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Examples from The Linux Kernel Programming Guide - test-driven with CppUTest - arstrube/lkmpg. New pull request. Find File. Clone or download. branch of the LKMPG for each new stable kernel version. . To avoid these two problems, I highly recommend that you download, compile and boot into a fresh, . home | HOWTOs | Guides | FAQs | man pages | Linux Gazette | LinuxFocus. We now offer multiple branches (corresponding to kernel versions) for The Linux.
The requested module has a dependency on another module if the other module defines symbols variables or functions that the requested module uses. Lastly, modprobe uses insmod to first load any prerequisite modules into the kernel, and then the requested module. Linux distros provide modprobe, insmod and depmod as a package called module-init-tools.
In previous versions that package was called modutils. Some distros also set up some wrappers that allow both packages to be installed in parallel and do the right thing in order to be able to deal with 2.
Users should not need to care about the details, as long as they're running recent versions of those tools. Now you know how modules get into the kernel. But this will have to wait for a future chapter.
We have a lot to cover before addressing this relatively high-level issue. Before We Begin Before we delve into code, there are a few issues we need to cover.
Everyone's system is different and everyone has their own groove.
Getting your first "hello world" program to compile and load correctly can sometimes be a trick. Rest assured, after you get over the initial hurdle of doing it for the first time, it will be smooth sailing thereafter. We won't go into module versioning until later in this guide. Until we cover modversions, the examples in the guide may not work if you're running a kernel with modversioning turned on. However, most stock Linux distro kernels come with it turned on. If you're having trouble loading the modules because of versioning errors, compile a kernel with modversioning turned off.
Using X It is highly recommended that you type in, compile and load all the examples this guide discusses. It's also highly recommended you do this from a console. You should not be working on this stuff in X.
Modules can't print to the screen like printf can, but they can log information and warnings, which ends up being printed on your screen, but only on a console.
If you insmod a module from an xterm, the information and warnings will be logged, but only to your log files. You won't see it unless you look through your log files. To have immediate access to this information, do all your work from the console. Compiling Issues and Kernel Version Very often, Linux distros will distribute kernel source that has been patched in various non-standard ways, which may cause trouble.
A more common problem is that some Linux distros distribute incomplete kernel headers. You'll need to compile your code using various header files from the Linux kernel. Murphy's Law states that the headers that are missing are exactly the ones that you'll need for your module work. To avoid these two problems, I highly recommend that you download, compile and boot into a fresh, stock Linux kernel which can be downloaded from any of the Linux kernel mirror sites.
Ironically, this can also cause a problem. This can be fixed by using gcc's -I switch. Chapter 2. Hello World 2. I don't know what happens to people who break with this tradition, but I think it's safer not to find out.
We'll start with a series of hello world programs that demonstrate the different aspects of the basics of writing a kernel module. Here's the simplest module possible.
Index of /pub/ldp/LDP/lkmpg/
Don't compile it yet; we'll cover module compilation in the next section. Example Actually, things have changed starting with kernel 2.
You can now use whatever name you like for the start and end functions of a module, and you'll learn how to do this in Section 2.
In fact, the new method is the preferred method. Introducing printk Despite what you might think, printk was not meant to communicate information to the user, even though we used it for exactly this purpose in hello-1! It happens to be a logging mechanism for the kernel, and is used to log information or give warnings.
Take time to read through the priority macros. The header file also describes what each priority means. When you write real modules, you'll want to use priorities that are meaningful for the situation at hand. Compiling Kernel Modules 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.
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So, let's look at a simple Makefile for compiling a module named hello Now you can compile the module by issuing the command make. The reason for this is that they contain an additional. We'll soon see what this information is good for. That changes once we're using modinfo on one of our the later examples, hello An author string for bugreports, license information, even a short description of the parameters it accepts.
Be sure to read this and the related files before starting to hack Makefiles. It'll probably save you lots of work.
Now it is time to insert your freshly-compiled module it into the kernel with insmod.
Go ahead and cat that file to see that your module is really a part of the kernel. Congratulations, you are now the author of Linux kernel code!
When the novelty wears off, remove your module from the kernel by using rmmod hello Here's another exercise for the reader. Change the return value to something negative, recompile and load the module again. What happens? Hello World part 2 As of Linux 2. The only caveat is that your init and cleanup functions must be defined before calling the macros, otherwise you'll get compilation errors.
Here's an example of this technique: Example Adding another module is as simple as this: Example As you can see, some things get hardwired into the kernel obj-y but where are all those obj-m gone? Those familiar with shell scripts will easily be able to spot them. Notice the change in the definitions of the init and cleanup functions.
If you think about when the init function is invoked, this makes perfect sense. Again, if you consider when the cleanup function runs, this makes complete sense; built-in drivers don't need a cleanup function, while loadable modules do.
When you boot your kernel and see something like Freeing unused kernel memory: k freed, this is precisely what the kernel is freeing.
By setting the license to GPL, you can keep the warning from being printed. They're simply for documentation and can be viewed by a tool like objdump. People unfamiliar with command line tools will probably like some web base solution, search for sites that offer kernel trees that got indexed with LXR.
Users of traditional Unix editors, like emacs or vi will also find tag files useful. Once you've got such a tagfile in your kerneltree you can put the cursor on some function call and use some key combination to directly jump to the definition function.
At runtime, insmod will fill the variables with any command line arguments that are given, like. The variable declarations and macros should be placed at the beginning of the module for clarity. The example code should clear up my admittedly lousy explanation. Integer types can be signed as usual or unsigned.
To keep track of the number of parameters you need to pass a pointer to a count variable as third parameter. At your option, you could also ignore the count and pass NULL instead.
If the variables contain the default values, then perform autodetection explained elsewhere. Otherwise, keep the current value. This will be made clear later on. It takes two parameters: a variable name and a free form string describing that variable. Modules Spanning Multiple Files Sometimes it makes sense to divide a kernel module between several source files. Here's an example of such a kernel module.
The first five lines are nothing special, but for the last example we'll need two lines. First we invent an object name for our combined module, second we tell make what object files are part of that module. This option can save you a lot of time and a number of reboots during the development of a module. Nevertheless, there is a number of cases in which you may want to load your module into a precompiled running kernel, such as the ones shipped with common Linux distributions, or a kernel you have compiled in the past.
Linux Kernel Module Programming Guide
In certain circumstances you could require to compile and insert a module into a running kernel which you are not allowed to recompile, or on a machine that you prefer not to reboot. If you can't think of a case that will force you to use modules for a precompiled kernel you might want to skip this and treat the rest of this chapter as a big footnote.
Incidentally, version magics are stored in the module object in the form of a static string, starting with vermagic:. To inspect version magics and other strings stored in a given module, issue the modinfo module. Consequently, we want to compile our module in an environment which was identical to the one in which our precompiled kernel was built. How to do this, is the subject of the remainder of this chapter. First of all, make sure that a kernel source tree is available, having exactly the same version as your current kernel.
Then, find the configuration file which was used to compile your precompiled kernel. Let's focus again on the previous error message: a closer look at the version magic strings suggests that, even with two configuration files which are exactly the same, a slight difference in the version magic could be possible, and it is sufficient to prevent insertion of the module into the kernel. That slight difference, namely the custom string which appears in the module's version magic and not in the kernel's one, is due to a modification with respect to the original, in the makefile that some distribution include.
Now, please run make to update configuration and version headers and objects: [root pcsenonsrv linux Now you can turn back to the directory of your module and compile it: It will be built exactly according your current kernel settings, and it will load into it without any errors. Chapter 3. Preliminaries 3. Modules vs Programs 3. How modules begin and end A program usually begins with a main function, executes a bunch of instructions and terminates upon completion of those instructions.
Kernel modules work a bit differently. This is the entry function for modules; it tells the kernel what functionality the module provides and sets up the kernel to run the module's functions when they're needed.
Once it does this, entry function returns and the module does nothing until the kernel wants to do something with the code that the module provides.
This is the exit function for modules; it undoes whatever entry function did. It unregisters the functionality that the entry function registered. Every module must have an entry function and an exit function. Functions available to modules Programmers use functions they don't define all the time.
A prime example of this is printf. You use these library functions which are provided by the standard C library, libc. The definitions for these functions don't actually enter your program until the linking stage, which insures that the code for printf for example is available, and fixes the call instruction to point to that code.
Kernel modules are different here, too. That's because modules are object files whose symbols get resolved upon insmod'ing. The definition for the symbols comes from the kernel itself; the only external functions you can use are the ones provided by the kernel.
One point to keep in mind is the difference between library functions and system calls. Library functions are higher level, run completely in user space and provide a more convenient interface for the programmer to the functions that do the real worksystem calls.
System calls run in kernel mode on the user's behalf and are provided by the kernel itself. The library function printf may look like a very general printing function, but all it really does is format the data into strings and write the string data using the low-level system call write , which then sends the data to standard output. Would you like to see what system calls are made by printf? It's easy! Run the exectable with strace.
Are you impressed? Every line you see corresponds to a system call. It's an invaluable tool for figuring out things like what files a program is trying to access. Towards the end, you'll see a line which looks like write 1, "hello", 5hello.
There it is. The face behind the printf mask. If that's the case, try looking at man 2 write. The 2nd man section is devoted to system calls like kill and read.
The 3rd man section is devoted to library calls, which you would probably be more familiar with like cosh and random.
You can even write modules to replace the kernel's system calls, which we'll do shortly. Crackers often make use of this sort of thing for backdoors or trojans, but you can write your own modules to do more benign things, like have the kernel write Tee hee, that tickles!
User Space vs Kernel Space A kernel is all about access to resources, whether the resource in question happens to be a video card, a hard drive or even memory. Programs often compete for the same resource.
As I just saved this document, updatedb started updating the locate database. My vim session and updatedb are both using the hard drive concurrently. The kernel needs to keep things orderly, and not give users access to resources whenever they feel like it.
To this end, a CPU can run in different modes. Each mode gives a different level of freedom to do what you want on the system. The Intel architecture has 4 of these modes, which are called rings.
Recall the discussion about library functions vs system calls. Typically, you use a library function in user mode. The library function calls one or more system calls, and these system calls execute on the library function's behalf, but do so in supervisor mode since they are part of the kernel itself. Once the system call completes its task, it returns and execution gets transfered back to user mode.
Name Space When you write a small C program, you use variables which are convenient and make sense to the reader. If, on the other hand, you're writing routines which will be part of a bigger problem, any global variables you have are part of a community of other peoples' global variables; some of the variable names can clash.
When a program has lots of global variables which aren't meaningful enough to be distinguished, you get namespace pollution. Linux Kernel Module Programming.
Originally Posted by gauravholey. Open Source Consulting Domain Registration. The higher-level functions are on the left side. It features an interface that is arguably more intuitive than what Spotlight offers. Find More Posts by sarin View Blog. Pdf is a very well made app that will save you time and help you get more work done with your phone.
Find More Posts by superSmash They hope these examples will help you to get a better understanding of the Linux system and that you feel encouraged to try out things on your own. Had a quick look at the tut that you were mentioning. Registration is quick, simple and absolutely free. There's a lot to like about this app, save for its rather unintuitive interface. We were able to easily arrange our files by moving them horizontally and then determining the start position for each song.
Find More Posts by gauravholey. This free app is great for making music playlists out of YouTube videos once you get the hang of the interface. Are you new to LinuxQuestions. The first one is the name of the task structure and the next is the function that need to be run.
The program automatically analyzed the images for common points to combine them. Llkmpg Posted by gauravholey hi there, This is in reference with the my first post This function can schedule itself once called.
For the scheduler module You gave me the lead I needed, both working code and a point in the right direction as to where to look for such info. Just starting out and have a question? For the approach, look at the post by estabroo.This a big leap in for a newcomer. That's because modules are object files whose symbols get resolved upon insmod'ing.
Modules Spanning Multiple Files Sometimes it makes sense to divide a kernel module between several source files. The minor number is used by the driver to distinguish between the various hardware it controls. There it is. Pdf all your data.
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