This document provides an overview of walking around the Linux kernel. It begins with a brief history of Linux starting with Richard Stallman founding GNU in 1984. It then discusses why an operating system is needed and what a kernel is. The document outlines the basic facilities a kernel provides including process management, memory management, and device management. It describes different kernel design approaches such as monolithic kernels, microkernels, and hybrid kernels. Finally, it provides some tips for hacking the Linux kernel such as installing development packages, configuring and compiling the kernel, checking hardware, and loading modules.

1. Walking around Linux Kernel
Dharshana Kasun Warusavitharana.
Software Engineer,Test Automation Name
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2. How Story Begins
Birth of Linux
1984 Richard M Stallman found GNU (GNU is Not Unix).
Andy Tannenbaum Wrote Minux.
1991 Linus Torvalds wrote Linux

3. Why Operating System
Process management
Interrupts
Memory management
File system
Device drivers
Networking (TCP/IP, UDP)
Security (Process/Memory protection)
I/O

4. What is a Kernel
The kernel is a bridge between applications and the
actual data processing done at the hardware level.
The kernel's responsibilities include
Managing the system's resources
The communication between hardware and software
components
The kernel's primary function is to manage the
computer's resources and allow other programs to
run and use these resources.Typically, the resources
consist of:
The Central Processing Unit.
The computer's memory.
Any Input/Output (I/O)
Key aspects necessary in resource management.

5. Kernel basic facilities
Process management
Support to hardware abstractions (Hal Daemon).
Setting up an address space for the application, loads the
file containing the application's code into memory
Multi-tasking kernels are able to give the user the illusion
that the number of processes being run simultaneously on
the computer.
The kernel generally also provides these processes a way
to communicate; this is known as inter-process
communication (IPC) and the main approaches are
shared memory, message passing and remote procedure
calls (see concurrent computing).

6. Kernel basic facilities (Contd.....)
Memory management
Allows processes to safely access this memory .
Virtual addressing, usually achieved by paging and/or
segmentation.
Virtual addressing allows the operating system to use other
data stores
Virtual addressing also allows creation of virtual partitions of
memory in two disjointed areas.

7. Kernel basic facilities (Contd.....)
Device management
1. On the hardware side:
Interfacing directly.
Using a high level interface (Video BIOS).
Using a lower-level device driver (file drivers using disk drivers).
Simulating work with hardware, while doing something entirely
different.
2. On the software side:
Allowing the operating system direct access to hardware resources.
Implementing only primitives.
Implementing an interface for non-driver software (Example:
TWAIN).
Implementing a language, sometimes high-level (Example
Postscript).

8. Kernel basic facilities (Contd.....)
System calls
Using a software-simulated interrupt.
Using a call gate. A call gate is a special address stored
by the kernel in a list in kernel memory at a location
known to the processor.
Using a special system call instruction.
Using a memory-based queue.

9. Kernel-wide design approaches

10. Monolithic kernels
In a monolithic kernel, all OS services run
along with the main kernel thread.
Monolithic kernels, which have traditionally
been used by Unix-like operating systems.
Since there is less software involved it is
faster.
As it is one single piece of software it should
be smaller both in source and compiled
forms.
Less code generally means fewer bugs
which can translate to fewer security
problems.
Most work in the monolithic kernel is done
via system calls.
These types of kernels consist of the core
functions of the operating system and the
device drivers with the ability to load
modules at runtime.

11. Cons
Coding in kernel space is hard, since you cannot use
common libraries (like a full-featured libc), debugging is
harder.
Bugs in one part of the kernel have strong side effects.
Kernels often become very huge, and difficult to maintain.
Even if the modules servicing these operations are
separate from the whole, the code integration is tight and
difficult to do correctly.
Since the modules run in the same address space, a bug
can bring down the entire system.
The disadvantage cited for monolithic kernels is that they
are not portable; that is, they must be rewritten for each
new architecture that the operating system is to be used
on.

12. Microkernel
●
A microkernel that is designed for a specific platform
or device is only ever going to have what it needs to
operate.
The microkernel approach consists of defining a
simple abstraction over the hardware,
Maintenance is generally easier.
Patches can be tested in a separate instance, and then
swapped in to take over a production instance.
Rapid development time and new software can be
tested without having to reboot the kernel.
More persistence in general, if one instance goes hay-
wire, it is often possible to substitute it with an
operational mirror.
Most micro kernels use a message passing system of
some sort to handle requests from one server to
another.
A microkernel allows the implementation of the
remaining part of the operating system as a normal
application program written in a high-level language.

13. Cons
Disadvantages in the microkernel exist however. Some are:
Larger running memory footprint.
More software for interfacing is required, there is a potential for
performance loss.
Messaging bugs can be harder to fix due to the longer trip they have
to take versus the one off copy in a monolithic kernel.
Process management in general can be very complicated.
Extremely context based. As an example, they work well for small
single purpose (and critical) systems because if not many processes
need to run, then the complications of process management are
effectively mitigated.

14. Hybrid kernels
●
Hybrid kernels are part of the operating systems such as
Microsoft Windows NT, 2000 and XP. Dragonfly BSD etc.
They are similar to micro kernels, except they include
some additional code in kernel-space
Many traditionally monolithic kernels are now at least
adding (if not actively exploiting) the module capability.
On demand capability versus spending time recompiling a
whole kernel for things like new drivers or subsystems.
Faster integration of third party technology (related to
development but pertinent unto itself nonetheless).
Disadvantages of the modular approach are:
With more interfaces to pass through, the possibility of
increased bugs exists (which implies more security holes).
Maintaining modules can be confusing for some
administrators when dealing with problems like symbol
differences.

15. Linux Kernel

17. Lets Hack :)
Because a driver is not working as well as it
should, or is not working at all.
Because we have a school or work project.
Because the kernel is crashing, and we don’t
know why.
Because we want to learn how the kernel works.
Because it’s fun! Real men hack kernels ;-

18. You need to install the kernel development packages
•
e.g.: gcc, make, binutils, ncurses, qt
• Extract the kernel source in /usr/src
• Optionally create a symlink
• Quick configuration using an old configuration
•
linux:# tar jxvf /tmp/linux-2.6.16.8.tar.gz
linux:# ln
-s /usr/src/linux-2.6.16.8 /usr/src/linux
linux:# cp /boot/config-2.6.14 .config
linux:# make oldconfig
Compiling the kernel
linux:# make && make install && make modules_install

19. But WHAT if you make a MESS and want
to CLEAN UP?
Just use Mr. Proper
# make mrproper
This will clear all pre-compiled
binaries as well as remove the .config
file
# make clean
Just clean all pre-compiled binaries

20. Know your Hardware
To see whats plugged into your motherboard, or USB
ports.
linux:# lspci -v
linux:# lsusb
To check your processor and memory
linux:# cat /proc/cpuinfo
linux:# cat /proc/meminfo
• To see your hard drive partitions.
• To see your kernel log
linux:# fdisk -l
linux:# dmesg

21. Add Modules
To list modules that are currently loaded you can issue
• To list available modules (compiled) for the current
kernel
• To get a small description about a module ...
• To load a kernel module driver manually
•
#lsmod
#modprobe -l
#modinfo radeon
#modprobe radeon
To load a kernel drive automatically as the machine boots
consider
adding it to /etc/modules.conf (2.4) or /etc/modprobe.conf
(2.6)

22. Thank You
&
Happy
compiling ...