The Core of Linux: Hacking the Kernel
-- Dean C. Parsons
Intro to the Linux Kernel
The Linux Kernel is the center of what makes the Linux and Unix operating systems. The
kernel is the foreman or mediator over all your programs and hardware components.
As the core of the operating system, the kernel has access to certain services that
interact with the systems hardware which no other program has direct access to. If a
program wants to carry out a special task, then it must go though the kernel first.
When a program wants to run, write data to a disk, write to certain parts of memory, it
asks the kernel for permission first. The kernel considers the program's request and
proceeds to do a number of tasks for that program. The kernel is very strict and will not
let programs away with illegal operations. If a program goes against what the kernel says,
it must be punished for it's actions.
If a program that got the "ok" from the kernel and then decides to write some
data to memory outside it's designated block(s), the kernel will kill that program and do
a core dump. A "core dump" is an operation the kernel carries out when a program
tries to perform a task it is not allowed to do. The kernel dumps the address space in
memory the program was using to the hard drive before the kernel kills that program.
The paradigm of a strict kernel is not new, the idea has been around for a while. It's
a great idea, and i think has been implemented very well on a Linux os. The majority of
the kernel was written in C with partial implementations written in assembly language
which makes the it flexible and very fast.
A Conversation with the Kernel
- (program app) Hello Mr. Kernel, I was wondering if I could have some memory to
run and play in?
- (kernel) Hmm... Ok, but if you try to run outside the memory space allocate for
you then I am going to stop you!
- (program app) Ok then, it's a deal. . . . .
- (kernel) Hey! I never gave you permission to write data in that memory segment,
you will pay!
- (program app) Oh no??.... I didn't mean it.. I.. I.. AHHHH.... ***Dead*** (
core dump ).
- (kernel) Ask me next time you want to do something like that!
- (kernel) When will those programs ever listen?!
Usually the operation of killing a program that was "bad" does not bring down
or crash the whole Linux operating system, it just kills the program or process that has
stepped out side it's "play ground" the kernel initially allocated for it. After
a core dump the kernel just goes about it's business of answering requests from other
processes on the system.
(when a core dump occurs it is possible in some cases to start up the program that
caused the core dump to where it left off just before the dump.)
Hacking / Configuring the Kernel
The "make" command
The kernel is usually compiled like other C programs using the "make"
command. Make is a smart program that does most of the compilation processing using
configuration information you provide via ( xconfig, config, and menuconfig -- see below
for more details.). So you can actually compile the program in which your hacking the
kernel with.
There are a few ways in which to configure the kernel. Depending on what you want to do
with your kernel, you have the options of:
- Make config
A text based interactive program that helps you config the kernel by answering Y-yes N-no
questions.
- Make menuconfig
Interactive menu configuration program, selections in the kernel are made using toggle
switches.
- Make xconfig
An X Windows program. A graphical menu is displayed in X, and all possible options or
modules are enabled/disabled with radio buttons and other common GUI interactions. ( very
user friendly )
- There is help inside the xconfig program, just use the "help" button. if your
stuck.
- Be careful which options you disable when configuring the kernel.
- It's recommended you read more on hacking and configuring the kernel before you start
the actual hack, the kernel is NOT something you was to play carelessly with!
Most default installs of Linux have relatively big kernels. These kernels provide you
with great flexibility in regards to a wide variety of hardware driver support. They often
include all of the most commonly used hardware component device drivers and most of the
kernel options. Generic kernels like this are created to deal with what ever type hardware
you have installed, but they are inefficient and big. This means longer boot time, and
less memory for your applications to run in. Hacking and reconfiguring the kernel is used
for cleaning out, and getting rid of extra drivers and options not needed for your system.
Once you have hacked out the unused materials your kernel will be faster, cleaner and more
efficient to run.
Compiling the Kernel after the Hack
It is recommended to start building from a clean slate. You should first do a
"make mrproper" in the directory of your kernel ( usually
"/usr/src/linux" on most Linux systems. The "make mrproper" command
removes any configuration files along with the remains of any previous kernel builds or
scraps that may be laying about in the source tree. ( another way to start with a
"clean" build is to do a "make clean" )
Then a "make dep" should follow the "make clean/mrproper". - This
will figure out what drivers in your kernel is dependent on other drivers. ( see links
below for Kernel Hackers Guide ).
Then do:
- make in "/usr/src/linux/" will give a kernel called: "vmlinux"
- make bzImage in "/usr/src/linux" will give you a commpressed kernel called
"bzImage"
- What ever name your kernel is make sure it's correct in the lilo.conf file. ( see LILO
below for details )
- Before rebooting after the kernel compile make sure there is an entry for the new kernel
in your "/etc/lilo.conf" file, if no entry is listed there, then the new kernel
will not be directly accessed by the boot loader "LILO". If you have a new
kernel and want to use it you should tell LILO about it.
Updating LILO
LILO:
Lilo is a boot loader that most linux users use for booting there Linux boxes
Example of a "lilo.conf" file ( usually located in "/etc/" )
# This line is a comment line
#LILO global section
boot = /dev/hda2
timeout = 500
prompt
default = linuxbox #"linuxbox" is default kernel
vga = normal
read-only
#End of globol section ends
# bootable kernel "vmlinuz-2.0.36-1" in directory "/boot/"
# kernel number one
image = /boot/vmlinuz-2.0.36-1
label = linuxbox
vga = normal
root = /dev/hda2
#end of kernel one section
# bootable kernel "zImage" in directory "/usr/src/linux/"
# THE NEW HACKED KERNEL!
image = /usr/scr/linux/zImage
label = hackedkernel
vga = normal
root = /dev/hda2
# End of HACKED KERNEL section
Note:
After compiling your new kernel make sure the "HACKED KERNEL" section is added
to your LILO configuration file ( usually "/etc/lilo.conf" )
In Closing
Hacking the kernel can have it's advantages. As stated earlier the Kernel is the
mediator over the whole Linux OS. It is the master of the OS who gives permissions to
programs and can take those permissions away. You are able to modify the core of Linux,
make it faster cleaner and in some case recompiling the kernel fixes some things. For
example, if your kernel is configured for some hardware you don't have, the kernel may
want to prob for a hardware component to make sure it's there or not, that is if the
driver for that device is compiled into the kernel. This is some cases may cause some
problems.
The kernel handles memory management, system processes and controls and keeps tabs on
system resource making sure things are working correctly.
Read up as mush as you can on compiling kernels.
Hope this helps you along the way, good luck!
Great Linux Links
Linux online
General linux info
Linux Apps
Great place to find some cool applications.
The Linux Kernel Archives
The Linux Kernel Archives, source code and more.
Linuxberg Linux Distribution Archive
source, libs, docs...
RedHat
The offical RedHat site
Kernel Hackers Guide
A kernel hacker information site, questions and answers
LinuxPower Dot Org
"Helping users harness the Power of Linux"
Linux Application and Utilities.
Apps and Utilities.
Linux Kernel
Lots of information about the kernel, how is runs, memory management, demand paging,
process etc...
Lilo HowTo
Questions and answers regarding the LILO boot loader.
linuxberg.com
Great place to find apps, games, untilities and more.
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