How the kernel is compiled

Process of the Linux kernel building

Introduction

I won't tell you how to build and install a custom Linux kernel on your machine. If you need help with this, you can find many resources that will help you do it. Instead, we will learn what occurs when you execute make in the root directory of the Linux kernel source code.

When I started to study the source code of the Linux kernel, the makefile was the first file that I opened. And it was scary :). The makefile contained 1591 lines of code when I wrote this part and the kernel was the 4.2.0-rc3 release.

This makefile is the top makefile in the Linux kernel source code and the kernel building starts here. Yes, it is big, but moreover, if you've read the source code of the Linux kernel you may have noted that all directories containing source code has its own makefile. Of course it is not possible to describe how each source file is compiled and linked, so we will only study the standard compilation case. You will not find here building of the kernel's documentation, cleaning of the kernel source code, tags generation, cross-compilation related stuff, etc... We will start from the make execution with the standard kernel configuration file and will finish with the building of the bzImage.

It would be better if you're already familiar with the make util, but I will try to describe every piece of code in this part anyway.

So let's start.

Preparation before the kernel compilation

There are many things to prepare before the kernel compilation can be started. The main point here is to find and configure the type of compilation, to parse command line arguments that are passed to make, etc... So let's dive into the top Makefile of Linux kernel.

The top Makefile of Linux kernel is responsible for building two major products: vmlinux (the resident kernel image) and the modules (any module files). The Makefile of the Linux kernel starts with the definition of following variables:

VERSION = 4
PATCHLEVEL = 2
SUBLEVEL = 0
EXTRAVERSION = -rc3
NAME = Hurr durr I'm a sheep

These variables determine the current version of Linux kernel and are used in different places, for example in the forming of the KERNELVERSION variable in the same Makefile:

KERNELVERSION = $(VERSION)$(if $(PATCHLEVEL),.$(PATCHLEVEL)$(if $(SUBLEVEL),.$(SUBLEVEL)))$(EXTRAVERSION)

After this we can see a couple of ifeq conditions that check some of the parameters passed to make. The Linux kernel makefiles provides a special make help target that prints all available targets and some of the command line arguments that can be passed to make. For example : make V=1 => verbose build. The first ifeq checks whether the V=n option is passed to make:

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 make, we set the KBUILD_VERBOSE variable to the value of V option. Otherwise we set the KBUILD_VERBOSE variable to zero. After this we check the value of KBUILD_VERBOSE variable and set values of the quiet and Q variables depending on the value of KBUILD_VERBOSE variable. The @ symbols suppress the output of command. And if it is present before a command the output will be something like this: CC scripts/mod/empty.o instead of 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:

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 represents the top directory of the kernel source code and whether it is empty (it is empty when the makefile is executed for the first time). We then set the KBUILD_OUTPUT variable to the value passed with the O option (if this option was passed). In the next step we check this KBUILD_OUTPUT variable and if it is set, we do following things:

  • Store the value of KBUILD_OUTPUT in the temporary saved-output variable;

  • Try to create the given output directory;

  • Check that directory created, in other way print error message;

  • If the custom output directory was created successfully, execute make again with the new directory (see the -C option).

The next ifeq statements check that the C or M options passed to make:

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 C option tells the makefile that we need to check all c source code with a tool provided by the $CHECK environment variable, by default it is sparse. The second M option provides build for the external modules (will not see this case in this part). We also check whether the KBUILD_SRC variable is set, and if it isn't, we set the srctree variable to .:

ifeq ($(KBUILD_SRC),)
        srctree := .
endif

objtree	:= .
src		:= $(srctree)
obj		:= $(objtree)

export srctree objtree VPATH

That tells Makefile that the kernel source tree will be in the current directory where make was executed. We then set objtree and other variables to this directory and export them. The next step is to get value for the SUBARCH variable that represents what the underlying architecture is:

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 the uname util that prints information about machine, operating system and architecture. As it gets the output of uname, it parses the output and assigns the result to the SUBARCH variable. Now that we have SUBARCH, we set the SRCARCH variable that provides the directory of the certain architecture and hdr-arch that provides the directory for the header files:

ifeq ($(ARCH),i386)
        SRCARCH := x86
endif
ifeq ($(ARCH),x86_64)
        SRCARCH := x86
endif

hdr-arch  := $(SRCARCH)

Note ARCH is an alias for 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 is set to .config by default:

KCONFIG_CONFIG	?= .config
export KCONFIG_CONFIG

and the shell that will be used during kernel compilation:

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 are related to the compilers used during Linux kernel compilation. We set the host compilers for the c and c++ and the flags to be used with them:

HOSTCC       = gcc
HOSTCXX      = g++
HOSTCFLAGS   = -Wall -Wmissing-prototypes -Wstrict-prototypes -O2 -fomit-frame-pointer -std=gnu89
HOSTCXXFLAGS = -O2

Next we get to the CC variable that represents compiler too, so why do we need the HOST* variables? 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 the definition of KBUILD_MODULES and KBUILD_BUILTIN variables that are used to determine what to compile (modules, kernel, or both):

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 KBUILD_BUILTIN variable will depend on the CONFIG_MODVERSIONS kernel configuration parameter if we pass only modules to make. The next step is to include the kbuild file.

include scripts/Kbuild.include

The Kbuild or Kernel Build System is a special infrastructure to manage building the kernel and its modules. kbuild files have the same syntax as makefiles. The scripts/Kbuild.include file provides some generic definitions for the kbuild system. After including this kbuild file (back in makefile) we can see the definitions of the variables that are related to the different tools used during kernel and module compilation (like linker, compilers, utils from the binutils, etc...):

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
...
...
...

We then define two other variables: USERINCLUDE and LINUXINCLUDE, which specify paths to header file directories (public for users in the first case and for kernel in the second case):

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:

KBUILD_CFLAGS   := -Wall -Wundef -Wstrict-prototypes -Wno-trigraphs \
		   -fno-strict-aliasing -fno-common \
		   -Werror-implicit-function-declaration \
		   -Wno-format-security \
		   -std=gnu89

These are not the final compilation flags, as they can be updated in other makefiles (for example kbuilds from arch/). After all of these, all variables will be exported to be available in the other makefiles. The RCS_FIND_IGNORE and the RCS_TAR_IGNORE variables contain files that will be ignored in the version control system:

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

With that, we have finished all preparations. The next step is building the vmlinux target.

Directly to the kernel build

We have now finished all the preparations, and next step in the main makefile is related to the kernel build. Before this moment, nothing has been printed to the terminal by make. But now the first steps of the compilation are started. We need to go to line 598 of the Linux kernel top makefile and we will find the vmlinux target there:

all: vmlinux
	include arch/$(SRCARCH)/Makefile

Don't worry that we have missed many lines in Makefile that are between export RCS_FIND_IGNORE..... and 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). From this moment we will continue from this makefile. As we can see all target depends on the vmlinux target that is defined a little lower in the top makefile:

vmlinux: scripts/link-vmlinux.sh $(vmlinux-deps) FORCE

The vmlinux is the Linux kernel in a statically linked executable file format. The scripts/link-vmlinux.sh script links and combines different compiled subsystems into vmlinux. The second target is the vmlinux-deps that defined as:

vmlinux-deps := $(KBUILD_LDS) $(KBUILD_VMLINUX_INIT) $(KBUILD_VMLINUX_MAIN)

and consists from the set of the built-in.o from 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:

$(sort $(vmlinux-deps)): $(vmlinux-dirs) ;
$(vmlinux-dirs): prepare scripts
	$(Q)$(MAKE) $(build)=$@

As we can see vmlinux-dirs depends on two targets: prepare and scripts. prepare is defined in the top Makefile of the Linux kernel and executes three stages of preparations:

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 expands to the archheaders and archscripts that defined in the x86_64 specific Makefile. Let's look on it. The x86_64 specific makefile starts from the definition of the variables that are related to the architecture-specific configs (defconfig, etc...). After this it defines flags for the compiling of the 16-bit 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). The first target is archheaders in the makefile and it generates syscall table:

archheaders:
	$(Q)$(MAKE) $(build)=arch/x86/entry/syscalls all

And the second target is archscripts in this makefile is:

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. At the first we can see the scripts_basic target that executes make for the scripts/basic makefile:

scripts_basic:
	$(Q)$(MAKE) $(build)=scripts/basic

The scripts/basic/Makefile contains targets for compilation of the two host programs: fixdep and bin2:

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 gcc that tells make when to remake a source code file. The second program is bin2c, which depends on the value of the CONFIG_BUILD_BIN2C kernel configuration option and is a very little C program that allows to convert a binary on stdin to a C include on stdout. You can note here a strange notation: hostprogs-y, etc... This notation is used in the all kbuild files and you can read more about it in the documentation. In our case hostprogs-y tells kbuild that there is one host program named fixdep that will be built from fixdep.c that is located in the same directory where the Makefile is. The first output after we execute make 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 makefile with the relocs target:

$(Q)$(MAKE) $(build)=arch/x86/tools relocs

The relocs_32.c and the relocs_64.c will be compiled that will contain relocation information and we will see it in the make output:

  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:

$(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:

prepare0: archprepare FORCE
	$(Q)$(MAKE) $(build)=.

Note on the build. It defined in the scripts/Kbuild.include and looks like this:

build := -f $(srctree)/scripts/Makefile.build obj

Or in our case it is current source directory - .:

$(Q)$(MAKE) -f $(srctree)/scripts/Makefile.build obj=.

The scripts/Makefile.build tries to find the Kbuild file by the given directory via the obj parameter, include this Kbuild files:

include $(kbuild-file)

and build targets from it. In our case . contains the 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, 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 of the Linux kernel:

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:

$(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 (depends 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, Documentation, etc... but I will not describe it here as I wrote in the beginning of this part.

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 script is linking of the all built-in.o(s) to the one statically linked executable and creation of the 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.

Building bzImage

The bzImage file is the compressed Linux kernel image. We can get it by executing make bzImage after vmlinux is built. That, or we can just execute make without any argument and we will get bzImage anyway because it is default image:

all: bzImage

in the arch/x86/kernel/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 and looks like this:

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:

boot := arch/x86/boot

The main goal now is to build the source code in the arch/x86/boot and arch/x86/boot/compressed directories, build setup.bin and vmlinux.bin, and build the bzImage from them in the end. First target in the arch/x86/boot/Makefile is the $(obj)/setup.elf:

$(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 variable that expands to the all source files from the boot directory. We can see first output:

  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 file is arch/x86/boot/header.S, but we can't build it now because this target depends on the following two header files:

$(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:

#define VO__end 0xffffffff82ab0000
#define VO__text 0xffffffff81000000

They are the start and the end of the kernel. The second is zoffset.h depens on the vmlinux target from the arch/x86/boot/compressed/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 directory and generates vmlinux.bin, vmlinux.bin.bz2, and compiles program - mkpiggy. We can see this in the output:

  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 vmlinux.bin is the vmlinux file with debugging information and comments stripped 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:

  MKPIGGY arch/x86/boot/compressed/piggy.S
  AS      arch/x86/boot/compressed/piggy.o

This assembly files will contain the computed offset from the compressed kernel. After this we can see that zoffset generated:

  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 can be continued:

  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:

  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 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 is familiar to all who once built 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 steps 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 understand the process of building the Linux kernel.

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