access_ok() 和__put_user()用法

#include <linux/module.h>
#include <linux/init.h>
#include <linux/kernel.h>
#include <linux/fs.h>
#include <linux/kdev_t.h>
#include <linux/cdev.h>
#include <asm/uaccess.h>

#define DEVICE_NAME "chardev"

static ssize_t chardev_read(struct file *filp, char __user *buf, size_t count, loff_t *f_pos);

static struct file_operations chardev_fops = {
        .read = chardev_read,
};

static struct cdev *cdevp = NULL;
static dev_t devno;

static ssize_t chardev_read(struct file *filp, char __user *buf, size_t count, loff_t *f_pos)
{
        int var = 10;

        if (!access_ok(VERIFY_WRITE, buf, count))
                goto err;

        if (__put_user(var, (int __user *) buf))
                goto err;

        return sizeof(var);
err:
        return -EFAULT;
}

static int __init chardev_init(void)
{
        int ret;

        ret = alloc_chrdev_region(&devno, 0, 1, DEVICE_NAME);
        if (ret < 0)
                goto out;

        cdevp = cdev_alloc();
        if (!cdevp)
                goto alloc_err;

        cdev_init(cdevp, &chardev_fops);
        ret = cdev_add(cdevp, devno, 1);
        if (!ret)
                goto out;

        cdev_del(cdevp);
alloc_err:
        unregister_chrdev_region(devno, 1);
out:
        return ret;
}

static void __exit chardev_exit(void)
{
        cdev_del(cdevp);
        unregister_chrdev_region(devno, 1);
}

MODULE_LICENSE("GPL");

module_init(chardev_init);
module_exit(chardev_exit);