Write long to linux char device driver?

I'm trying to write a character device driver in linux. Unfortunately it's not working for any numbers greater than 255.

I want this driver specifically to work with value of type long. Anytime I input a value greater than 255, the numbers wrong. 256 goes to 0 etc.

I've written a simple character device driver that shows the problem, there might be a lot of unused include statements as I copied my full driver and deleted almost everything:

chartest.c

#include <linux/init.h>
#include <linux/module.h> /* I mean this is a module after all! */
#include <linux/moduleparam.h>
#include <linux/kernel.h>
#include <linux/sched.h> /* For current task information */
#include <linux/fs.h> /* For file operations */
#include <linux/types.h> /* dev_t: device number data type */
#include <linux/cdev.h> /* cdev is the module data type that the kernel sees */
#include <asm/uaccess.h> /* For routines to copy data to/from user space */
#include <linux/uaccess.h>
#include <linux/slab.h> /* kmalloc/kfree */


MODULE_LICENSE("GPL");

#define DRIVER_NAME "chartest"

#define MAJOR_NUM 230
#define MINOR_NUM 0

struct cdev *cdev;

int test_device_open(struct inode *inode, struct file *fp) {
    return 0;
}


int test_device_release(struct inode *inode, struct file *fp) {
    return 0;
}


ssize_t test_read(struct file *fp, char __user *buffer, size_t count, loff_t *f_pos) {
    return count;
}

ssize_t test_write(struct file *fp, const char __user *buffer, size_t count, loff_t *fpos) {
    // We must validate the user's buffer and convert it to a long long
    long userOperand;
    unsigned char *userInput = NULL;

    printk(KERN_NOTICE "Write Function Entered.\n");
    printk(KERN_ALERT "Write count: %ld, Write fp: %lld\n", count, *fpos);

    userInput = kmalloc(count, GFP_KERNEL); 
    get_user(*userInput, buffer);

    printk(KERN_NOTICE "Value before cast: %ld\n", (long) *userInput);

    userOperand = (long) *userInput;

    printk(KERN_NOTICE "Value after cast: %ld\n", userOperand);

    // Increment the file position pointer (in our case, always by 8)
    *fpos += count;

    kfree(userInput);
    return count;
}


/*
* Declaration of function for open file operations
*/
static struct file_operations test_fops = {
    .owner = THIS_MODULE,
    .read = test_read,
    .write = test_write,
    .open = test_device_open,
    .release = test_device_release,
};



// Initialization function
static int __init test_init(void)
{
    // Register device number:
    int err = 0;
    dev_t device_number = MKDEV(MAJOR_NUM, MINOR_NUM);

    err = register_chrdev_region(device_number, 1, DRIVER_NAME);

    if (err < 0) {
        printk(KERN_ALERT "Could not allocate device number.\n");
        return err;
    }

    cdev = cdev_alloc();
    cdev->owner = THIS_MODULE;
    cdev->ops = &test_fops;

    err = cdev_add(cdev, device_number, 1);
    if (err) {
        printk("Error allocating cdev.\n");
    }

    printk(KERN_ALERT "Test Initialized. Major Number: %d\n", MAJOR_NUM);

    return 0;
}

// Exit function:
static void __exit test_exit(void)
{
    dev_t device_number = MKDEV(MAJOR_NUM, MINOR_NUM);

    // Remove char device */
    cdev_del(cdev);

    /* Unregister Device Number: */
    unregister_chrdev_region(device_number, 1);
    printk(KERN_ALERT "TestDriver %d destroyed.\n", MAJOR_NUM);
}

module_init(test_init);
module_exit(test_exit);

Small test program:

maintest.c:

#include <unistd.h>
#include <fcntl.h>



int main(void) {
    long input = 256;


    int fd = open("/dev/chartest0", O_RDWR);

    write(fd, &input, sizeof(long));

    close(fd);

    return 0;

}

The printk statements gives the following output with the given input of 256:

Write Eunction Entered.
Write count: 8, Write fp: 0
Value before cast: 0
Value after cast: 0

This also fails with copy_from_user given an in put size of 8 bytes. It also fails when iterating through the buffer one byte at a time and copying the data. I've tried everything.

If you are graciously willing to help, compile with: MakeFile

ifeq ($(KERNELRELEASE),)

    # Assume the source tree is where the running kernel was built
    # You should set KERNELDIR in the environment if it's elsewhere
    KERNELDIR ?= /lib/modules/$(shell uname -r)/build
    # The current directory is passed to sub-makes as argument
    PWD := $(shell pwd)

modules:
    $(MAKE) -C $(KERNELDIR) M=$(PWD) modules

modules_install:
    $(MAKE) -C $(KERNELDIR) M=$(PWD) modules_install

clean:
    rm -rf *.o *~ core .depend .*.cmd *.ko *.mod.c .tmp_versions

.PHONY: modules modules_install clean

else
    # called from kernel build system: just declare what our modules are
    obj-m := chartest.o

endif

then in the same directory:

sudo insmod chartest.ko

finally:

sudo mknod -m 777 /dev/chartest0 c 230 0

Then you can compile maindriver.c and run it to test.

Can someone please help me fix this issue?

Solution

you can not use get_user the way you do:

from get_user doc

This macro copies a single simple variable from user space to kernel space. It supports simple types like char and int, but not larger data types like structures or arrays.
ptr must have pointer-to-simple-variable type, and the result of dereferencing ptr must be assignable to x without a cast.

With get_user, you will only copy the first character.

You need to use copy_from_user, this function can copy array and structure, not only simple types:

ssize_t test_write(struct file *fp, const char __user *buffer, size_t count, loff_t *fpos) {
    // We must validate the user's buffer and convert it to a long long
    long userOperand;
    unsigned char *userInput = NULL;

    userInput = kmalloc(count, GFP_KERNEL); 

    printk(KERN_NOTICE "Write Function Entered.\n");
    printk(KERN_ALERT "Write count: %ld, Write fp: %lld\n", count, *fpos);

    /* warning, here you should test that count is exactly sizeof userInput */
    copy_from_user(userInput, buffer, count);

    userOperand =  *(long*)userInput;

    printk(KERN_NOTICE "Value after cast: %ld\n", userOperand);

    // Increment the file position pointer (in our case, always by 8)
    *fpos += count;

    kfree(userInput);
    return count;
}

You can also copy from char * to long in copy_from_user (no memory alloc in that case):

ssize_t test_write(struct file *fp, const char __user *buffer, size_t count, loff_t *fpos) {
    // We must validate the user's buffer and convert it to a long long
    long userOperand;

    printk(KERN_NOTICE "Write Function Entered.\n");
    printk(KERN_ALERT "Write count: %ld, Write fp: %lld\n", count, *fpos);

    /* warning, here you should test that count is exactly sizeof userOperand */
    copy_from_user(&userOperand, buffer, sizeof userOperand);

    printk(KERN_NOTICE "Value after reading: %ld\n", userOperand);

    // Increment the file position pointer (in our case, always by 8)
    *fpos += count;

    return count;
}