201.2. Compiling a kernel

201.2 Compiling a kernel

Weight: 3

Description: Candidates should be able to properly configure a kernel to include or disable specific features of the Linux kernel as necessary. This objective includes compiling and recompiling the Linux kernel as needed, updating and noting changes in a new kernel, creating an initrd image and installing new kernels.
Key Knowledge Areas:
  • /usr/src/linux/
  • Kernel Makefiles
  • Kernel 2.6.x/3.x make targets
  • Customize the current kernel configuration.
  • Build a new kernel and appropriate kernel modules.
  • Install a new kernel and any modules.
  • Ensure that the boot manager can locate the new kernel and associated files.
  • Module configuration files
  • Use DKMS to compile kernel modules.
  • Awareness of dracut
Terms and Utilities:
  • mkinitrd
  • mkinitramfs
  • make
  • make targets (all, config, xconfig, menuconfig, gconfig, oldconfig, mrproper, zImage, bzImage, modules, modules_install, rpm-pkg, binrpm-pkg, deb-pkg)
  • gzip
  • bzip2
  • module tools
  • /usr/src/linux/.config
  • /lib/modules/kernel-version/
  • depmod
  • dkms


Why we should compile our linux kernel or upgrade it ? to tell the truth as linux kernel is so modular and flexible, most of the time there is no need to manipulate kernel. Specially When we are talking about commercial versions of linux like redhat, manipulating kernel cause support issues. In contrast in embedded linux system, you should try to make kernel as small as possible. So compile or upgrade kernel just if there is a good reason.
Upgrading kernel and compiling it is very distro specific. The way that we traverse to achive that might be different in Ubuntu, Open Suse or cent OS. Upgrading kernel include compiling, so here we start with downloading new version of kernel and compiling it, but as an administrator you might recompile current version of your linux inorder to add / remove some modules and futures.

Upgrading kernel

First go to https://kernel.org inorder to get the desired kernel version:
We chose latest stable kernel version, do not forget we have to be in /usr/src/ and from now on work in this directory only, in this demonstartion we use CentOS7 machine:
[root@server1 ~]# cd /usr/src/
[root@server1 src]# ls -l
total 0
drwxr-xr-x. 2 root root 6 Nov 5 2016 debug
drwxr-xr-x. 2 root root 6 Nov 5 2016 kernels
[root@server1 src]# wget https://cdn.kernel.org/pub/linux/kernel/v4.x/linux-4.14.3.tar.xz
--2017-12-04 08:34:00-- https://cdn.kernel.org/pub/linux/kernel/v4.x/linux-4.14.3.tar.xz
Resolving cdn.kernel.org (cdn.kernel.org)...,,, ...
Connecting to cdn.kernel.org (cdn.kernel.org)||:443... connected.
HTTP request sent, awaiting response... 200 OK
Length: 100778240 (96M) [application/x-xz]
Saving to: ‘linux-4.14.3.tar.xz’
100%[=============================================>] 100,778,240 247KB/s in 8m 15s
2017-12-04 08:42:20 (199 KB/s) - ‘linux-4.14.3.tar.xz’ saved [100778240/100778240]
[root@server1 src]# ls -l
total 98420
drwxr-xr-x. 2 root root 6 Nov 5 2016 debug
drwxr-xr-x. 2 root root 6 Nov 5 2016 kernels
-rw-r--r--. 1 root root 100778240 Nov 30 03:52 linux-4.14.3.tar.xz
Now lets extract .xz file. Also we should make a symbolic link as linux which point to downloaded kernel:
[root@server1 src]# tar -Jxvf linux-4.14.3.tar.xz
[root@server1 src]# ls -l
total 98424
drwxr-xr-x. 2 root root 6 Nov 5 2016 debug
drwxr-xr-x. 2 root root 6 Nov 5 2016 kernels
drwxrwxr-x. 24 root root 4096 Nov 30 03:41 linux-4.14.3
-rw-r--r--. 1 root root 100778240 Nov 30 03:52 linux-4.14.3.tar.xz
[root@server1 src]# ln -s linux-4.14.3 linux
[root@server1 src]# ls -l
total 98424
drwxr-xr-x. 2 root root 6 Nov 5 2016 debug
drwxr-xr-x. 2 root root 6 Nov 5 2016 kernels
lrwxrwxrwx. 1 root root 12 Dec 4 08:48 linux -> linux-4.14.3
drwxrwxr-x. 24 root root 4096 Nov 30 03:41 linux-4.14.3
-rw-r--r--. 1 root root 100778240 Nov 30 03:52 linux-4.14.3.tar.xz
Before continue there is document folder inside downloaded kernel, There are amazing help text file:
[root@server1 src]# cd linux/Documentation/
[root@server1 Documentation]# ls
00-INDEX gpio parport-lowlevel.txt
accounting hid pcmcia
acpi highuid.txt percpu-rw-semaphore.txt
admin-guide hwmon perf
aoe hw_random.txt phy
arm hwspinlock.txt phy.txt
arm64 i2c pi-futex.txt
atomic_bitops.txt ia64 platform
atomic_t.txt ide pnp.txt
auxdisplay iio power
backlight index.rst powerpc
bcache.txt infiniband pps
blackfin input preempt-locking.txt
block Intel-IOMMU.txt printk-formats.txt
blockdev intel_txt.txt process
bt8xxgpio.txt ioctl pti
btmrvl.txt io-mapping.txt ptp
bus-devices io_ordering.txt pwm.txt
bus-virt-phys-mapping.txt iostats.txt rapidio
cachetlb.txt IPMI.txt rbtree.txt
cdrom IRQ-affinity.txt RCU
cgroup-v1 IRQ-domain.txt remoteproc.txt
cgroup-v2.txt irqflags-tracing.txt rfkill.txt
Changes IRQ.txt robust-futex-ABI.txt
circular-buffers.txt isapnp.txt robust-futexes.txt
clk.txt isa.txt rpmsg.txt
cma isdn rtc.txt
CodingStyle kbuild s390
conf.py kdump SAK.txt
connector kernel-doc-nano-HOWTO.txt scheduler
console kernel-hacking scsi
core-api kernel-per-CPU-kthreads.txt security
cpu-freq kobject.txt serial
cpuidle kprobes.txt sgi-ioc4.txt
cpu-load.txt kref.txt sh
cputopology.txt laptops siphash.txt
crc32.txt ldm.txt SM501.txt
cris leds smsc_ece1099.txt
crypto lightnvm sound
dcdbas.txt livepatch sparc
debugging-modules.txt locking sphinx
debugging-via-ohci1394.txt lockup-watchdogs.txt sphinx-static
dell_rbu.txt logo.gif spi
device-mapper logo.txt static-keys.txt
devicetree lsm.txt SubmittingPatches
dev-tools lzo.txt svga.txt
digsig.txt m68k switchtec.txt
DMA-API-HOWTO.txt mailbox.txt sync_file.txt
DMA-API.txt Makefile sysctl
DMA-attributes.txt md target
dmaengine media tee.txt
DMA-ISA-LPC.txt memory-barriers.txt thermal
doc-guide memory-devices this_cpu_ops.txt
docutils.conf memory-hotplug.txt timers
dontdiff men-chameleon-bus.txt trace
driver-api metag translations
driver-model mic unaligned-memory-access.txt
early-userspace mips usb
EDID misc-devices userspace-api
efi-stub.txt mmc vfio-mediated-device.txt
eisa.txt mn10300 vfio.txt
errseq.rst mtd video-output.txt
extcon namespaces virtual
fault-injection netlabel vm
fb networking w1
features nfc watchdog
filesystems nios2 wimax
firmware_class nommu-mmap.txt x86
flexible-arrays.txt ntb.txt xillybus.txt
fmc numastat.txt xtensa
fpga nvdimm xz.txt
frv nvmem zorro.txt
futex-requeue-pi.txt padata.txt
gcc-plugins.txt parisc
Before continue there are some Development tools which is need to compile the kernel lets install them first:
[root@server1 linux]# yum groupinstall "Development tools"
[root@server1 ~]# yum install elfutils-libelf-devel
Next step is making kernel with modules that we like, compile it and make other kernel required components. to do do that we use make. make is a command that make a program from the source code. Make can do somany thing and is used for many targets.
[root@server1 ~]# cd /usr/src/linux
[root@server1 linux]# make help
./scripts/gcc-version.sh: line 26: gcc: command not found
./scripts/gcc-version.sh: line 27: gcc: command not found
make: gcc: Command not found
make: gcc: Command not found
Cleaning targets:
clean - Remove most generated files but keep the config and
enough build support to build external modules
mrproper - Remove all generated files + config + various backup files
distclean - mrproper + remove editor backup and patch files
Configuration targets:
config - Update current config utilising a line-oriented program
nconfig - Update current config utilising a ncurses menu based
menuconfig - Update current config utilising a menu based program
xconfig - Update current config utilising a Qt based front-end
gconfig - Update current config utilising a GTK+ based front-end
oldconfig - Update current config utilising a provided .config as base
localmodconfig - Update current config disabling modules not loaded
localyesconfig - Update current config converting local mods to core
silentoldconfig - Same as oldconfig, but quietly, additionally update deps
defconfig - New config with default from ARCH supplied defconfig
savedefconfig - Save current config as ./defconfig (minimal config)
allnoconfig - New config where all options are answered with no
allyesconfig - New config where all options are accepted with yes
allmodconfig - New config selecting modules when possible
alldefconfig - New config with all symbols set to default
randconfig - New config with random answer to all options
listnewconfig - List new options
olddefconfig - Same as silentoldconfig but sets new symbols to their
default value
kvmconfig - Enable additional options for kvm guest kernel support
xenconfig - Enable additional options for xen dom0 and guest kernel support
tinyconfig - Configure the tiniest possible kernel
Other generic targets:
all - Build all targets marked with [*]
* vmlinux - Build the bare kernel
* modules - Build all modules
modules_install - Install all modules to INSTALL_MOD_PATH (default: /)
dir/ - Build all files in dir and below
dir/file.[ois] - Build specified target only
dir/file.ll - Build the LLVM assembly file
(requires compiler support for LLVM assembly generation)
dir/file.lst - Build specified mixed source/assembly target only
(requires a recent binutils and recent build (System.map))
dir/file.ko - Build module including final link
modules_prepare - Set up for building external modules
tags/TAGS - Generate tags file for editors
cscope - Generate cscope index
gtags - Generate GNU GLOBAL index
kernelrelease - Output the release version string (use with make -s)
kernelversion - Output the version stored in Makefile (use with make -s)
image_name - Output the image name (use with make -s)
headers_install - Install sanitised kernel headers to INSTALL_HDR_PATH
(default: ./usr)
Static analysers:
checkstack - Generate a list of stack hogs
namespacecheck - Name space analysis on compiled kernel
versioncheck - Sanity check on version.h usage
includecheck - Check for duplicate included header files
export_report - List the usages of all exported symbols
headers_check - Sanity check on exported headers
headerdep - Detect inclusion cycles in headers
coccicheck - Check with Coccinelle.
Kernel selftest:
kselftest - Build and run kernel selftest (run as root)
Build, install, and boot kernel before
running kselftest on it
kselftest-clean - Remove all generated kselftest files
kselftest-merge - Merge all the config dependencies of kselftest to existing
Userspace tools targets:
use "make tools/help"
or "cd tools; make help"
Kernel packaging:
rpm-pkg - Build both source and binary RPM kernel packages
binrpm-pkg - Build only the binary kernel RPM package
deb-pkg - Build both source and binary deb kernel packages
bindeb-pkg - Build only the binary kernel deb package
tar-pkg - Build the kernel as an uncompressed tarball
targz-pkg - Build the kernel as a gzip compressed tarball
tarbz2-pkg - Build the kernel as a bzip2 compressed tarball
tarxz-pkg - Build the kernel as a xz compressed tarball
perf-tar-src-pkg - Build perf-4.14.3.tar source tarball
perf-targz-src-pkg - Build perf-4.14.3.tar.gz source tarball
perf-tarbz2-src-pkg - Build perf-4.14.3.tar.bz2 source tarball
perf-tarxz-src-pkg - Build perf-4.14.3.tar.xz source tarball
Documentation targets:
Linux kernel internal documentation in different formats from ReST:
htmldocs - HTML
latexdocs - LaTeX
pdfdocs - PDF
epubdocs - EPUB
xmldocs - XML
linkcheckdocs - check for broken external links (will connect to external hosts)
cleandocs - clean all generated files
make SPHINXDIRS="s1 s2" [target] Generate only docs of folder s1, s2
valid values for SPHINXDIRS are: networking input core-api userspace-api media process sound crypto sh dev-tools doc-guide driver-api filesystems kernel-hacking admin-guide gpu
make SPHINX_CONF={conf-file} [target] use *additional* sphinx-build
configuration. This is e.g. useful to build with nit-picking config.
Architecture specific targets (x86):
* bzImage - Compressed kernel image (arch/x86/boot/bzImage)
install - Install kernel using
(your) ~/bin/installkernel or
(distribution) /sbin/installkernel or
install to $(INSTALL_PATH) and run lilo
fdimage - Create 1.4MB boot floppy image (arch/x86/boot/fdimage)
fdimage144 - Create 1.4MB boot floppy image (arch/x86/boot/fdimage)
fdimage288 - Create 2.8MB boot floppy image (arch/x86/boot/fdimage)
isoimage - Create a boot CD-ROM image (arch/x86/boot/image.iso)
bzdisk/fdimage*/isoimage also accept:
FDARGS="..." arguments for the booted kernel
FDINITRD=file initrd for the booted kernel
i386_defconfig - Build for i386
x86_64_defconfig - Build for x86_64
make V=0|1 [targets] 0 => quiet build (default), 1 => verbose build
make V=2 [targets] 2 => give reason for rebuild of target
make O=dir [targets] Locate all output files in "dir", including .config
make C=1 [targets] Check re-compiled c source with $CHECK (sparse by default)
make C=2 [targets] Force check of all c source with $CHECK
make RECORDMCOUNT_WARN=1 [targets] Warn about ignored mcount sections
make W=n [targets] Enable extra gcc checks, n=1,2,3 where
1: warnings which may be relevant and do not occur too often
2: warnings which occur quite often but may still be relevant
3: more obscure warnings, can most likely be ignored
Multiple levels can be combined with W=12 or W=123
Execute "make" or "make all" to build all targets marked with [*]
For further info see the ./README file
Lets classified each target with it usefull commands:
Cleaning Targets (used if you have tried to compile before)
Remove partially Generated Files From Previous compile and refresh the code but keeps config file
clean everything + config + backup
Configuration Targets(To configure desired kernel modules)
line oriented program, take days (very hard tool)
ncurses based menu program (hard tool) [need to nstall ncurses-devel]
menu base program (recommanded tool)
QT-Based menu program, like menu config [need to install qt-devel]
use the current setting of current kernel for the next one
The thing we are going to do is making kernel image (zImage / BigzImage if you remember :) )and we need to make modules that should be available in the kernel. Final step is installation process. Because we need to put what we have made in proper place. Let take a look at make xconfig , make menuconfig:
No matter what you choose, a module can have different states:
Condition To choose
<>:Not Available
not even compiled to the kernel
<M>:Loadable as module
compiled with the new kernel, but not loaded as default
Compiled and loaded , Built into the kernel
We are happy with current kernel configuration so lets try:
[root@server1 src]#make oldconfig
Wow after answering tons of questions finall:
# configuration written to .config

Make bzImage

[root@server1 linux]# make bzImage
scripts/kconfig/conf --silentoldconfig Kconfig
SYSTBL arch/x86/entry/syscalls/../../include/generated/asm/syscalls_32.h
SYSHDR arch/x86/entry/syscalls/../../include/generated/asm/unistd_32_ia32.h
SYSHDR arch/x86/entry/syscalls/../../include/generated/asm/unistd_64_x32.h
SYSTBL arch/x86/entry/syscalls/../../include/generated/asm/syscalls_64.h
HYPERCALLS arch/x86/entry/syscalls/../../include/generated/asm/xen-hypercalls.h
SYSHDR arch/x86/entry/syscalls/../../include/generated/uapi/asm/unistd_32.h
SYSHDR arch/x86/entry/syscalls/../../include/generated/uapi/asm/unistd_64.h
SYSHDR arch/x86/entry/syscalls/../../include/generated/uapi/asm/unistd_x32.h
HOSTCC scripts/basic/bin2c
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
CHK include/config/kernel.release
UPD include/config/kernel.release
WRAP arch/x86/include/generated/asm/clkdev.h
WRAP arch/x86/include/generated/asm/dma-contiguous.h
WRAP arch/x86/include/generated/asm/early_ioremap.h
WRAP arch/x86/include/generated/asm/mcs_spinlock.h
WRAP arch/x86/include/generated/asm/mm-arch-hooks.h
CHK include/generated/uapi/linux/version.h
UPD include/generated/uapi/linux/version.h
CHK include/generated/utsrelease.h
UPD include/generated/utsrelease.h
CC arch/x86/purgatory/purgatory.o
AS arch/x86/purgatory/stack.o
AS arch/x86/purgatory/setup-x86_64.o
CC arch/x86/purgatory/sha256.o
AS arch/x86/purgatory/entry64.o
CC arch/x86/purgatory/string.o
LD arch/x86/purgatory/purgatory.ro
BIN2C arch/x86/purgatory/kexec-purgatory.c
CC kernel/bounds.s
CHK include/generated/bounds.h
UPD include/generated/bounds.h
CHK include/generated/timeconst.h
UPD include/generated/timeconst.h
CC arch/x86/kernel/asm-offsets.s
CHK include/generated/asm-offsets.h
UPD include/generated/asm-offsets.h
CALL scripts/checksyscalls.sh
HOSTCC scripts/genksyms/genksyms.o
SHIPPED scripts/genksyms/parse.tab.c
HOSTCC scripts/genksyms/parse.tab.o
SHIPPED scripts/genksyms/lex.lex.c
SHIPPED scripts/genksyms/parse.tab.h
HOSTCC scripts/genksyms/lex.lex.o
HOSTLD scripts/genksyms/genksyms
CC scripts/mod/empty.o
HOSTCC scripts/mod/mk_elfconfig
MKELF scripts/mod/elfconfig.h
HOSTCC scripts/mod/modpost.o
CC scripts/mod/devicetable-offsets.s
CHK scripts/mod/devicetable-offsets.h
UPD scripts/mod/devicetable-offsets.h
HOSTCC scripts/mod/file2alias.o
HOSTCC scripts/mod/sumversion.o
HOSTLD scripts/mod/modpost
HOSTCC scripts/selinux/genheaders/genheaders
HOSTCC scripts/selinux/mdp/mdp
HOSTCC scripts/kallsyms
HOSTCC scripts/pnmtologo
HOSTCC scripts/conmakehash
HOSTCC scripts/recordmcount
HOSTCC scripts/sortextable
HOSTCC scripts/asn1_compiler
HOSTCC scripts/sign-file
scripts/sign-file.c:25:30: fatal error: openssl/opensslv.h: No such file or directory
#include <openssl/opensslv.h>
compilation terminated.
make[1]: *** [scripts/sign-file] Error 1
make: *** [scripts] Error 2
ops we encounter a problem, lets resolve it:
[root@server1 linux]# yum install openssl-devel
And Finally our bzImage is ready:
Setup is 16860 bytes (padded to 16896 bytes).
System is 6386 kB
CRC e17db93b
Kernel: arch/x86/boot/bzImage is ready (#1)

make modules

Modules need to be compiled too, so do the same thing for them:
[root@server1 linux]# make modules
CC sound/usb/snd-usbmidi-lib.mod.o
LD [M] sound/usb/snd-usbmidi-lib.ko
CC sound/usb/usx2y/snd-usb-us122l.mod.o
LD [M] sound/usb/usx2y/snd-usb-us122l.ko
CC sound/usb/usx2y/snd-usb-usx2y.mod.o
LD [M] sound/usb/usx2y/snd-usb-usx2y.ko
CC virt/lib/irqbypass.mod.o
LD [M] virt/lib/irqbypass.ko

make modules_install

Now that every thing is prepared, its time to put all part to the their right places, First let put compiled modules to /lib/modules/
INSTALL sound/soc/snd-soc-core.ko
INSTALL sound/soundcore.ko
INSTALL sound/synth/emux/snd-emux-synth.ko
INSTALL sound/synth/snd-util-mem.ko
INSTALL sound/usb/6fire/snd-usb-6fire.ko
INSTALL sound/usb/bcd2000/snd-bcd2000.ko
INSTALL sound/usb/caiaq/snd-usb-caiaq.ko
INSTALL sound/usb/hiface/snd-usb-hiface.ko
INSTALL sound/usb/line6/snd-usb-line6.ko
INSTALL sound/usb/line6/snd-usb-pod.ko
INSTALL sound/usb/line6/snd-usb-podhd.ko
INSTALL sound/usb/line6/snd-usb-toneport.ko
INSTALL sound/usb/line6/snd-usb-variax.ko
INSTALL sound/usb/misc/snd-ua101.ko
INSTALL sound/usb/snd-usb-audio.ko
INSTALL sound/usb/snd-usbmidi-lib.ko
INSTALL sound/usb/usx2y/snd-usb-us122l.ko
INSTALL sound/usb/usx2y/snd-usb-usx2y.ko
INSTALL virt/lib/irqbypass.ko
DEPMOD 4.14.3
to confirm what has been done:
[root@server1 linux]# ls -lrth /lib/modules
total 8.0K
drwxr-xr-x. 7 root root 4.0K Oct 28 11:25 3.10.0-693.el7.x86_64
drwxr-xr-x. 3 root root 4.0K Dec 5 07:14 4.14.3

make install

make install write required files into the /boot and modified grub and set everything for us
[root@server1 linux]# make install
sh ./arch/x86/boot/install.sh 4.14.3 arch/x86/boot/bzImage \
System.map "/boot"
[root@server1 linux]# uname -r
and reboot and enjoy your new kernel!
[root@server1 ~]# uname -r


As we have seen kernel needs RAM to be populated with some drivers and modules.In old days each distro tries to implement its own taste of initrd/initramfs. It made hard for users and developers to work with each distro. dracut is a tool which has tried to standardize the process of making initrd/initramfs.
dracut uses udev deamon events (uevents) and based on them create general purpose initramfs. All we need to know is that dracut is a program wich is used behind kernel installation process to create initrd/initramfs. It considers current kernel and settings and generates desired initramfs/initrd base on them and put initramfs/initrd on the right place and modified grub.
dracut also can be used to just modifying initrd/initramfs:
###in ubuntu
root@server1:~# dracut
The program 'dracut' is currently not installed. You can install it by typing:
apt install dracut-core
root@server1:~# apt install dracut-core
dracut reads its configurations from /etc/dracut.conf :
[root@server1 ~]# vi /etc/dracut.conf
# PUT YOUR CONFIG HERE OR IN separate files named *.conf
# in /etc/dracut.conf.d
# SEE man dracut.conf(5)
# Sample dracut config file
# Exact list of dracut modules to use. Modules not listed here are not going
# to be included. If you only want to add some optional modules use
# add_dracutmodules option instead.
# dracut modules to omit
# dracut modules to add to the default
# additional kernel modules to the default
# list of kernel filesystem modules to be included in the generic initramfs
# build initrd only to boot current hardware
# install local /etc/mdadm.conf
# install local /etc/lvm/lvm.conf
# A list of fsck tools to install. If it's not specified, module's hardcoded
# default is used, currently: "umount mount /sbin/fsck* xfs_db xfs_check
# xfs_repair e2fsck jfs_fsck reiserfsck btrfsck". The installation is
# opportunistic, so non-existing tools are just ignored.
# inhibit installation of any fsck tools
# mount / and /usr read-only by default
# set the directory for temporary files
# default: /var/tmp
in dracut.conf you can commented out module/driver that you like and then run dracut --force to make your falavor of initrd/initramfs.