Sarah Diesburg
CS 3430
Operating Systems
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Kernel Coding
 The next project involves compiling a new kernel!

Fun but challenging….
 Each person/team will get their own virtual machine(s)
 See website instructions for how to install/use the virtual machine
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
Booting with the Bootloader
(grub2)
The newest version of the kernel will boot by default
 Look in the submenu (or further down the list) for any older version
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Logging In
 Username: os
 Password: os
 Username: root
 Password: iamr00t
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No GUI/Desktop?
 I disabled it by default (to run faster)
 If you really want it, issue (as root)

#> systemctl set-default multi-user.target
 If you want to turn it off again, issue (as root)

#> systemctl set-default graphical.target
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
Communicating with your fresh virtual machine
Use ssh
1.
Log into the terminal, and issue /sbin/ifconfig to get the second IP address
Type the IP address directly into PuTTY
2.
 Transfer files

Must use Winscp or Filezilla with the IP address found above
 Basically, communicate with it the same way you communicated with the student.cs.uni.edu machine
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SSH/WinSCP Demo…
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
If you created your own virtual machine….
Good for you!
 You will want to install these packages:
 gcc
 make
 ssh
 vim / emacs / (your choice of editor)
 sudo
 libncurses5-dev
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Basic System Commands
 $> passwd

Change your password
 $> su

Become root directly. Use root’s password when prompted.
 $> sudo

Do something with root’s privileges without having to become root
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Basic System Commands
 $> chown –R os:os <dir>

Changes ownership of dir to user/group os
 $> adduser <username>

Adds user specified by username
 $> userdel <username>

Deletes user specified by username
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Example of Kernel Compilation….
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Choosing a Kernel
 The next project will involve kernel development

We all need to be in the same boat…
 The class version will be 4.4.2 (bleeding edge stable version)
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Kernel Repository
 http://kernel.org
– place to download latest and greatest Linux kernels!
 Traditional place for kernel is in /usr/src/
 It is already in there for you

…That is, unless you created your own virtual machine. Then download it from kernel.org, and extract it with the command

$> tar xv linux-4.4.2.tar.xz
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Compiling the Kernel
 Compiles based on configuration of .config file located in the kernel source directory
 How can I correctly configure my kernel?
1.
2.
Use the ‘make defconfig’ command
Use the ‘make menuconfig’ command
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‘make defconfig’
 This new command makes the default configuration file for Linux.

It creates a smaller configuration file than previous methods, so the kernel is faster to compile
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‘make menuconfig’
 Only do this if you are trying to make the kernel smaller
 Manually select and deselect drivers and options for your kernel


* -- compiled directly into kernel
M – compiled into a module (driver) that can be dynamically added
 Can be tough to figure out what you need and don’t need

Command ‘lspci’ shows hardware on computer
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‘make menuconfig’
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‘lspci’
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General Linux Source Organization
 Memory Management
 Networking Layer
 Security
 Sound Processing
 Architecture-specific
Code
 Disk Drivers
 Devices
 File Systems
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Config Example (Inside Source Dir)
$> cd /usr/src/linux-4.4.2
$> make defconfig
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Compiling the Kernel

Issue ‘make’ inside the source directory
$> make

Grab some coffee….
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Installing the Kernel
1.

Install the modules (drivers)
$> sudo make modules_install
2.

Install the actual kernel
$> sudo make install

What does ‘sudo’ do?
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Final Step
 Create the ramfs image
 Why do we need an extra ramfs image?
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Boot Driver Problem
 Suppose you compiled the disk drivers and root file system drivers as modules

Boot process loads the kernel image

Kernel image does not contain disk drivers
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Boot Driver Solutions
 Hard : Figure out all drivers needed to read the root file system and compile into kernel

May take a few tries…
 Easy : Create a root file system image file that will load the right modules for you
 initramfs
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initramfs
 Create the initramfs image
$> cd /boot
$> sudo mkinitramfs –o /boot/initrd.img-4.4.2 4.4.2
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Configure the Boot Loader (Grub)
$> sudo update-grub
 Now you can issue a reboot and cross your fingers!
$> sudo reboot
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Telling Which Kernel You are In
 Select the new kernel from the grub menu!
 Watch your virtual machine

If something goes wrong, reboot and go back into your original kernel to try again
 The following command will tell you which kernel you are in:
$> uname -r
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Uh oh….

If it doesn’t boot, go back into the original kernel


Always important to have one working kernel!
Make sure you didn’t skip a step…
 You may have taken too much out of the kernel…

Start with bloated kernel, take things out one by one
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Next Recitation
 More on Project 2

Loading a kernel module and playing with proc files
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