linux boot loader

8/8/2019 Linux Boot Loader

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LINUX BOOTLOADER

Presented By

Abhiram AnnangiUSC


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TOPICS AT A GLANCE

Brief introduction to Linux kernel

Linux executable format (ELF) Linux boot process

Linux bootloader (GRUB) Linux kernel compile procedure


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LINUX OS (SYSTEM PROGRAMMERS VIEW)

Any Operating System is implemented as two modulesat a macro level.

Module 1- User interface functionality (Shell) Module 2- Request Processing functionality (kernel)

Shell is responsible for interaction with the user, which

collects user requests and forward it to the kernel toprocess.

Shell scripting/programming is customizing the user

interface of an OS and nothing to do with the kernel. Kernel itself is a module, which has different sub

modules


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LINUX KERNEL (OVERVIEW)

Process management Access IPC

LOADER

Memory management

KERNEL

HARDWARE PERIPHERALS

ARCHITECTURESPECIFIC (AS)

NETWORK SERVICESI/O File System andDevice Drivers


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CONTD..

Architecture Specific (AS) , Kernel and Loader are thekey units which together are called the Engine of an OS

Architecture Specific AS layer contains the CPU initialization , Board

initialization code.

In Embedded Systems terminology, it is referred to asthe Board Support Package (BSP)

AS of Linux is very much specific to the hardware

architecture. AS has all the startup drivers which initialize the MMU

and on board devices.


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KERNEL

Within Kernel, there is a layer called In-Kernel which is thecore of the kernel.

In-Kernel is responsible for the boot process. When system is initialized and ready to use, Loader starts

up. Loader is started by the Linux Kernel.

The rest of the boxes in the figure are just used to provideServices, called Service Layers.

For a general purpose OS, all the services needed, where asfor a RT-OS, depending on the requirement the services are

provided. Hence because of this modular nature of the Linux kernel,

the size of Linux kernel varies a lot making it highly scalable.


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KERNEL CONTD..

Loader contains routines to receive requests from the shell andprocess them by forwarding to the respective service layers.

Kernel processes include :- Process Management

Access

IPC

Memory Management

N/W services

I/O file system

Device drivers (High level drivers, as Initial drivers initialized by AS) File system


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KERNEL CONTD..

At start up time, memory is divided into two parts, userspace and kernel space.

Shell and applications are resided on the user space. Shell and applications cannot access the K space where

as kernel space can access the whole of memory,

including the user space. Windows being a user-friendly OS puts the shell as

one of the kernel service.

But the disadvantage is that both the shell and kernelcannot be upgraded/customized independently.

It also saves kernel from many application level bugs,making the kernel more safe.


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KERNEL INTEGRATION

Kernel is an aggregation of many source codes (Loader, Services etc..)which have to be compiled and built to convert to an executable imagein order to use it. This is called Kernel Integration.

There are two types of Kernel Integration

Monolithic integration

Micro integration

In Monolithic integration, the entire kernel including the services arecompiled and built into the memory.

In Micro integration, only the kernel, AS and the loader are compiled andbuilt into the memory where as the services are installed only whenrequired.

Hence Monolithic kernels occupy more memory but are much faster asthey avoid the compiling and loading time where as micro kernelsoccupy less memory but have high response time.

What does Linux kernel support??


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MODULAR INTEGRATION

Linux defines Modular integration. It gathers the

components which are needed frequently and makesthem Static and keeps rest as modules.

In Linux the static ones are the Loader, BSP and some

services like Process and memory management, Rest of the dynamic modules/processes can also be

made Static depending on the environment. This

flexibility gives Linux to be used as embedded and

RTOS.


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EXECUTABLES IN LINUX

Any executable file when formed consists of two parts: Functionality

and Runtime.

When an executable gets loaded into memory (process addressspace), it requires key resource allocation to support the execution of

functionality. Runtime code is responsible to acquire such resources.

Runtime code involves making system calls to acquire resources, assystem calls are OS specific, this makes Runtime and hence the

executable to be OS specific.

Runtime code in Linux is called GNU runtime, which consists of three

functions: Init, Start and Fini.

Any application starts with an Init which does the required resource

allocation. Functionality of an executable is triggered with Start. Fini

is called after the execution of application for freeing up resources.


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CONTD..

Format is a standard to organize data with in a file. Binary file format specifies how binary code is to be organized and

saved on disk. Assembler and linker should make use of this fileformat to create an executable.

Binary file format is platform specific. Linux follows ELF 32 fileformat.

The combination of Binary file format and Runtime called ApplicationBinary Interface (ABI) makes an executable platform dependent. Hence Linux ABI is a combination of GNU runtime plus ELF 32.

There are 3 types of ELF files:

shared objects (loadables) - .so Executables (runnables)

Relocatables (linkables) - .o


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LINUX BOOT PROCESS (OVERVIEW)

System boot up

BIOS/Boot monitor

Stage 1 boot loader

MBR

Stage 2 boot loader LILO/GRUB

Kernel - Linux

Init user space

Gettys trigger

Power

up/reset

Linux

LILO/GRUB

MBR

BIOS/Bootmonitor

operation

User-space


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SYSTEM BOOT UP

There is a slight difference in the start up between PCand embedded platform.

Embedded platform Embedded platforms have a bootstrap environment

used when system is powered on or reset.

Embedded platforms also have something called BootMonitor. It is a small interactive computer programwhich supports an interaction for the user to select a listof Oss, do system test and also cover abilities of firstand second level boot loaders.

This boot monitor loads the Linux (strip down) kernel toflash and execute it.


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SYSTEM BOOT UP - PC

BIOS begins at address 0xFFFF0 and performs POST (Poweron Self Test) which includes the following

Initialization of CPU ( checking CPU type, speed etc.) andprobing the CPU to real mode.

Probe the memory. Allocate real mode address space i.earchitecture specific segments. No paging and virtualmemory.

Initialize basic I/O ( standard I/O,O/P, IDE controllers, harddisk, graphic cards)

Jump to sector 0 of configured boot device. Commonly, Linux is booted from a hard disk, where the

Master Boot Record (MBR) contains the primary boot loader.


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MBR

446

bytes

64

bytes

2

bytes


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MBR

The primary boot loader that resides in the MBR is a 512-byte image containing both program code and a small

partition table The first 446 bytes are the primary boot loader, which

contains both executable code and error message text.

The next sixty-four bytes are the partition table, whichcontains a record for each of four partitions (sixteen byteseach).

The job of the primary boot loader is to find and load the

secondary boot loader (GRUB). It does this by looking through the partition table for an active partition.

When it finds an active partition, it scans the remainingpartitions in the table to ensure that they're all inactive.


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GRUB ( GRAND UNIFIED BOOTLOADER)

When MBR calls GRUB, it reads the target of the bzimage from

/boot/grub, pulls it from there and loads into memory.

Bzimage is a compressed kernel image. Bzimage is a

combination of vmlinuz and bootstrap code.

BZIMAGEBOOTSTRAP

VMLINUZ


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CONTD..

When kernel image (bzimage) is loaded, first the bootstrapcode is loaded.

Bootstrap performs the following functions Puts the CPU to protected mode

Sets up page tables

Uncompresses Vmlinuz Calls start_kernel function

With the call to start_kernel, a long list of initializationfunctions are called to set up interrupts, perform furthermemory configuration.

Vmlinuz starts executing. This is the process 0, also calledSwapper.


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WHAT IS VMLINUZ

Vmlinuz is the name of the Linux kernelexecutable.

It is a compressed kernel image which is in thebootable form which means it has the capability toload OS into the memory.

Vmlinux on the other hand is the non-compressedversion of Vmlinuz, a non bootable form. It is anintermediate step to produce Vmlinuz.

Vmlinuz is located in/boot directory which is thedirectory required that needs files needed to beginbooting the system.


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SWAPPER

Swapper does the following

Initializes CPU and probes BOGOMIPS Initializes the onboard controllers, timers, and the

start up drivers (called BSP drivers in embedded

systems lexicon) Initialize interrupt descriptor tables

Initialize the file system

Swapper finally starts a kernel thread called Init,seen in/bin/init.


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INIT

Init runs as a daemon with PID as 1.

Init then runs the Inittab file present in/etc/init.d Inittab file describes which process should be started

at boot up and during the normal operation.

Finally, the idle task is started and the scheduler can

now take control (after the call to cpu_idle). With

interrupts enabled, the pre-emptive scheduler

periodically takes control to provide multitasking.


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GETTYS

Init then runs the mingetty application ( the

one which shows the login prompt). Now mingetty validates by running a task called

passwd present in/etc/passwd. If it fails, it

then respawns the login prompt. If it validates,it execs the shell.

All the steps of Linux boot process can be seenusing the command dmesg|more, rightimmediately after boot up.


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STEPS TO COMPILE/RECOMPILE KERNEL Configuring the kernel

Build dependencies Build the kernel

Build the modules Store it in the boot folder

Grub configuration


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CONFIGURING THE KERNEL (2.6.X VERSIONS)

On the 2.6.x kernels there are three main frontend programs: config,menuconfig, andxconfig.

config is the least user-friendly option as it merely presents a seriesof questions that must be answered sequentially. menuconfig is an ncurses based frontend. The system must have the

ncurses-devel libraries installed in order to use this utility.

Y for static and include it during the kernel compile, N for notincluding in the kernel and M for making it as module.

In the kernel source tree, use either of these, preferrably [2] and [3]

1) make config

2) make menuconfig3) make xconfig

Make mproper should be done on the source tree beforeperforming kernel recompile.


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MENUCONFIG


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XCONFIG


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BUILD DEPENDENCIES

The next step is to create the necessary includefiles and generate dependency information. This

step is only required for the 2.4.x kernel tree. make dep (only for 2.4.x kernel versions)

Lots of messages will scroll by. Depending on thespeed of your machine and on what options youchose, this may take several minutes to complete.

Once the dependency information is created wecan clean up some miscellaneous object files. Thisstep is required for all versions of the kernel.

make clean (for all versions)


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BUILD THE KERNEL

(finally) ready to start the actual kernel build. At

the prompt type: make bzimage Some computers do not work with make

bzimage either due to some hardware

incompatibilities or due to older versions ofbootloaders or due to smaller kernel images.Use make zimage or make zdisk

If everything went correctly then the new kernelshould exist in ./arch/$ARCH/boot.


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BUILD THE MODULES

There is one more step needed for the build process i.e tocreate all the loadable modules if we have them configured.

make modules Once the modules are built they can be installed. If we were

building as a non-privileged user you will now need to switchto root to complete this next step:

$ su password:

$ make modules_install The freshly baked modules will be copied into

/lib/modules/KERNEL_VERSION.


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CHANGING LOCATION

Once your kernel is created, we can prepare it foruse. From the ./linux directory, copy the kernel andSystem.map file to /boot. In the followingexamples change KERNEL_VERSION to the versionof the new kernel.

$ cp arch/i386/boot/bzImage /boot/bzImage-KERNEL_VERSION $ cp System.map /boot/System.map-KERNEL_VERSION The next step is to configure our bootloader.


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CONFIGURING THE BOOTLOADER

This forms the final step in the Linux compile procedure.The bootloader is the first program that runs when a

computer is booted. Once we have copied the bzImage and System.map to

/boot, edit the grub configuration file located in/boot/grub/menu.lst. On some distributions/etc/grub.conf is a symbolic link to this file.

# Note that we do not have to rerun grub after makingchanges to this file

title Test Kernel (2.6.0)root (hd0,1) kernel /boot/bzImage-2.6.0 roroot=LABEL=/ initrd /boot/initrd-2.6.0.img


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REFERENCES

Links:

1. http://www.ibm.com/developerworks/linux/library/l-

linuxboot/2. http://www.linfo.org/vmlinuz.html

3. http://www.yolinux.com/TUTORIALS/LinuxTutorialInitProcess.html

4. http://www.digitalhermit.com/linux/Kernel-Build-HOWTO.html#BUILDING

Books:

1. Understanding the Linux Kernel, by Daniel. P. Bovet andMarco Cesati, OReilly third edition.

2. Running Linux by Matt Wesh, OReilly fourth edition.
http://www.ibm.com/developerworks/linux/library/l-linuxboot/http://www.ibm.com/developerworks/linux/library/l-linuxboot/http://www.linfo.org/vmlinuz.htmlhttp://www.yolinux.com/TUTORIALS/LinuxTutorialInitProcess.htmlhttp://www.yolinux.com/TUTORIALS/LinuxTutorialInitProcess.htmlhttp://www.digitalhermit.com/linux/Kernel-Build-HOWTO.htmlhttp://www.digitalhermit.com/linux/Kernel-Build-HOWTO.htmlhttp://www.digitalhermit.com/linux/Kernel-Build-HOWTO.htmlhttp://www.digitalhermit.com/linux/Kernel-Build-HOWTO.htmlhttp://www.yolinux.com/TUTORIALS/LinuxTutorialInitProcess.htmlhttp://www.yolinux.com/TUTORIALS/LinuxTutorialInitProcess.htmlhttp://www.linfo.org/vmlinuz.htmlhttp://www.ibm.com/developerworks/linux/library/l-linuxboot/http://www.ibm.com/developerworks/linux/library/l-linuxboot/


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THANK YOU

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