ioctlシステムコールについて解説

#include <sys/ioctl.h>
int ioctl(int fd, unsigned long request, ...);

#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/fs.h>
#include <linux/uaccess.h>
#include "ioctl_sample.h"

// デバイスのメジャーナンバー
int major_number;
// デバイス名
#define DEVICE_NAME "ioctl_sample"

// 仮のデータストレージ
static ioctl_sample_data sample_data = {
    .number = 0,
    .string = "initial data",
};

static long device_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
{
    switch (cmd)
    {
    case IOCTL_SAMPLE_WRITE:
        if (copy_from_user(&sample_data, (ioctl_sample_data *)arg, sizeof(ioctl_sample_data)))
        {
            return -EACCES;
        }
        printk(KERN_INFO "Received data from user space - number: %d, string: %s\n", sample_data.number, sample_data.string);
        break;
    case IOCTL_SAMPLE_READ:
        if (copy_to_user((ioctl_sample_data *)arg, &sample_data, sizeof(ioctl_sample_data)))
        {
            return -EACCES;
        }
        printk(KERN_INFO "Sent data to user space - number: %d, string: %s\n", sample_data.number, sample_data.string);
        break;
    default:
        return -EINVAL;
    }
    return 0;
}

// file_operations構造体
static struct file_operations fops = {
    .unlocked_ioctl = device_ioctl,
};

// モジュール初期化
static int __init ioctl_sample_init(void)
{
    major_number = register_chrdev(0, DEVICE_NAME, &fops);
    if (major_number < 0)
    {
        printk(KERN_ALERT "Failed to register a major number\n");
        return major_number;
    }
    printk(KERN_INFO "Registered correctly with major number %d\n", major_number);
    return 0;
}

// モジュール終了
static void __exit ioctl_sample_exit(void)
{
    unregister_chrdev(major_number, DEVICE_NAME);
    printk(KERN_INFO "Goodbye!\n");
}

module_init(ioctl_sample_init);
module_exit(ioctl_sample_exit);

MODULE_LICENSE("GPL");

#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <unistd.h>
#include <string.h>
#include <errno.h>
#include <sys/ioctl.h>
#include "ioctl_sample.h"

int main()
{
    int fd;
    ioctl_sample_data data;

    fd = open("/dev/ioctl_sample", O_RDWR);
    if (fd < 0)
    {
        perror("Failed to open the device");
        return errno;
    }

    data.number = 42;
    strcpy(data.string, "Hello from user space");

    // データをカーネルモジュールに書き込む
    if (ioctl(fd, IOCTL_SAMPLE_WRITE, &data) < 0)
    {
        perror("Failed to write to the device");
        return errno;
    }

    printf("Sent data to kernel space - number: %d, string: %s\n", data.number, data.string);

    // データをカーネルモジュールから読み込む
    if (ioctl(fd, IOCTL_SAMPLE_READ, &data) < 0)
    {
        perror("Failed to read from the device");
        return errno;
    }

    printf("Received data from kernel space - number: %d, string: %s\n", data.number, data.string);

    close(fd);
    return 0;
}

#ifndef IOCTL_SAMPLE_H
#define IOCTL_SAMPLE_H

#include <linux/ioctl.h>

typedef struct
{
    int number;
    char string[256];
} ioctl_sample_data;

#define IOCTL_SAMPLE_WRITE _IOW('k', 1, ioctl_sample_data *)
#define IOCTL_SAMPLE_READ _IOR('k', 2, ioctl_sample_data *)

#endif

obj-m += ioctl_kernel_sample.o
all:
	make -C /lib/modules/$(shell uname -r)/build M=$(PWD) modules
clean:
	make -C /lib/modules/$(shell uname -r)/build M=$(PWD) clean

$ make

$ sudo insmod ioctl_kernel_sample.ko

$ sudo dmesg | tail
...
[  151.755276] ioctl_kernel_sample: loading out-of-tree module taints kernel.
[  151.755288] ioctl_kernel_sample: module verification failed: signature and/or required key missing - tainting kernel
[  151.756081] Registered correctly with major number 234

$ sudo mknod /dev/ioctl_sample c 234 0

$ sudo chmod 666 /dev/ioctl_sample

gcc ioctl_user_sample.c -o ioctl_user_sample

$ ./ioctl_user_sample
Sent data to kernel space - number: 42, string: Hello from user space
Received data from kernel space - number: 42, string: Hello from user space

$ sudo dmesg | tail
...
[  278.723423] Received data from user space - number: 42, string: Hello from user space
[  278.723531] Sent data to user space - number: 42, string: Hello from user space