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Process of the Linux kernel building
================================================================================
Introduction
--------------------------------------------------------------------------------
I will not tell you how to build and install custom Linux kernel on your machine, you can find many many [resources](https://encrypted.google.com/search?q=building+linux+kernel#q=building+linux+kernel+from+source+code) that will help you to do it. Instead, we will know what does occur when you are typed `make` in the directory with Linux kernel source code in this part. When I just started to learn source code of the Linux kernel, the [Makefile](https://github.com/torvalds/linux/blob/master/Makefile) file was a first file that I've opened. And it was scary :) This [makefile](https://en.wikipedia.org/wiki/Make_%28software%29) contains `1591` lines of code at the time when I wroute this part and it was [third](https://github.com/torvalds/linux/commit/52721d9d3334c1cb1f76219a161084094ec634dc) release candidate.
This makefile is the the top makefile in the Linux kernel source code and kernel build starts here. Yes, it is big, but moreover, if you've read the source code of the Linux kernel you can noted that all directories with a source code has an own makefile. Of course it is not real to describe how each source files compiled and linked. So, we will see compilation only for the standard case. You will not find here building of the kernel's documentation, cleaning of the kernel source code, [tags](https://en.wikipedia.org/wiki/Ctags) generation, [cross-compilation](https://en.wikipedia.org/wiki/Cross_compiler) related stuff and etc. We will start from the `make` execution with the standard kernel configuration file and will finish with the building of the [bzImage](https://en.wikipedia.org/wiki/Vmlinux#bzImage).
Will be good if you already familar with the [make](https://en.wikipedia.org/wiki/Make_%28software%29) util, but anyway I will try to describe all code that will be in this part.
So let's start.
Preparation before the kernel compilation
---------------------------------------------------------------------------------
Yes, there are need to do many preparations before the kernel compilation will be started. The main point here is to find and configure type of compilation, to parse command line arguments that are passed to the `make` util and etc. So let's dive into the top `Makefile` of the Linux kernel.
The Linux kernel top `Makefile` is responsible for building two major products: [vmlinux](https://en.wikipedia.org/wiki/Vmlinux) (the resident kernel image) and modules (any module files). The [Makefile](https://github.com/torvalds/linux/blob/master/Makefile) of the Linux kernel starts from the definition of the following variables:
```Makefile
VERSION = 4
PATCHLEVEL = 2
SUBLEVEL = 0
EXTRAVERSION = -rc3
NAME = Hurr durr I'ma sheep
```
As you can understand from the names of these variables they determine the current version of the Linux kernel and will be used in the different places, first is forming of the `KERNELVERSION` variable:
```Makefile
KERNELVERSION = $(VERSION)$(if $(PATCHLEVEL),.$(PATCHLEVEL)$(if $(SUBLEVEL),.$(SUBLEVEL)))$(EXTRAVERSION)
```
After this we can see a couple of the `ifeq` condition that makes a check of some parameters passed to the `make`. The Linux kernel `makefiles` provides special `make help` target that prints available targets and some command line arguments that can be passed to the `make`. For example: `make V=1` - provides verbose builds. First `ifeq` condition checks that `V=n` option is passed to the make:
```Makefile
ifeq ("$(origin V)", "command line")
KBUILD_VERBOSE = $(V)
endif
ifndef KBUILD_VERBOSE
KBUILD_VERBOSE = 0
endif
ifeq ($(KBUILD_VERBOSE),1)
quiet =
Q =
else
quiet=quiet_
Q = @
endif
export quiet Q KBUILD_VERBOSE
```
If this option is passed to the `make` we set `KBUILD_VERBOSE` variable to the value of the `V` option and set it to zero in other way. After this we check value of the `KBUILD_VERBOSE` variable and set values of the `quiet` and `Q` variables depends on the `KBUILD_VERBOSE` value. The `@` symbols suppress the output of the command and if it will be set before a command we will see something like this: `CC scripts/mod/empty.o` instead of the `Compiling .... scripts/mod/empty.o`. In the end we just export all of these variables. The next `ifeq` statement checks that `O=/dir` option was passed to the `make`. This option allows to locate all output files in the given `dir`:
```Makefile
ifeq ($(KBUILD_SRC),)
ifeq ("$(origin O)", "command line")
KBUILD_OUTPUT := $(O)
endif
ifneq ($(KBUILD_OUTPUT),)
saved-output := $(KBUILD_OUTPUT)
KBUILD_OUTPUT := $(shell mkdir -p $(KBUILD_OUTPUT) && cd $(KBUILD_OUTPUT) \
&& /bin/pwd)
$(if $(KBUILD_OUTPUT),, \
$(error failed to create output directory "$(saved-output)"))
sub-make: FORCE
$(Q)$(MAKE) -C $(KBUILD_OUTPUT) KBUILD_SRC=$(CURDIR) \
-f $(CURDIR)/Makefile $(filter-out _all sub-make,$(MAKECMDGOALS))
skip-makefile := 1
endif # ifneq ($(KBUILD_OUTPUT),)
endif # ifeq ($(KBUILD_SRC),)
```
We check the `KBUILD_SRC` that represent top directory of the source code of the linux kernel and if it is empty (it is empty every time while makefile executes first time) and the set the `KBUILD_OUTPUT` variable to the value that passed with the `O` option (if this option was passed). In the next step we check this `KBUILD_OUTPUT` variable and if we set it, we do following things:
* Store value of the `KBUILD_OUTPUT` in the temp `saved-output` variable;
* Try to create given output directory;
* Check that directory created, in other way print error;
* If custom output directory created sucessfully, execute `make` again with the new directory (see `-C` option).
The next `ifeq` statements checks that `C` or `M` options was passed to the make:
```Makefile
ifeq ("$(origin C)", "command line")
KBUILD_CHECKSRC = $(C)
endif
ifndef KBUILD_CHECKSRC
KBUILD_CHECKSRC = 0
endif
ifeq ("$(origin M)", "command line")
KBUILD_EXTMOD := $(M)
endif
```
The first `C` option tells to the `makefile` that need to check all `c` source code with a tool provided by the `$CHECK` environment variable, by default it is [sparse](https://en.wikipedia.org/wiki/Sparse). The second `M` option provides build for the external modules (will not see this case in this part). As we set this variables we make a check of the `KBUILD_SRC` variable and if it is not set we set `srctree` variable to `.`:
```Makefile
ifeq ($(KBUILD_SRC),)
srctree := .
endif
objtree := .
src := $(srctree)
obj := $(objtree)
export srctree objtree VPATH
```
That tells to `Makefile` that source tree of the Linux kernel will be in the current directory where `make` command was executed. After this we set `objtree` and other variables to this directory and export these variables. The next step is the getting value for the `SUBARCH` variable that will represent tewhat the underlying archicecture is:
```Makefile
SUBARCH := $(shell uname -m | sed -e s/i.86/x86/ -e s/x86_64/x86/ \
-e s/sun4u/sparc64/ \
-e s/arm.*/arm/ -e s/sa110/arm/ \
-e s/s390x/s390/ -e s/parisc64/parisc/ \
-e s/ppc.*/powerpc/ -e s/mips.*/mips/ \
-e s/sh[234].*/sh/ -e s/aarch64.*/arm64/ )
```
As you can see it executes [uname](https://en.wikipedia.org/wiki/Uname) utils that prints information about machine, operating system and architecture. As it will get output of the `uname` util, it will parse it and assign to the `SUBARCH` variable. As we got `SUBARCH`, we set the `SRCARCH` variable that provides directory of the certain architecture and `hfr-arch` that provides directory for the header files:
```Makefile
ifeq ($(ARCH),i386)
SRCARCH := x86
endif
ifeq ($(ARCH),x86_64)
SRCARCH := x86
endif
hdr-arch := $(SRCARCH)
```
Note that `ARCH` is the alias for the `SUBARCH`. In the next step we set the `KCONFIG_CONFIG` variable that represents path to the kernel configuration file and if it was not set before, it will be `.config` by default:
```Makefile
KCONFIG_CONFIG ?= .config
export KCONFIG_CONFIG
```
and the [shell](https://en.wikipedia.org/wiki/Shell_%28computing%29) that will be used during kernel compilation:
```Makefile
CONFIG_SHELL := $(shell if [ -x "$$BASH" ]; then echo $$BASH; \
else if [ -x /bin/bash ]; then echo /bin/bash; \
else echo sh; fi ; fi)
```
The next set of variables related to the compiler that will be used during Linux kernel compilation. We set the host compilers for the `c` and `c++` and flags for it:
```Makefile
HOSTCC = gcc
HOSTCXX = g++
HOSTCFLAGS = -Wall -Wmissing-prototypes -Wstrict-prototypes -O2 -fomit-frame-pointer -std=gnu89
HOSTCXXFLAGS = -O2
```
Next we will meet the `CC` variable that represent compiler too, so why do we need in the `HOST*` variables? The `CC` is the target compiler that will be used during kernel compilation, but `HOSTCC` will be used during compilation of the set of the `host` programs (we will see it soon). After this we can see definition of the `KBUILD_MODULES` and `KBUILD_BUILTIN` variables that are used for the determination of the what to compile (kernel, modules or both):
```Makefile
KBUILD_MODULES :=
KBUILD_BUILTIN := 1
ifeq ($(MAKECMDGOALS),modules)
KBUILD_BUILTIN := $(if $(CONFIG_MODVERSIONS),1)
endif
```
Here we can see definition of these variables and the value of the `KBUILD_BUILTIN` will depens on the `CONFIG_MODVERSIONS` kernel configuration parameter if we pass only `modules` to the `make`. The next step is including of the:
```Makefile
include scripts/Kbuild.include
```
`kbuild` file. The [Kbuild](https://github.com/torvalds/linux/blob/master/Documentation/kbuild/kbuild.txt) or `Kernel Build System` is the special infrastructure to manage building of the kernel and its modules. The `kbuild` files has the same syntax that makefiles. The [scripts/Kbuild.include](https://github.com/torvalds/linux/blob/master/scripts/Kbuild.include) file provides some generic definitions for the `kbuild` system. As we included this `kbuild` files we can see definition of the variables that are related to the different tools that will be used during kernel and modules compilation (like linker, compilers, utils from the [binutils](http://www.gnu.org/software/binutils/) and etc...):
```Makefile
AS = $(CROSS_COMPILE)as
LD = $(CROSS_COMPILE)ld
CC = $(CROSS_COMPILE)gcc
CPP = $(CC) -E
AR = $(CROSS_COMPILE)ar
NM = $(CROSS_COMPILE)nm
STRIP = $(CROSS_COMPILE)strip
OBJCOPY = $(CROSS_COMPILE)objcopy
OBJDUMP = $(CROSS_COMPILE)objdump
AWK = awk
...
...
...
```
After definition of these variables we define two variables: `USERINCLUDE` and `LINUXINCLUDE`. They will contain paths of the directories with headers (public for users in the first case and for kernel in the second case):
```Makefile
USERINCLUDE := \
-I$(srctree)/arch/$(hdr-arch)/include/uapi \
-Iarch/$(hdr-arch)/include/generated/uapi \
-I$(srctree)/include/uapi \
-Iinclude/generated/uapi \
-include $(srctree)/include/linux/kconfig.h
LINUXINCLUDE := \
-I$(srctree)/arch/$(hdr-arch)/include \
...
```
And the standard flags for the C compiler:
```Makefile
KBUILD_CFLAGS := -Wall -Wundef -Wstrict-prototypes -Wno-trigraphs \
-fno-strict-aliasing -fno-common \
-Werror-implicit-function-declaration \
-Wno-format-security \
-std=gnu89
```
It is the not last compiler flags, they can be updated by the other makefiles (for example kbuilds from `arch/`). After all of these, all variables will be exported to be available in the other makefiles. The following two the `RCS_FIND_IGNORE` and the `RCS_TAR_IGNORE` variables will contain files that will be ignored in the version control system:
```Makefile
export RCS_FIND_IGNORE := \( -name SCCS -o -name BitKeeper -o -name .svn -o \
-name CVS -o -name .pc -o -name .hg -o -name .git \) \
-prune -o
export RCS_TAR_IGNORE := --exclude SCCS --exclude BitKeeper --exclude .svn \
--exclude CVS --exclude .pc --exclude .hg --exclude .git
```
That's all. We have finsihed with the all preparations, next point is the building of `vmlinux`.
Directly to the kernel build
--------------------------------------------------------------------------------
As we have finished all preparations, next step in the root makefile is related to the kernel build. Before this moment we will not see in the our terminal after the execution of the `make` command. But now first steps of the compilation are started. In this moment we need to go on the [598](https://github.com/torvalds/linux/blob/master/Makefile#L598) line of the Linux kernel top makefile and we will see `vmlinux` target there:
```Makefile
all: vmlinux
include arch/$(SRCARCH)/Makefile
```
Don't worry that we have missed many lines in Makefile that are placed after `export RCS_FIND_IGNORE.....` and before `all: vmlinux.....`. This part of the makefile is responsible for the `make *.config` targets and as I wrote in the beginning of this part we will see only building of the kernel in a general way.
The `all:` target is the default when no target is given on the command line. You can see here that we include architecture specific makefile there (in our case it will be [arch/x86/Makefile](https://github.com/torvalds/linux/blob/master/arch/x86/Makefile)). From this moment we will continue from this makefile. As we can see `all` target depends on the `vmlinux` target that defined a little lower in the top makefile:
```Makefile
vmlinux: scripts/link-vmlinux.sh $(vmlinux-deps) FORCE
```
The `vmlinux` is is the Linux kernel in an statically linked executable file format. The [scripts/link-vmlinux.sh](https://github.com/torvalds/linux/blob/master/scripts/link-vmlinux.sh) script links combines different compiled subsystems into vmlinux. The second target is the `vmlinux-deps` that defined as:
```Makefile
vmlinux-deps := $(KBUILD_LDS) $(KBUILD_VMLINUX_INIT) $(KBUILD_VMLINUX_MAIN)
```
and consists from the set of the `built-in.o` from the each top directory of the Linux kernel. Later, when we will go through all directories in the Linux kernel, the `Kbuild` will compile all the `$(obj-y)` files. It then calls `$(LD) -r` to merge these files into one `built-in.o` file. For this moment we have no `vmlinux-deps`, so the `vmlinux` target will not be executed now. For me `vmlinux-deps` contains following files:
```
arch/x86/kernel/vmlinux.lds arch/x86/kernel/head_64.o
arch/x86/kernel/head64.o arch/x86/kernel/head.o
init/built-in.o usr/built-in.o
arch/x86/built-in.o kernel/built-in.o
mm/built-in.o fs/built-in.o
ipc/built-in.o security/built-in.o
crypto/built-in.o block/built-in.o
lib/lib.a arch/x86/lib/lib.a
lib/built-in.o arch/x86/lib/built-in.o
drivers/built-in.o sound/built-in.o
firmware/built-in.o arch/x86/pci/built-in.o
arch/x86/power/built-in.o arch/x86/video/built-in.o
net/built-in.o
```
The next target that can be executed is following:
```Makefile
$(sort $(vmlinux-deps)): $(vmlinux-dirs) ;
$(vmlinux-dirs): prepare scripts
$(Q)$(MAKE) $(build)=$@
```
As we can see the `vmlinux-dirs` depends on the two targets: `prepare` and `scripts`. The first `prepare` defined in the top `Makefile` of the Linux kernel and executes three stages of preparations:
```Makefile
prepare: prepare0
prepare0: archprepare FORCE
$(Q)$(MAKE) $(build)=.
archprepare: archheaders archscripts prepare1 scripts_basic
prepare1: prepare2 $(version_h) include/generated/utsrelease.h \
include/config/auto.conf
$(cmd_crmodverdir)
prepare2: prepare3 outputmakefile asm-generic
```
The first `prepare0` expands to the `archprepare` that exapnds to the `archheaders` and `archscripts` that defined in the `x86_64` specific [Makefile](https://github.com/torvalds/linux/blob/master/arch/x86/Makefile). Let's look on it. The `x86_64` specific makefile starts from the definition of the variables that are related to the archicteture-specific configs ([defconfig](https://github.com/torvalds/linux/tree/master/arch/x86/configs) and etc.). After this it defines flags for the compiling of the [16-bit](https://en.wikipedia.org/wiki/Real_mode) code, calculating of the `BITS` variable that can be `32` for `i386` or `64` for the `x86_64` flags for the assembly source code, flags for the linker and many many more (all definitions you can find in the [arch/x86/Makefile](https://github.com/torvalds/linux/blob/master/arch/x86/Makefile)). The first target is `archheaders` in the makefile generates syscall table:
```Makefile
archheaders:
$(Q)$(MAKE) $(build)=arch/x86/entry/syscalls all
```
And the second target is `archscripts` in this makefile is:
```Makefile
archscripts: scripts_basic
$(Q)$(MAKE) $(build)=arch/x86/tools relocs
```
We can see that it depends on the `scripts_basic` target from the top [Makefile](https://github.com/torvalds/linux/blob/master/Makefile). At the first we can see the `scripts_basic` target that executes make for the [scripts/basic](https://github.com/torvalds/linux/blob/master/scripts/basic/Makefile) makefile:
```Maklefile
scripts_basic:
$(Q)$(MAKE) $(build)=scripts/basic
```
The `scripts/basic/Makefile` contains targets for compilation of the two host programs: `fixdep` and `bin2`:
```Makefile
hostprogs-y := fixdep
hostprogs-$(CONFIG_BUILD_BIN2C) += bin2c
always := $(hostprogs-y)
$(addprefix $(obj)/,$(filter-out fixdep,$(always))): $(obj)/fixdep
```
First program is `fixdep` - optimizes list of dependencies generated by the [gcc](https://gcc.gnu.org/) that tells make when to remake a source code file. The second program is `bin2c` depends on the value of the `CONFIG_BUILD_BIN2C` kernel configuration option and very little C program that allows to convert a binary on stdin to a C include on stdout. You can note here strange notation: `hostprogs-y` and etc. This notation is used in the all `kbuild` files and more about it you can read in the [documentation](https://github.com/torvalds/linux/blob/master/Documentation/kbuild/makefiles.txt). In our case the `hostprogs-y` tells to the `kbuild` that there is one host program named `fixdep` that will be built from the will be built from `fixdep.c` that located in the same directory that `Makefile`. The first output after we will execute `make` command in our terminal will be result of this `kbuild` file:
```
$ make
HOSTCC scripts/basic/fixdep
```
As `script_basic` target was executed, the `archscripts` target will execute `make` for the [arch/x86/tools](https://github.com/torvalds/linux/blob/master/arch/x86/tools/Makefile) makefile with the `relocs` target:
```Makefile
$(Q)$(MAKE) $(build)=arch/x86/tools relocs
```
The `relocs_32.c` and the `relocs_64.c` will be compiled that will contain [relocation](https://en.wikipedia.org/wiki/Relocation_%28computing%29) information and we will see it in the `make` output:
```Makefile
HOSTCC arch/x86/tools/relocs_32.o
HOSTCC arch/x86/tools/relocs_64.o
HOSTCC arch/x86/tools/relocs_common.o
HOSTLD arch/x86/tools/relocs
```
There is checking of the `version.h` after compiling of the `relocs.c`:
```Makefile
$(version_h): $(srctree)/Makefile FORCE
$(call filechk,version.h)
$(Q)rm -f $(old_version_h)
```
We can see it in the output:
```
CHK include/config/kernel.release
```
and the building of the `generic` assembly headers with the `asm-generic` target from the `arch/x86/include/generated/asm` that generated in the top Makefile of the Linux kernel. After the `asm-generic` target the `archprepare` will be done, so the `prepare0` target will be executed. As I wrote above:
```Makefile
prepare0: archprepare FORCE
$(Q)$(MAKE) $(build)=.
```
Note on the `build`. It defined in the [scripts/Kbuild.include](https://github.com/torvalds/linux/blob/master/scripts/Kbuild.include) and looks like this:
```Makefile
build := -f $(srctree)/scripts/Makefile.build obj
```
or in our case it is current source directory - `.`:
```Makefile
$(Q)$(MAKE) -f $(srctree)/scripts/Makefile.build obj=.
```
The [scripts/Makefile.build](https://github.com/torvalds/linux/blob/master/scripts/Makefile.build) tries to find the `Kbuild` file by the given directory via the `obj` parameter, include this `Kbuild` files:
```Makefile
include $(kbuild-file)
```
and build targets from it. In our case `.` contains the [Kbuild](https://github.com/torvalds/linux/blob/master/Kbuild) file that generates the `kernel/bounds.s` and the `arch/x86/kernel/asm-offsets.s`. After this the `prepare` target finished to work. The `vmlinux-dirs` also depends on the second target - `scripts` that compiles following programs: `file2alias`, `mk_elfconfig`, `modpost` and etc... After scripts/host-programs compilation our `vmlinux-dirs` target can be executed. First of all let's try to understand what does `vmlinux-dirs` contain. For my case it contains paths of the following kernel directories:
```
init usr arch/x86 kernel mm fs ipc security crypto block
drivers sound firmware arch/x86/pci arch/x86/power
arch/x86/video net lib arch/x86/lib
```
We can find definition of the `vmlinux-dirs` in the top [Makefile](https://github.com/torvalds/linux/blob/master/Makefile) of the Linux kernel:
```Makefile
vmlinux-dirs := $(patsubst %/,%,$(filter %/, $(init-y) $(init-m) \
$(core-y) $(core-m) $(drivers-y) $(drivers-m) \
$(net-y) $(net-m) $(libs-y) $(libs-m)))
init-y := init/
drivers-y := drivers/ sound/ firmware/
net-y := net/
libs-y := lib/
...
...
...
```
Here we remove the `/` symbol from the each directory with the help of the `patsubst` and `filter` functions and put it to the `vmlinux-dirs`. So we have list of directories in the `vmlinux-dirs` and the following code:
```Makefile
$(vmlinux-dirs): prepare scripts
$(Q)$(MAKE) $(build)=$@
```
The `$@` represents `vmlinux-dirs` here that means that it will go recursively over all directories from the `vmlinux-dirs` and its internal directories (depens on configuration) and will execute `make` in there. We can see it in the output:
```
CC init/main.o
CHK include/generated/compile.h
CC init/version.o
CC init/do_mounts.o
...
CC arch/x86/crypto/glue_helper.o
AS arch/x86/crypto/aes-x86_64-asm_64.o
CC arch/x86/crypto/aes_glue.o
...
AS arch/x86/entry/entry_64.o
AS arch/x86/entry/thunk_64.o
CC arch/x86/entry/syscall_64.o
```
Source code in each directory will be compiled and linked to the `built-in.o`:
```
$ find . -name built-in.o
./arch/x86/crypto/built-in.o
./arch/x86/crypto/sha-mb/built-in.o
./arch/x86/net/built-in.o
./init/built-in.o
./usr/built-in.o
...
...
```
Ok, all buint-in.o(s) built, now we can back to the `vmlinux` target. As you remember, the `vmlinux` target is in the top Makefile of the Linux kernel. Before the linking of the `vmlinux` it builds [samples](https://github.com/torvalds/linux/tree/master/samples), [Documentation](https://github.com/torvalds/linux/tree/master/Documentation) and etc., but I will not describe it in this part as I wrote in the beginning of this part.
```Makefile
vmlinux: scripts/link-vmlinux.sh $(vmlinux-deps) FORCE
...
...
+$(call if_changed,link-vmlinux)
```
As you can see main purpose of it is a call of the [scripts/link-vmlinux.sh](https://github.com/torvalds/linux/blob/master/scripts/link-vmlinux.sh) script is linking of the all `built-in.o`(s) to the one statically linked executable and creation of the [System.map](https://en.wikipedia.org/wiki/System.map). In the end we will see following output:
```
LINK vmlinux
LD vmlinux.o
MODPOST vmlinux.o
GEN .version
CHK include/generated/compile.h
UPD include/generated/compile.h
CC init/version.o
LD init/built-in.o
KSYM .tmp_kallsyms1.o
KSYM .tmp_kallsyms2.o
LD vmlinux
SORTEX vmlinux
SYSMAP System.map
```
and `vmlinux` and `System.map` in the root of the Linux kernel source tree:
```
$ ls vmlinux System.map
System.map vmlinux
```
That's all, `vmlinux` is ready. The next step is creation of the [bzImage](https://en.wikipedia.org/wiki/Vmlinux#bzImage).
Building bzImage
--------------------------------------------------------------------------------
The `bzImage` is the compressed Linux kernel image. We can get it with the execution of the `make bzImage` after the `vmlinux` built. In other way we can just execute `make` without arguments and will get `bzImage` anyway because it is default image:
```Makefile
all: bzImage
```
in the [arch/x86/kernel/Makefile](https://github.com/torvalds/linux/blob/master/arch/x86/Makefile). Let's look on this target, it will help us to understand how this image builds. As I already said the `bzImage` target defined in the [arch/x86/kernel/Makefile](https://github.com/torvalds/linux/blob/master/arch/x86/Makefile) and looks like this:
```Makefile
bzImage: vmlinux
$(Q)$(MAKE) $(build)=$(boot) $(KBUILD_IMAGE)
$(Q)mkdir -p $(objtree)/arch/$(UTS_MACHINE)/boot
$(Q)ln -fsn ../../x86/boot/bzImage $(objtree)/arch/$(UTS_MACHINE)/boot/$@
```
We can see here, that first of all called `make` for the boot directory, in our case it is:
```Makefile
boot := arch/x86/boot
```
The main goal now to build source code in the `arch/x86/boot` and `arch/x86/boot/compressed` directories, build `setup.bin` and `vmlinux.bin`, and build the `bzImage` from they in the end. First target in the [arch/x86/boot/Makefile](https://github.com/torvalds/linux/blob/master/arch/x86/boot/Makefile) is the `$(obj)/setup.elf`:
```Makefile
$(obj)/setup.elf: $(src)/setup.ld $(SETUP_OBJS) FORCE
$(call if_changed,ld)
```
We already have the `setup.ld` linker script in the `arch/x86/boot` directory and the `SETUP_OBJS` expands to the all source files from the `boot` directory. We can see first output:
```Makefile
AS arch/x86/boot/bioscall.o
CC arch/x86/boot/cmdline.o
AS arch/x86/boot/copy.o
HOSTCC arch/x86/boot/mkcpustr
CPUSTR arch/x86/boot/cpustr.h
CC arch/x86/boot/cpu.o
CC arch/x86/boot/cpuflags.o
CC arch/x86/boot/cpucheck.o
CC arch/x86/boot/early_serial_console.o
CC arch/x86/boot/edd.o
```
The next source code file is the [arch/x86/boot/header.S](https://github.com/torvalds/linux/blob/master/arch/x86/boot/header.S), but we can't build it now because this target depends on the following two header files:
```Makefile
$(obj)/header.o: $(obj)/voffset.h $(obj)/zoffset.h
```
The first is `voffset.h` generated by the `sed` script that gets two addresses from the `vmlinux` with the `nm` util:
```C
#define VO__end 0xffffffff82ab0000
#define VO__text 0xffffffff81000000
```
They are start and end of the kernel. The second is `zoffset.h` depens on the `vmlinux` target from the [arch/x86/boot/compressed/Makefile](https://github.com/torvalds/linux/blob/master/arch/x86/boot/compressed/Makefile):
```Makefile
$(obj)/zoffset.h: $(obj)/compressed/vmlinux FORCE
$(call if_changed,zoffset)
```
The `$(obj)/compressed/vmlinux` target depends on the `vmlinux-objs-y` that compiles source code files from the [arch/x86/boot/compressed](https://github.com/torvalds/linux/tree/master/arch/x86/boot/compressed) directory and generates `vmlinux.bin`, `vmlinux.bin.bz2`, and compiles programm - `mkpiggy`. We can see this in the output:
```Makefile
LDS arch/x86/boot/compressed/vmlinux.lds
AS arch/x86/boot/compressed/head_64.o
CC arch/x86/boot/compressed/misc.o
CC arch/x86/boot/compressed/string.o
CC arch/x86/boot/compressed/cmdline.o
OBJCOPY arch/x86/boot/compressed/vmlinux.bin
BZIP2 arch/x86/boot/compressed/vmlinux.bin.bz2
HOSTCC arch/x86/boot/compressed/mkpiggy
```
Where the `vmlinux.bin` is the `vmlinux` with striped debuging information and comments and the `vmlinux.bin.bz2` compressed `vmlinux.bin.all` + `u32` size of `vmlinux.bin.all`. The `vmlinux.bin.all` is `vmlinux.bin + vmlinux.relocs`, where `vmlinux.relocs` is the `vmlinux` that was handled by the `relocs` program (see above). As we got these files, the `piggy.S` assembly files will be generated with the `mkpiggy` program and compiled:
```Makefile
MKPIGGY arch/x86/boot/compressed/piggy.S
AS arch/x86/boot/compressed/piggy.o
```
This assembly files will contain computed offset from a compressed kernel. After this we can see that `zoffset` generated:
```Makefile
ZOFFSET arch/x86/boot/zoffset.h
```
As the `zoffset.h` and the `voffset.h` are generated, compilation of the source code files from the [arch/x86/boot](https://github.com/torvalds/linux/tree/master/arch/x86/boot/) can be continued:
```Makefile
AS arch/x86/boot/header.o
CC arch/x86/boot/main.o
CC arch/x86/boot/mca.o
CC arch/x86/boot/memory.o
CC arch/x86/boot/pm.o
AS arch/x86/boot/pmjump.o
CC arch/x86/boot/printf.o
CC arch/x86/boot/regs.o
CC arch/x86/boot/string.o
CC arch/x86/boot/tty.o
CC arch/x86/boot/video.o
CC arch/x86/boot/video-mode.o
CC arch/x86/boot/video-vga.o
CC arch/x86/boot/video-vesa.o
CC arch/x86/boot/video-bios.o
```
As all source code files will be compiled, they will be linked to the `setup.elf`:
```Makefile
LD arch/x86/boot/setup.elf
```
or:
```
ld -m elf_x86_64 -T arch/x86/boot/setup.ld arch/x86/boot/a20.o arch/x86/boot/bioscall.o arch/x86/boot/cmdline.o arch/x86/boot/copy.o arch/x86/boot/cpu.o arch/x86/boot/cpuflags.o arch/x86/boot/cpucheck.o arch/x86/boot/early_serial_console.o arch/x86/boot/edd.o arch/x86/boot/header.o arch/x86/boot/main.o arch/x86/boot/mca.o arch/x86/boot/memory.o arch/x86/boot/pm.o arch/x86/boot/pmjump.o arch/x86/boot/printf.o arch/x86/boot/regs.o arch/x86/boot/string.o arch/x86/boot/tty.o arch/x86/boot/video.o arch/x86/boot/video-mode.o arch/x86/boot/version.o arch/x86/boot/video-vga.o arch/x86/boot/video-vesa.o arch/x86/boot/video-bios.o -o arch/x86/boot/setup.elf
```
The last two things is the creation of the `setup.bin` that will contain compiled code from the `arch/x86/boot/*` directory:
```
objcopy -O binary arch/x86/boot/setup.elf arch/x86/boot/setup.bin
```
and the creation of the `vmlinux.bin` from the `vmlinux`:
```
objcopy -O binary -R .note -R .comment -S arch/x86/boot/compressed/vmlinux arch/x86/boot/vmlinux.bin
```
In the end we compile host program: [arch/x86/boot/tools/build.c](https://github.com/torvalds/linux/blob/master/arch/x86/boot/tools/build.c) that will create our `bzImage` from the `setup.bin` and the `vmlinux.bin`:
```
arch/x86/boot/tools/build arch/x86/boot/setup.bin arch/x86/boot/vmlinux.bin arch/x86/boot/zoffset.h arch/x86/boot/bzImage
```
Actually the `bzImage` is the concatenated `setup.bin` and the `vmlinux.bin`. In the end we will see the output which familiar to all who once build the Linux kernel from source:
```
Setup is 16268 bytes (padded to 16384 bytes).
System is 4704 kB
CRC 94a88f9a
Kernel: arch/x86/boot/bzImage is ready (#5)
```
That's all.
Conclusion
================================================================================
It is the end of this part and here we saw all stesp from the execution of the `make` command to the generation of the `bzImage`. I know, the Linux kernel makefiles and process of the Linux kernel building may seem confusing at first glance, but it is not so hard. Hope this part will help you to understand process of the Linux kernel building.
Links
================================================================================
* [GNU make util](https://en.wikipedia.org/wiki/Make_%28software%29)
* [Linux kernel top Makefile](https://github.com/torvalds/linux/blob/master/Makefile)
* [cross-compilation](https://en.wikipedia.org/wiki/Cross_compiler)
* [Ctags](https://en.wikipedia.org/wiki/Ctags)
* [sparse](https://en.wikipedia.org/wiki/Sparse)
* [bzImage](https://en.wikipedia.org/wiki/Vmlinux#bzImage)
* [uname](https://en.wikipedia.org/wiki/Uname)
* [shell](https://en.wikipedia.org/wiki/Shell_%28computing%29)
* [Kbuild](https://github.com/torvalds/linux/blob/master/Documentation/kbuild/kbuild.txt)
* [binutils](http://www.gnu.org/software/binutils/)
* [gcc](https://gcc.gnu.org/)
* [Documentation](https://github.com/torvalds/linux/blob/master/Documentation/kbuild/makefiles.txt)
* [System.map](https://en.wikipedia.org/wiki/System.map)
* [Relocation](https://en.wikipedia.org/wiki/Relocation_%28computing%29)
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