#include <linux/init.h>
#include <linux/module.h>
#include <linux/fs.h>
#include <linux/uaccess.h>
#include <linux/debugfs.h>
#include <linux/mm.h>
#include <linux/mutex.h>

extern struct dentry *module_dir;

ssize_t foo_read(struct file *f, char __user *buf, size_t len, loff_t *off);
ssize_t foo_write(struct file *f, const char __user *buf, size_t len, loff_t *off);
int init_foo(void);

char buffer[PAGE_SIZE];
size_t buffer_len = 0;
DEFINE_MUTEX(rw_lock);

static const struct file_operations fops = {
	.owner = THIS_MODULE,
	.read = foo_read,
	.write = foo_write,
};


ssize_t foo_read(struct file *f, char __user *buf, size_t len, loff_t *off)
{
	size_t read;
	
	mutex_lock(&rw_lock);
	read = simple_read_from_buffer(buf, len, off, buffer, buffer_len);
	mutex_unlock(&rw_lock);
	return read;
}


ssize_t foo_write(struct file *f, const char __user *buf, size_t len, loff_t *off)
{
	if (len > sizeof(buffer))
		return -EINVAL;
	mutex_lock(&rw_lock);
	if (copy_from_user(buffer, buf, len))
	{
		mutex_unlock(&rw_lock);
		return -EINVAL;
	}
	buffer_len = len;
	mutex_unlock(&rw_lock);
	return len;
}

int init_foo(void)
{
	struct dentry *ret;

	mutex_init(&rw_lock);
	ret = debugfs_create_file("foo", 0644, module_dir, NULL, &fops);
	if (IS_ERR(ret)) return PTR_ERR(ret);
	return 0;
}