Linux Kernel Threads : How to pass the Linux module write function as the function that the thread has to execute?

Question

I am developing Linux kernel module that communicate with user space program. This module waits for a message which is being sent from user space in order to print it in kernel mode.

This is the module :

#include <linux/init.h>
#include <linux/module.h>
#include <linux/fs.h>
#include <linux/device.h>  
#include <linux/kernel.h>
#include <linux/uaccess.h>

MODULE_LICENSE("GPL");      
MODULE_AUTHOR("Gaston");  
MODULE_DESCRIPTION("A simple Linux char driver"); 
MODULE_VERSION("0.1"); 

#define MAX 256
static char message[MAX] ="";      ///< Memory for the string that is passed from userspace

ssize_t exer_open(struct inode *pinode, struct file *pfile) {

    printk(KERN_INFO "Device has been opened\n");
    return 0;
}


ssize_t exer_read(struct file *pfile, char __user *buffer, size_t length, loff_t *offset) {

    return 0;
}


ssize_t exer_write(struct file *pfile, const char __user *buffer, size_t length, loff_t *offset) {
    if (length > MAX)
        return -EINVAL;

    if (copy_from_user(message, buffer, length) != 0)
        return -EFAULT;

    printk(KERN_INFO "Received %s characters from the user\n", message);
    return 0;

}   


ssize_t exer_close(struct inode *pinode, struct file *pfile) {

    printk(KERN_INFO "Device successfully closed\n");
    return 0;
}


struct file_operations exer_file_operations = { 
    .owner = THIS_MODULE,
    .open = exer_open,
    .read = exer_read,
    .write = exer_write,
    .release = exer_close,
};


int exer_simple_module_init(void) {

    printk(KERN_INFO "Initializing the LKM\n");
    register_chrdev(240, "Simple Char Drv", &exer_file_operations);
    return 0;
}


void exer_simple_module_exit(void) {

    unregister_chrdev(240, "Simple Char Drv");
}


module_init(exer_simple_module_init);
module_exit(exer_simple_module_exit);

This is the user-space program :

#include<stdio.h>
#include<stdlib.h>
#include<errno.h>
#include<fcntl.h>
#include<string.h>
#include<unistd.h>


#define BUFFER_LENGTH 256 

int main()

{

int ret, fd;
char stringToSend[BUFFER_LENGTH];


fd = open("/dev/char_device", O_RDWR);             // Open the device with read/write access

if (fd < 0)
    {
            perror("Failed to open the device...");
            return errno;
    }


printf("Type in a short string to send to the kernel module:\n");

scanf("%s", stringToSend);                // Read in a string (with spaces)

printf("Writing message to the device [%s].\n", stringToSend);

ret = write(fd, stringToSend, strlen(stringToSend)); // Send the string to the LKM

if (ret < 0)
    {
            perror("Failed to write the message to the device.");
            return errno;
    }

return 0;

}

This works fine and the message that I enter from user-space is printed in kernel space.

Now, I am using this example in order to learn kernel thread programming.

I am trying to make the LKM write function as a function that kernel thread has to execute, so I modified my module like this :

#include <linux/init.h>
#include <linux/module.h>
#include <linux/fs.h>
#include <linux/device.h>  
#include <linux/kernel.h>
#include <linux/uaccess.h>
#include <linux/kthread.h>

MODULE_LICENSE("GPL");      
MODULE_AUTHOR("Gaston");  
MODULE_DESCRIPTION("A simple Linux char driver"); 
MODULE_VERSION("0.1"); 


#define MAX 256
static struct task_struct *thread1;
static char message[MAX] ="";           ///< Memory for the string that is passed from userspace


ssize_t exer_open(struct inode *pinode, struct file *pfile) {

    printk(KERN_INFO "Device has been opened\n");
    return 0;
}



ssize_t exer_write(struct file *pfile, const char __user *buffer, size_t length, loff_t *offset) {

    if (length > MAX)
        return -EINVAL;

    printk("Thread_fuction is running ...\n");

    if (copy_from_user(message, buffer, length) != 0)
        return -EFAULT;

    printk(KERN_INFO "Received this message : %s ,from the user\n", message);

    return 0;

}   




struct file_operations exer_file_operations = { 
    .owner = THIS_MODULE,
    .open = exer_open,
    .write = exer_write,    
};




int exer_simple_module_init(void) {

    char our_thread[8]="thread1";

    printk(KERN_INFO "Initializing the LKM\n");
    register_chrdev(240, "Simple Char Drv", &exer_file_operations);

    thread1 = kthread_create(exer_write,NULL,our_thread);
    if((thread1))
            {
                printk(KERN_INFO "Thread is created");
                wake_up_process(thread1);
            }

    return 0;
}



void exer_simple_module_exit(void) {

    int ret;    

    unregister_chrdev(240, "Simple Char Drv");

    ret = kthread_stop(thread1);
    if(!ret)
        printk(KERN_INFO "Thread stopped");
}


module_init(exer_simple_module_init);
module_exit(exer_simple_module_exit);

The problem here, when I compile the module, an error occurs :

/exer_simple_char_drv.c:63:27: error: passing argument 1 of ‘kthread_create_on_node’ from incompatible pointer type [-Werror=incompatible-pointer-types] thread1 = kthread_create(exer_write,NULL,our_thread);

Searching the net, I find that a thread is created by the call to the function :

struct task_struct *kthread_create(int (*function)(void *data), void *data, const char name[], ...)

This function has as first argument the function that the thread has to execute and has as argument (void *data), but, my write kernel function has not the same arguments :

ssize_t exer_write(struct file *pfile, const char __user *buffer, size_t length, loff_t *offset)

Is there a way to solve this problem please ? How can I modify my Linux kernel in order to execute the write function as a thread ?

Thank you.

Answer 1

You can't. Your thread function has to use this format:

int my_thread_function(void *data)

You can call it whatever you want - it doesn't have to be called my_thread_function - and the parameter doesn't have to be called data but it does have to be a void *.

This will not work:

ssize_t exer_write(struct file *pfile, const char __user *buffer, size_t length, loff_t *offset)

I suggest writing a new function to be your thread function:

int exer_write_in_thread(void *data) {
    exer_write(???, ???, ???, ???);
    return 0;
}

Obviously you have to figure out what arguments you want to call exer_write with.