Learn Using copy_to_user and copy_from_user | Writing Your First Character Driver

When writing device drivers in C, it is critical to understand that direct access to user memory from kernel space is unsafe. The main reason is that user-space memory addresses may not be valid or accessible from the kernel context, and attempting to read or write directly can cause faults, crashes, or security vulnerabilities. The Linux kernel provides special functions to safely transfer data between user space and kernel space, ensuring proper access checks and avoiding these issues.

char_driver_read_example.c


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#include <linux/module.h>
#include <linux/fs.h>
#include <linux/uaccess.h>

#define DEVICE_NAME "mychardev"
#define BUF_LEN 80

static char msg[BUF_LEN] = "Hello from kernel space!";
static int device_open = 0;

static int dev_open(struct inode *inode, struct file *file) {
    if (device_open)
        return -EBUSY;
    device_open++;
    try_module_get(THIS_MODULE);
    return 0;
}

static int dev_release(struct inode *inode, struct file *file) {
    device_open--;
    module_put(THIS_MODULE);
    return 0;
}

static ssize_t dev_read(struct file *filp, char __user *buffer, size_t length, loff_t *offset) {
    int bytes_read = 0;
    if (*msg == 0)
        return 0;
    while (length && *msg) {
        if (copy_to_user(buffer++, msg++, 1)) {
            return -EFAULT;
        }
        bytes_read++;
        length--;
    }
    return bytes_read;
}

static struct file_operations fops = {
    .read = dev_read,
    .open = dev_open,
    .release = dev_release,
};

static int major;

static int __init chardev_init(void) {
    major = register_chrdev(0, DEVICE_NAME, &fops);
    if (major < 0) {
        printk(KERN_ALERT "Registering char device failed\n");
        return major;
    }
    printk(KERN_INFO "I was assigned major number %d\n", major);
    return 0;
}

static void __exit chardev_exit(void) {
    unregister_chrdev(major, DEVICE_NAME);
}

module_init(chardev_init);
module_exit(chardev_exit);

MODULE_LICENSE("GPL");

To safely receive data from user space in your driver's write operation, you use the copy_from_user function. In the previous code, you saw how copy_to_user safely copies data from the kernel buffer to user space in the read method. Similarly, copy_from_user is used in the write method to transfer data from a user-space buffer into a kernel-space buffer, ensuring no unsafe memory access occurs.

char_driver_write_example.c


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#include <linux/module.h>
#include <linux/fs.h>
#include <linux/uaccess.h>

#define DEVICE_NAME "mychardev"
#define BUF_LEN 80

static char msg[BUF_LEN];
static int device_open = 0;

static int dev_open(struct inode *inode, struct file *file) {
    if (device_open)
        return -EBUSY;
    device_open++;
    try_module_get(THIS_MODULE);
    return 0;
}

static int dev_release(struct inode *inode, struct file *file) {
    device_open--;
    module_put(THIS_MODULE);
    return 0;
}

static ssize_t dev_write(struct file *filp, const char __user *buffer, size_t length, loff_t *offset) {
    if (length > BUF_LEN - 1)
        length = BUF_LEN - 1;
    if (copy_from_user(msg, buffer, length)) {
        return -EFAULT;
    }
    msg[length] = '\0';
    return length;
}

static struct file_operations fops = {
    .write = dev_write,
    .open = dev_open,
    .release = dev_release,
};

static int major;

static int __init chardev_init(void) {
    major = register_chrdev(0, DEVICE_NAME, &fops);
    if (major < 0) {
        printk(KERN_ALERT "Registering char device failed\n");
        return major;
    }
    printk(KERN_INFO "I was assigned major number %d\n", major);
    return 0;
}

static void __exit chardev_exit(void) {
    unregister_chrdev(major, DEVICE_NAME);
}

module_init(chardev_init);
module_exit(chardev_exit);

MODULE_LICENSE("GPL");

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Section 2. Chapter 5

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char_driver_read_example.c


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#include <linux/module.h>
#include <linux/fs.h>
#include <linux/uaccess.h>

#define DEVICE_NAME "mychardev"
#define BUF_LEN 80

static char msg[BUF_LEN] = "Hello from kernel space!";
static int device_open = 0;

static int dev_open(struct inode *inode, struct file *file) {
    if (device_open)
        return -EBUSY;
    device_open++;
    try_module_get(THIS_MODULE);
    return 0;
}

static int dev_release(struct inode *inode, struct file *file) {
    device_open--;
    module_put(THIS_MODULE);
    return 0;
}

static ssize_t dev_read(struct file *filp, char __user *buffer, size_t length, loff_t *offset) {
    int bytes_read = 0;
    if (*msg == 0)
        return 0;
    while (length && *msg) {
        if (copy_to_user(buffer++, msg++, 1)) {
            return -EFAULT;
        }
        bytes_read++;
        length--;
    }
    return bytes_read;
}

static struct file_operations fops = {
    .read = dev_read,
    .open = dev_open,
    .release = dev_release,
};

static int major;

static int __init chardev_init(void) {
    major = register_chrdev(0, DEVICE_NAME, &fops);
    if (major < 0) {
        printk(KERN_ALERT "Registering char device failed\n");
        return major;
    }
    printk(KERN_INFO "I was assigned major number %d\n", major);
    return 0;
}

static void __exit chardev_exit(void) {
    unregister_chrdev(major, DEVICE_NAME);
}

module_init(chardev_init);
module_exit(chardev_exit);

MODULE_LICENSE("GPL");

char_driver_write_example.c


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#include <linux/module.h>
#include <linux/fs.h>
#include <linux/uaccess.h>

#define DEVICE_NAME "mychardev"
#define BUF_LEN 80

static char msg[BUF_LEN];
static int device_open = 0;

static int dev_open(struct inode *inode, struct file *file) {
    if (device_open)
        return -EBUSY;
    device_open++;
    try_module_get(THIS_MODULE);
    return 0;
}

static int dev_release(struct inode *inode, struct file *file) {
    device_open--;
    module_put(THIS_MODULE);
    return 0;
}

static ssize_t dev_write(struct file *filp, const char __user *buffer, size_t length, loff_t *offset) {
    if (length > BUF_LEN - 1)
        length = BUF_LEN - 1;
    if (copy_from_user(msg, buffer, length)) {
        return -EFAULT;
    }
    msg[length] = '\0';
    return length;
}

static struct file_operations fops = {
    .write = dev_write,
    .open = dev_open,
    .release = dev_release,
};

static int major;

static int __init chardev_init(void) {
    major = register_chrdev(0, DEVICE_NAME, &fops);
    if (major < 0) {
        printk(KERN_ALERT "Registering char device failed\n");
        return major;
    }
    printk(KERN_INFO "I was assigned major number %d\n", major);
    return 0;
}

static void __exit chardev_exit(void) {
    unregister_chrdev(major, DEVICE_NAME);
}

module_init(chardev_init);
module_exit(chardev_exit);

MODULE_LICENSE("GPL");

Everything was clear?

Thanks for your feedback!

Section 2. Chapter 5