Difference between revisions of "Embedded-Linux-Device-Driver/C2/Simple-Loadable-Module/English"

Welcome slide:

Learning objectives:

• Implement a simple loadable module.
• Create the Makefile.
• Compile the module.

• Implement a simple loadable module.
• Create a Makefile.
• Compile a module.

System Requirements:

• VirtualBox 5.2
• Ubuntu Linux 18.04 LTS operating system.
• Linux kernel version 5.0.0-31 generic.
• gedit Text Editor.

• VirtualBox 5.2
• Ubuntu Linux 18.04 LTS operating system.
• Linux kernel version 5.0.0-31 generic
• gedit Text Editor.

Prerequisites:

To follow this tutorial, you should be familiar with:

• C programming language and
• basics of Linux kernel

• C programming language and
• basics of Linux kernel

If not, then go through the C/C++ and Linux spoken tutorials on this website

Gedit Installation:

• To proceed further, make sure that gedit is installed on your system.
• If not, to do the installation, follow the instructions in gedit series on this https://spoken-tutorial.org

• To proceed further, make sure that Gedit is installed on your system.
• If not, to do the installation, follow the instructions in Gedit series on this website.

What is a kernel module?

• Code that is added to the kernel at runtime is called a module.
• It extends the functionality of the kernel without the need to reboot the system.

• It communicates with the base kernel to complete their functions.

Code file - sample_module.c

Make use of this file while practising.

Remember the file path.

Text box

Open the terminal by pressing Ctrl+Alt+T keys simultaneously.

Press the Enter key after every command.

Go to the folder where simple_module dot c is saved.

Type gedit simple_module dot c

Highlight <linux/init.h>

Highlight <linux/module.h>

Highlight <linux/kernel.h>

Init dot h file is used to start the init process.

The module dot h file contains functions related to the modules.

The kernel dot h file contains functions such as printk.

Highlight GPL

GPL means GNU Public License.

It is otherwise called as GNU, which is a free and open-source license.

I have given the description as “First Linux kernel module”.

Highlight return(0)

Highlight module_init()

So it is called as an initialization function of a module.

If this function fails then the error will be returned.

It will return 0 to the kernel on successful execution.

Initialization function prints a simple message and returns zero.

The module init macro is used to register the init function.

Highlight module_exit()

It is mostly used to cleanup the resources acquired by the module

The exit function will undo whatever the init function did.

The module underscore exit macro is used to register the exit function.

Highlight KERN_INFO

Press >> Ctrl + S

Press Enter

Click on close option.

It is similar to printf function in C programming.

Using this function, string data will get loaded in the kernel log buffer.

printk has an optional prefix string, Loglevel such as KERN_INFO.

Loglevel specifies the type of message being sent to the kernel message log.

KERN underscore INFO is used for informational messages.

For more log levels, go through the Additional reading material link of this tutorial.

Press Ctrl + S keys to save the program .

Close the file.

Highlight the makefile

Press Enter

Type gedit Makefile

The Makefile is a special file containing shell commands.

Press Enter.

Create a Makefile where you have saved the simple_module dot c .

Highlight simple_module.o

Highlight all:

Highlight clean:

When you run the make all command, then commands under all section will be executed.

If you run the make clean command, it executes commands under the clean section.

Highlight uname -r Highlight (PWD) Highlight M=$(PWD) Highlight modules

As you know, uname hyphen r finds the current Linux kernel version of a system.

PWD means the present working directory.

This option tells the kernel makefile that the source code for the driver is in PWD.

The word modules tell the kernel makefile to build the modules.

But it will not build the complete kernel source code.

Click on the close.

Type >> make all

Press Enter

Let us compile the program.

Type make space all.

Highlight ls

Show the output

This command lists the files in the current directory.

Here, you can see the number of files created in this directory.

Highlight mod.c

These symbols are exported from the kernel build.

The mod.c file contains information about the module.

Press Enter

Press Enter

Highlight modinfo

Highlight filename

Highlight description

Highlight author, license

Type modinfo space simple_module dot ko.

This command displays information about the Linux kernel module.

The filename field contains an absolute path to the .ko kernel object file.

This field displays a short description of a module.

Author field shows author and License field shows the license of the module.

It is calculated automatically at build time.

It is shown empty here as this module does not depend on any other module.

Modules must also be compiled with a retpoline-aware compiler.

Otherwise the kernel can be vulnerable.

Retpoline technique protects the kernel from the hacker’s attacks.

This version magic number is used while loading the kernel module.

The version magic number of module and current kernel version should be the same.

Otherwise it will fail to load in the current kernel.

Press Enter

This command removes all object files that are created after the compilation.

Type ls.

Now there are no object files.

Summary:

• Implement a simple loadable module.
• Create a Makefile.
• Compile a module.

Assignment:

1. Open the simple_module.c file
2. Change the module description using MODULE_DESCRIPTION macro
3. Run the modinfo command
4. See the output of description.

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Acknowledgment:

Thank you slide:

This is Mayuri Panchakshari signing off.

Thanks for watching.