CS4540 – Operating Systems Assignment 6. Linux Kernel Modules


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In this project, you will learn how to create a kernel module and load it into the Linux kernel. The project
can be completed using the Ubuntu Linux running on a virtual machine. Although you may use an editor
to write C programs, you will have to use the terminal application to compile the programs, and you will
have to enter commands on the command line to manage the modules in the kernel.
As you’ll discover, the advantage of developing kernel modules is that it is a relatively easy method of
interacting with the kernel, thus allowing you to write programs that directly invoke kernel functions. It is
important for you to keep in mind that you are indeed writing kernel code that directly interacts with the
kernel. That normally means that any errors in the code could crash the system! However, since you will
be using a virtual machine, any failures will at worst only require rebooting the system.
Part I—Creating a Simple Kernel Module
The first part of this project involves following a series of steps for creating and inserting a module into
the Linux kernel.
I.1. An Illustrative Program for a Simple Kernel Module
Program code. The following program (named simple.c) is used to illustrate creating a very basic
kernel module; this module prints two simple messages: one when the kernel module is loaded, and one
when the kernel module is unloaded.
#include #include #include /* This function is called when the module is loaded. */
int simple_init(void)
printk(KERN INFO “Loading Module\n”);
return 0;
/* This function is called when the module is removed. */
void simple_exit(void)
printk(KERN INFO “Removing Module\n”);
/* Macros for registering module entry and exit points. */
module init(simple_init);
module exit(simple_exit);
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Program explanation. The function simple_init() is the module entry point, which represents the
function that is invoked when the module is loaded into the kernel. Similarly, the simple_exit()
function is the module exit point—the function that is called when the module is removed from the kernel.
The module entry point function must return an integer value, with 0 representing success and any other
value representing failure. The module exit point function returns void. Neither the module entry point
nor the module exit point is passed any parameters. The two following macros are used for registering the
module entry and exit points with the kernel:
Notice how both the module entry and exit point functions make calls to the printk() function.
printk() is the kernel equivalent of printf(), yet its output is sent to a kernel log buffer whose
contents can be read by the dmesg command. One difference between printf() and printk() is that
printk() allows us to specify a priority flag whose values are given in the include
file. In this instance, the priority is KERN_INFO, which is defined as an informational message.
details regarding the software license, description of the module, and author. For our purposes, we do not
depend on this information, but we include it because it is standard practice in developing kernel modules.
Compilation. This kernel module simple.c is compiled using the Makefile accompanying the source
code with this project. To compile the module, enter the following on the command line:
The compilation produces several files. The file simple.ko represents the compiled kernel module. The
following step illustrates inserting this module into the Linux kernel.
I.2. Loading and Removing Kernel Modules
Kernel modules are loaded using the insmod command, which is run as follows:
sudo insmod simple.ko
To check whether the module has loaded, enter the lsmod command and search for the module simple.
Recall that the module entry point is invoked when the module is inserted into the kernel. To check the
contents of this message in the kernel log buffer, enter the command
You should see the message “Loading Module.”
Removing the kernel module involves invoking the rmmod command (notice that the .ko suffix is
sudo rmmod simple
Be sure to check with the dmesg command to ensure the module has been removed.
Because the kernel log buffer can fill up quickly, it often makes sense to clear the buffer periodically. This
can be accomplished as follows:
sudo dmesg -c
– 3 –
Part II—Creating a Kernel Module Using Kernel Data Structures
The second part of this project involves modifying the kernel module so that it uses the kernel linked-list
data structure.
We explore using the circular, doubly linked list that is available to kernel developers. Much of what we
discuss is available in the Linux source code— in this instance, the include file —and
you should examine this file to understand the following steps (list.h should be in the directory:
/usr/src/linux-headers-3.13.0-32/include/linux ).
II.1. A Kernel struct and its Description
Initially, you must define a struct containing the elements that are to be inserted in the linked list. The
following C struct defines students:
struct student {
int studentNumber;
int courseCredit;
int grade;
struct list_head list;
Notice the member struct list_head list. The list_head structure is defined in the include file (types.h should be in the same directory as list.h). Its intention is to embed the
linked list within the nodes that comprise the list. This list_head structure is quite simple—it merely
holds two members, next and prev, that point to the next and previous entries in the list. By embedding
the linked list within the structure, Linux makes it possible to manage the data structure with a series of
macro functions (defined in list.h).
II.2. Inserting Elements into the Kernel Linked List
We can declare a list_head object, which we use as a reference to the head of the list by using the
LIST_HEAD() macro
static LIST_HEAD(student_list);
This macro defines and initializes the variable student_list, which is of type struct list_head.
We create and initialize instances of struct student as follows:
struct student *studentDemo;
studentDemo = kmalloc(sizeof(*studentDemo), GFP KERNEL);
studentDemo->studentNumber = 760120495;
studentDemo->courseCredit = 3;
studentDemo->grade = 5;
– 4 –
The kmalloc() function is the kernel equivalent of the user-level malloc() function for allocating
memory, except that kernel memory is being allocated. (The GFP_KERNEL flag indicates routine kernel
memory allocation.) The macro INIT_LIST_HEAD() initializes the list member in struct student.
We can then add this instance to the end of the linked list using the list_add_tail() macro:
list_add_tail(&studentDemo->list, &student_list);
II.3. Traversing the Kernel Linked List
Traversing the list involves using the list_for_each_entry() macro (see list.h), which accepts
three parameters:
• A pointer to the structure being iterated over
• A pointer to the head of the list being iterated over
• The name of the variable containing the list_head structure
The following code illustrates this macro:
struct student *ptr;
list_for_each_entry(ptr, &student_list, list) {
/* on each iteration ptr points */
/* to the next student struct */
II.4. Removing Elements from the Kernel Linked List
Careful memory management—which includes releasing memory to prevent memory leaks—is crucial
when developing kernel-level code. Before your program terminates, it must remove all list elements
Removing elements from the list involves using the list_del() macro, which is passed a pointer to
struct list_head
list_del(struct list_head *element)
This removes element from the list while maintaining the structure of the remainder of the list.
Perhaps the simplest approach for removing all elements from a linked list is to remove each element as
you traverse the list. The macro list_for_each_entry_safe() behaves much like
list_for_each_entry()except that it is passed an additional argument that maintains the value of the
next pointer of the item being deleted. (This is necessary for preserving the structure of the list.) The
following code example illustrates this macro:
struct student *ptr, *next
list for_each_entry_safe(ptr, next, &student_list, list) {
/* on each iteration ptr points */
/* to the next student struct */
– 5 –
Notice that after deleting each element, we return memory that was previously allocated with kmalloc()
back to the kernel with the call to kfree(). Make sure that you print contents of each list element before
removing it (this will play the role of confirming—to some extent—of the removal).
Part III—Implement Simple Kernel Module Using Illustrative Program
Use the code from Section I.1 to create the kernel module and to load and unload the module (proceed
through the steps described above).
Be sure to display the contents of the kernel log buffer (using dmesg) in your report for this assignment.
Part IV— Implement Linked List Kernel Module
Entry point. In the module entry point, create a linked list containing six struct student elements
for a single students, that is with the same value of studentNumber (and different values of
courseCredit and grade). Traverse the linked list and output its contents to the kernel log buffer.
Invoke the dmesg command to ensure the list is properly constructed once the kernel module has been
Exit point. In the module exit point, delete the elements from the linked list and return the free memory
back to the kernel. Again, invoke the dmesg command to check that the list has been removed once the
kernel module has been unloaded.
Part V—Hints
H1) You need to use #include for kmalloc()
> If you are using kmalloc or kzalloc() for memory allocation you have to include
> #include > They are called […] slab allocators […].
H2) Updating Package Lists and Installing the Linux Kernel Source in Ubuntu
Download the package lists from the repositories and “update” them to get information on the newest
versions of packages and their dependencies [1]:
sudo apt-get update
– 6 –
Note: APT, or Advanced Package Tool is a free software user interface that works with core
libraries to handle the installation and removal of software on the Debian GNU/Linux
distribution and its variants. [http://en.wikipedia.org/wiki/Advanced_Packaging_Tool]
Fetch new versions of packages existing on the machine if APT knows about these new versions by way
of apt-get update, and handle intelligently the dependencies, so it might remove obsolete packages or add
new ones [1]:
sudo apt-get dist-upgrade
Install just the headers in Ubuntu [2]:
sudo apt-get install linux-headers-$(uname -r)
Install the entire Linux kernel source in Ubuntu [2]:
sudo apt-get install linux-source
References (cf. VI below):
[1] http://askubuntu.com/questions/222348/what-does-sudo-apt-get-update-do
[2] http://stackoverflow.com/questions/16919512/linux-module-h-no-such-file-or-directory
H3) Makefiles for Linux Kernel
Source: “The Linux Kernel Module Programming Guide” by Peter Jay Salzman, available at:
CRITICAL: Generally, ‘make’ is used to build a kernel module and not a bare cc. More on this:
Kernel modules need to be compiled a bit differently from regular userspace apps. Former
kernel versions required us to care much about these settings, which are usually stored in
Makefiles. Although hierarchically organized, many redundant settings accumulated in sublevel
Makefiles and made them large and rather difficult to maintain.
Fortunately, there is a new way of doing these things, called kbuild, and the build process for
external loadable modules is now fully integrated into the standard kernel build mechanism. To
learn more on how to compile modules which are not part of the official kernel (such as all the
examples you’ll find in this guide), see file linux/Documentation/kbuild/modules.txt.
See the whole Makefile example there—very important!
See also the Hello World example in Section 2 — at least Subsections 2.1 – 2.5.
Here is Makefile that you must use:
obj-m += ProjectA6.o
make -C /lib/modules/$(shell uname -r)/build M=”$(PWD)” modules
make -C /lib/modules/$(shell uname -r)/build M=”$(PWD)” clean
H4) Summary of Commands for Compiling and Loading Kernel Modules
Compile the module (using the above Makefile):
– 7 –
List loaded modules:
See that the module ‘projectA6’ is not loaded into the Ubuntu kernel.
Load your module into the kernel:
sudo insmod ProjectA6.ko
Note that ‘.ko’ denotes a kernel .o file.
List loaded modules again:
See that the module ‘ProjectA6’ is shown now (that is, that it was successfully loaded into the kernel).
See the contents of the message that your module ‘ProjectA6’ sent to the kernel log buffer:
H5) Expected Output of dmesg.
In the output produced by dmesg, you might see a long list of messages. Look for the expected output of
your kernel module. It should similar to this (starting with ‘Loading Module’ and with all the
student dates from the program in the list of students):
[ 8641.136737] Loading Module
[ 8641.136741] Student: 7601204958 Course Credit 3 Grade 5
[ 8641.136742] Student: 7601204958 Course Credit 2 Grade 4
[ 8641.136743] Student: 7601204958 Course Credit 4 Grade 4
[ 8641.136744] Student: 7601204958 Course Credit 3 Grade 2
Part VI—Additional Background Information
VI.1. Copy of Text from Reference [1] (cf. VI.1 above)
I am wondering what sudo apt-get update does? What does it update?
edited Aug 31 at 5:34 Pandya
asked Nov 27 ’12 at 0:07 Elysium
In a nutshell, apt-get update doesn’t actually install new versions of software.
 apt-get update downloads the package lists from the repositories and “updates” them to get
information on the newest versions of packages and their dependencies. It will do this for all
repositories and PPAs. From http://linux.die.net/man/8/apt-get:
– 8 –
Used to re-synchronize the package index files from their sources. The indexes of available packages
are fetched from the location(s) specified in /etc/apt/sources.list(5). An update should always be
performed before an upgrade or dist-upgrade.
 apt-get upgrade will fetch new versions of packages existing on the machine if APT knows
about these new versions by way of apt-get update.
From http://linux.die.net/man/8/apt-get:
Used to install the newest versions of all packages currently installed on the system from the sources
enumerated in /etc/apt/sources.list(5). Packages currently installed with new versions available are
retrieved and upgraded; under no circumstances are currently installed packages removed, nor are
packages that are not already installed retrieved and installed. New versions of currently installed
packages that cannot be upgraded without changing the install status of another package will
be left at their current version. [Emphasis mine] An update must be performed first so that apt-get
knows that new versions of packages are available.
 apt-get dist-upgrade will do the same job which is done by apt-get upgrade, plus it will
also intelligently handle the dependencies, so it might remove obsolete packages or add new ones.
See here: What is “dist-upgrade” and why does it upgrade more than “upgrade”?
From http://linux.die.net/man/8/apt-get:
In addition to performing the function of upgrade, this option also intelligently handles changing
dependencies with new versions of packages; apt-get has a “smart” conflict resolution system, and it
will attempt to upgrade the most important packages at the expense of less important ones, if
necessary. The /etc/apt/sources.list(5) file contains a list of locations from which to retrieve desired
package files. See also apt_preferences(5) for a mechanism for over-riding the general settings for
individual packages.
You can combine commands with && as follows:
sudo apt-get update&&sudo apt-get install foo bar baz foo-dev foo-dbg
or to get newest versions possible as per version requirements of dependencies:
sudo apt-get update&&sudo apt-get dist-upgrade
You need sudo both times, but since sudo by default doesn’t prompt you within 5 or so minutes since the
last sudo operation, you will be prompted for your password only once (or not at all).
edited May 25 at 18:57
answered Nov 27 ’12 at 0:17 hexafraction
VI.2. Copy of Text from Reference [2] (cf. VI.1 above)
For my thesis I am creating a Manet using the protocol ARAN. To install the protocol I’m using this
manual, but the first step, the creation of trace_route, I received errors such as:
 linux/module.h: No such file or directory
 linux/procs_Fs: No such file or directory
– 9 –
 linux/skbuff: No such file or directory
I searched the web and found out that the problem is in the headers, but I do not find the solution …
P.S. I am using Ubuntu 10.04 LTS Kernel 2.6.33 recompiled
edited Jun 6 ’13 at 19:15 Vilhelm Gray
asked Jun 4 ’13 at 13:43 Peppe Cook
You’re missing the Linux kernel headers which allow you to compile code against the Linux kernel.
To install just the headers in Ubuntu:
$ sudo apt-get install linux-headers-$(uname -r)
To install the entire Linux kernel source in Ubuntu:
$ sudo apt-get install linux-source
Note that you should use the kernel headers that match the kernel you are running.
answered Jun 4 ’13 at 17:32 Vilhelm Gray