Linux Boot Process Tree V1.0
======================
By Multazim Abdul Rashid Deshmukh
Power on
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SMPS (Switched Mode Power Supply)-Generates power for all hardware devices in PC
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CPU RESETS – Program Counter of CPU points to Address of BIOS
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BIOS from ROM loaded into RAM {boot strapping}
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BIOS do POST – checks all hardware devices are working properly
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If Yes If not
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—————– | boot process stops. Gives Beep codes
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BIOS loads CMOS program into RAM–checks for 1st boot device (say HDD)
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Cylinder 0 is read from backwards
BIOS reaches first 512 bytes of cylinder 0
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MBR loads into RAM (from cylinder 0 sector 1 of HDD)
| MBR has 3 parts: ——————————————–
| Boot Sector | Partition Table | Magic No |
| | | | | | | |
| | 446 bytes | 64 bytes | 2 bytes |
| ——————————————–
|
Magic no is checked ————————————–|
| |
| If it says no (other than 0x55AA)
If it says ok (hex form-0x55 AA) BIOS gives Message
Means Boot sector & Partition Table “Non-bootable disk” or “No-System”
are written properly
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BIOS checks for Active Partition in Partition Table
Goes to BS in AP
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LILO Byte code from Boot sector loads into RAM-1st stage of boot loading. LILO byte code is /etc/lilo.conf file in binary form placed by anaconda.
| |
| During installation anaconda does 5 things
| |
| 1) creates /etc/lilo.conf and puts a binry
| bytecode of it in bootsector.
| 2) puts an exe at 0x228 for redirecting BIOS
| to lilo bytecode in dual boot system.
| 3) puts CHS nos of /boot/boot.b, /boot/message
| /boot/map, /boot/initrd-***.img in bootsector
| 4) Writes Partition table in MBR.
| 5) checks sanity of BS & PT and writes ok i.e.
| 0x55AA in magic no.
|
|—|
|–>LILO byte code gives CHS no. of /boot/boot.b to BIOS,
| BIOS do int 13h fn8
|
|–>BIOS loads Binary file /boot/boot.b{2nd stage of boot loading}
|
|–> boot.b gives CHS no. of /boot/message to BIOS
| BIOS do int 13h fn2
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|–> Bios loads /boot/message –> splash screen is displayed
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|->OS Labels will be shown
|->after timeout default OS will boot
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|————————————|———-|
| |
If it is linux then if it is windows then
| Control goes to ntldr
|———–|
|
|–> boot.b gives CHS no. of /boot/map to BIOS
| BIOS do int 13h fn2
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|–> Bios loads /boot/map -> map file contains CHS no. and name of kernel
| i.e. vmlinuz-****
|
|–> boot.b gives CHS no. of kernel to BIOS
| BIOS do int 13h fn2
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|–> BIOS loads kernel into RAM –now kernel takes control
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|–>kernel gives CHS no. of initrd-***.img to BIOS
| BIOS do int 13h fn2
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|–>initrd is extracted into memory
| |
| |-> to see contents to /boot/initrd-***.img,run following cmnds:
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| # cp /boot/initrd-2.6.18-194.el5.img /tmp/
| # cd /tmp
| # mv initrd-2.6.18-194.el5.img initrd-2.6.18-194.el5.img.gz
| # gunzip initrd-2.6.18-194.el5.img.gz
| # mkdir funny
| # cd funny
| # cpio -id < /tmp/initrd-2.6.18-194.el5.img
| # ll
| # tree
|
|-> content of initrd file varies depending on Linux version
| On Centos 5.5 I got following list:
| .
| |– bin
| | |– dmraid
| | |– insmod
| | |– kpartx
| | |– modprobe -> /sbin/nash
| | `– nash —–> this is the mini shell
| |– dev
| | |– console
| | |– mapper
| | |– null
| | |– ptmx
| | |– ram -> ram1
| | |– ram0
| | |– ram1
| | |– rtc
| | |– systty
| | |– tty
| | |– tty0
| | |– tty1
| | |– tty10
| | |– tty11
| | |– tty12
| | |– tty2
| | |– tty3
| | |– tty4
| | |– tty5
| | |– tty6
| | |– tty7
| | |– tty8
| | |– tty9
| | |– ttyS0
| | |– ttyS1
| | |– ttyS2
| | |– ttyS3
| | `– zero
| |– etc
| |– init –> this is an executable i.e. linuxrc script as in
| | Old versions of linux e.g. RH9. Now renamed.
| |– lib
| | |– ata_piix.ko
| | |– dm-log.ko
| | |– dm-mem-cache.ko
| | |– dm-message.ko
| | |– dm-mod.ko
| | |– dm-raid45.ko
| | |– dm-region_hash.ko
| | |– ehci-hcd.ko
| | |– ext3.ko ————-|
| | |– firmware |–> important modules
| | |– jbd.ko ————-|
| | |– libata.ko
| | |– mptbase.ko
| | |– mptscsih.ko
| | |– mptspi.ko
| | |– ohci-hcd.ko
| | |– scsi_mod.ko
| | |– scsi_transport_spi.ko
| | |– sd_mod.ko
| | `– uhci-hcd.ko
| |– proc
| |– sbin -> bin
| |– sys
| `– sysroot
|
|
— Four files are very important: /bin/nash, ext3.ko, jbd.ko and
| init script (linuxrc script)
|
| 1) nash – Nash is a Minimal shell, it takes low memory. It is
| used to run linuxrc script.
| 2) ext3.ko – kernel needs a driver file called ‘ext3.ko’ for
| filesystems without which you cannot mount any partition.
| 3) jbd.ko – It’s a HDD disk driver file.
| 4) linuxrc (init) – It’s a Script file having commands using
| ext3.ko and jbd.ko and function to ro mount / partition.
| Go through init script.
|
|
|–> after linuxrc script is run,now kernel land is ready.
|
init |
|——–| –> now kernel calls init daemon i.e. /sbin/init
|
|–>/etc/inittab is loaded by init into memory and executed.
|
|———-> default runlevel
|—-|—–> /etc/rc.d/rc.sysinit {as a sub-shell}
| |
| |–> HOSTNAME, HOSTTYPE is set
| |–> /etc/sysconfig/network –> networking is enabled.
| |–> mount –n –t proc /proc /proc
| |–> mount –n –t sysfs /sys /sys >/dev/null 2>&1
| |–> /etc/init.d/functions {same env}
| | |
| | |–> global umask
| | |–> global PATH
| | |–> defines 27 shell functions
| | to get the below list, run command:
| | cat /etc/init.d/functions | grep \(\)
| | success() ————–|
| | failure() |
| | passed() |
| | warning() |
| | echo_success() |
| | echo_failure() |
| | echo_passed() |
| | echo_warning() |–These 17 functions were
| | Killproc() | present in RH9 as well.
| | pidofproc() | rest 10 were added in
| | Pidfileofproc() | new versions of RHEL.
| | action() |
| | Checkpid() |
| | confirm() |
| | Status() |
| | strstr() |
| | Daemon() —————-|
| | fstab_decode_str()
| | __readlink()
| | __umount_loop()
| | __umount_loopback_loop()
| | __pids_var_run()
| | __pids_pidof()
| | update_boot_stage()
| | get_numeric_dev()
| | resolve_dm_name()
| | is_ignored_file()
| |
| |–> /etc/redhat-release
| |–> press I to enter into interactive mode
| |–> /etc/sysconfig/clock – sets system clock
| |–> /sbin/start_udev – initializes device drivers
| |–> /etc/sysconfig/i18n – set language
| |–> /etc/sysctl.conf – Configure kernel parameters
| |–> /etc/sysconfig/keyboard – set keyboard mapping
| |–> /proc/acpi – Initialize ACPI bits
| |–> /etc/mdadm.conf – RAID setup
| |–> Device mapper & related initialization
| |–> Setting up LVM:
| | /sbin/lvm.static vgchange –a y ignorelocking failure
| |–> /fastboot – if present fast booting occurs skipping fsck
| |–> /forcefsck – if present fsck is forcefully done even if not
| | mentioned in fstab
| |–> /.autofsck – /etc/sysconfig/autofsck
| |–> /etc/sysconfig/readonly-root –if present / is mounted as
| | read-only
| |–> /etc/fstab – persistently mount filesystems
| |–> /etc/rwtab – exception to readonly-root, you can give rw
| | permission to certain dirs even if you have mounted whole
| | filesystem as readonly.
| |–> /etc/mtab – create this file and keep info of all mounted FS.
| |–> /sbin/quotacheck
| |–> Remounting root filesystem in rw mode:
| | mount -n -o remount,rw /
| |–> mount –a: Mount all other filesystems (except NFS &
| | /proc -> already mounted)
| |–> /sbin/quotaon – Enabling local filesystem quotas
| |–> rm -f /fastboot /fsckoptions /forcefsck /.autofsck
| | /forcequotacheck /halt \ /poweroff /.suspended &> /dev/null
| |–> enabling swaps: swapon -a –e
| |–> dmesg -s 131072 > /var/log/dmesg
|
|
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|—->/etc/rc.d/rc {as a sub shell}
| |
| |–> checks present and previous runlevel
| |–> set runlevel – /sbin/runlevel
| |–> calls /etc/init.d/functions {in same shell}
| |–> checks user confirmation mode
| |–> checks if /etc/rc.d/rc$runlevel.d exists?
| |–> /etc/rc.d/rc$runlevel.d/K* – first run kill scripts
| |–> /etc/rc.d/rc$runlevel.d/S* – then run start scripts
| |–> all scripts in /etc/rc.d/rc$runlevel.d call
| | /etc/init.d/functions in same shell. Also these scripts
| | have 5 functions defined in them i.e start, stop, restart,
| | condrestart, status. Kill scripts use only stop function
| | while S* scripts use only start function.
| |
| |–> /etc/rc.d/rc.local – if you want to run any script before
| login then put it here.
|
|————> shutdown CAD magic keys are set
| /sbin/shutdown -t3 -r now
|————> power failure considerations
| /sbin/shutdown -f -h +2
|————> power restore considerations
| /sbin/shutdown –c
|
|
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|————> 6 mingettys are spawned by init with tty* device drivers
| All enter sleep state except one
| 1:2345: respawn: /sbin/mingetty tty1
|————> IF default runlevel is 5 then
| /etc/X11/predfm –nodaemon (brings graphical login)
|
|——–>first mingetty
| |–> loads /dev/tty1
| |–> displays /etc/issue
| |–> calls /bin/login – enter Username/login
| |–> /usr/bin/passwd – enter password
| |
| |–>PAM for authentication ———–
| PAM uses passwd,group,shadow files from /etc dir|
| | |
| | |
| Valid user invalid user
| | |
| | prompts that login/passwd
|–> puts login daemon in sleep state is incorrect
|
|
|–>if $USER/.hushlogin exists ? DO NOT RUN mail,motd,lastlog!
| |
| Else –> /etc/motd – message of the day put by you
| |–> lastlog* using /var/log/lastlog
| |–> Checks user’s mail – /var/spool/mail/$USER
|
|– init then wakes /bin/login process
| mingetty then goes into zombie state and is killed by init
| Login then loads /bin/bash as a monitored child process
| /bin/bash takes over and login goes into sleep state
|
|
|–> /bin/bash
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|–> /etc/profile {sets HOSTNAME, HISTSIZE, PATH etc}
| |–> /etc/inputrc {sets keyboard mappings,
\ | terminal Characteristics}
\ |–> /etc/termcap {sets terminal capabilities}
\ |–> /etc/profile.d/*.sh {14 files}
\___ / {
| / colorls.sh
| / cvs.sh
|———–|———/ glib2.sh
| | gnome-ssh-askpass.sh
\|/ | kde.sh
if you want to run any | krb5-devel.sh
script after login then | krb5-workstation.sh
put it here. | lang.sh
| less.sh
| mc.sh
| mpi-selector.sh
| qt.sh
| vim.sh
| which-2.sh
| }
|
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|–> /etc/bashrc (set umask and system wide variables)
|
Local settings |— |–> .bash_profile—-|
For a particular <–| |–> .bashrc |___ present under
User | |–> .bash_history | /home/$UNAME
|— |–> .bash_logout—–|
Finally, the bash shell login prompt appears.
**************
