chardev2.c

/*
 * chardev2.c - Create an input/output character device
 */

#include <linux/atomic.h>
#include <linux/cdev.h>
#include <linux/delay.h>
#include <linux/device.h>
#include <linux/fs.h>
#include <linux/init.h>
#include <linux/module.h> /* Specifically, a module */
#include <linux/printk.h>
#include <linux/types.h>
#include <linux/uaccess.h> /* for get_user and put_user */
#include <linux/version.h>

#include <asm/errno.h>

#include "chardev.h"
#define SUCCESS 0
#define DEVICE_NAME "char_dev"
#define BUF_LEN 80

enum {
    CDEV_NOT_USED,
    CDEV_EXCLUSIVE_OPEN,
};

/* Is the device open right now? Used to prevent concurrent access into
 * the same device
 */
static atomic_t already_open = ATOMIC_INIT(CDEV_NOT_USED);

/* The message the device will give when asked */
static char message[BUF_LEN + 1];

static struct class *cls;

/* This is called whenever a process attempts to open the device file */
static int device_open(struct inode *inode, struct file *file)
{
    pr_info("device_open(%p)\n", file);

    try_module_get(THIS_MODULE);
    return SUCCESS;
}

static int device_release(struct inode *inode, struct file *file)
{
    pr_info("device_release(%p,%p)\n", inode, file);

    module_put(THIS_MODULE);
    return SUCCESS;
}

/* This function is called whenever a process which has already opened the
 * device file attempts to read from it.
 */
static ssize_t device_read(struct file *file, /* see include/linux/fs.h   */
                           char __user *buffer, /* buffer to be filled  */
                           size_t length, /* length of the buffer     */
                           loff_t *offset)
{
    /* Number of bytes actually written to the buffer */
    int bytes_read = 0;
    /* How far did the process reading the message get? Useful if the message
     * is larger than the size of the buffer we get to fill in device_read.
     */
    const char *message_ptr = message;

    if (!*(message_ptr + *offset)) { /* we are at the end of message */
        *offset = 0; /* reset the offset */
        return 0; /* signify end of file */
    }

    message_ptr += *offset;

    /* Actually put the data into the buffer */
    while (length && *message_ptr) {
        /* Because the buffer is in the user data segment, not the kernel
         * data segment, assignment would not work. Instead, we have to
         * use put_user which copies data from the kernel data segment to
         * the user data segment.
         */
        put_user(*(message_ptr++), buffer++);
        length--;
        bytes_read++;
    }

    pr_info("Read %d bytes, %ld left\n", bytes_read, length);

    *offset += bytes_read;

    /* Read functions are supposed to return the number of bytes actually
     * inserted into the buffer.
     */
    return bytes_read;
}

/* called when somebody tries to write into our device file. */
static ssize_t device_write(struct file *file, const char __user *buffer,
                            size_t length, loff_t *offset)
{
    int i;

    pr_info("device_write(%p,%p,%ld)", file, buffer, length);

    for (i = 0; i < length && i < BUF_LEN; i++)
        get_user(message[i], buffer + i);

    /* Again, return the number of input characters used. */
    return i;
}

/* This function is called whenever a process tries to do an ioctl on our
 * device file. We get two extra parameters (additional to the inode and file
 * structures, which all device functions get): the number of the ioctl called
 * and the parameter given to the ioctl function.
 *
 * If the ioctl is write or read/write (meaning output is returned to the
 * calling process), the ioctl call returns the output of this function.
 */
static long
device_ioctl(struct file *file, /* ditto */
             unsigned int ioctl_num, /* number and param for ioctl */
             unsigned long ioctl_param)
{
    int i;
    long ret = SUCCESS;

    /* We don't want to talk to two processes at the same time. */
    if (atomic_cmpxchg(&already_open, CDEV_NOT_USED, CDEV_EXCLUSIVE_OPEN))
        return -EBUSY;

    /* Switch according to the ioctl called */
    switch (ioctl_num) {
    case IOCTL_SET_MSG: {
        /* Receive a pointer to a message (in user space) and set that to
         * be the device's message. Get the parameter given to ioctl by
         * the process.
         */
        char __user *tmp = (char __user *)ioctl_param;
        char ch;

        /* Find the length of the message */
        get_user(ch, tmp);
        for (i = 0; ch && i < BUF_LEN; i++, tmp++)
            get_user(ch, tmp);

        device_write(file, (char __user *)ioctl_param, i, NULL);
        break;
    }
    case IOCTL_GET_MSG: {
        loff_t offset = 0;

        /* Give the current message to the calling process - the parameter
         * we got is a pointer, fill it.
         */
        i = device_read(file, (char __user *)ioctl_param, 99, &offset);

        /* Put a zero at the end of the buffer, so it will be properly
         * terminated.
         */
        put_user('\0', (char __user *)ioctl_param + i);
        break;
    }
    case IOCTL_GET_NTH_BYTE:
        /* This ioctl is both input (ioctl_param) and output (the return
         * value of this function).
         */
        ret = (long)message[ioctl_param];
        break;
    }

    /* We're now ready for our next caller */
    atomic_set(&already_open, CDEV_NOT_USED);

    return ret;
}

/* Module Declarations */

/* This structure will hold the functions to be called when a process does
 * something to the device we created. Since a pointer to this structure
 * is kept in the devices table, it can't be local to init_module. NULL is
 * for unimplemented functions.
 */
static struct file_operations fops = {
    .read = device_read,
    .write = device_write,
    .unlocked_ioctl = device_ioctl,
    .open = device_open,
    .release = device_release, /* a.k.a. close */
};

/* Initialize the module - Register the character device */
static int __init chardev2_init(void)
{
    /* Register the character device (at least try) */
    int ret_val = register_chrdev(MAJOR_NUM, DEVICE_NAME, &fops);

    /* Negative values signify an error */
    if (ret_val < 0) {
        pr_alert("%s failed with %d\n",
                 "Sorry, registering the character device ", ret_val);
        return ret_val;
    }

#if LINUX_VERSION_CODE >= KERNEL_VERSION(6, 4, 0)
    cls = class_create(DEVICE_FILE_NAME);
#else
    cls = class_create(THIS_MODULE, DEVICE_FILE_NAME);
#endif
    device_create(cls, NULL, MKDEV(MAJOR_NUM, 0), NULL, DEVICE_FILE_NAME);

    pr_info("Device created on /dev/%s\n", DEVICE_FILE_NAME);

    return 0;
}

/* Cleanup - unregister the appropriate file from /proc */
static void __exit chardev2_exit(void)
{
    device_destroy(cls, MKDEV(MAJOR_NUM, 0));
    class_destroy(cls);

    /* Unregister the device */
    unregister_chrdev(MAJOR_NUM, DEVICE_NAME);
}

module_init(chardev2_init);
module_exit(chardev2_exit);

MODULE_LICENSE("GPL");