ADVANCED is an advanced feature of +# CORE_BELL_A so naturally it depends on CORE_BELL_A. So if CORE_BELL_A is 'y' +# we know CORE_BELL_A_ADVANCED can be 'y' too. +# +# * What influences CORE_BELL_A ? +# +# CORE_BELL_A depends on CORE, so CORE influences CORE_BELL_A. +# +# But that is a problem, because this means that in order to determine +# what values are possible for CORE we ended up needing to address questions +# regarding possible values of CORE itself again. Answering the original +# question of what are the possible values of CORE would make the kconfig +# tools run in a loop. When this happens Kconfig exits and complains about +# the "recursive dependency detected" error. +# +# Reading the Documentation/kbuild/Kconfig.recursion-issue-01 file it may be +# obvious that an easy to solution to this problem should just be the removal +# of the "select CORE" from CORE_BELL_A_ADVANCED as that is implicit already +# since CORE_BELL_A depends on CORE. Recursive dependency issues are not always +# so trivial to resolve, we provide another example below of practical +# implications of this recursive issue where the solution is perhaps not so +# easy to understand. Note that matching semantics on the dependency on +# CORE also consist of a solution to this recursive problem. + +mainmenu "Simple example to demo kconfig recursive dependency issue" + +config CORE + tristate + +config CORE_BELL_A + tristate + depends on CORE + +config CORE_BELL_A_ADVANCED + tristate + depends on CORE_BELL_A + select CORE diff --git a/Documentation/kbuild/Kconfig.recursion-issue-02 b/Documentation/kbuild/Kconfig.recursion-issue-02 new file mode 100644 index 0000000000..09dcb92d9b --- /dev/null +++ b/Documentation/kbuild/Kconfig.recursion-issue-02 @@ -0,0 +1,63 @@ +# Cumulative Kconfig recursive issue +# ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +# +# Test with: +# +# make KBUILD_KCONFIG=Documentation/kbuild/Kconfig.recursion-issue-02 allnoconfig +# +# The recursive limitations with Kconfig has some non intuitive implications on +# kconfig semantics which are documented here. One known practical implication +# of the recursive limitation is that drivers cannot negate features from other +# drivers if they share a common core requirement and use disjoint semantics to +# annotate those requirements, ie, some drivers use "depends on" while others +# use "select". For instance it means if a driver A and driver B share the same +# core requirement, and one uses "select" while the other uses "depends on" to +# annotate this, all features that driver A selects cannot now be negated by +# driver B. +# +# A perhaps not so obvious implication of this is that, if semantics on these +# core requirements are not carefully synced, as drivers evolve features +# they select or depend on end up becoming shared requirements which cannot be +# negated by other drivers. +# +# The example provided in Documentation/kbuild/Kconfig.recursion-issue-02 +# describes a simple driver core layout of example features a kernel might +# have. Let's assume we have some CORE functionality, then the kernel has a +# series of bells and whistles it desires to implement, its not so advanced so +# it only supports bells at this time: CORE_BELL_A and CORE_BELL_B. If +# CORE_BELL_A has some advanced feature CORE_BELL_A_ADVANCED which selects +# CORE_BELL_A then CORE_BELL_A ends up becoming a common BELL feature which +# other bells in the system cannot negate. The reason for this issue is +# due to the disjoint use of semantics on expressing each bell's relationship +# with CORE, one uses "depends on" while the other uses "select". Another +# more important reason is that kconfig does not check for dependencies listed +# under 'select' for a symbol, when such symbols are selected kconfig them +# as mandatory required symbols. For more details on the heavy handed nature +# of select refer to Documentation/kbuild/Kconfig.select-break +# +# To fix this the "depends on CORE" must be changed to "select CORE", or the +# "select CORE" must be changed to "depends on CORE". +# +# For an example real world scenario issue refer to the attempt to remove +# "select FW_LOADER" [0], in the end the simple alternative solution to this +# problem consisted on matching semantics with newly introduced features. +# +# [0] https://lore.kernel.org/r/1432241149-8762-1-git-send-email-mcgrof@do-not-panic.com + +mainmenu "Simple example to demo cumulative kconfig recursive dependency implication" + +config CORE + tristate + +config CORE_BELL_A + tristate + depends on CORE + +config CORE_BELL_A_ADVANCED + tristate + select CORE_BELL_A + +config CORE_BELL_B + tristate + depends on !CORE_BELL_A + select CORE diff --git a/Documentation/kbuild/Kconfig.select-break b/Documentation/kbuild/Kconfig.select-break new file mode 100644 index 0000000000..365ceb3424 --- /dev/null +++ b/Documentation/kbuild/Kconfig.select-break @@ -0,0 +1,33 @@ +# Select broken dependency issue +# ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +# +# Test with: +# +# make KBUILD_KCONFIG=Documentation/kbuild/Kconfig.select-break menuconfig +# +# kconfig will not complain and enable this layout for configuration. This is +# currently a feature of kconfig, given select was designed to be heavy handed. +# Kconfig currently does not check the list of symbols listed on a symbol's +# "select" list, this is done on purpose to help load a set of known required +# symbols. Because of this use of select should be used with caution. An +# example of this issue is below. +# +# The option B and C are clearly contradicting with respect to A. +# However, when A is set, C can be set as well because Kconfig does not +# visit the dependencies of the select target (in this case B). And since +# Kconfig does not visit the dependencies, it breaks the dependencies of B +# (!A). + +mainmenu "Simple example to demo kconfig select broken dependency issue" + +config A + bool "CONFIG A" + +config B + bool "CONFIG B" + depends on !A + +config C + bool "CONFIG C" + depends on A + select B diff --git a/Documentation/kbuild/gcc-plugins.rst b/Documentation/kbuild/gcc-plugins.rst new file mode 100644 index 0000000000..c578c6ba3e --- /dev/null +++ b/Documentation/kbuild/gcc-plugins.rst @@ -0,0 +1,133 @@ +========================= +GCC plugin infrastructure +========================= + + +Introduction +============ + +GCC plugins are loadable modules that provide extra features to the +compiler [1]_. They are useful for runtime instrumentation and static analysis. +We can analyse, change and add further code during compilation via +callbacks [2]_, GIMPLE [3]_, IPA [4]_ and RTL passes [5]_. + +The GCC plugin infrastructure of the kernel supports building out-of-tree +modules, cross-compilation and building in a separate directory. +Plugin source files have to be compilable by a C++ compiler. + +Currently the GCC plugin infrastructure supports only some architectures. +Grep "select HAVE_GCC_PLUGINS" to find out which architectures support +GCC plugins. + +This infrastructure was ported from grsecurity [6]_ and PaX [7]_. + +-- + +.. [1] https://gcc.gnu.org/onlinedocs/gccint/Plugins.html +.. [2] https://gcc.gnu.org/onlinedocs/gccint/Plugin-API.html#Plugin-API +.. [3] https://gcc.gnu.org/onlinedocs/gccint/GIMPLE.html +.. [4] https://gcc.gnu.org/onlinedocs/gccint/IPA.html +.. [5] https://gcc.gnu.org/onlinedocs/gccint/RTL.html +.. [6] https://grsecurity.net/ +.. [7] https://pax.grsecurity.net/ + + +Purpose +======= + +GCC plugins are designed to provide a place to experiment with potential +compiler features that are neither in GCC nor Clang upstream. Once +their utility is proven, the goal is to upstream the feature into GCC +(and Clang), and then to finally remove them from the kernel once the +feature is available in all supported versions of GCC. + +Specifically, new plugins should implement only features that have no +upstream compiler support (in either GCC or Clang). + +When a feature exists in Clang but not GCC, effort should be made to +bring the feature to upstream GCC (rather than just as a kernel-specific +GCC plugin), so the entire ecosystem can benefit from it. + +Similarly, even if a feature provided by a GCC plugin does *not* exist +in Clang, but the feature is proven to be useful, effort should be spent +to upstream the feature to GCC (and Clang). + +After a feature is available in upstream GCC, the plugin will be made +unbuildable for the corresponding GCC version (and later). Once all +kernel-supported versions of GCC provide the feature, the plugin will +be removed from the kernel. + + +Files +===== + +**$(src)/scripts/gcc-plugins** + + This is the directory of the GCC plugins. + +**$(src)/scripts/gcc-plugins/gcc-common.h** + + This is a compatibility header for GCC plugins. + It should be always included instead of individual gcc headers. + +**$(src)/scripts/gcc-plugins/gcc-generate-gimple-pass.h, +$(src)/scripts/gcc-plugins/gcc-generate-ipa-pass.h, +$(src)/scripts/gcc-plugins/gcc-generate-simple_ipa-pass.h, +$(src)/scripts/gcc-plugins/gcc-generate-rtl-pass.h** + + These headers automatically generate the registration structures for + GIMPLE, SIMPLE_IPA, IPA and RTL passes. + They should be preferred to creating the structures by hand. + + +Usage +===== + +You must install the gcc plugin headers for your gcc version, +e.g., on Ubuntu for gcc-10:: + + apt-get install gcc-10-plugin-dev + +Or on Fedora:: + + dnf install gcc-plugin-devel libmpc-devel + +Or on Fedora when using cross-compilers that include plugins:: + + dnf install libmpc-devel + +Enable the GCC plugin infrastructure and some plugin(s) you want to use +in the kernel config:: + + CONFIG_GCC_PLUGINS=y + CONFIG_GCC_PLUGIN_LATENT_ENTROPY=y + ... + +Run gcc (native or cross-compiler) to ensure plugin headers are detected:: + + gcc -print-file-name=plugin + CROSS_COMPILE=arm-linux-gnu- ${CROSS_COMPILE}gcc -print-file-name=plugin + +The word "plugin" means they are not detected:: + + plugin + +A full path means they are detected:: + + /usr/lib/gcc/x86_64-redhat-linux/12/plugin + +To compile the minimum tool set including the plugin(s):: + + make scripts + +or just run the kernel make and compile the whole kernel with +the cyclomatic complexity GCC plugin. + + +4. How to add a new GCC plugin +============================== + +The GCC plugins are in scripts/gcc-plugins/. You need to put plugin source files +right under scripts/gcc-plugins/. Creating subdirectories is not supported. +It must be added to scripts/gcc-plugins/Makefile, scripts/Makefile.gcc-plugins +and a relevant Kconfig file. diff --git a/Documentation/kbuild/headers_install.rst b/Documentation/kbuild/headers_install.rst new file mode 100644 index 0000000000..f6c6b74a60 --- /dev/null +++ b/Documentation/kbuild/headers_install.rst @@ -0,0 +1,44 @@ +============================================= +Exporting kernel headers for use by userspace +============================================= + +The "make headers_install" command exports the kernel's header files in a +form suitable for use by userspace programs. + +The linux kernel's exported header files describe the API for user space +programs attempting to use kernel services. These kernel header files are +used by the system's C library (such as glibc or uClibc) to define available +system calls, as well as constants and structures to be used with these +system calls. The C library's header files include the kernel header files +from the "linux" subdirectory. The system's libc headers are usually +installed at the default location /usr/include and the kernel headers in +subdirectories under that (most notably /usr/include/linux and +/usr/include/asm). + +Kernel headers are backwards compatible, but not forwards compatible. This +means that a program built against a C library using older kernel headers +should run on a newer kernel (although it may not have access to new +features), but a program built against newer kernel headers may not work on an +older kernel. + +The "make headers_install" command can be run in the top level directory of the +kernel source code (or using a standard out-of-tree build). It takes two +optional arguments:: + + make headers_install ARCH=i386 INSTALL_HDR_PATH=/usr + +ARCH indicates which architecture to produce headers for, and defaults to the +current architecture. The linux/asm directory of the exported kernel headers +is platform-specific, to see a complete list of supported architectures use +the command:: + + ls -d include/asm-* | sed 's/.*-//' + +INSTALL_HDR_PATH indicates where to install the headers. It defaults to +"./usr". + +An 'include' directory is automatically created inside INSTALL_HDR_PATH and +headers are installed in 'INSTALL_HDR_PATH/include'. + +The kernel header export infrastructure is maintained by David Woodhouse +<dwmw2@infradead.org>. diff --git a/Documentation/kbuild/index.rst b/Documentation/kbuild/index.rst new file mode 100644 index 0000000000..cee2f99f73 --- /dev/null +++ b/Documentation/kbuild/index.rst @@ -0,0 +1,30 @@ +.. SPDX-License-Identifier: GPL-2.0 + +=================== +Kernel Build System +=================== + +.. toctree:: + :maxdepth: 1 + + kconfig-language + kconfig-macro-language + + kbuild + kconfig + makefiles + modules + + headers_install + + issues + reproducible-builds + gcc-plugins + llvm + +.. only:: subproject and html + + Indices + ======= + + * :ref:`genindex` diff --git a/Documentation/kbuild/issues.rst b/Documentation/kbuild/issues.rst new file mode 100644 index 0000000000..bdab01f733 --- /dev/null +++ b/Documentation/kbuild/issues.rst @@ -0,0 +1,15 @@ +================ +Recursion issues +================ + +issue #1 +-------- + +.. literalinclude:: Kconfig.recursion-issue-01 + :language: kconfig + +issue #2 +-------- + +.. literalinclude:: Kconfig.recursion-issue-02 + :language: kconfig diff --git a/Documentation/kbuild/kbuild.rst b/Documentation/kbuild/kbuild.rst new file mode 100644 index 0000000000..bd906407e3 --- /dev/null +++ b/Documentation/kbuild/kbuild.rst @@ -0,0 +1,310 @@ +====== +Kbuild +====== + + +Output files +============ + +modules.order +------------- +This file records the order in which modules appear in Makefiles. This +is used by modprobe to deterministically resolve aliases that match +multiple modules. + +modules.builtin +--------------- +This file lists all modules that are built into the kernel. This is used +by modprobe to not fail when trying to load something builtin. + +modules.builtin.modinfo +----------------------- +This file contains modinfo from all modules that are built into the kernel. +Unlike modinfo of a separate module, all fields are prefixed with module name. + + +Environment variables +===================== + +KCPPFLAGS +--------- +Additional options to pass when preprocessing. The preprocessing options +will be used in all cases where kbuild does preprocessing including +building C files and assembler files. + +KAFLAGS +------- +Additional options to the assembler (for built-in and modules). + +AFLAGS_MODULE +------------- +Additional assembler options for modules. + +AFLAGS_KERNEL +------------- +Additional assembler options for built-in. + +KCFLAGS +------- +Additional options to the C compiler (for built-in and modules). + +KRUSTFLAGS +---------- +Additional options to the Rust compiler (for built-in and modules). + +CFLAGS_KERNEL +------------- +Additional options for $(CC) when used to compile +code that is compiled as built-in. + +CFLAGS_MODULE +------------- +Additional module specific options to use for $(CC). + +RUSTFLAGS_KERNEL +---------------- +Additional options for $(RUSTC) when used to compile +code that is compiled as built-in. + +RUSTFLAGS_MODULE +---------------- +Additional module specific options to use for $(RUSTC). + +LDFLAGS_MODULE +-------------- +Additional options used for $(LD) when linking modules. + +HOSTCFLAGS +---------- +Additional flags to be passed to $(HOSTCC) when building host programs. + +HOSTCXXFLAGS +------------ +Additional flags to be passed to $(HOSTCXX) when building host programs. + +HOSTRUSTFLAGS +------------- +Additional flags to be passed to $(HOSTRUSTC) when building host programs. + +HOSTLDFLAGS +----------- +Additional flags to be passed when linking host programs. + +HOSTLDLIBS +---------- +Additional libraries to link against when building host programs. + +.. _userkbuildflags: + +USERCFLAGS +---------- +Additional options used for $(CC) when compiling userprogs. + +USERLDFLAGS +----------- +Additional options used for $(LD) when linking userprogs. userprogs are linked +with CC, so $(USERLDFLAGS) should include "-Wl," prefix as applicable. + +KBUILD_KCONFIG +-------------- +Set the top-level Kconfig file to the value of this environment +variable. The default name is "Kconfig". + +KBUILD_VERBOSE +-------------- +Set the kbuild verbosity. Can be assigned same values as "V=...". + +See make help for the full list. + +Setting "V=..." takes precedence over KBUILD_VERBOSE. + +KBUILD_EXTMOD +------------- +Set the directory to look for the kernel source when building external +modules. + +Setting "M=..." takes precedence over KBUILD_EXTMOD. + +KBUILD_OUTPUT +------------- +Specify the output directory when building the kernel. + +The output directory can also be specified using "O=...". + +Setting "O=..." takes precedence over KBUILD_OUTPUT. + +KBUILD_EXTRA_WARN +----------------- +Specify the extra build checks. The same value can be assigned by passing +W=... from the command line. + +See `make help` for the list of the supported values. + +Setting "W=..." takes precedence over KBUILD_EXTRA_WARN. + +KBUILD_DEBARCH +-------------- +For the deb-pkg target, allows overriding the normal heuristics deployed by +deb-pkg. Normally deb-pkg attempts to guess the right architecture based on +the UTS_MACHINE variable, and on some architectures also the kernel config. +The value of KBUILD_DEBARCH is assumed (not checked) to be a valid Debian +architecture. + +KDOCFLAGS +--------- +Specify extra (warning/error) flags for kernel-doc checks during the build, +see scripts/kernel-doc for which flags are supported. Note that this doesn't +(currently) apply to documentation builds. + +ARCH +---- +Set ARCH to the architecture to be built. + +In most cases the name of the architecture is the same as the +directory name found in the arch/ directory. + +But some architectures such as x86 and sparc have aliases. + +- x86: i386 for 32 bit, x86_64 for 64 bit +- parisc: parisc64 for 64 bit +- sparc: sparc32 for 32 bit, sparc64 for 64 bit + +CROSS_COMPILE +------------- +Specify an optional fixed part of the binutils filename. +CROSS_COMPILE can be a part of the filename or the full path. + +CROSS_COMPILE is also used for ccache in some setups. + +CF +-- +Additional options for sparse. + +CF is often used on the command-line like this:: + + make CF=-Wbitwise C=2 + +INSTALL_PATH +------------ +INSTALL_PATH specifies where to place the updated kernel and system map +images. Default is /boot, but you can set it to other values. + +INSTALLKERNEL +------------- +Install script called when using "make install". +The default name is "installkernel". + +The script will be called with the following arguments: + + - $1 - kernel version + - $2 - kernel image file + - $3 - kernel map file + - $4 - default install path (use root directory if blank) + +The implementation of "make install" is architecture specific +and it may differ from the above. + +INSTALLKERNEL is provided to enable the possibility to +specify a custom installer when cross compiling a kernel. + +MODLIB +------ +Specify where to install modules. +The default value is:: + + $(INSTALL_MOD_PATH)/lib/modules/$(KERNELRELEASE) + +The value can be overridden in which case the default value is ignored. + +INSTALL_MOD_PATH +---------------- +INSTALL_MOD_PATH specifies a prefix to MODLIB for module directory +relocations required by build roots. This is not defined in the +makefile but the argument can be passed to make if needed. + +INSTALL_MOD_STRIP +----------------- +INSTALL_MOD_STRIP, if defined, will cause modules to be +stripped after they are installed. If INSTALL_MOD_STRIP is '1', then +the default option --strip-debug will be used. Otherwise, +INSTALL_MOD_STRIP value will be used as the options to the strip command. + +INSTALL_HDR_PATH +---------------- +INSTALL_HDR_PATH specifies where to install user space headers when +executing "make headers_*". + +The default value is:: + + $(objtree)/usr + +$(objtree) is the directory where output files are saved. +The output directory is often set using "O=..." on the commandline. + +The value can be overridden in which case the default value is ignored. + +KBUILD_ABS_SRCTREE +-------------------------------------------------- +Kbuild uses a relative path to point to the tree when possible. For instance, +when building in the source tree, the source tree path is '.' + +Setting this flag requests Kbuild to use absolute path to the source tree. +There are some useful cases to do so, like when generating tag files with +absolute path entries etc. + +KBUILD_SIGN_PIN +--------------- +This variable allows a passphrase or PIN to be passed to the sign-file +utility when signing kernel modules, if the private key requires such. + +KBUILD_MODPOST_WARN +------------------- +KBUILD_MODPOST_WARN can be set to avoid errors in case of undefined +symbols in the final module linking stage. It changes such errors +into warnings. + +KBUILD_MODPOST_NOFINAL +---------------------- +KBUILD_MODPOST_NOFINAL can be set to skip the final link of modules. +This is solely useful to speed up test compiles. + +KBUILD_EXTRA_SYMBOLS +-------------------- +For modules that use symbols from other modules. +See more details in modules.rst. + +ALLSOURCE_ARCHS +--------------- +For tags/TAGS/cscope targets, you can specify more than one arch +to be included in the databases, separated by blank space. E.g.:: + + $ make ALLSOURCE_ARCHS="x86 mips arm" tags + +To get all available archs you can also specify all. E.g.:: + + $ make ALLSOURCE_ARCHS=all tags + +IGNORE_DIRS +----------- +For tags/TAGS/cscope targets, you can choose which directories won't +be included in the databases, separated by blank space. E.g.:: + + $ make IGNORE_DIRS="drivers/gpu/drm/radeon tools" cscope + +KBUILD_BUILD_TIMESTAMP +---------------------- +Setting this to a date string overrides the timestamp used in the +UTS_VERSION definition (uname -v in the running kernel). The value has to +be a string that can be passed to date -d. The default value +is the output of the date command at one point during build. + +KBUILD_BUILD_USER, KBUILD_BUILD_HOST +------------------------------------ +These two variables allow to override the user@host string displayed during +boot and in /proc/version. The default value is the output of the commands +whoami and host, respectively. + +LLVM +---- +If this variable is set to 1, Kbuild will use Clang and LLVM utilities instead +of GCC and GNU binutils to build the kernel. diff --git a/Documentation/kbuild/kconfig-language.rst b/Documentation/kbuild/kconfig-language.rst new file mode 100644 index 0000000000..0135905c0a --- /dev/null +++ b/Documentation/kbuild/kconfig-language.rst @@ -0,0 +1,772 @@ +================ +Kconfig Language +================ + +Introduction +------------ + +The configuration database is a collection of configuration options +organized in a tree structure:: + + +- Code maturity level options + | +- Prompt for development and/or incomplete code/drivers + +- General setup + | +- Networking support + | +- System V IPC + | +- BSD Process Accounting + | +- Sysctl support + +- Loadable module support + | +- Enable loadable module support + | +- Set version information on all module symbols + | +- Kernel module loader + +- ... + +Every entry has its own dependencies. These dependencies are used +to determine the visibility of an entry. Any child entry is only +visible if its parent entry is also visible. + +Menu entries +------------ + +Most entries define a config option; all other entries help to organize +them. A single configuration option is defined like this:: + + config MODVERSIONS + bool "Set version information on all module symbols" + depends on MODULES + help + Usually, modules have to be recompiled whenever you switch to a new + kernel. ... + +Every line starts with a key word and can be followed by multiple +arguments. "config" starts a new config entry. The following lines +define attributes for this config option. Attributes can be the type of +the config option, input prompt, dependencies, help text and default +values. A config option can be defined multiple times with the same +name, but every definition can have only a single input prompt and the +type must not conflict. + +Menu attributes +--------------- + +A menu entry can have a number of attributes. Not all of them are +applicable everywhere (see syntax). + +- type definition: "bool"/"tristate"/"string"/"hex"/"int" + + Every config option must have a type. There are only two basic types: + tristate and string; the other types are based on these two. The type + definition optionally accepts an input prompt, so these two examples + are equivalent:: + + bool "Networking support" + + and:: + + bool + prompt "Networking support" + +- input prompt: "prompt" <prompt> ["if" <expr>] + + Every menu entry can have at most one prompt, which is used to display + to the user. Optionally dependencies only for this prompt can be added + with "if". + +- default value: "default" <expr> ["if" <expr>] + + A config option can have any number of default values. If multiple + default values are visible, only the first defined one is active. + Default values are not limited to the menu entry where they are + defined. This means the default can be defined somewhere else or be + overridden by an earlier definition. + The default value is only assigned to the config symbol if no other + value was set by the user (via the input prompt above). If an input + prompt is visible the default value is presented to the user and can + be overridden by him. + Optionally, dependencies only for this default value can be added with + "if". + + The default value deliberately defaults to 'n' in order to avoid bloating the + build. With few exceptions, new config options should not change this. The + intent is for "make oldconfig" to add as little as possible to the config from + release to release. + + Note: + Things that merit "default y/m" include: + + a) A new Kconfig option for something that used to always be built + should be "default y". + + b) A new gatekeeping Kconfig option that hides/shows other Kconfig + options (but does not generate any code of its own), should be + "default y" so people will see those other options. + + c) Sub-driver behavior or similar options for a driver that is + "default n". This allows you to provide sane defaults. + + d) Hardware or infrastructure that everybody expects, such as CONFIG_NET + or CONFIG_BLOCK. These are rare exceptions. + +- type definition + default value:: + + "def_bool"/"def_tristate" <expr> ["if" <expr>] + + This is a shorthand notation for a type definition plus a value. + Optionally dependencies for this default value can be added with "if". + +- dependencies: "depends on" <expr> + + This defines a dependency for this menu entry. If multiple + dependencies are defined, they are connected with '&&'. Dependencies + are applied to all other options within this menu entry (which also + accept an "if" expression), so these two examples are equivalent:: + + bool "foo" if BAR + default y if BAR + + and:: + + depends on BAR + bool "foo" + default y + +- reverse dependencies: "select" <symbol> ["if" <expr>] + + While normal dependencies reduce the upper limit of a symbol (see + below), reverse dependencies can be used to force a lower limit of + another symbol. The value of the current menu symbol is used as the + minimal value <symbol> can be set to. If <symbol> is selected multiple + times, the limit is set to the largest selection. + Reverse dependencies can only be used with boolean or tristate + symbols. + + Note: + select should be used with care. select will force + a symbol to a value without visiting the dependencies. + By abusing select you are able to select a symbol FOO even + if FOO depends on BAR that is not set. + In general use select only for non-visible symbols + (no prompts anywhere) and for symbols with no dependencies. + That will limit the usefulness but on the other hand avoid + the illegal configurations all over. + +- weak reverse dependencies: "imply" <symbol> ["if" <expr>] + + This is similar to "select" as it enforces a lower limit on another + symbol except that the "implied" symbol's value may still be set to n + from a direct dependency or with a visible prompt. + + Given the following example:: + + config FOO + tristate "foo" + imply BAZ + + config BAZ + tristate "baz" + depends on BAR + + The following values are possible: + + === === ============= ============== + FOO BAR BAZ's default choice for BAZ + === === ============= ============== + n y n N/m/y + m y m M/y/n + y y y Y/m/n + n m n N/m + m m m M/n + y m m M/n + y n * N + === === ============= ============== + + This is useful e.g. with multiple drivers that want to indicate their + ability to hook into a secondary subsystem while allowing the user to + configure that subsystem out without also having to unset these drivers. + + Note: If the combination of FOO=y and BAR=m causes a link error, + you can guard the function call with IS_REACHABLE():: + + foo_init() + { + if (IS_REACHABLE(CONFIG_BAZ)) + baz_register(&foo); + ... + } + + Note: If the feature provided by BAZ is highly desirable for FOO, + FOO should imply not only BAZ, but also its dependency BAR:: + + config FOO + tristate "foo" + imply BAR + imply BAZ + +- limiting menu display: "visible if" <expr> + + This attribute is only applicable to menu blocks, if the condition is + false, the menu block is not displayed to the user (the symbols + contained there can still be selected by other symbols, though). It is + similar to a conditional "prompt" attribute for individual menu + entries. Default value of "visible" is true. + +- numerical ranges: "range" <symbol> <symbol> ["if" <expr>] + + This allows to limit the range of possible input values for int + and hex symbols. The user can only input a value which is larger than + or equal to the first symbol and smaller than or equal to the second + symbol. + +- help text: "help" + + This defines a help text. The end of the help text is determined by + the indentation level, this means it ends at the first line which has + a smaller indentation than the first line of the help text. + +- module attribute: "modules" + This declares the symbol to be used as the MODULES symbol, which + enables the third modular state for all config symbols. + At most one symbol may have the "modules" option set. + +Menu dependencies +----------------- + +Dependencies define the visibility of a menu entry and can also reduce +the input range of tristate symbols. The tristate logic used in the +expressions uses one more state than normal boolean logic to express the +module state. Dependency expressions have the following syntax:: + + <expr> ::= <symbol> (1) + <symbol> '=' <symbol> (2) + <symbol> '!=' <symbol> (3) + <symbol1> '<' <symbol2> (4) + <symbol1> '>' <symbol2> (4) + <symbol1> '<=' <symbol2> (4) + <symbol1> '>=' <symbol2> (4) + '(' <expr> ')' (5) + '!' <expr> (6) + <expr> '&&' <expr> (7) + <expr> '||' <expr> (8) + +Expressions are listed in decreasing order of precedence. + +(1) Convert the symbol into an expression. Boolean and tristate symbols + are simply converted into the respective expression values. All + other symbol types result in 'n'. +(2) If the values of both symbols are equal, it returns 'y', + otherwise 'n'. +(3) If the values of both symbols are equal, it returns 'n', + otherwise 'y'. +(4) If value of <symbol1> is respectively lower, greater, lower-or-equal, + or greater-or-equal than value of <symbol2>, it returns 'y', + otherwise 'n'. +(5) Returns the value of the expression. Used to override precedence. +(6) Returns the result of (2-/expr/). +(7) Returns the result of min(/expr/, /expr/). +(8) Returns the result of max(/expr/, /expr/). + +An expression can have a value of 'n', 'm' or 'y' (or 0, 1, 2 +respectively for calculations). A menu entry becomes visible when its +expression evaluates to 'm' or 'y'. + +There are two types of symbols: constant and non-constant symbols. +Non-constant symbols are the most common ones and are defined with the +'config' statement. Non-constant symbols consist entirely of alphanumeric +characters or underscores. +Constant symbols are only part of expressions. Constant symbols are +always surrounded by single or double quotes. Within the quote, any +other character is allowed and the quotes can be escaped using '\'. + +Menu structure +-------------- + +The position of a menu entry in the tree is determined in two ways. First +it can be specified explicitly:: + + menu "Network device support" + depends on NET + + config NETDEVICES + ... + + endmenu + +All entries within the "menu" ... "endmenu" block become a submenu of +"Network device support". All subentries inherit the dependencies from +the menu entry, e.g. this means the dependency "NET" is added to the +dependency list of the config option NETDEVICES. + +The other way to generate the menu structure is done by analyzing the +dependencies. If a menu entry somehow depends on the previous entry, it +can be made a submenu of it. First, the previous (parent) symbol must +be part of the dependency list and then one of these two conditions +must be true: + +- the child entry must become invisible, if the parent is set to 'n' +- the child entry must only be visible, if the parent is visible:: + + config MODULES + bool "Enable loadable module support" + + config MODVERSIONS + bool "Set version information on all module symbols" + depends on MODULES + + comment "module support disabled" + depends on !MODULES + +MODVERSIONS directly depends on MODULES, this means it's only visible if +MODULES is different from 'n'. The comment on the other hand is only +visible when MODULES is set to 'n'. + + +Kconfig syntax +-------------- + +The configuration file describes a series of menu entries, where every +line starts with a keyword (except help texts). The following keywords +end a menu entry: + +- config +- menuconfig +- choice/endchoice +- comment +- menu/endmenu +- if/endif +- source + +The first five also start the definition of a menu entry. + +config:: + + "config" <symbol> + <config options> + +This defines a config symbol <symbol> and accepts any of above +attributes as options. + +menuconfig:: + + "menuconfig" <symbol> + <config options> + +This is similar to the simple config entry above, but it also gives a +hint to front ends, that all suboptions should be displayed as a +separate list of options. To make sure all the suboptions will really +show up under the menuconfig entry and not outside of it, every item +from the <config options> list must depend on the menuconfig symbol. +In practice, this is achieved by using one of the next two constructs:: + + (1): + menuconfig M + if M + config C1 + config C2 + endif + + (2): + menuconfig M + config C1 + depends on M + config C2 + depends on M + +In the following examples (3) and (4), C1 and C2 still have the M +dependency, but will not appear under menuconfig M anymore, because +of C0, which doesn't depend on M:: + + (3): + menuconfig M + config C0 + if M + config C1 + config C2 + endif + + (4): + menuconfig M + config C0 + config C1 + depends on M + config C2 + depends on M + +choices:: + + "choice" [symbol] + <choice options> + <choice block> + "endchoice" + +This defines a choice group and accepts any of the above attributes as +options. A choice can only be of type bool or tristate. If no type is +specified for a choice, its type will be determined by the type of +the first choice element in the group or remain unknown if none of the +choice elements have a type specified, as well. + +While a boolean choice only allows a single config entry to be +selected, a tristate choice also allows any number of config entries +to be set to 'm'. This can be used if multiple drivers for a single +hardware exists and only a single driver can be compiled/loaded into +the kernel, but all drivers can be compiled as modules. + +A choice accepts another option "optional", which allows to set the +choice to 'n' and no entry needs to be selected. +If no [symbol] is associated with a choice, then you can not have multiple +definitions of that choice. If a [symbol] is associated to the choice, +then you may define the same choice (i.e. with the same entries) in another +place. + +comment:: + + "comment" <prompt> + <comment options> + +This defines a comment which is displayed to the user during the +configuration process and is also echoed to the output files. The only +possible options are dependencies. + +menu:: + + "menu" <prompt> + <menu options> + <menu block> + "endmenu" + +This defines a menu block, see "Menu structure" above for more +information. The only possible options are dependencies and "visible" +attributes. + +if:: + + "if" <expr> + <if block> + "endif" + +This defines an if block. The dependency expression <expr> is appended +to all enclosed menu entries. + +source:: + + "source" <prompt> + +This reads the specified configuration file. This file is always parsed. + +mainmenu:: + + "mainmenu" <prompt> + +This sets the config program's title bar if the config program chooses +to use it. It should be placed at the top of the configuration, before any +other statement. + +'#' Kconfig source file comment: + +An unquoted '#' character anywhere in a source file line indicates +the beginning of a source file comment. The remainder of that line +is a comment. + + +Kconfig hints +------------- +This is a collection of Kconfig tips, most of which aren't obvious at +first glance and most of which have become idioms in several Kconfig +files. + +Adding common features and make the usage configurable +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +It is a common idiom to implement a feature/functionality that are +relevant for some architectures but not all. +The recommended way to do so is to use a config variable named HAVE_* +that is defined in a common Kconfig file and selected by the relevant +architectures. +An example is the generic IOMAP functionality. + +We would in lib/Kconfig see:: + + # Generic IOMAP is used to ... + config HAVE_GENERIC_IOMAP + + config GENERIC_IOMAP + depends on HAVE_GENERIC_IOMAP && FOO + +And in lib/Makefile we would see:: + + obj-$(CONFIG_GENERIC_IOMAP) += iomap.o + +For each architecture using the generic IOMAP functionality we would see:: + + config X86 + select ... + select HAVE_GENERIC_IOMAP + select ... + +Note: we use the existing config option and avoid creating a new +config variable to select HAVE_GENERIC_IOMAP. + +Note: the use of the internal config variable HAVE_GENERIC_IOMAP, it is +introduced to overcome the limitation of select which will force a +config option to 'y' no matter the dependencies. +The dependencies are moved to the symbol GENERIC_IOMAP and we avoid the +situation where select forces a symbol equals to 'y'. + +Adding features that need compiler support +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +There are several features that need compiler support. The recommended way +to describe the dependency on the compiler feature is to use "depends on" +followed by a test macro:: + + config STACKPROTECTOR + bool "Stack Protector buffer overflow detection" + depends on $(cc-option,-fstack-protector) + ... + +If you need to expose a compiler capability to makefiles and/or C source files, +`CC_HAS_` is the recommended prefix for the config option:: + + config CC_HAS_FOO + def_bool $(success,$(srctree)/scripts/cc-check-foo.sh $(CC)) + +Build as module only +~~~~~~~~~~~~~~~~~~~~ +To restrict a component build to module-only, qualify its config symbol +with "depends on m". E.g.:: + + config FOO + depends on BAR && m + +limits FOO to module (=m) or disabled (=n). + +Compile-testing +~~~~~~~~~~~~~~~ +If a config symbol has a dependency, but the code controlled by the config +symbol can still be compiled if the dependency is not met, it is encouraged to +increase build coverage by adding an "|| COMPILE_TEST" clause to the +dependency. This is especially useful for drivers for more exotic hardware, as +it allows continuous-integration systems to compile-test the code on a more +common system, and detect bugs that way. +Note that compile-tested code should avoid crashing when run on a system where +the dependency is not met. + +Architecture and platform dependencies +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +Due to the presence of stubs, most drivers can now be compiled on most +architectures. However, this does not mean it makes sense to have all drivers +available everywhere, as the actual hardware may only exist on specific +architectures and platforms. This is especially true for on-SoC IP cores, +which may be limited to a specific vendor or SoC family. + +To prevent asking the user about drivers that cannot be used on the system(s) +the user is compiling a kernel for, and if it makes sense, config symbols +controlling the compilation of a driver should contain proper dependencies, +limiting the visibility of the symbol to (a superset of) the platform(s) the +driver can be used on. The dependency can be an architecture (e.g. ARM) or +platform (e.g. ARCH_OMAP4) dependency. This makes life simpler not only for +distro config owners, but also for every single developer or user who +configures a kernel. + +Such a dependency can be relaxed by combining it with the compile-testing rule +above, leading to: + + config FOO + bool "Support for foo hardware" + depends on ARCH_FOO_VENDOR || COMPILE_TEST + +Optional dependencies +~~~~~~~~~~~~~~~~~~~~~ + +Some drivers are able to optionally use a feature from another module +or build cleanly with that module disabled, but cause a link failure +when trying to use that loadable module from a built-in driver. + +The most common way to express this optional dependency in Kconfig logic +uses the slightly counterintuitive:: + + config FOO + tristate "Support for foo hardware" + depends on BAR || !BAR + +This means that there is either a dependency on BAR that disallows +the combination of FOO=y with BAR=m, or BAR is completely disabled. +For a more formalized approach if there are multiple drivers that have +the same dependency, a helper symbol can be used, like:: + + config FOO + tristate "Support for foo hardware" + depends on BAR_OPTIONAL + + config BAR_OPTIONAL + def_tristate BAR || !BAR + +Kconfig recursive dependency limitations +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +If you've hit the Kconfig error: "recursive dependency detected" you've run +into a recursive dependency issue with Kconfig, a recursive dependency can be +summarized as a circular dependency. The kconfig tools need to ensure that +Kconfig files comply with specified configuration requirements. In order to do +that kconfig must determine the values that are possible for all Kconfig +symbols, this is currently not possible if there is a circular relation +between two or more Kconfig symbols. For more details refer to the "Simple +Kconfig recursive issue" subsection below. Kconfig does not do recursive +dependency resolution; this has a few implications for Kconfig file writers. +We'll first explain why this issues exists and then provide an example +technical limitation which this brings upon Kconfig developers. Eager +developers wishing to try to address this limitation should read the next +subsections. + +Simple Kconfig recursive issue +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +Read: Documentation/kbuild/Kconfig.recursion-issue-01 + +Test with:: + + make KBUILD_KCONFIG=Documentation/kbuild/Kconfig.recursion-issue-01 allnoconfig + +Cumulative Kconfig recursive issue +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +Read: Documentation/kbuild/Kconfig.recursion-issue-02 + +Test with:: + + make KBUILD_KCONFIG=Documentation/kbuild/Kconfig.recursion-issue-02 allnoconfig + +Practical solutions to kconfig recursive issue +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +Developers who run into the recursive Kconfig issue have two options +at their disposal. We document them below and also provide a list of +historical issues resolved through these different solutions. + + a) Remove any superfluous "select FOO" or "depends on FOO" + b) Match dependency semantics: + + b1) Swap all "select FOO" to "depends on FOO" or, + + b2) Swap all "depends on FOO" to "select FOO" + +The resolution to a) can be tested with the sample Kconfig file +Documentation/kbuild/Kconfig.recursion-issue-01 through the removal +of the "select CORE" from CORE_BELL_A_ADVANCED as that is implicit already +since CORE_BELL_A depends on CORE. At times it may not be possible to remove +some dependency criteria, for such cases you can work with solution b). + +The two different resolutions for b) can be tested in the sample Kconfig file +Documentation/kbuild/Kconfig.recursion-issue-02. + +Below is a list of examples of prior fixes for these types of recursive issues; +all errors appear to involve one or more "select" statements and one or more +"depends on". + +============ =================================== +commit fix +============ =================================== +06b718c01208 select A -> depends on A +c22eacfe82f9 depends on A -> depends on B +6a91e854442c select A -> depends on A +118c565a8f2e select A -> select B +f004e5594705 select A -> depends on A +c7861f37b4c6 depends on A -> (null) +80c69915e5fb select A -> (null) (1) +c2218e26c0d0 select A -> depends on A (1) +d6ae99d04e1c select A -> depends on A +95ca19cf8cbf select A -> depends on A +8f057d7bca54 depends on A -> (null) +8f057d7bca54 depends on A -> select A +a0701f04846e select A -> depends on A +0c8b92f7f259 depends on A -> (null) +e4e9e0540928 select A -> depends on A (2) +7453ea886e87 depends on A > (null) (1) +7b1fff7e4fdf select A -> depends on A +86c747d2a4f0 select A -> depends on A +d9f9ab51e55e select A -> depends on A +0c51a4d8abd6 depends on A -> select A (3) +e98062ed6dc4 select A -> depends on A (3) +91e5d284a7f1 select A -> (null) +============ =================================== + +(1) Partial (or no) quote of error. +(2) That seems to be the gist of that fix. +(3) Same error. + +Future kconfig work +~~~~~~~~~~~~~~~~~~~ + +Work on kconfig is welcomed on both areas of clarifying semantics and on +evaluating the use of a full SAT solver for it. A full SAT solver can be +desirable to enable more complex dependency mappings and / or queries, +for instance one possible use case for a SAT solver could be that of handling +the current known recursive dependency issues. It is not known if this would +address such issues but such evaluation is desirable. If support for a full SAT +solver proves too complex or that it cannot address recursive dependency issues +Kconfig should have at least clear and well defined semantics which also +addresses and documents limitations or requirements such as the ones dealing +with recursive dependencies. + +Further work on both of these areas is welcomed on Kconfig. We elaborate +on both of these in the next two subsections. + +Semantics of Kconfig +~~~~~~~~~~~~~~~~~~~~ + +The use of Kconfig is broad, Linux is now only one of Kconfig's users: +one study has completed a broad analysis of Kconfig use in 12 projects [0]_. +Despite its widespread use, and although this document does a reasonable job +in documenting basic Kconfig syntax a more precise definition of Kconfig +semantics is welcomed. One project deduced Kconfig semantics through +the use of the xconfig configurator [1]_. Work should be done to confirm if +the deduced semantics matches our intended Kconfig design goals. +Another project formalized a denotational semantics of a core subset of +the Kconfig language [10]_. + +Having well defined semantics can be useful for tools for practical +evaluation of dependencies, for instance one such case was work to +express in boolean abstraction of the inferred semantics of Kconfig to +translate Kconfig logic into boolean formulas and run a SAT solver on this to +find dead code / features (always inactive), 114 dead features were found in +Linux using this methodology [1]_ (Section 8: Threats to validity). +The kismet tool, based on the semantics in [10]_, finds abuses of reverse +dependencies and has led to dozens of committed fixes to Linux Kconfig files [11]_. + +Confirming this could prove useful as Kconfig stands as one of the leading +industrial variability modeling languages [1]_ [2]_. Its study would help +evaluate practical uses of such languages, their use was only theoretical +and real world requirements were not well understood. As it stands though +only reverse engineering techniques have been used to deduce semantics from +variability modeling languages such as Kconfig [3]_. + +.. [0] https://www.eng.uwaterloo.ca/~shshe/kconfig_semantics.pdf +.. [1] https://gsd.uwaterloo.ca/sites/default/files/vm-2013-berger.pdf +.. [2] https://gsd.uwaterloo.ca/sites/default/files/ase241-berger_0.pdf +.. [3] https://gsd.uwaterloo.ca/sites/default/files/icse2011.pdf + +Full SAT solver for Kconfig +~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +Although SAT solvers [4]_ haven't yet been used by Kconfig directly, as noted +in the previous subsection, work has been done however to express in boolean +abstraction the inferred semantics of Kconfig to translate Kconfig logic into +boolean formulas and run a SAT solver on it [5]_. Another known related project +is CADOS [6]_ (former VAMOS [7]_) and the tools, mainly undertaker [8]_, which +has been introduced first with [9]_. The basic concept of undertaker is to +extract variability models from Kconfig and put them together with a +propositional formula extracted from CPP #ifdefs and build-rules into a SAT +solver in order to find dead code, dead files, and dead symbols. If using a SAT +solver is desirable on Kconfig one approach would be to evaluate repurposing +such efforts somehow on Kconfig. There is enough interest from mentors of +existing projects to not only help advise how to integrate this work upstream +but also help maintain it long term. Interested developers should visit: + +https://kernelnewbies.org/KernelProjects/kconfig-sat + +.. [4] https://www.cs.cornell.edu/~sabhar/chapters/SATSolvers-KR-Handbook.pdf +.. [5] https://gsd.uwaterloo.ca/sites/default/files/vm-2013-berger.pdf +.. [6] https://cados.cs.fau.de +.. [7] https://vamos.cs.fau.de +.. [8] https://undertaker.cs.fau.de +.. [9] https://www4.cs.fau.de/Publications/2011/tartler_11_eurosys.pdf +.. [10] https://paulgazzillo.com/papers/esecfse21.pdf +.. [11] https://github.com/paulgazz/kmax diff --git a/Documentation/kbuild/kconfig-macro-language.rst b/Documentation/kbuild/kconfig-macro-language.rst new file mode 100644 index 0000000000..6163467f6a --- /dev/null +++ b/Documentation/kbuild/kconfig-macro-language.rst @@ -0,0 +1,247 @@ +====================== +Kconfig macro language +====================== + +Concept +------- + +The basic idea was inspired by Make. When we look at Make, we notice sort of +two languages in one. One language describes dependency graphs consisting of +targets and prerequisites. The other is a macro language for performing textual +substitution. + +There is clear distinction between the two language stages. For example, you +can write a makefile like follows:: + + APP := foo + SRC := foo.c + CC := gcc + + $(APP): $(SRC) + $(CC) -o $(APP) $(SRC) + +The macro language replaces the variable references with their expanded form, +and handles as if the source file were input like follows:: + + foo: foo.c + gcc -o foo foo.c + +Then, Make analyzes the dependency graph and determines the targets to be +updated. + +The idea is quite similar in Kconfig - it is possible to describe a Kconfig +file like this:: + + CC := gcc + + config CC_HAS_FOO + def_bool $(shell, $(srctree)/scripts/gcc-check-foo.sh $(CC)) + +The macro language in Kconfig processes the source file into the following +intermediate:: + + config CC_HAS_FOO + def_bool y + +Then, Kconfig moves onto the evaluation stage to resolve inter-symbol +dependency as explained in kconfig-language.rst. + + +Variables +--------- + +Like in Make, a variable in Kconfig works as a macro variable. A macro +variable is expanded "in place" to yield a text string that may then be +expanded further. To get the value of a variable, enclose the variable name in +$( ). The parentheses are required even for single-letter variable names; $X is +a syntax error. The curly brace form as in ${CC} is not supported either. + +There are two types of variables: simply expanded variables and recursively +expanded variables. + +A simply expanded variable is defined using the := assignment operator. Its +righthand side is expanded immediately upon reading the line from the Kconfig +file. + +A recursively expanded variable is defined using the = assignment operator. +Its righthand side is simply stored as the value of the variable without +expanding it in any way. Instead, the expansion is performed when the variable +is used. + +There is another type of assignment operator; += is used to append text to a +variable. The righthand side of += is expanded immediately if the lefthand +side was originally defined as a simple variable. Otherwise, its evaluation is +deferred. + +The variable reference can take parameters, in the following form:: + + $(name,arg1,arg2,arg3) + +You can consider the parameterized reference as a function. (more precisely, +"user-defined function" in contrast to "built-in function" listed below). + +Useful functions must be expanded when they are used since the same function is +expanded differently if different parameters are passed. Hence, a user-defined +function is defined using the = assignment operator. The parameters are +referenced within the body definition with $(1), $(2), etc. + +In fact, recursively expanded variables and user-defined functions are the same +internally. (In other words, "variable" is "function with zero argument".) +When we say "variable" in a broad sense, it includes "user-defined function". + + +Built-in functions +------------------ + +Like Make, Kconfig provides several built-in functions. Every function takes a +particular number of arguments. + +In Make, every built-in function takes at least one argument. Kconfig allows +zero argument for built-in functions, such as $(filename), $(lineno). You could +consider those as "built-in variable", but it is just a matter of how we call +it after all. Let's say "built-in function" here to refer to natively supported +functionality. + +Kconfig currently supports the following built-in functions. + + - $(shell,command) + + The "shell" function accepts a single argument that is expanded and passed + to a subshell for execution. The standard output of the command is then read + and returned as the value of the function. Every newline in the output is + replaced with a space. Any trailing newlines are deleted. The standard error + is not returned, nor is any program exit status. + + - $(info,text) + + The "info" function takes a single argument and prints it to stdout. + It evaluates to an empty string. + + - $(warning-if,condition,text) + + The "warning-if" function takes two arguments. If the condition part is "y", + the text part is sent to stderr. The text is prefixed with the name of the + current Kconfig file and the current line number. + + - $(error-if,condition,text) + + The "error-if" function is similar to "warning-if", but it terminates the + parsing immediately if the condition part is "y". + + - $(filename) + + The 'filename' takes no argument, and $(filename) is expanded to the file + name being parsed. + + - $(lineno) + + The 'lineno' takes no argument, and $(lineno) is expanded to the line number + being parsed. + + +Make vs Kconfig +--------------- + +Kconfig adopts Make-like macro language, but the function call syntax is +slightly different. + +A function call in Make looks like this:: + + $(func-name arg1,arg2,arg3) + +The function name and the first argument are separated by at least one +whitespace. Then, leading whitespaces are trimmed from the first argument, +while whitespaces in the other arguments are kept. You need to use a kind of +trick to start the first parameter with spaces. For example, if you want +to make "info" function print " hello", you can write like follows:: + + empty := + space := $(empty) $(empty) + $(info $(space)$(space)hello) + +Kconfig uses only commas for delimiters, and keeps all whitespaces in the +function call. Some people prefer putting a space after each comma delimiter:: + + $(func-name, arg1, arg2, arg3) + +In this case, "func-name" will receive " arg1", " arg2", " arg3". The presence +of leading spaces may matter depending on the function. The same applies to +Make - for example, $(subst .c, .o, $(sources)) is a typical mistake; it +replaces ".c" with " .o". + +In Make, a user-defined function is referenced by using a built-in function, +'call', like this:: + + $(call my-func,arg1,arg2,arg3) + +Kconfig invokes user-defined functions and built-in functions in the same way. +The omission of 'call' makes the syntax shorter. + +In Make, some functions treat commas verbatim instead of argument separators. +For example, $(shell echo hello, world) runs the command "echo hello, world". +Likewise, $(info hello, world) prints "hello, world" to stdout. You could say +this is _useful_ inconsistency. + +In Kconfig, for simpler implementation and grammatical consistency, commas that +appear in the $( ) context are always delimiters. It means:: + + $(shell, echo hello, world) + +is an error because it is passing two parameters where the 'shell' function +accepts only one. To pass commas in arguments, you can use the following trick:: + + comma := , + $(shell, echo hello$(comma) world) + + +Caveats +------- + +A variable (or function) cannot be expanded across tokens. So, you cannot use +a variable as a shorthand for an expression that consists of multiple tokens. +The following works:: + + RANGE_MIN := 1 + RANGE_MAX := 3 + + config FOO + int "foo" + range $(RANGE_MIN) $(RANGE_MAX) + +But, the following does not work:: + + RANGES := 1 3 + + config FOO + int "foo" + range $(RANGES) + +A variable cannot be expanded to any keyword in Kconfig. The following does +not work:: + + MY_TYPE := tristate + + config FOO + $(MY_TYPE) "foo" + default y + +Obviously from the design, $(shell command) is expanded in the textual +substitution phase. You cannot pass symbols to the 'shell' function. + +The following does not work as expected:: + + config ENDIAN_FLAG + string + default "-mbig-endian" if CPU_BIG_ENDIAN + default "-mlittle-endian" if CPU_LITTLE_ENDIAN + + config CC_HAS_ENDIAN_FLAG + def_bool $(shell $(srctree)/scripts/gcc-check-flag ENDIAN_FLAG) + +Instead, you can do like follows so that any function call is statically +expanded:: + + config CC_HAS_ENDIAN_FLAG + bool + default $(shell $(srctree)/scripts/gcc-check-flag -mbig-endian) if CPU_BIG_ENDIAN + default $(shell $(srctree)/scripts/gcc-check-flag -mlittle-endian) if CPU_LITTLE_ENDIAN diff --git a/Documentation/kbuild/kconfig.rst b/Documentation/kbuild/kconfig.rst new file mode 100644 index 0000000000..c946eb44bd --- /dev/null +++ b/Documentation/kbuild/kconfig.rst @@ -0,0 +1,326 @@ +=================== +Kconfig make config +=================== + +This file contains some assistance for using `make *config`. + +Use "make help" to list all of the possible configuration targets. + +The xconfig ('qconf'), menuconfig ('mconf'), and nconfig ('nconf') +programs also have embedded help text. Be sure to check that for +navigation, search, and other general help text. + +The gconfig ('gconf') program has limited help text. + +General +------- + +New kernel releases often introduce new config symbols. Often more +important, new kernel releases may rename config symbols. When +this happens, using a previously working .config file and running +"make oldconfig" won't necessarily produce a working new kernel +for you, so you may find that you need to see what NEW kernel +symbols have been introduced. + +To see a list of new config symbols, use:: + + cp user/some/old.config .config + make listnewconfig + +and the config program will list any new symbols, one per line. + +Alternatively, you can use the brute force method:: + + make oldconfig + scripts/diffconfig .config.old .config | less + +---------------------------------------------------------------------- + +Environment variables for `*config` + +KCONFIG_CONFIG +-------------- +This environment variable can be used to specify a default kernel config +file name to override the default name of ".config". + +KCONFIG_DEFCONFIG_LIST +---------------------- + +This environment variable specifies a list of config files which can be used +as a base configuration in case the .config does not exist yet. Entries in +the list are separated with whitespaces to each other, and the first one +that exists is used. + +KCONFIG_OVERWRITECONFIG +----------------------- +If you set KCONFIG_OVERWRITECONFIG in the environment, Kconfig will not +break symlinks when .config is a symlink to somewhere else. + +KCONFIG_WARN_UNKNOWN_SYMBOLS +---------------------------- +This environment variable makes Kconfig warn about all unrecognized +symbols in the config input. + +KCONFIG_WERROR +-------------- +If set, Kconfig treats warnings as errors. + +`CONFIG_` +--------- +If you set `CONFIG_` in the environment, Kconfig will prefix all symbols +with its value when saving the configuration, instead of using the default, +`CONFIG_`. + +---------------------------------------------------------------------- + +Environment variables for '{allyes/allmod/allno/rand}config' + +KCONFIG_ALLCONFIG +----------------- +(partially based on lkml email from/by Rob Landley, re: miniconfig) + +-------------------------------------------------- + +The allyesconfig/allmodconfig/allnoconfig/randconfig variants can also +use the environment variable KCONFIG_ALLCONFIG as a flag or a filename +that contains config symbols that the user requires to be set to a +specific value. If KCONFIG_ALLCONFIG is used without a filename where +KCONFIG_ALLCONFIG == "" or KCONFIG_ALLCONFIG == "1", `make *config` +checks for a file named "all{yes/mod/no/def/random}.config" +(corresponding to the `*config` command that was used) for symbol values +that are to be forced. If this file is not found, it checks for a +file named "all.config" to contain forced values. + +This enables you to create "miniature" config (miniconfig) or custom +config files containing just the config symbols that you are interested +in. Then the kernel config system generates the full .config file, +including symbols of your miniconfig file. + +This 'KCONFIG_ALLCONFIG' file is a config file which contains +(usually a subset of all) preset config symbols. These variable +settings are still subject to normal dependency checks. + +Examples:: + + KCONFIG_ALLCONFIG=custom-notebook.config make allnoconfig + +or:: + + KCONFIG_ALLCONFIG=mini.config make allnoconfig + +or:: + + make KCONFIG_ALLCONFIG=mini.config allnoconfig + +These examples will disable most options (allnoconfig) but enable or +disable the options that are explicitly listed in the specified +mini-config files. + +---------------------------------------------------------------------- + +Environment variables for 'randconfig' + +KCONFIG_SEED +------------ +You can set this to the integer value used to seed the RNG, if you want +to somehow debug the behaviour of the kconfig parser/frontends. +If not set, the current time will be used. + +KCONFIG_PROBABILITY +------------------- +This variable can be used to skew the probabilities. This variable can +be unset or empty, or set to three different formats: + + ======================= ================== ===================== + KCONFIG_PROBABILITY y:n split y:m:n split + ======================= ================== ===================== + unset or empty 50 : 50 33 : 33 : 34 + N N : 100-N N/2 : N/2 : 100-N + [1] N:M N+M : 100-(N+M) N : M : 100-(N+M) + [2] N:M:L N : 100-N M : L : 100-(M+L) + ======================= ================== ===================== + +where N, M and L are integers (in base 10) in the range [0,100], and so +that: + + [1] N+M is in the range [0,100] + + [2] M+L is in the range [0,100] + +Examples:: + + KCONFIG_PROBABILITY=10 + 10% of booleans will be set to 'y', 90% to 'n' + 5% of tristates will be set to 'y', 5% to 'm', 90% to 'n' + KCONFIG_PROBABILITY=15:25 + 40% of booleans will be set to 'y', 60% to 'n' + 15% of tristates will be set to 'y', 25% to 'm', 60% to 'n' + KCONFIG_PROBABILITY=10:15:15 + 10% of booleans will be set to 'y', 90% to 'n' + 15% of tristates will be set to 'y', 15% to 'm', 70% to 'n' + +---------------------------------------------------------------------- + +Environment variables for 'syncconfig' + +KCONFIG_NOSILENTUPDATE +---------------------- +If this variable has a non-blank value, it prevents silent kernel +config updates (requires explicit updates). + +KCONFIG_AUTOCONFIG +------------------ +This environment variable can be set to specify the path & name of the +"auto.conf" file. Its default value is "include/config/auto.conf". + +KCONFIG_AUTOHEADER +------------------ +This environment variable can be set to specify the path & name of the +"autoconf.h" (header) file. +Its default value is "include/generated/autoconf.h". + + +---------------------------------------------------------------------- + +menuconfig +---------- + +SEARCHING for CONFIG symbols + +Searching in menuconfig: + + The Search function searches for kernel configuration symbol + names, so you have to know something close to what you are + looking for. + + Example:: + + /hotplug + This lists all config symbols that contain "hotplug", + e.g., HOTPLUG_CPU, MEMORY_HOTPLUG. + + For search help, enter / followed by TAB-TAB (to highlight + <Help>) and Enter. This will tell you that you can also use + regular expressions (regexes) in the search string, so if you + are not interested in MEMORY_HOTPLUG, you could try:: + + /^hotplug + + When searching, symbols are sorted thus: + + - first, exact matches, sorted alphabetically (an exact match + is when the search matches the complete symbol name); + - then, other matches, sorted alphabetically. + + For example: ^ATH.K matches: + + ATH5K ATH9K ATH5K_AHB ATH5K_DEBUG [...] ATH6KL ATH6KL_DEBUG + [...] ATH9K_AHB ATH9K_BTCOEX_SUPPORT ATH9K_COMMON [...] + + of which only ATH5K and ATH9K match exactly and so are sorted + first (and in alphabetical order), then come all other symbols, + sorted in alphabetical order. + + In this menu, pressing the key in the (#) prefix will jump + directly to that location. You will be returned to the current + search results after exiting this new menu. + +---------------------------------------------------------------------- + +User interface options for 'menuconfig' + +MENUCONFIG_COLOR +---------------- +It is possible to select different color themes using the variable +MENUCONFIG_COLOR. To select a theme use:: + + make MENUCONFIG_COLOR=<theme> menuconfig + +Available themes are:: + + - mono => selects colors suitable for monochrome displays + - blackbg => selects a color scheme with black background + - classic => theme with blue background. The classic look + - bluetitle => a LCD friendly version of classic. (default) + +MENUCONFIG_MODE +--------------- +This mode shows all sub-menus in one large tree. + +Example:: + + make MENUCONFIG_MODE=single_menu menuconfig + +---------------------------------------------------------------------- + +nconfig +------- + +nconfig is an alternate text-based configurator. It lists function +keys across the bottom of the terminal (window) that execute commands. +You can also just use the corresponding numeric key to execute the +commands unless you are in a data entry window. E.g., instead of F6 +for Save, you can just press 6. + +Use F1 for Global help or F3 for the Short help menu. + +Searching in nconfig: + + You can search either in the menu entry "prompt" strings + or in the configuration symbols. + + Use / to begin a search through the menu entries. This does + not support regular expressions. Use <Down> or <Up> for + Next hit and Previous hit, respectively. Use <Esc> to + terminate the search mode. + + F8 (SymSearch) searches the configuration symbols for the + given string or regular expression (regex). + + In the SymSearch, pressing the key in the (#) prefix will + jump directly to that location. You will be returned to the + current search results after exiting this new menu. + +NCONFIG_MODE +------------ +This mode shows all sub-menus in one large tree. + +Example:: + + make NCONFIG_MODE=single_menu nconfig + +---------------------------------------------------------------------- + +xconfig +------- + +Searching in xconfig: + + The Search function searches for kernel configuration symbol + names, so you have to know something close to what you are + looking for. + + Example:: + + Ctrl-F hotplug + + or:: + + Menu: File, Search, hotplug + + lists all config symbol entries that contain "hotplug" in + the symbol name. In this Search dialog, you may change the + config setting for any of the entries that are not grayed out. + You can also enter a different search string without having + to return to the main menu. + + +---------------------------------------------------------------------- + +gconfig +------- + +Searching in gconfig: + + There is no search command in gconfig. However, gconfig does + have several different viewing choices, modes, and options. diff --git a/Documentation/kbuild/llvm.rst b/Documentation/kbuild/llvm.rst new file mode 100644 index 0000000000..b1d97fafdd --- /dev/null +++ b/Documentation/kbuild/llvm.rst @@ -0,0 +1,222 @@ +.. _kbuild_llvm: + +============================== +Building Linux with Clang/LLVM +============================== + +This document covers how to build the Linux kernel with Clang and LLVM +utilities. + +About +----- + +The Linux kernel has always traditionally been compiled with GNU toolchains +such as GCC and binutils. Ongoing work has allowed for `Clang +<https://clang.llvm.org/>`_ and `LLVM <https://llvm.org/>`_ utilities to be +used as viable substitutes. Distributions such as `Android +<https://www.android.com/>`_, `ChromeOS +<https://www.chromium.org/chromium-os>`_, `OpenMandriva +<https://www.openmandriva.org/>`_, and `Chimera Linux +<https://chimera-linux.org/>`_ use Clang built kernels. Google's and Meta's +datacenter fleets also run kernels built with Clang. + +`LLVM is a collection of toolchain components implemented in terms of C++ +objects <https://www.aosabook.org/en/llvm.html>`_. Clang is a front-end to LLVM +that supports C and the GNU C extensions required by the kernel, and is +pronounced "klang," not "see-lang." + +Building with LLVM +------------------ + +Invoke ``make`` via:: + + make LLVM=1 + +to compile for the host target. For cross compiling:: + + make LLVM=1 ARCH=arm64 + +The LLVM= argument +------------------ + +LLVM has substitutes for GNU binutils utilities. They can be enabled +individually. The full list of supported make variables:: + + make CC=clang LD=ld.lld AR=llvm-ar NM=llvm-nm STRIP=llvm-strip \ + OBJCOPY=llvm-objcopy OBJDUMP=llvm-objdump READELF=llvm-readelf \ + HOSTCC=clang HOSTCXX=clang++ HOSTAR=llvm-ar HOSTLD=ld.lld + +``LLVM=1`` expands to the above. + +If your LLVM tools are not available in your PATH, you can supply their +location using the LLVM variable with a trailing slash:: + + make LLVM=/path/to/llvm/ + +which will use ``/path/to/llvm/clang``, ``/path/to/llvm/ld.lld``, etc. The +following may also be used:: + + PATH=/path/to/llvm:$PATH make LLVM=1 + +If your LLVM tools have a version suffix and you want to test with that +explicit version rather than the unsuffixed executables like ``LLVM=1``, you +can pass the suffix using the ``LLVM`` variable:: + + make LLVM=-14 + +which will use ``clang-14``, ``ld.lld-14``, etc. + +To support combinations of out of tree paths with version suffixes, we +recommend:: + + PATH=/path/to/llvm/:$PATH make LLVM=-14 + +``LLVM=0`` is not the same as omitting ``LLVM`` altogether, it will behave like +``LLVM=1``. If you only wish to use certain LLVM utilities, use their +respective make variables. + +The same value used for ``LLVM=`` should be set for each invocation of ``make`` +if configuring and building via distinct commands. ``LLVM=`` should also be set +as an environment variable when running scripts that will eventually run +``make``. + +Cross Compiling +--------------- + +A single Clang compiler binary (and corresponding LLVM utilities) will +typically contain all supported back ends, which can help simplify cross +compiling especially when ``LLVM=1`` is used. If you use only LLVM tools, +``CROSS_COMPILE`` or target-triple-prefixes become unnecessary. Example:: + + make LLVM=1 ARCH=arm64 + +As an example of mixing LLVM and GNU utilities, for a target like ``ARCH=s390`` +which does not yet have ``ld.lld`` or ``llvm-objcopy`` support, you could +invoke ``make`` via:: + + make LLVM=1 ARCH=s390 LD=s390x-linux-gnu-ld.bfd \ + OBJCOPY=s390x-linux-gnu-objcopy + +This example will invoke ``s390x-linux-gnu-ld.bfd`` as the linker and +``s390x-linux-gnu-objcopy``, so ensure those are reachable in your ``$PATH``. + +``CROSS_COMPILE`` is not used to prefix the Clang compiler binary (or +corresponding LLVM utilities) as is the case for GNU utilities when ``LLVM=1`` +is not set. + +The LLVM_IAS= argument +---------------------- + +Clang can assemble assembler code. You can pass ``LLVM_IAS=0`` to disable this +behavior and have Clang invoke the corresponding non-integrated assembler +instead. Example:: + + make LLVM=1 LLVM_IAS=0 + +``CROSS_COMPILE`` is necessary when cross compiling and ``LLVM_IAS=0`` +is used in order to set ``--prefix=`` for the compiler to find the +corresponding non-integrated assembler (typically, you don't want to use the +system assembler when targeting another architecture). Example:: + + make LLVM=1 ARCH=arm LLVM_IAS=0 CROSS_COMPILE=arm-linux-gnueabi- + + +Ccache +------ + +``ccache`` can be used with ``clang`` to improve subsequent builds, (though +KBUILD_BUILD_TIMESTAMP_ should be set to a deterministic value between builds +in order to avoid 100% cache misses, see Reproducible_builds_ for more info): + + KBUILD_BUILD_TIMESTAMP='' make LLVM=1 CC="ccache clang" + +.. _KBUILD_BUILD_TIMESTAMP: kbuild.html#kbuild-build-timestamp +.. _Reproducible_builds: reproducible-builds.html#timestamps + +Supported Architectures +----------------------- + +LLVM does not target all of the architectures that Linux supports and +just because a target is supported in LLVM does not mean that the kernel +will build or work without any issues. Below is a general summary of +architectures that currently work with ``CC=clang`` or ``LLVM=1``. Level +of support corresponds to "S" values in the MAINTAINERS files. If an +architecture is not present, it either means that LLVM does not target +it or there are known issues. Using the latest stable version of LLVM or +even the development tree will generally yield the best results. +An architecture's ``defconfig`` is generally expected to work well, +certain configurations may have problems that have not been uncovered +yet. Bug reports are always welcome at the issue tracker below! + +.. list-table:: + :widths: 10 10 10 + :header-rows: 1 + + * - Architecture + - Level of support + - ``make`` command + * - arm + - Supported + - ``LLVM=1`` + * - arm64 + - Supported + - ``LLVM=1`` + * - hexagon + - Maintained + - ``LLVM=1`` + * - loongarch + - Maintained + - ``LLVM=1`` + * - mips + - Maintained + - ``LLVM=1`` + * - powerpc + - Maintained + - ``LLVM=1`` + * - riscv + - Supported + - ``LLVM=1`` + * - s390 + - Maintained + - ``CC=clang`` + * - um (User Mode) + - Maintained + - ``LLVM=1`` + * - x86 + - Supported + - ``LLVM=1`` + +Getting Help +------------ + +- `Website <https://clangbuiltlinux.github.io/>`_ +- `Mailing List <https://lore.kernel.org/llvm/>`_: <llvm@lists.linux.dev> +- `Old Mailing List Archives <https://groups.google.com/g/clang-built-linux>`_ +- `Issue Tracker <https://github.com/ClangBuiltLinux/linux/issues>`_ +- IRC: #clangbuiltlinux on irc.libera.chat +- `Telegram <https://t.me/ClangBuiltLinux>`_: @ClangBuiltLinux +- `Wiki <https://github.com/ClangBuiltLinux/linux/wiki>`_ +- `Beginner Bugs <https://github.com/ClangBuiltLinux/linux/issues?q=is%3Aopen+is%3Aissue+label%3A%22good+first+issue%22>`_ + +.. _getting_llvm: + +Getting LLVM +------------- + +We provide prebuilt stable versions of LLVM on `kernel.org +<https://kernel.org/pub/tools/llvm/>`_. These have been optimized with profile +data for building Linux kernels, which should improve kernel build times +relative to other distributions of LLVM. + +Below are links that may be useful for building LLVM from source or procuring +it through a distribution's package manager. + +- https://releases.llvm.org/download.html +- https://github.com/llvm/llvm-project +- https://llvm.org/docs/GettingStarted.html +- https://llvm.org/docs/CMake.html +- https://apt.llvm.org/ +- https://www.archlinux.org/packages/extra/x86_64/llvm/ +- https://github.com/ClangBuiltLinux/tc-build +- https://github.com/ClangBuiltLinux/linux/wiki/Building-Clang-from-source +- https://android.googlesource.com/platform/prebuilts/clang/host/linux-x86/ diff --git a/Documentation/kbuild/makefiles.rst b/Documentation/kbuild/makefiles.rst new file mode 100644 index 0000000000..e67eb261c9 --- /dev/null +++ b/Documentation/kbuild/makefiles.rst @@ -0,0 +1,1659 @@ +====================== +Linux Kernel Makefiles +====================== + +This document describes the Linux kernel Makefiles. + +Overview +======== + +The Makefiles have five parts:: + + Makefile the top Makefile. + .config the kernel configuration file. + arch/$(SRCARCH)/Makefile the arch Makefile. + scripts/Makefile.* common rules etc. for all kbuild Makefiles. + kbuild Makefiles exist in every subdirectory + +The top Makefile reads the .config file, which comes from the kernel +configuration process. + +The top Makefile is responsible for building two major products: vmlinux +(the resident kernel image) and modules (any module files). +It builds these goals by recursively descending into the subdirectories of +the kernel source tree. + +The list of subdirectories which are visited depends upon the kernel +configuration. The top Makefile textually includes an arch Makefile +with the name arch/$(SRCARCH)/Makefile. The arch Makefile supplies +architecture-specific information to the top Makefile. + +Each subdirectory has a kbuild Makefile which carries out the commands +passed down from above. The kbuild Makefile uses information from the +.config file to construct various file lists used by kbuild to build +any built-in or modular targets. + +scripts/Makefile.* contains all the definitions/rules etc. that +are used to build the kernel based on the kbuild makefiles. + +Who does what +============= + +People have four different relationships with the kernel Makefiles. + +*Users* are people who build kernels. These people type commands such as +``make menuconfig`` or ``make``. They usually do not read or edit +any kernel Makefiles (or any other source files). + +*Normal developers* are people who work on features such as device +drivers, file systems, and network protocols. These people need to +maintain the kbuild Makefiles for the subsystem they are +working on. In order to do this effectively, they need some overall +knowledge about the kernel Makefiles, plus detailed knowledge about the +public interface for kbuild. + +*Arch developers* are people who work on an entire architecture, such +as sparc or ia64. Arch developers need to know about the arch Makefile +as well as kbuild Makefiles. + +*Kbuild developers* are people who work on the kernel build system itself. +These people need to know about all aspects of the kernel Makefiles. + +This document is aimed towards normal developers and arch developers. + + +The kbuild files +================ + +Most Makefiles within the kernel are kbuild Makefiles that use the +kbuild infrastructure. This chapter introduces the syntax used in the +kbuild makefiles. + +The preferred name for the kbuild files are ``Makefile`` but ``Kbuild`` can +be used and if both a ``Makefile`` and a ``Kbuild`` file exists, then the ``Kbuild`` +file will be used. + +Section `Goal definitions`_ is a quick intro; further chapters provide +more details, with real examples. + +Goal definitions +---------------- + +Goal definitions are the main part (heart) of the kbuild Makefile. +These lines define the files to be built, any special compilation +options, and any subdirectories to be entered recursively. + +The most simple kbuild makefile contains one line: + +Example:: + + obj-y += foo.o + +This tells kbuild that there is one object in that directory, named +foo.o. foo.o will be built from foo.c or foo.S. + +If foo.o shall be built as a module, the variable obj-m is used. +Therefore the following pattern is often used: + +Example:: + + obj-$(CONFIG_FOO) += foo.o + +$(CONFIG_FOO) evaluates to either y (for built-in) or m (for module). +If CONFIG_FOO is neither y nor m, then the file will not be compiled +nor linked. + +Built-in object goals - obj-y +----------------------------- + +The kbuild Makefile specifies object files for vmlinux +in the $(obj-y) lists. These lists depend on the kernel +configuration. + +Kbuild compiles all the $(obj-y) files. It then calls +``$(AR) rcSTP`` to merge these files into one built-in.a file. +This is a thin archive without a symbol table. It will be later +linked into vmlinux by scripts/link-vmlinux.sh + +The order of files in $(obj-y) is significant. Duplicates in +the lists are allowed: the first instance will be linked into +built-in.a and succeeding instances will be ignored. + +Link order is significant, because certain functions +(module_init() / __initcall) will be called during boot in the +order they appear. So keep in mind that changing the link +order may e.g. change the order in which your SCSI +controllers are detected, and thus your disks are renumbered. + +Example:: + + #drivers/isdn/i4l/Makefile + # Makefile for the kernel ISDN subsystem and device drivers. + # Each configuration option enables a list of files. + obj-$(CONFIG_ISDN_I4L) += isdn.o + obj-$(CONFIG_ISDN_PPP_BSDCOMP) += isdn_bsdcomp.o + +Loadable module goals - obj-m +----------------------------- + +$(obj-m) specifies object files which are built as loadable +kernel modules. + +A module may be built from one source file or several source +files. In the case of one source file, the kbuild makefile +simply adds the file to $(obj-m). + +Example:: + + #drivers/isdn/i4l/Makefile + obj-$(CONFIG_ISDN_PPP_BSDCOMP) += isdn_bsdcomp.o + +Note: In this example $(CONFIG_ISDN_PPP_BSDCOMP) evaluates to "m" + +If a kernel module is built from several source files, you specify +that you want to build a module in the same way as above; however, +kbuild needs to know which object files you want to build your +module from, so you have to tell it by setting a $(<module_name>-y) +variable. + +Example:: + + #drivers/isdn/i4l/Makefile + obj-$(CONFIG_ISDN_I4L) += isdn.o + isdn-y := isdn_net_lib.o isdn_v110.o isdn_common.o + +In this example, the module name will be isdn.o. Kbuild will +compile the objects listed in $(isdn-y) and then run +``$(LD) -r`` on the list of these files to generate isdn.o. + +Due to kbuild recognizing $(<module_name>-y) for composite objects, +you can use the value of a ``CONFIG_`` symbol to optionally include an +object file as part of a composite object. + +Example:: + + #fs/ext2/Makefile + obj-$(CONFIG_EXT2_FS) += ext2.o + ext2-y := balloc.o dir.o file.o ialloc.o inode.o ioctl.o \ + namei.o super.o symlink.o + ext2-$(CONFIG_EXT2_FS_XATTR) += xattr.o xattr_user.o \ + xattr_trusted.o + +In this example, xattr.o, xattr_user.o and xattr_trusted.o are only +part of the composite object ext2.o if $(CONFIG_EXT2_FS_XATTR) +evaluates to "y". + +Note: Of course, when you are building objects into the kernel, +the syntax above will also work. So, if you have CONFIG_EXT2_FS=y, +kbuild will build an ext2.o file for you out of the individual +parts and then link this into built-in.a, as you would expect. + +Library file goals - lib-y +-------------------------- + +Objects listed with obj-* are used for modules, or +combined in a built-in.a for that specific directory. +There is also the possibility to list objects that will +be included in a library, lib.a. +All objects listed with lib-y are combined in a single +library for that directory. +Objects that are listed in obj-y and additionally listed in +lib-y will not be included in the library, since they will +be accessible anyway. +For consistency, objects listed in lib-m will be included in lib.a. + +Note that the same kbuild makefile may list files to be built-in +and to be part of a library. Therefore the same directory +may contain both a built-in.a and a lib.a file. + +Example:: + + #arch/x86/lib/Makefile + lib-y := delay.o + +This will create a library lib.a based on delay.o. For kbuild to +actually recognize that there is a lib.a being built, the directory +shall be listed in libs-y. + +See also `List directories to visit when descending`_. + +Use of lib-y is normally restricted to ``lib/`` and ``arch/*/lib``. + +Descending down in directories +------------------------------ + +A Makefile is only responsible for building objects in its own +directory. Files in subdirectories should be taken care of by +Makefiles in these subdirs. The build system will automatically +invoke make recursively in subdirectories, provided you let it know of +them. + +To do so, obj-y and obj-m are used. +ext2 lives in a separate directory, and the Makefile present in fs/ +tells kbuild to descend down using the following assignment. + +Example:: + + #fs/Makefile + obj-$(CONFIG_EXT2_FS) += ext2/ + +If CONFIG_EXT2_FS is set to either "y" (built-in) or "m" (modular) +the corresponding obj- variable will be set, and kbuild will descend +down in the ext2 directory. + +Kbuild uses this information not only to decide that it needs to visit +the directory, but also to decide whether or not to link objects from +the directory into vmlinux. + +When Kbuild descends into the directory with "y", all built-in objects +from that directory are combined into the built-in.a, which will be +eventually linked into vmlinux. + +When Kbuild descends into the directory with "m", in contrast, nothing +from that directory will be linked into vmlinux. If the Makefile in +that directory specifies obj-y, those objects will be left orphan. +It is very likely a bug of the Makefile or of dependencies in Kconfig. + +Kbuild also supports dedicated syntax, subdir-y and subdir-m, for +descending into subdirectories. It is a good fit when you know they +do not contain kernel-space objects at all. A typical usage is to let +Kbuild descend into subdirectories to build tools. + +Examples:: + + # scripts/Makefile + subdir-$(CONFIG_GCC_PLUGINS) += gcc-plugins + subdir-$(CONFIG_MODVERSIONS) += genksyms + subdir-$(CONFIG_SECURITY_SELINUX) += selinux + +Unlike obj-y/m, subdir-y/m does not need the trailing slash since this +syntax is always used for directories. + +It is good practice to use a ``CONFIG_`` variable when assigning directory +names. This allows kbuild to totally skip the directory if the +corresponding ``CONFIG_`` option is neither "y" nor "m". + +Non-builtin vmlinux targets - extra-y +------------------------------------- + +extra-y specifies targets which are needed for building vmlinux, +but not combined into built-in.a. + +Examples are: + +1) vmlinux linker script + + The linker script for vmlinux is located at + arch/$(SRCARCH)/kernel/vmlinux.lds + +Example:: + + # arch/x86/kernel/Makefile + extra-y += vmlinux.lds + +$(extra-y) should only contain targets needed for vmlinux. + +Kbuild skips extra-y when vmlinux is apparently not a final goal. +(e.g. ``make modules``, or building external modules) + +If you intend to build targets unconditionally, always-y (explained +in the next section) is the correct syntax to use. + +Always built goals - always-y +----------------------------- + +always-y specifies targets which are literally always built when +Kbuild visits the Makefile. + +Example:: + + # ./Kbuild + offsets-file := include/generated/asm-offsets.h + always-y += $(offsets-file) + +Compilation flags +----------------- + +ccflags-y, asflags-y and ldflags-y + These three flags apply only to the kbuild makefile in which they + are assigned. They are used for all the normal cc, as and ld + invocations happening during a recursive build. + Note: Flags with the same behaviour were previously named: + EXTRA_CFLAGS, EXTRA_AFLAGS and EXTRA_LDFLAGS. + They are still supported but their usage is deprecated. + + ccflags-y specifies options for compiling with $(CC). + + Example:: + + # drivers/acpi/acpica/Makefile + ccflags-y := -Os -D_LINUX -DBUILDING_ACPICA + ccflags-$(CONFIG_ACPI_DEBUG) += -DACPI_DEBUG_OUTPUT + + This variable is necessary because the top Makefile owns the + variable $(KBUILD_CFLAGS) and uses it for compilation flags for the + entire tree. + + asflags-y specifies assembler options. + + Example:: + + #arch/sparc/kernel/Makefile + asflags-y := -ansi + + ldflags-y specifies options for linking with $(LD). + + Example:: + + #arch/cris/boot/compressed/Makefile + ldflags-y += -T $(srctree)/$(src)/decompress_$(arch-y).lds + +subdir-ccflags-y, subdir-asflags-y + The two flags listed above are similar to ccflags-y and asflags-y. + The difference is that the subdir- variants have effect for the kbuild + file where they are present and all subdirectories. + Options specified using subdir-* are added to the commandline before + the options specified using the non-subdir variants. + + Example:: + + subdir-ccflags-y := -Werror + +ccflags-remove-y, asflags-remove-y + These flags are used to remove particular flags for the compiler, + assembler invocations. + + Example:: + + ccflags-remove-$(CONFIG_MCOUNT) += -pg + +CFLAGS_$@, AFLAGS_$@ + CFLAGS_$@ and AFLAGS_$@ only apply to commands in current + kbuild makefile. + + $(CFLAGS_$@) specifies per-file options for $(CC). The $@ + part has a literal value which specifies the file that it is for. + + CFLAGS_$@ has the higher priority than ccflags-remove-y; CFLAGS_$@ + can re-add compiler flags that were removed by ccflags-remove-y. + + Example:: + + # drivers/scsi/Makefile + CFLAGS_aha152x.o = -DAHA152X_STAT -DAUTOCONF + + This line specify compilation flags for aha152x.o. + + $(AFLAGS_$@) is a similar feature for source files in assembly + languages. + + AFLAGS_$@ has the higher priority than asflags-remove-y; AFLAGS_$@ + can re-add assembler flags that were removed by asflags-remove-y. + + Example:: + + # arch/arm/kernel/Makefile + AFLAGS_head.o := -DTEXT_OFFSET=$(TEXT_OFFSET) + AFLAGS_crunch-bits.o := -Wa,-mcpu=ep9312 + AFLAGS_iwmmxt.o := -Wa,-mcpu=iwmmxt + +Dependency tracking +------------------- + +Kbuild tracks dependencies on the following: + +1) All prerequisite files (both ``*.c`` and ``*.h``) +2) ``CONFIG_`` options used in all prerequisite files +3) Command-line used to compile target + +Thus, if you change an option to $(CC) all affected files will +be re-compiled. + +Custom Rules +------------ + +Custom rules are used when the kbuild infrastructure does +not provide the required support. A typical example is +header files generated during the build process. +Another example are the architecture-specific Makefiles which +need custom rules to prepare boot images etc. + +Custom rules are written as normal Make rules. +Kbuild is not executing in the directory where the Makefile is +located, so all custom rules shall use a relative +path to prerequisite files and target files. + +Two variables are used when defining custom rules: + +$(src) + $(src) is a relative path which points to the directory + where the Makefile is located. Always use $(src) when + referring to files located in the src tree. + +$(obj) + $(obj) is a relative path which points to the directory + where the target is saved. Always use $(obj) when + referring to generated files. + + Example:: + + #drivers/scsi/Makefile + $(obj)/53c8xx_d.h: $(src)/53c7,8xx.scr $(src)/script_asm.pl + $(CPP) -DCHIP=810 - < $< | ... $(src)/script_asm.pl + + This is a custom rule, following the normal syntax + required by make. + + The target file depends on two prerequisite files. References + to the target file are prefixed with $(obj), references + to prerequisites are referenced with $(src) (because they are not + generated files). + +$(kecho) + echoing information to user in a rule is often a good practice + but when execution ``make -s`` one does not expect to see any output + except for warnings/errors. + To support this kbuild defines $(kecho) which will echo out the + text following $(kecho) to stdout except if ``make -s`` is used. + + Example:: + + # arch/arm/Makefile + $(BOOT_TARGETS): vmlinux + $(Q)$(MAKE) $(build)=$(boot) MACHINE=$(MACHINE) $(boot)/$@ + @$(kecho) ' Kernel: $(boot)/$@ is ready' + + When kbuild is executing with KBUILD_VERBOSE unset, then only a shorthand + of a command is normally displayed. + To enable this behaviour for custom commands kbuild requires + two variables to be set:: + + quiet_cmd_<command> - what shall be echoed + cmd_<command> - the command to execute + + Example:: + + # lib/Makefile + quiet_cmd_crc32 = GEN $@ + cmd_crc32 = $< > $@ + + $(obj)/crc32table.h: $(obj)/gen_crc32table + $(call cmd,crc32) + + When updating the $(obj)/crc32table.h target, the line:: + + GEN lib/crc32table.h + + will be displayed with ``make KBUILD_VERBOSE=``. + +Command change detection +------------------------ + +When the rule is evaluated, timestamps are compared between the target +and its prerequisite files. GNU Make updates the target when any of the +prerequisites is newer than that. + +The target should be rebuilt also when the command line has changed +since the last invocation. This is not supported by Make itself, so +Kbuild achieves this by a kind of meta-programming. + +if_changed is the macro used for this purpose, in the following form:: + + quiet_cmd_<command> = ... + cmd_<command> = ... + + <target>: <source(s)> FORCE + $(call if_changed,<command>) + +Any target that utilizes if_changed must be listed in $(targets), +otherwise the command line check will fail, and the target will +always be built. + +If the target is already listed in the recognized syntax such as +obj-y/m, lib-y/m, extra-y/m, always-y/m, hostprogs, userprogs, Kbuild +automatically adds it to $(targets). Otherwise, the target must be +explicitly added to $(targets). + +Assignments to $(targets) are without $(obj)/ prefix. if_changed may be +used in conjunction with custom rules as defined in `Custom Rules`_. + +Note: It is a typical mistake to forget the FORCE prerequisite. +Another common pitfall is that whitespace is sometimes significant; for +instance, the below will fail (note the extra space after the comma):: + + target: source(s) FORCE + +**WRONG!** $(call if_changed, objcopy) + +Note: + if_changed should not be used more than once per target. + It stores the executed command in a corresponding .cmd + file and multiple calls would result in overwrites and + unwanted results when the target is up to date and only the + tests on changed commands trigger execution of commands. + +$(CC) support functions +----------------------- + +The kernel may be built with several different versions of +$(CC), each supporting a unique set of features and options. +kbuild provides basic support to check for valid options for $(CC). +$(CC) is usually the gcc compiler, but other alternatives are +available. + +as-option + as-option is used to check if $(CC) -- when used to compile + assembler (``*.S``) files -- supports the given option. An optional + second option may be specified if the first option is not supported. + + Example:: + + #arch/sh/Makefile + cflags-y += $(call as-option,-Wa$(comma)-isa=$(isa-y),) + + In the above example, cflags-y will be assigned the option + -Wa$(comma)-isa=$(isa-y) if it is supported by $(CC). + The second argument is optional, and if supplied will be used + if first argument is not supported. + +as-instr + as-instr checks if the assembler reports a specific instruction + and then outputs either option1 or option2 + C escapes are supported in the test instruction + Note: as-instr-option uses KBUILD_AFLAGS for assembler options + +cc-option + cc-option is used to check if $(CC) supports a given option, and if + not supported to use an optional second option. + + Example:: + + #arch/x86/Makefile + cflags-y += $(call cc-option,-march=pentium-mmx,-march=i586) + + In the above example, cflags-y will be assigned the option + -march=pentium-mmx if supported by $(CC), otherwise -march=i586. + The second argument to cc-option is optional, and if omitted, + cflags-y will be assigned no value if first option is not supported. + Note: cc-option uses KBUILD_CFLAGS for $(CC) options + +cc-option-yn + cc-option-yn is used to check if gcc supports a given option + and return "y" if supported, otherwise "n". + + Example:: + + #arch/ppc/Makefile + biarch := $(call cc-option-yn, -m32) + aflags-$(biarch) += -a32 + cflags-$(biarch) += -m32 + + In the above example, $(biarch) is set to y if $(CC) supports the -m32 + option. When $(biarch) equals "y", the expanded variables $(aflags-y) + and $(cflags-y) will be assigned the values -a32 and -m32, + respectively. + + Note: cc-option-yn uses KBUILD_CFLAGS for $(CC) options + +cc-disable-warning + cc-disable-warning checks if gcc supports a given warning and returns + the commandline switch to disable it. This special function is needed, + because gcc 4.4 and later accept any unknown -Wno-* option and only + warn about it if there is another warning in the source file. + + Example:: + + KBUILD_CFLAGS += $(call cc-disable-warning, unused-but-set-variable) + + In the above example, -Wno-unused-but-set-variable will be added to + KBUILD_CFLAGS only if gcc really accepts it. + +gcc-min-version + gcc-min-version tests if the value of $(CONFIG_GCC_VERSION) is greater than + or equal to the provided value and evaluates to y if so. + + Example:: + + cflags-$(call gcc-min-version, 70100) := -foo + + In this example, cflags-y will be assigned the value -foo if $(CC) is gcc and + $(CONFIG_GCC_VERSION) is >= 7.1. + +clang-min-version + clang-min-version tests if the value of $(CONFIG_CLANG_VERSION) is greater + than or equal to the provided value and evaluates to y if so. + + Example:: + + cflags-$(call clang-min-version, 110000) := -foo + + In this example, cflags-y will be assigned the value -foo if $(CC) is clang + and $(CONFIG_CLANG_VERSION) is >= 11.0.0. + +cc-cross-prefix + cc-cross-prefix is used to check if there exists a $(CC) in path with + one of the listed prefixes. The first prefix where there exist a + prefix$(CC) in the PATH is returned - and if no prefix$(CC) is found + then nothing is returned. + + Additional prefixes are separated by a single space in the + call of cc-cross-prefix. + + This functionality is useful for architecture Makefiles that try + to set CROSS_COMPILE to well-known values but may have several + values to select between. + + It is recommended only to try to set CROSS_COMPILE if it is a cross + build (host arch is different from target arch). And if CROSS_COMPILE + is already set then leave it with the old value. + + Example:: + + #arch/m68k/Makefile + ifneq ($(SUBARCH),$(ARCH)) + ifeq ($(CROSS_COMPILE),) + CROSS_COMPILE := $(call cc-cross-prefix, m68k-linux-gnu-) + endif + endif + +$(LD) support functions +----------------------- + +ld-option + ld-option is used to check if $(LD) supports the supplied option. + ld-option takes two options as arguments. + + The second argument is an optional option that can be used if the + first option is not supported by $(LD). + + Example:: + + #Makefile + LDFLAGS_vmlinux += $(call ld-option, -X) + +Script invocation +----------------- + +Make rules may invoke scripts to build the kernel. The rules shall +always provide the appropriate interpreter to execute the script. They +shall not rely on the execute bits being set, and shall not invoke the +script directly. For the convenience of manual script invocation, such +as invoking ./scripts/checkpatch.pl, it is recommended to set execute +bits on the scripts nonetheless. + +Kbuild provides variables $(CONFIG_SHELL), $(AWK), $(PERL), +and $(PYTHON3) to refer to interpreters for the respective +scripts. + +Example:: + + #Makefile + cmd_depmod = $(CONFIG_SHELL) $(srctree)/scripts/depmod.sh $(DEPMOD) \ + $(KERNELRELEASE) + +Host Program support +==================== + +Kbuild supports building executables on the host for use during the +compilation stage. + +Two steps are required in order to use a host executable. + +The first step is to tell kbuild that a host program exists. This is +done utilising the variable ``hostprogs``. + +The second step is to add an explicit dependency to the executable. +This can be done in two ways. Either add the dependency in a rule, +or utilise the variable ``always-y``. +Both possibilities are described in the following. + +Simple Host Program +------------------- + +In some cases there is a need to compile and run a program on the +computer where the build is running. + +The following line tells kbuild that the program bin2hex shall be +built on the build host. + +Example:: + + hostprogs := bin2hex + +Kbuild assumes in the above example that bin2hex is made from a single +c-source file named bin2hex.c located in the same directory as +the Makefile. + +Composite Host Programs +----------------------- + +Host programs can be made up based on composite objects. +The syntax used to define composite objects for host programs is +similar to the syntax used for kernel objects. +$(<executable>-objs) lists all objects used to link the final +executable. + +Example:: + + #scripts/lxdialog/Makefile + hostprogs := lxdialog + lxdialog-objs := checklist.o lxdialog.o + +Objects with extension .o are compiled from the corresponding .c +files. In the above example, checklist.c is compiled to checklist.o +and lxdialog.c is compiled to lxdialog.o. + +Finally, the two .o files are linked to the executable, lxdialog. +Note: The syntax <executable>-y is not permitted for host-programs. + +Using C++ for host programs +--------------------------- + +kbuild offers support for host programs written in C++. This was +introduced solely to support kconfig, and is not recommended +for general use. + +Example:: + + #scripts/kconfig/Makefile + hostprogs := qconf + qconf-cxxobjs := qconf.o + +In the example above the executable is composed of the C++ file +qconf.cc - identified by $(qconf-cxxobjs). + +If qconf is composed of a mixture of .c and .cc files, then an +additional line can be used to identify this. + +Example:: + + #scripts/kconfig/Makefile + hostprogs := qconf + qconf-cxxobjs := qconf.o + qconf-objs := check.o + +Using Rust for host programs +---------------------------- + +Kbuild offers support for host programs written in Rust. However, +since a Rust toolchain is not mandatory for kernel compilation, +it may only be used in scenarios where Rust is required to be +available (e.g. when ``CONFIG_RUST`` is enabled). + +Example:: + + hostprogs := target + target-rust := y + +Kbuild will compile ``target`` using ``target.rs`` as the crate root, +located in the same directory as the ``Makefile``. The crate may +consist of several source files (see ``samples/rust/hostprogs``). + +Controlling compiler options for host programs +---------------------------------------------- + +When compiling host programs, it is possible to set specific flags. +The programs will always be compiled utilising $(HOSTCC) passed +the options specified in $(KBUILD_HOSTCFLAGS). + +To set flags that will take effect for all host programs created +in that Makefile, use the variable HOST_EXTRACFLAGS. + +Example:: + + #scripts/lxdialog/Makefile + HOST_EXTRACFLAGS += -I/usr/include/ncurses + +To set specific flags for a single file the following construction +is used: + +Example:: + + #arch/ppc64/boot/Makefile + HOSTCFLAGS_piggyback.o := -DKERNELBASE=$(KERNELBASE) + +It is also possible to specify additional options to the linker. + +Example:: + + #scripts/kconfig/Makefile + HOSTLDLIBS_qconf := -L$(QTDIR)/lib + +When linking qconf, it will be passed the extra option +``-L$(QTDIR)/lib``. + +When host programs are actually built +------------------------------------- + +Kbuild will only build host-programs when they are referenced +as a prerequisite. + +This is possible in two ways: + +(1) List the prerequisite explicitly in a custom rule. + + Example:: + + #drivers/pci/Makefile + hostprogs := gen-devlist + $(obj)/devlist.h: $(src)/pci.ids $(obj)/gen-devlist + ( cd $(obj); ./gen-devlist ) < $< + + The target $(obj)/devlist.h will not be built before + $(obj)/gen-devlist is updated. Note that references to + the host programs in custom rules must be prefixed with $(obj). + +(2) Use always-y + + When there is no suitable custom rule, and the host program + shall be built when a makefile is entered, the always-y + variable shall be used. + + Example:: + + #scripts/lxdialog/Makefile + hostprogs := lxdialog + always-y := $(hostprogs) + + Kbuild provides the following shorthand for this:: + + hostprogs-always-y := lxdialog + + This will tell kbuild to build lxdialog even if not referenced in + any rule. + +Userspace Program support +========================= + +Just like host programs, Kbuild also supports building userspace executables +for the target architecture (i.e. the same architecture as you are building +the kernel for). + +The syntax is quite similar. The difference is to use ``userprogs`` instead of +``hostprogs``. + +Simple Userspace Program +------------------------ + +The following line tells kbuild that the program bpf-direct shall be +built for the target architecture. + +Example:: + + userprogs := bpf-direct + +Kbuild assumes in the above example that bpf-direct is made from a +single C source file named bpf-direct.c located in the same directory +as the Makefile. + +Composite Userspace Programs +---------------------------- + +Userspace programs can be made up based on composite objects. +The syntax used to define composite objects for userspace programs is +similar to the syntax used for kernel objects. +$(<executable>-objs) lists all objects used to link the final +executable. + +Example:: + + #samples/seccomp/Makefile + userprogs := bpf-fancy + bpf-fancy-objs := bpf-fancy.o bpf-helper.o + +Objects with extension .o are compiled from the corresponding .c +files. In the above example, bpf-fancy.c is compiled to bpf-fancy.o +and bpf-helper.c is compiled to bpf-helper.o. + +Finally, the two .o files are linked to the executable, bpf-fancy. +Note: The syntax <executable>-y is not permitted for userspace programs. + +Controlling compiler options for userspace programs +--------------------------------------------------- + +When compiling userspace programs, it is possible to set specific flags. +The programs will always be compiled utilising $(CC) passed +the options specified in $(KBUILD_USERCFLAGS). + +To set flags that will take effect for all userspace programs created +in that Makefile, use the variable userccflags. + +Example:: + + # samples/seccomp/Makefile + userccflags += -I usr/include + +To set specific flags for a single file the following construction +is used: + +Example:: + + bpf-helper-userccflags += -I user/include + +It is also possible to specify additional options to the linker. + +Example:: + + # net/bpfilter/Makefile + bpfilter_umh-userldflags += -static + +When linking bpfilter_umh, it will be passed the extra option -static. + +From command line, :ref:`USERCFLAGS and USERLDFLAGS <userkbuildflags>` will also be used. + +When userspace programs are actually built +------------------------------------------ + +Kbuild builds userspace programs only when told to do so. +There are two ways to do this. + +(1) Add it as the prerequisite of another file + + Example:: + + #net/bpfilter/Makefile + userprogs := bpfilter_umh + $(obj)/bpfilter_umh_blob.o: $(obj)/bpfilter_umh + + $(obj)/bpfilter_umh is built before $(obj)/bpfilter_umh_blob.o + +(2) Use always-y + + Example:: + + userprogs := binderfs_example + always-y := $(userprogs) + + Kbuild provides the following shorthand for this:: + + userprogs-always-y := binderfs_example + + This will tell Kbuild to build binderfs_example when it visits this + Makefile. + +Kbuild clean infrastructure +=========================== + +``make clean`` deletes most generated files in the obj tree where the kernel +is compiled. This includes generated files such as host programs. +Kbuild knows targets listed in $(hostprogs), $(always-y), $(always-m), +$(always-), $(extra-y), $(extra-) and $(targets). They are all deleted +during ``make clean``. Files matching the patterns ``*.[oas]``, ``*.ko``, plus +some additional files generated by kbuild are deleted all over the kernel +source tree when ``make clean`` is executed. + +Additional files or directories can be specified in kbuild makefiles by use of +$(clean-files). + +Example:: + + #lib/Makefile + clean-files := crc32table.h + +When executing ``make clean``, the file ``crc32table.h`` will be deleted. +Kbuild will assume files to be in the same relative directory as the +Makefile. + +To exclude certain files or directories from make clean, use the +$(no-clean-files) variable. + +Usually kbuild descends down in subdirectories due to ``obj-* := dir/``, +but in the architecture makefiles where the kbuild infrastructure +is not sufficient this sometimes needs to be explicit. + +Example:: + + #arch/x86/boot/Makefile + subdir- := compressed + +The above assignment instructs kbuild to descend down in the +directory compressed/ when ``make clean`` is executed. + +Note 1: arch/$(SRCARCH)/Makefile cannot use ``subdir-``, because that file is +included in the top level makefile. Instead, arch/$(SRCARCH)/Kbuild can use +``subdir-``. + +Note 2: All directories listed in core-y, libs-y, drivers-y and net-y will +be visited during ``make clean``. + +Architecture Makefiles +====================== + +The top level Makefile sets up the environment and does the preparation, +before starting to descend down in the individual directories. + +The top level makefile contains the generic part, whereas +arch/$(SRCARCH)/Makefile contains what is required to set up kbuild +for said architecture. + +To do so, arch/$(SRCARCH)/Makefile sets up a number of variables and defines +a few targets. + +When kbuild executes, the following steps are followed (roughly): + +1) Configuration of the kernel => produce .config + +2) Store kernel version in include/linux/version.h + +3) Updating all other prerequisites to the target prepare: + + - Additional prerequisites are specified in arch/$(SRCARCH)/Makefile + +4) Recursively descend down in all directories listed in + init-* core* drivers-* net-* libs-* and build all targets. + + - The values of the above variables are expanded in arch/$(SRCARCH)/Makefile. + +5) All object files are then linked and the resulting file vmlinux is + located at the root of the obj tree. + The very first objects linked are listed in scripts/head-object-list.txt. + +6) Finally, the architecture-specific part does any required post processing + and builds the final bootimage. + + - This includes building boot records + - Preparing initrd images and the like + +Set variables to tweak the build to the architecture +---------------------------------------------------- + +KBUILD_LDFLAGS + Generic $(LD) options + + Flags used for all invocations of the linker. + Often specifying the emulation is sufficient. + + Example:: + + #arch/s390/Makefile + KBUILD_LDFLAGS := -m elf_s390 + + Note: ldflags-y can be used to further customise + the flags used. See `Non-builtin vmlinux targets - extra-y`_. + +LDFLAGS_vmlinux + Options for $(LD) when linking vmlinux + + LDFLAGS_vmlinux is used to specify additional flags to pass to + the linker when linking the final vmlinux image. + + LDFLAGS_vmlinux uses the LDFLAGS_$@ support. + + Example:: + + #arch/x86/Makefile + LDFLAGS_vmlinux := -e stext + +OBJCOPYFLAGS + objcopy flags + + When $(call if_changed,objcopy) is used to translate a .o file, + the flags specified in OBJCOPYFLAGS will be used. + + $(call if_changed,objcopy) is often used to generate raw binaries on + vmlinux. + + Example:: + + #arch/s390/Makefile + OBJCOPYFLAGS := -O binary + + #arch/s390/boot/Makefile + $(obj)/image: vmlinux FORCE + $(call if_changed,objcopy) + + In this example, the binary $(obj)/image is a binary version of + vmlinux. The usage of $(call if_changed,xxx) will be described later. + +KBUILD_AFLAGS + Assembler flags + + Default value - see top level Makefile. + + Append or modify as required per architecture. + + Example:: + + #arch/sparc64/Makefile + KBUILD_AFLAGS += -m64 -mcpu=ultrasparc + +KBUILD_CFLAGS + $(CC) compiler flags + + Default value - see top level Makefile. + + Append or modify as required per architecture. + + Often, the KBUILD_CFLAGS variable depends on the configuration. + + Example:: + + #arch/x86/boot/compressed/Makefile + cflags-$(CONFIG_X86_32) := -march=i386 + cflags-$(CONFIG_X86_64) := -mcmodel=small + KBUILD_CFLAGS += $(cflags-y) + + Many arch Makefiles dynamically run the target C compiler to + probe supported options:: + + #arch/x86/Makefile + + ... + cflags-$(CONFIG_MPENTIUMII) += $(call cc-option,\ + -march=pentium2,-march=i686) + ... + # Disable unit-at-a-time mode ... + KBUILD_CFLAGS += $(call cc-option,-fno-unit-at-a-time) + ... + + + The first example utilises the trick that a config option expands + to "y" when selected. + +KBUILD_RUSTFLAGS + $(RUSTC) compiler flags + + Default value - see top level Makefile. + + Append or modify as required per architecture. + + Often, the KBUILD_RUSTFLAGS variable depends on the configuration. + + Note that target specification file generation (for ``--target``) + is handled in ``scripts/generate_rust_target.rs``. + +KBUILD_AFLAGS_KERNEL + Assembler options specific for built-in + + $(KBUILD_AFLAGS_KERNEL) contains extra C compiler flags used to compile + resident kernel code. + +KBUILD_AFLAGS_MODULE + Assembler options specific for modules + + $(KBUILD_AFLAGS_MODULE) is used to add arch-specific options that + are used for assembler. + + From commandline AFLAGS_MODULE shall be used (see kbuild.rst). + +KBUILD_CFLAGS_KERNEL + $(CC) options specific for built-in + + $(KBUILD_CFLAGS_KERNEL) contains extra C compiler flags used to compile + resident kernel code. + +KBUILD_CFLAGS_MODULE + Options for $(CC) when building modules + + $(KBUILD_CFLAGS_MODULE) is used to add arch-specific options that + are used for $(CC). + + From commandline CFLAGS_MODULE shall be used (see kbuild.rst). + +KBUILD_RUSTFLAGS_KERNEL + $(RUSTC) options specific for built-in + + $(KBUILD_RUSTFLAGS_KERNEL) contains extra Rust compiler flags used to + compile resident kernel code. + +KBUILD_RUSTFLAGS_MODULE + Options for $(RUSTC) when building modules + + $(KBUILD_RUSTFLAGS_MODULE) is used to add arch-specific options that + are used for $(RUSTC). + + From commandline RUSTFLAGS_MODULE shall be used (see kbuild.rst). + +KBUILD_LDFLAGS_MODULE + Options for $(LD) when linking modules + + $(KBUILD_LDFLAGS_MODULE) is used to add arch-specific options + used when linking modules. This is often a linker script. + + From commandline LDFLAGS_MODULE shall be used (see kbuild.rst). + +KBUILD_LDS + The linker script with full path. Assigned by the top-level Makefile. + +KBUILD_VMLINUX_OBJS + All object files for vmlinux. They are linked to vmlinux in the same + order as listed in KBUILD_VMLINUX_OBJS. + + The objects listed in scripts/head-object-list.txt are exceptions; + they are placed before the other objects. + +KBUILD_VMLINUX_LIBS + All .a ``lib`` files for vmlinux. KBUILD_VMLINUX_OBJS and + KBUILD_VMLINUX_LIBS together specify all the object files used to + link vmlinux. + +Add prerequisites to archheaders +-------------------------------- + +The archheaders: rule is used to generate header files that +may be installed into user space by ``make header_install``. + +It is run before ``make archprepare`` when run on the +architecture itself. + +Add prerequisites to archprepare +-------------------------------- + +The archprepare: rule is used to list prerequisites that need to be +built before starting to descend down in the subdirectories. + +This is usually used for header files containing assembler constants. + +Example:: + + #arch/arm/Makefile + archprepare: maketools + +In this example, the file target maketools will be processed +before descending down in the subdirectories. + +See also chapter XXX-TODO that describes how kbuild supports +generating offset header files. + +List directories to visit when descending +----------------------------------------- + +An arch Makefile cooperates with the top Makefile to define variables +which specify how to build the vmlinux file. Note that there is no +corresponding arch-specific section for modules; the module-building +machinery is all architecture-independent. + +core-y, libs-y, drivers-y + $(libs-y) lists directories where a lib.a archive can be located. + + The rest list directories where a built-in.a object file can be + located. + + Then the rest follows in this order: + + $(core-y), $(libs-y), $(drivers-y) + + The top level Makefile defines values for all generic directories, + and arch/$(SRCARCH)/Makefile only adds architecture-specific + directories. + + Example:: + + # arch/sparc/Makefile + core-y += arch/sparc/ + + libs-y += arch/sparc/prom/ + libs-y += arch/sparc/lib/ + + drivers-$(CONFIG_PM) += arch/sparc/power/ + +Architecture-specific boot images +--------------------------------- + +An arch Makefile specifies goals that take the vmlinux file, compress +it, wrap it in bootstrapping code, and copy the resulting files +somewhere. This includes various kinds of installation commands. +The actual goals are not standardized across architectures. + +It is common to locate any additional processing in a boot/ +directory below arch/$(SRCARCH)/. + +Kbuild does not provide any smart way to support building a +target specified in boot/. Therefore arch/$(SRCARCH)/Makefile shall +call make manually to build a target in boot/. + +The recommended approach is to include shortcuts in +arch/$(SRCARCH)/Makefile, and use the full path when calling down +into the arch/$(SRCARCH)/boot/Makefile. + +Example:: + + #arch/x86/Makefile + boot := arch/x86/boot + bzImage: vmlinux + $(Q)$(MAKE) $(build)=$(boot) $(boot)/$@ + +``$(Q)$(MAKE) $(build)=<dir>`` is the recommended way to invoke +make in a subdirectory. + +There are no rules for naming architecture-specific targets, +but executing ``make help`` will list all relevant targets. +To support this, $(archhelp) must be defined. + +Example:: + + #arch/x86/Makefile + define archhelp + echo '* bzImage - Compressed kernel image (arch/x86/boot/bzImage)' + endif + +When make is executed without arguments, the first goal encountered +will be built. In the top level Makefile the first goal present +is all:. + +An architecture shall always, per default, build a bootable image. +In ``make help``, the default goal is highlighted with a ``*``. + +Add a new prerequisite to all: to select a default goal different +from vmlinux. + +Example:: + + #arch/x86/Makefile + all: bzImage + +When ``make`` is executed without arguments, bzImage will be built. + +Commands useful for building a boot image +----------------------------------------- + +Kbuild provides a few macros that are useful when building a +boot image. + +ld + Link target. Often, LDFLAGS_$@ is used to set specific options to ld. + + Example:: + + #arch/x86/boot/Makefile + LDFLAGS_bootsect := -Ttext 0x0 -s --oformat binary + LDFLAGS_setup := -Ttext 0x0 -s --oformat binary -e begtext + + targets += setup setup.o bootsect bootsect.o + $(obj)/setup $(obj)/bootsect: %: %.o FORCE + $(call if_changed,ld) + + In this example, there are two possible targets, requiring different + options to the linker. The linker options are specified using the + LDFLAGS_$@ syntax - one for each potential target. + + $(targets) are assigned all potential targets, by which kbuild knows + the targets and will: + + 1) check for commandline changes + 2) delete target during make clean + + The ``: %: %.o`` part of the prerequisite is a shorthand that + frees us from listing the setup.o and bootsect.o files. + + Note: + It is a common mistake to forget the ``targets :=`` assignment, + resulting in the target file being recompiled for no + obvious reason. + +objcopy + Copy binary. Uses OBJCOPYFLAGS usually specified in + arch/$(SRCARCH)/Makefile. + + OBJCOPYFLAGS_$@ may be used to set additional options. + +gzip + Compress target. Use maximum compression to compress target. + + Example:: + + #arch/x86/boot/compressed/Makefile + $(obj)/vmlinux.bin.gz: $(vmlinux.bin.all-y) FORCE + $(call if_changed,gzip) + +dtc + Create flattened device tree blob object suitable for linking + into vmlinux. Device tree blobs linked into vmlinux are placed + in an init section in the image. Platform code *must* copy the + blob to non-init memory prior to calling unflatten_device_tree(). + + To use this command, simply add ``*.dtb`` into obj-y or targets, or make + some other target depend on ``%.dtb`` + + A central rule exists to create ``$(obj)/%.dtb`` from ``$(src)/%.dts``; + architecture Makefiles do no need to explicitly write out that rule. + + Example:: + + targets += $(dtb-y) + DTC_FLAGS ?= -p 1024 + +Preprocessing linker scripts +---------------------------- + +When the vmlinux image is built, the linker script +arch/$(SRCARCH)/kernel/vmlinux.lds is used. + +The script is a preprocessed variant of the file vmlinux.lds.S +located in the same directory. + +kbuild knows .lds files and includes a rule ``*lds.S`` -> ``*lds``. + +Example:: + + #arch/x86/kernel/Makefile + extra-y := vmlinux.lds + +The assignment to extra-y is used to tell kbuild to build the +target vmlinux.lds. + +The assignment to $(CPPFLAGS_vmlinux.lds) tells kbuild to use the +specified options when building the target vmlinux.lds. + +When building the ``*.lds`` target, kbuild uses the variables:: + + KBUILD_CPPFLAGS : Set in top-level Makefile + cppflags-y : May be set in the kbuild makefile + CPPFLAGS_$(@F) : Target-specific flags. + Note that the full filename is used in this + assignment. + +The kbuild infrastructure for ``*lds`` files is used in several +architecture-specific files. + +Generic header files +-------------------- + +The directory include/asm-generic contains the header files +that may be shared between individual architectures. + +The recommended approach how to use a generic header file is +to list the file in the Kbuild file. + +See `generic-y`_ for further info on syntax etc. + +Post-link pass +-------------- + +If the file arch/xxx/Makefile.postlink exists, this makefile +will be invoked for post-link objects (vmlinux and modules.ko) +for architectures to run post-link passes on. Must also handle +the clean target. + +This pass runs after kallsyms generation. If the architecture +needs to modify symbol locations, rather than manipulate the +kallsyms, it may be easier to add another postlink target for +.tmp_vmlinux? targets to be called from link-vmlinux.sh. + +For example, powerpc uses this to check relocation sanity of +the linked vmlinux file. + +Kbuild syntax for exported headers +================================== + +The kernel includes a set of headers that is exported to userspace. +Many headers can be exported as-is but other headers require a +minimal pre-processing before they are ready for user-space. + +The pre-processing does: + +- drop kernel-specific annotations +- drop include of compiler.h +- drop all sections that are kernel internal (guarded by ``ifdef __KERNEL__``) + +All headers under include/uapi/, include/generated/uapi/, +arch/<arch>/include/uapi/ and arch/<arch>/include/generated/uapi/ +are exported. + +A Kbuild file may be defined under arch/<arch>/include/uapi/asm/ and +arch/<arch>/include/asm/ to list asm files coming from asm-generic. + +See subsequent chapter for the syntax of the Kbuild file. + +no-export-headers +----------------- + +no-export-headers is essentially used by include/uapi/linux/Kbuild to +avoid exporting specific headers (e.g. kvm.h) on architectures that do +not support it. It should be avoided as much as possible. + +generic-y +--------- + +If an architecture uses a verbatim copy of a header from +include/asm-generic then this is listed in the file +arch/$(SRCARCH)/include/asm/Kbuild like this: + +Example:: + + #arch/x86/include/asm/Kbuild + generic-y += termios.h + generic-y += rtc.h + +During the prepare phase of the build a wrapper include +file is generated in the directory:: + + arch/$(SRCARCH)/include/generated/asm + +When a header is exported where the architecture uses +the generic header a similar wrapper is generated as part +of the set of exported headers in the directory:: + + usr/include/asm + +The generated wrapper will in both cases look like the following: + +Example: termios.h:: + + #include <asm-generic/termios.h> + +generated-y +----------- + +If an architecture generates other header files alongside generic-y +wrappers, generated-y specifies them. + +This prevents them being treated as stale asm-generic wrappers and +removed. + +Example:: + + #arch/x86/include/asm/Kbuild + generated-y += syscalls_32.h + +mandatory-y +----------- + +mandatory-y is essentially used by include/(uapi/)asm-generic/Kbuild +to define the minimum set of ASM headers that all architectures must have. + +This works like optional generic-y. If a mandatory header is missing +in arch/$(SRCARCH)/include/(uapi/)/asm, Kbuild will automatically +generate a wrapper of the asm-generic one. + +Kbuild Variables +================ + +The top Makefile exports the following variables: + +VERSION, PATCHLEVEL, SUBLEVEL, EXTRAVERSION + These variables define the current kernel version. A few arch + Makefiles actually use these values directly; they should use + $(KERNELRELEASE) instead. + + $(VERSION), $(PATCHLEVEL), and $(SUBLEVEL) define the basic + three-part version number, such as "2", "4", and "0". These three + values are always numeric. + + $(EXTRAVERSION) defines an even tinier sublevel for pre-patches + or additional patches. It is usually some non-numeric string + such as "-pre4", and is often blank. + +KERNELRELEASE + $(KERNELRELEASE) is a single string such as "2.4.0-pre4", suitable + for constructing installation directory names or showing in + version strings. Some arch Makefiles use it for this purpose. + +ARCH + This variable defines the target architecture, such as "i386", + "arm", or "sparc". Some kbuild Makefiles test $(ARCH) to + determine which files to compile. + + By default, the top Makefile sets $(ARCH) to be the same as the + host system architecture. For a cross build, a user may + override the value of $(ARCH) on the command line:: + + make ARCH=m68k ... + +SRCARCH + This variable specifies the directory in arch/ to build. + + ARCH and SRCARCH may not necessarily match. A couple of arch + directories are biarch, that is, a single ``arch/*/`` directory supports + both 32-bit and 64-bit. + + For example, you can pass in ARCH=i386, ARCH=x86_64, or ARCH=x86. + For all of them, SRCARCH=x86 because arch/x86/ supports both i386 and + x86_64. + +INSTALL_PATH + This variable defines a place for the arch Makefiles to install + the resident kernel image and System.map file. + Use this for architecture-specific install targets. + +INSTALL_MOD_PATH, MODLIB + $(INSTALL_MOD_PATH) specifies a prefix to $(MODLIB) for module + installation. This variable is not defined in the Makefile but + may be passed in by the user if desired. + + $(MODLIB) specifies the directory for module installation. + The top Makefile defines $(MODLIB) to + $(INSTALL_MOD_PATH)/lib/modules/$(KERNELRELEASE). The user may + override this value on the command line if desired. + +INSTALL_MOD_STRIP + If this variable is specified, it will cause modules to be stripped + after they are installed. If INSTALL_MOD_STRIP is "1", then the + default option --strip-debug will be used. Otherwise, the + INSTALL_MOD_STRIP value will be used as the option(s) to the strip + command. + +Makefile language +================= + +The kernel Makefiles are designed to be run with GNU Make. The Makefiles +use only the documented features of GNU Make, but they do use many +GNU extensions. + +GNU Make supports elementary list-processing functions. The kernel +Makefiles use a novel style of list building and manipulation with few +``if`` statements. + +GNU Make has two assignment operators, ``:=`` and ``=``. ``:=`` performs +immediate evaluation of the right-hand side and stores an actual string +into the left-hand side. ``=`` is like a formula definition; it stores the +right-hand side in an unevaluated form and then evaluates this form each +time the left-hand side is used. + +There are some cases where ``=`` is appropriate. Usually, though, ``:=`` +is the right choice. + +Credits +======= + +- Original version made by Michael Elizabeth Chastain, <mailto:mec@shout.net> +- Updates by Kai Germaschewski <kai@tp1.ruhr-uni-bochum.de> +- Updates by Sam Ravnborg <sam@ravnborg.org> +- Language QA by Jan Engelhardt <jengelh@gmx.de> + +TODO +==== + +- Describe how kbuild supports shipped files with _shipped. +- Generating offset header files. +- Add more variables to chapters 7 or 9? diff --git a/Documentation/kbuild/modules.rst b/Documentation/kbuild/modules.rst new file mode 100644 index 0000000000..a1f3eb7a43 --- /dev/null +++ b/Documentation/kbuild/modules.rst @@ -0,0 +1,561 @@ +========================= +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.rst. + +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/$(SRCARCH)/include/. + +4.1 Kernel Includes +------------------- + + To include a header file located under include/linux/, simply + use:: + + #include <linux/module.h> + + kbuild will add options to "gcc" so the relevant directories + are searched. + +4.2 Single Subdirectory +----------------------- + + External modules tend to place header files in a separate + include/ directory where their source is located, although this + is not the usual kernel style. To inform kbuild of the + directory, use either ccflags-y or CFLAGS_<filename>.o. + + Using the example from section 3, if we moved 8123_if.h to a + subdirectory named include, the resulting kbuild file would + look like:: + + --> filename: Kbuild + obj-m := 8123.o + + ccflags-y := -Iinclude + 8123-y := 8123_if.o 8123_pci.o 8123_bin.o + + Note that in the assignment there is no space between -I and + the path. This is a limitation of kbuild: there must be no + space present. + +4.3 Several Subdirectories +-------------------------- + + kbuild can handle files that are spread over several directories. + Consider the following example:: + + . + |__ src + | |__ complex_main.c + | |__ hal + | |__ hardwareif.c + | |__ include + | |__ hardwareif.h + |__ include + |__ complex.h + + To build the module complex.ko, we then need the following + kbuild file:: + + --> filename: Kbuild + obj-m := complex.o + complex-y := src/complex_main.o + complex-y += src/hal/hardwareif.o + + ccflags-y := -I$(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> <Export Type> <Namespace> + + 0xe1cc2a05 usb_stor_suspend drivers/usb/storage/usb-storage EXPORT_SYMBOL_GPL USB_STORAGE + + The fields are separated by tabs and values may be empty (e.g. + if no namespace is defined for an exported symbol). + + 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. During the MODPOST step, a new Module.symvers file will be + written containing all exported symbols from that external module. + +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. Two 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-m := 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 "make" variable KBUILD_EXTRA_SYMBOLS + If it is impractical to add a top-level kbuild file, + 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. diff --git a/Documentation/kbuild/reproducible-builds.rst b/Documentation/kbuild/reproducible-builds.rst new file mode 100644 index 0000000000..f2dcc39044 --- /dev/null +++ b/Documentation/kbuild/reproducible-builds.rst @@ -0,0 +1,137 @@ +=================== +Reproducible builds +=================== + +It is generally desirable that building the same source code with +the same set of tools is reproducible, i.e. the output is always +exactly the same. This makes it possible to verify that the build +infrastructure for a binary distribution or embedded system has not +been subverted. This can also make it easier to verify that a source +or tool change does not make any difference to the resulting binaries. + +The `Reproducible Builds project`_ has more information about this +general topic. This document covers the various reasons why building +the kernel may be unreproducible, and how to avoid them. + +Timestamps +---------- + +The kernel embeds timestamps in three places: + +* The version string exposed by ``uname()`` and included in + ``/proc/version`` + +* File timestamps in the embedded initramfs + +* If enabled via ``CONFIG_IKHEADERS``, file timestamps of kernel + headers embedded in the kernel or respective module, + exposed via ``/sys/kernel/kheaders.tar.xz`` + +By default the timestamp is the current time and in the case of +``kheaders`` the various files' modification times. This must +be overridden using the `KBUILD_BUILD_TIMESTAMP`_ variable. +If you are building from a git commit, you could use its commit date. + +The kernel does *not* use the ``__DATE__`` and ``__TIME__`` macros, +and enables warnings if they are used. If you incorporate external +code that does use these, you must override the timestamp they +correspond to by setting the `SOURCE_DATE_EPOCH`_ environment +variable. + +User, host +---------- + +The kernel embeds the building user and host names in +``/proc/version``. These must be overridden using the +`KBUILD_BUILD_USER and KBUILD_BUILD_HOST`_ variables. If you are +building from a git commit, you could use its committer address. + +Absolute filenames +------------------ + +When the kernel is built out-of-tree, debug information may include +absolute filenames for the source files. This must be overridden by +including the ``-fdebug-prefix-map`` option in the `KCFLAGS`_ variable. + +Depending on the compiler used, the ``__FILE__`` macro may also expand +to an absolute filename in an out-of-tree build. Kbuild automatically +uses the ``-fmacro-prefix-map`` option to prevent this, if it is +supported. + +The Reproducible Builds web site has more information about these +`prefix-map options`_. + +Generated files in source packages +---------------------------------- + +The build processes for some programs under the ``tools/`` +subdirectory do not completely support out-of-tree builds. This may +cause a later source package build using e.g. ``make rpm-pkg`` to +include generated files. You should ensure the source tree is +pristine by running ``make mrproper`` or ``git clean -d -f -x`` before +building a source package. + +Module signing +-------------- + +If you enable ``CONFIG_MODULE_SIG_ALL``, the default behaviour is to +generate a different temporary key for each build, resulting in the +modules being unreproducible. However, including a signing key with +your source would presumably defeat the purpose of signing modules. + +One approach to this is to divide up the build process so that the +unreproducible parts can be treated as sources: + +1. Generate a persistent signing key. Add the certificate for the key + to the kernel source. + +2. Set the ``CONFIG_SYSTEM_TRUSTED_KEYS`` symbol to include the + signing key's certificate, set ``CONFIG_MODULE_SIG_KEY`` to an + empty string, and disable ``CONFIG_MODULE_SIG_ALL``. + Build the kernel and modules. + +3. Create detached signatures for the modules, and publish them as + sources. + +4. Perform a second build that attaches the module signatures. It + can either rebuild the modules or use the output of step 2. + +Structure randomisation +----------------------- + +If you enable ``CONFIG_RANDSTRUCT``, you will need to pre-generate +the random seed in ``scripts/basic/randstruct.seed`` so the same +value is used by each build. See ``scripts/gen-randstruct-seed.sh`` +for details. + +Debug info conflicts +-------------------- + +This is not a problem of unreproducibility, but of generated files +being *too* reproducible. + +Once you set all the necessary variables for a reproducible build, a +vDSO's debug information may be identical even for different kernel +versions. This can result in file conflicts between debug information +packages for the different kernel versions. + +To avoid this, you can make the vDSO different for different +kernel versions by including an arbitrary string of "salt" in it. +This is specified by the Kconfig symbol ``CONFIG_BUILD_SALT``. + +Git +--- + +Uncommitted changes or different commit ids in git can also lead +to different compilation results. For example, after executing +``git reset HEAD^``, even if the code is the same, the +``include/config/kernel.release`` generated during compilation +will be different, which will eventually lead to binary differences. +See ``scripts/setlocalversion`` for details. + +.. _KBUILD_BUILD_TIMESTAMP: kbuild.html#kbuild-build-timestamp +.. _KBUILD_BUILD_USER and KBUILD_BUILD_HOST: kbuild.html#kbuild-build-user-kbuild-build-host +.. _KCFLAGS: kbuild.html#kcflags +.. _prefix-map options: https://reproducible-builds.org/docs/build-path/ +.. _Reproducible Builds project: https://reproducible-builds.org/ +.. _SOURCE_DATE_EPOCH: https://reproducible-builds.org/docs/source-date-epoch/ |