How to call exported kernel module functions from another module?

Question

I\'m writing an API as a kernel module that provides device drivers with various functions. I wrote three functions in mycode.c. I then built and loaded the

Answer 1

Minimal QEMU + Buildroot example

I have tested the following in a fully reproducible QEMU + Buildroot environment, so maybe having this working version version will help you find out what is wong with your code.

GitHub upstream is centered on the files:

• dep.c
• dep2.c
• Makefile

dep.c

#include <linux/delay.h> /* usleep_range */
#include <linux/kernel.h>
#include <linux/kthread.h>
#include <linux/module.h>

MODULE_LICENSE("GPL");

int lkmc_dep = 0;
EXPORT_SYMBOL(lkmc_dep);
static struct task_struct *kthread;

static int work_func(void *data)
{
    while (!kthread_should_stop()) {
        printk(KERN_INFO "%d\n", lkmc_dep);
        usleep_range(1000000, 1000001);
    }
    return 0;
}

static int myinit(void)
{
    kthread = kthread_create(work_func, NULL, "mykthread");
    wake_up_process(kthread);
    return 0;
}

static void myexit(void)
{
    kthread_stop(kthread);
}

module_init(myinit)
module_exit(myexit)

dep2.c

#include <linux/delay.h> /* usleep_range */
#include <linux/kernel.h>
#include <linux/kthread.h>
#include <linux/module.h>

MODULE_LICENSE("GPL");

extern int lkmc_dep;
static struct task_struct *kthread;

static int work_func(void *data)
{
    while (!kthread_should_stop()) {
        usleep_range(1000000, 1000001);
        lkmc_dep++;
    }
    return 0;
}

static int myinit(void)
{
    kthread = kthread_create(work_func, NULL, "mykthread");
    wake_up_process(kthread);
    return 0;
}

static void myexit(void)
{
    kthread_stop(kthread);
}

module_init(myinit)
module_exit(myexit)

And now you can do:

insmod dep.ko
insmod dep2.ko

With that Buildroot setup, things are already configuring depmod /lib/module/*/depmod with the dependency, so just this is enough to load both:

modprobe dep

Also, if you built your kernel with CONFIG_KALLSYMS_ALL=y, then the exported symbol can be seen with:

grep lkmc_dep /proc/kallsyms

see also: Does kallsyms have all the symbol of kernel functions?

Answer 2

From my research, it seems that those are the only three ways to handle this situation, and I've gotten each of them to work, so I think I'll just pick my favorite out of those.

Answer 3

OK: You have one module where the function is and one place what wants to import it right?

You must use "EXPORT_SYMBOL("name of the function") such as foo in the place where the function is. So in the "c" file have the function "foo" defined and put in: EXPORT_SYMBOL(foo)

Make sure you have the prototype for "foo" in a common header file (you can have it in separate places for each module and it will work but you are asking for trouble if the signatures change). So say: void foo(void *arg);

Then the other module that wants it just invoke "foo" and you are good.

Also: Make sure that you load the module with foo first. If you have cross dependencies like module2 needs foo from module1 and module1 needs bar from module2 you need to have one register functions with another. If you want to know please ask a separate Q.