Declaring and using a global variable in Kernel

naftilos · 06-05-2014, 08:05 PM

Hi all! I need to add a new function somewhere in the kernel (in mm/page_alloc.c but I guess it doesn't really matter). Then from the userspace (through a module) I want to enable or disable some parts of the code. My idea was to use a global variable. I have this:

linux/my_file.h:

Code:

extern int do_extra_enabled;

mm/my_file.c:

Code:

int do_extra_enabled;

mm/page_alloc.c

Code:

#include <linux/my_file.h>

void do_some_work() {
	printk(KERN_WARNING "normal work\n");
	if (do_extra_enabled)
		printk(KERN_WARNING "extra work!\n");
}

my_module:

Code:

#include <linux/my_file.h>

static int __init module_init(void)
{
	do_extra_enabled = 1;
}

static void __exit module_exit(void)
{
	do_extra_enabled = 0;
}

1. When I tried to compile the kernel, I got "page_alloc.c: undefined reference to `do_extra_enabled'".

2. Is there a better way to do what I want to achieve?

dijetlo · 06-08-2014, 10:43 AM

Quote:

in mm/page_alloc.c but I guess it doesn't really matter

That would be an incorrect guess.
If you start changing the kernel header files and libraries, you will end up re-writing a significant part of the kernel to accommodate the changes. You also will not end up with anything of value unless your ultimate goal is to replace Linus Torvalds and his team as the maintainers of the Linux kernel.

Quote:

Is there a better way to do what I want to achieve?

Most people write modules and have them load and unload against the standard Linux kernel. If this is your first attempt, I'd strongly encourage you to move your new static function into a module and try to compile it there. Then, when an application needs that functionality, it can dynamically load it in the kernel and then unload it when it's finished.

sundialsvcs · 06-10-2014, 06:32 AM

You should construct a self-contained kernel module which initializes its own data structure pointers when it is loaded, and tears them all down correctly when it is unloaded ... with all of the necessary protocols to ensure that this takes place correctly on an incredibly-busy multi-CPU system.

Your kernel module should contain whatever APIs are necessary to access the functionality of whatever-it-is you are doing, from user-space. Nothing else in the kernel should know or care.

Treat the main-line kernel source files as sacrosanct, reserved only for the gods. Let this Eleventh Commandment be carved upon your door ... or upon your brow ... "Thou Shalt Not Touch."

(In all seriousness, "you could

up a lot of things." Go find the existing kernel module that is most similar to what you are already doing, and hack it into something else very carefully base your own efforts upon its precedents. It's a time-honored tradition to do that.)

naftilos · 06-15-2014, 07:43 PM

For what is worth, I have found the answer I was looking for. I just had to export the global variable:

mm/my_file.c:

Code:

#include <linux/my_file.h>
#include <linux/module.h>

int do_extra_enabled;
EXPORT_SYMBOL(do_extra_enabled);