Linux Kernel Compilation

Understand the Linux Kernel compilation process

Man

An interface to the on-line reference manuals

user@workstation:~$ man man
...
MAN(1)                        Manual pager utils                        MAN(1)

NAME
       man - an interface to the on-line reference manuals

SYNOPSIS
       man  [-C  file]  [-d]  [-D]  [--warnings[=warnings]]  [-R encoding] [-L
       locale] [-m system[,...]] [-M path] [-S list]  [-e  extension]  [-i|-I]
...

Uname

Print system information

Print all information

user@workstation:~$ uname -a
Linux kali 4.12.0-kali2-amd64 #1 SMP Debian 4.12.13-1kali2 (2017-10-03) x86_64 GNU/Linux

Print the Kernel Release

user@workstation:~$ uname -r
4.12.0-kali2-amd64

Directory /boot

This directory contains everything required for the boot process except for configuration files not needed at boot time (the most notable of those being those that belong to the GRUB boot-loader) and the map installer. LDP Linux Filesystem Hierarchy /boot

user@workstation:~$ cd /boot/
user@workstation:~/boot$

user@workstation:~/boot$ ls
config-4.12.0-kali1-amd64      initrd.img-4.12.0-kali2-amd64  vmlinuz-4.12.0-kali1-amd64
config-4.12.0-kali2-amd64      initrd.img-4.15.0-kali1-amd64  vmlinuz-4.12.0-kali2-amd64
config-4.15.0-kali1-amd64      System.map-4.12.0-kali1-amd64  vmlinuz-4.15.0-kali1-amd64
grub                           System.map-4.12.0-kali2-amd64
initrd.img-4.12.0-kali1-amd64  System.map-4.15.0-kali1-amd64

config

Installed kernel configuration

user@workstation:~/boot$ ls /boot/config-$(uname -r)
/boot/config-4.12.0-kali2-amd64

user@workstation:~/boot$ file config-4.12.0-kali2-amd64
config-4.12.0-kali2-amd64: Linux make config build file, ASCII text

user@workstation:~/boot$ ls -alh config-4.12.0-kali2-amd64 
-rw-r--r-- 1 root root 190K Oct  3 01:58 config-4.12.0-kali2-amd64
user@workstation:~/boot$

user@workstation:~/boot$ cat config-4.12.0-kali2-amd64 | less
#
# Automatically generated file; DO NOT EDIT.
# Linux/x86 4.12.13 Kernel Configuration
#
CONFIG_64BIT=y
CONFIG_X86_64=y
CONFIG_X86=y
CONFIG_INSTRUCTION_DECODER=y
CONFIG_OUTPUT_FORMAT="elf64-x86-64"
CONFIG_ARCH_DEFCONFIG="arch/x86/configs/x86_64_defconfig"
CONFIG_LOCKDEP_SUPPORT=y
CONFIG_STACKTRACE_SUPPORT=y
CONFIG_MMU=y
CONFIG_ARCH_MMAP_RND_BITS_MIN=28
CONFIG_ARCH_MMAP_RND_BITS_MAX=32
CONFIG_ARCH_MMAP_RND_COMPAT_BITS_MIN=8
CONFIG_ARCH_MMAP_RND_COMPAT_BITS_MAX=16
CONFIG_NEED_DMA_MAP_STATE=y
CONFIG_NEED_SG_DMA_LENGTH=y
...

initrd

In computing, initrd (initial ramdisk) is a scheme for loading a temporary root file system into memory, which may be used as part of the Linux startup process. initrd and initramfs refer to two different methods of achieving this. Both are commonly used to make preparations before the real root file system can be mounted. Wikipedia

user@workstation:~/boot$ ls /boot/initrd.img-$(uname -r)
/boot/initrd.img-4.12.0-kali2-amd64

user@workstation:~/boot$ file initrd.img-4.12.0-kali2-amd64
initrd.img-4.12.0-kali2-amd64: ASCII cpio archive (SVR4 with no CRC)

user@workstation:~/boot$ ls -alh initrd.img-4.12.0-kali2-amd64
-rw-r--r-- 1 root root 26M Oct  8 22:54 initrd.img-4.12.0-kali2-amd64

user@workstation:~/boot$ mkdir /tmp/temp
user@workstation:~/boot$ cd /tmp/temp/
user@workstation:/tmp/temp$ cp /boot/initrd.img-4.12.0-kali2-amd64 .
user@workstation:/tmp/temp$ (cpio -id; zcat | cpio -id) < initrd.img-4.12.0-kali2-amd64
36 blocks
148321 blocks
user@workstation:/tmp/temp$ ls
bin  conf  etc    init  initrd.img-4.12.0-kali2-amd64  kernel  lib  lib64  run  sbin  scripts
user@workstation:/tmp/temp$

user@workstation:/tmp/temp$ find . -type f | wc -l
1117
user@workstation:/tmp/temp$ find lib/modules/4.12.0-kali2-amd64/kernel/ -type f | wc -l
662
xe1gyq@kali:/tmp/temp$

user@workstation:/tmp/temp$ cd /boot
user@workstation:~/boot$ rm -rf /tmp/temp/
user@workstation:~/boot$

System

In Linux, the System.map file is a symbol table used by the kernel. A symbol table is a look-up between symbol names and their addresses in memory. A symbol name may be the name of a variable or the name of a function. The System.map is required when the address of a symbol name, or the symbol name of an address, is needed. It is especially useful for debugging kernel panics and kernel oopses. The kernel does the address-to-name translation itself when CONFIG_KALLSYMS is enabled so that tools like ksymoops are not required. Wikipedia

user@workstation:~/boot$ ls /boot/System.map-$(uname -r)
/boot/System.map-4.12.0-kali2-amd64

user@workstation:~/boot$ file System.map-4.12.0-kali2-amd64 
System.map-4.12.0-kali2-amd64: ASCII text
user@workstation:~/boot$

user@workstation:~/boot$ ls -alh System.map-4.12.0-kali2-amd64
-rw-r--r-- 1 root root 2.9M Oct  3 01:58 System.map-4.12.0-kali2-amd64

user@workstation:~/boot$ cat System.map-4.12.0-kali2-amd64 | less
0000000000000000 D __per_cpu_start
0000000000000000 D irq_stack_union
0000000000000000 A xen_irq_disable_direct_reloc
0000000000000000 A xen_save_fl_direct_reloc
00000000000001c5 A kexec_control_code_size
0000000000004000 d exception_stacks
0000000000009000 D gdt_page
000000000000a000 D espfix_waddr
000000000000a008 D espfix_stack
000000000000a020 D cpu_llc_id
000000000000a040 D cpu_llc_shared_map
000000000000a080 D cpu_core_map
000000000000a0c0 D cpu_sibling_map
000000000000a100 D cpu_info
000000000000a1e8 D cpu_number
000000000000a1f0 D this_cpu_off
000000000000a1f8 D x86_cpu_to_acpiid
000000000000a1fc D x86_cpu_to_apicid
000000000000a1fe D x86_bios_cpu_apicid
000000000000a200 D sched_core_priority
000000000000a240 d cpu_loops_per_jiffy
000000000000a280 d pmc_prev_left
000000000000a480 D cpu_hw_events
000000000000b740 d bts_ctx
000000000000b800 D pqr_state
000000000000b810 d insn_buffer
...

user@workstation:~/boot$ cat /proc/kallsyms | less
0000000000000000 A irq_stack_union
0000000000000000 A __per_cpu_start
0000000000004000 A exception_stacks
0000000000009000 A gdt_page
000000000000a000 A espfix_waddr
000000000000a008 A espfix_stack
000000000000a020 A cpu_llc_id
000000000000a040 A cpu_llc_shared_map
000000000000a080 A cpu_core_map
000000000000a0c0 A cpu_sibling_map
000000000000a100 A cpu_info
000000000000a1e8 A cpu_number
000000000000a1f0 A this_cpu_off
000000000000a1f8 A x86_cpu_to_acpiid
000000000000a1fc A x86_cpu_to_apicid
000000000000a1fe A x86_bios_cpu_apicid
000000000000a200 A sched_core_priority
000000000000a240 A cpu_loops_per_jiffy
000000000000a280 A pmc_prev_left
000000000000a480 A cpu_hw_events
000000000000b740 A bts_ctx
000000000000b800 A pqr_state
000000000000b810 A insn_buffer
000000000000b820 A pt_ctx
000000000000b900 A xen_vcpu_info
000000000000b940 A xen_vcpu_id
...

user@workstation:~/boot$ cat System.map-4.12.0-kali2-amd64 | grep exception_stacks
0000000000004000 d exception_stacks
user@workstation:~/boot$ cat /proc/kallsyms | grep exception_stacks
0000000000004000 A exception_stacks

Vmlinuz

On Linux systems, vmlinux is a statically linked executable file that contains the Linux kernel in one of the object file formats supported by Linux, which includes ELF, COFF and a.out. The vmlinux file might be required for kernel debugging, symbol table generation or other operations, but must be made bootable before being used as an operating system kernel by adding a multiboot header, bootsector and setup routines. Wikipedia

user@workstation:~/boot$ ls /boot/vmlinuz-$(uname -r)
/boot/vmlinuz-4.12.0-kali2-amd64
user@workstation:~/boot$

user@workstation:~/boot$ file vmlinuz-4.12.0-kali2-amd64
vmlinuz-4.12.0-kali2-amd64: Linux kernel x86 boot executable bzImage, version 4.12.0-kali2-amd64 ([e-mail]) #1 SMP Debian 4.12.13-1kali2 (2017-10-03), RO-rootFS, swap_dev 0x4, Normal VGA
user@workstation:~/boot$

user@workstation:~/boot$ ls -alh vmlinuz-4.12.0-kali2-amd64
-rw-r--r-- 1 root root 4.2M Oct  3 01:58 vmlinuz-4.12.0-kali2-amd64
user@workstation:~/boot$

user@workstation:~/boot$ mkdir /tmp/vmlinuz
user@workstation:~/boot$ cd /tmp/vmlinuz/
xe1gyq@kali:/tmp/vmlinuz$ cp /boot/vmlinuz-4.12.0-kali2-amd64 .
...

user@workstation:~/boot$ cd
user@workstation:~$

Directory /lib

The /lib directory contains kernel modules and those shared library images (the C programming code library) needed to boot the system and run the commands in the root filesystem. LDP Linux Filesystem Hierarchy /lib

user@workstation:~$ ls -al /lib/modules/
total 24K
drwxr-xr-x  7 root root 4.0K Dec 23 09:05 .
drwxr-xr-x 18 root root 4.0K Sep  9 19:05 ..
drwxr-xr-x  3 root root 4.0K Sep 10 09:26 4.12.0-kali1-amd64
drwxr-xr-x  3 root root 4.0K Oct  8 22:54 4.12.0-kali2-amd64
drwxr-xr-x  2 root root 4.0K Oct  8 22:59 4.6.0-kali1-amd64
drwxr-xr-x  2 root root 4.0K Oct  8 22:59 4.9.0-kali2-amd64
user@workstation:~$

user@workstation:~$ ls -al /lib/modules/4.12.0-kali2-amd64/
total 4176
drwxr-xr-x  3 root root    4096 Oct  8 22:54 .
drwxr-xr-x  7 root root    4096 Dec 23 09:05 ..
lrwxrwxrwx  1 root root      41 Oct  3 01:58 build -> /usr/src/linux-headers-4.12.0-kali2-amd64
drwxr-xr-x 12 root root    4096 Sep 26 00:17 kernel
-rw-r--r--  1 root root 1031395 Oct  8 22:54 modules.alias
-rw-r--r--  1 root root  988746 Oct  8 22:54 modules.alias.bin
-rw-r--r--  1 root root    4402 Oct  3 01:58 modules.builtin
-rw-r--r--  1 root root    5907 Oct  8 22:54 modules.builtin.bin
-rw-r--r--  1 root root  399937 Oct  8 22:54 modules.dep
-rw-r--r--  1 root root  551428 Oct  8 22:54 modules.dep.bin
-rw-r--r--  1 root root     402 Oct  8 22:54 modules.devname
-rw-r--r--  1 root root  133633 Oct  3 01:58 modules.order
-rw-r--r--  1 root root     772 Oct  8 22:54 modules.softdep
-rw-r--r--  1 root root  501957 Oct  8 22:54 modules.symbols
-rw-r--r--  1 root root  618714 Oct  8 22:54 modules.symbols.bin
lrwxrwxrwx  1 root root      42 Oct  3 01:58 source -> /usr/src/linux-headers-4.12.0-kali2-common
user@workstation:~$

user@workstation:~$ ls /usr/src/linux-headers-4.12.0-kali2-common
arch  include  Makefile  scripts
user@workstation:~$ ls /usr/src/linux-headers-4.12.0-kali2-amd64/
arch  include  Makefile  Module.symvers  scripts
user@workstation:~$

user@workstation:~$ find /lib/modules/4.12.0-kali2-amd64/kernel/ -type f | wc -l
3403
user@workstation:~$

Directory /usr/src

user@workstation:~$ ls /usr/src
aufs-4.12+20170904           linux-kbuild-4.12
linux-headers-4.12.0-kali2-amd64   volatility-tools
linux-headers-4.12.0-kali2-common
user@workstation:~$

Mainline

user@workstation:~$ cd linux
user@workstation:~/linux$

user@workstation:~/linux$ ls
arch     CREDITS    firmware  ipc       lib        net     security  virt
block     crypto        fs      Kbuild   MAINTAINERS    README     sound
certs     Documentation    include   Kconfig  Makefile    samples  tools
COPYING  drivers    init      kernel   mm        scripts  usr
user@workstation:~/linux$

Configuration Setup

user@workstation:~/linux$ cp /boot/config-4.12.0-kali2-amd64 .config
user@workstation:~/linux$ make oldconfig
<You will be asked configuration questions not answered, for now hit Enter for all of them>
DesignWare Cores SATA support (SATA_DWC) [N/m/?] (NEW)
...
#
# configuration written to .config
#

Compilation Kernel and Modules

user@workstation:~/linux$ make

user@workstation:~/linux$ ls
arch     crypto        include  kernel       modules.builtin  scripts     virt
block     Documentation    init     lib          modules.order    security
certs     drivers    ipc     MAINTAINERS  net           sound
COPYING  firmware    Kbuild     Makefile     README           tools
CREDITS  fs        Kconfig  mm          samples           usr
user@workstation:~/linux$

Modules Installation

root@workstation:~/linux# make modules_install

Kernel Installation

root@workstation:~/linux# make install

Uname

Reboot your workstation and confirm the new version has been installed, print the Kernel Release

user@workstation:~$ uname -a
Linux kali config-4.15.0-rc4+ #1 SMP Debian 4.12.13-1kali2 (2017-10-03) x86_64 GNU/Linux