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The Linux Kernel

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Presenter’s Name

The Linux Kernel

Course objectives

n  Talk about kernel types and Linux architecture. Tanenbaum’s delight.

n  Manage modules and parameters. Make your devices to work.

n  Apply patches. Add third-party features, or even your own !

n  Configure, compile, and install a new kernel. Dive into menuconfig.

By completing this course, you will:

The Linux Kernel

Course topics

n  About kernels. One concept, different implementations.

n  Linux. A focus on your favorite operating system.

n  Module management. Work with modules and parameters.

n  Patching. Apply patches to the kernel sources.

n  Configure and compile. Your very own kernel.

Course’s plan:

The Linux Kernel

About Kernels

One concept, different implementations

The Linux Kernel

What’s a kernel ? About Kernels

n  Provide abstractions

n  files / filesystem

n  Network sockets

n  …

n  Manage hardware

n  Allocate CPU time

§ Multitasking

n  Manage memory

§  Process isolation

§  Virtual memory

Turn expensive paperweight into a computer.

Kernel families About Kernels

n  Monolithic kernels

n  everything run in kernel mode (ring 0 on x86)

§  Crash on fault

n  Faster

n  Easier to implement

n  Micro kernel

n  Most services as userland processes

§  No crash on fault, just restart faulty “server”

n  Slower (context switches)

n  Hard to keep in sync

Different implementations to provide the same services.

Monolithic kernel About Kernels

Hardware Drivers

Network stack

Virtual Memory Management

Micro kernel About Kernels

Hardware Drivers

Network stack

Filesystem Micro kernel

Stop-and-think About Kernels

Do you have any questions ?

Stop-and-think About Kernels

Micro kernel

Monolithic

Something else

Both

What do you think about Linux ?

Stop-and-think About Kernels

Micro kernel

Monolithic

Something else

Both

What do you think about Linux ?

Linux

A focus on your favorite operating system

The Linux Kernel

History Linux

n  Started as a hobby

n  Minix-like

n  Monolithic

n  Released in (19)91

n  To run GNU tools on x86

n  “Never gonna be professional like Hurd”

n  Joined by

n  Individuals

n  Companies

Linus Torvalds’ “Toy OS”.

Linux or GNU/Linux ? Linux

n  Running controversy

n  Considered trivial

n  “Linux” systems

n  GNU Userland

n  Linux kernel

n  Stallman insists on GNU/Linux

n  Getting old

n  FSF “Newspeak”

n  “Linux” more widespread

Vi or Emacs ?

Linux system stack Linux

Development and versioning Linux

n  Sources available from kernel.org

n  Patches

n  Full versions

n  Versioning

n  Version.major.minor-rev

n  Main trunk: 2.6.x

n  Old trunk: 2.4.x

§  Still used for embedded devices

Where is the latest kernel ?

What’s the running kernel version ? Linux

The uname command

Options Definitions

-a

-s

-r

All information

Kernel name

Kernel release (version no)

[user@linux ~]$ uname [options]

[user@linux ~]$ uname –r 2.6.38.2

Example:

Architecture Linux

n  Monolithic

n  Everything in kernel land

n  Privileged mode

n  Modular

n  Load code at runtime

§  Dynamic linking

n  No difference between loaded and core code

n  Reduce memory footprint

Modules doesn’t mean microkernel

Architecture Linux

n  Modules

n  Functional units

n  Provide features

n  Hardware drivers

n  Wide range

n  Gigantic footprint

n  Load only what’s needed

§  Connected hardware

Modules ?

Stop-and-think Linux

Do you have any questions ?

Stop-and-think Linux

True

False

Linux kernel modules are running in userland.

Stop-and-think Linux

True

False

Linux kernel modules are running in userland.

Modules management

Work with modules and parameters

The Linux Kernel

Module files Modules management

n  Modules are stored as

n  .ko (kernel objects)

n  .ko.gz (compressed)

n  Per kernel

n  /lib/modules/kernel-version n  Per category

n  /lib/modules/2.6.18/kernel/drivers/char/drm/radeon.ko

n  Tools work

n  Files

n  Names

Where are they stored ?

List currently loaded modules Modules management

With the lsmod command

n  Get a list of currently loaded modules and deps:

root@localhost # lsmod Module Size Used by radeon 108832 1 drm 72852 2 radeon ...

List currently loaded modules Modules management

With the lsmod command

n  Get a list of currently loaded modules and deps:

root@localhost # lsmod Module Size Used by radeon 108832 1 drm 72852 2 radeon ...

Loading modules Modules management

The modprobe command

Arguments Definitions

options

module

params

man 8 modprobe

module name

Module parameters (see modinfo)

[root@linux ~]# modprobe [options] module [params]

[root@linux ~]# modprobe snd-had-intel model=7c

Example:

Loading modules Modules management

The insmod command

Arguments Definitions

module-file

params

Path to the kernel object

module parameters (see modinfo)

[root@linux ~]# insmod module-file [params]

[root@linux ~]# insmod ./testmod.ko idx=2

Example:

Information about modules Modules management

The modinfo command

[user@linux ~]$ modinfo [options] module|module-file

[user@linux ~]$ modinfo snd-had-intel […]

Example:

Arguments Definitions

options

module

module-file

man 8 modinfo

module name

Module file to read from

Modules dependencies Modules management

The depmod command

[root@linux ~]# depmod [options] [version] [filenames]

[root@linux ~]# depmod

Example:

Arguments Definitions

-a

version

filenames

Probe all modules. Default

Kernel version to work with. Defaults to $(uname -r)

Specific modules to work on

Module unloading Modules management

The rmmod command

[root@linux ~]# rmmod [options] module-name

[root@linux ~]# rmmod pcnet32

Example:

Arguments Definitions

-f

-w

Force module unload. Use with caution.

Prevent new usage of the in-use module. When usage count == 0 proceed with unloading

Stop-and-think Modules management

Do you have any questions ?

Stop-and-think Modules management

You want to insert a module in the running kernel to support your wireless card. You need to set a custom value for the “prefix” parameter. Which command will you use ?

insmod

modprobe

depmod

rmmod

modinfo

Stop-and-think Modules management

You want to insert a module in the running kernel to support your wireless card. You need to set a custom value for the “prefix” parameter. Which command will you use ?

insmod

modprobe

depmod

rmmod

modinfo

Patching

Apply patches to the kernel sources

The Linux Kernel

Kernel flavors Patching

n  Vanilla

n  Official version from kernel.org

n  Distro

n  Customized vanilla

§  Distro patches

n  Your flavor

n  Vanilla/Distro

§  Your patches

§  Third-party patches

Kernels in the wild

Why patching ? Patching

n  New features

n  Not in Vanilla

n  Not in your distro

n  Bugfixes

n  Kernel panics solved

n  Enhancements

§  Performance

n  Supported hardware

n  Various hacks

Customized kernels

About patches Patching

Patches are released in ”diff" format: text file composed of code to add or to remove

diff command generate diff’s file:

# diff –Nur original modified > diff_file

--- soft-orig/src/amazing.c 2008-01-13 22:27:33.000000000 +0100

+++ soft/src/amazingV2.c 2008-01-13 22:16:32.000000000 +0100

@@ -3,5 +3,6 @@

int main(int argc, char **argv)

{

int result = 0;

+ result =+ 1;

return result;

}

Patch level Patching

n  Patches are creating and applied in different contexts

n  --- soft-orig/src/main.c n  +++ soft-modified/src/main.c

n  Will be applied to

n  soft/src/main.c §  Need to rebase

§  Strip X path components

n  Rebase 1 level

§  src/main.c

§  Apply from soft “root” dir

Rebasing paths.

Working with patches Patching

Apply patch

In the parent directory :

Remove patch :

[user@linux soft-1.2.3]$ patch -p1 < mypatch.diff

[user@linux soft-1.2.3]$ patch -R < mypatch.diff

Stop-and-think Patching

Do you have any questions ?

Stop-and-think Patching

1

2

3

4

5

The kernel source is in /usr/src/linux. You are in this directory. You want to apply a (poorly written) patch. The patch begins with “--- usr/src/linux/fs/aio.c”. Which patch level are you going to use ?

Stop-and-think Patching

1

2

3

4

5

The kernel source is in /usr/src/linux. You are in this directory. You want to apply a (poorly written) patch. The patch begins with “--- usr/src/linux/fs/aio.c”. Which patch level are you going to use ?

Configure and compile

Your very own kernel

The Linux Kernel

Why ? Configure and compile

n  Optimize

n  Only needed features

n  Smaller binary

n  Faster kernel

n  Compile for your arch

n  New features

n  Patched version

n  New version not in your distro repositories

n  The geek touch

Two good reasons and the real one.

How ? Configure and compile

n  Download the sources

n  kernel.org

n  Be sure to select the ‘F’ull version

n  Unpack in /usr/src

n  You get /usr/src/linux-x.y.z

n  Create /usr/src/linux symlink

n  Configure

n  Compile

n  Install

n  You may need to manually update your bootloader

Quick howto.

Tools Configure and compile

n  Working toolchain

n  Compiler (gcc)

n  Assembler (gas)

n  Linker (ld)

n  The make tool

n  libncurses to do menuconfig

n  development headers

n  Various graphic libs for other *config targets

What you’ll need.

Configure Configure and compile

n  make menuconfig

n  Navigate with arrows

n  Select with enter

n  Read help

n  Search with ‘/’

n  Feature status with space

n  ‘*’ Builtin/enabled

n  ‘M’ Module

n  ‘ ‘ Not build/disabled

n  Use ‘exit’ button to save

Most important step.

Guided tour Configure and compile

Compile and install Configure and compile

n  You may now have to configure the bootloader

n  Many distros have hooks for “make install” stage

# make # make install

# make modules_install

n  Kernel compilation and installation

Stop-and-think Configure and compile

Do you have any questions ?

Stop-and-think Configure and compile

True

False

The “make install” command copies the kernel image to /boot and the modules to /lib/modules/kernel-version.

Stop-and-think Configure and compile

True

False

The “make install” command copies the kernel image to /boot and the modules to /lib/modules/kernel-version.

Applying patches

Kernel configuration

dialog Kernel types

Compile and install

Course summary

Linux kernel architecture &

modules management

The Linux Kernel

For more The Linux Kernel

If you want to go into these subjects more deeply, …

Courses Publications

Web sites

www.labo-linux.com

www.blackbeltfactory.com

Linux Technologies: Edge Computing

Conferences

FOSDEM

RMLL

Solutions Linux

If you want to go into these subjects more deeply, …

www.supinfo.com

Understanding the Linux kernel

Congratulations

You have successfully completed the SUPINFO course module n°22

The Linux kernel

The end

n  Read all config items help text and don’t hesitate to try n  Always keep a working kernel to boot your system

The Linux Kernel