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fops.c
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151 lines (137 loc) · 3.46 KB
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/* https://cirosantilli.com/linux-kernel-module-cheat#fops */
#include <linux/debugfs.h>
#include <linux/errno.h> /* EFAULT */
#include <linux/fs.h> /* file_operations */
#include <linux/kernel.h> /* min */
#include <linux/module.h>
#include <linux/printk.h> /* printk */
#include <linux/uaccess.h> /* copy_from_user, copy_to_user */
#include <uapi/linux/stat.h> /* S_IRUSR */
static struct dentry *debugfs_file;
// The buffer can be stored in two ways: static module data or kmalloc.
#define STATIC 1
#if STATIC
static char data[] = {'a', 'b', 'c', 'd'};
#define BUFLEN sizeof(data)
#else
static char *data;
#define BUFLEN 4
#endif
static int open(struct inode *inode, struct file *filp)
{
pr_info("open\n");
return 0;
}
/* @param[in,out] off: gives the initial position into the buffer.
* We must increment this by the amount of bytes read.
* Then when userland reads the same file descriptor again,
* we start from that point instead.
*/
static ssize_t read(struct file *filp, char __user *buf, size_t len, loff_t *off)
{
ssize_t ret;
pr_info("read len=%zu off=%lld\n", len, (long long)*off);
if (BUFLEN <= *off) {
ret = 0;
} else {
ret = min(len, BUFLEN - (size_t)*off);
if (copy_to_user(buf, data + *off, ret)) {
ret = -EFAULT;
} else {
*off += ret;
}
}
pr_info("ret=%lld\n", (long long)ret);
return ret;
}
/* Similar to read, but with one notable difference:
* we must return ENOSPC if the user tries to write more
* than the size of our buffer. Otherwise, Bash > just
* keeps trying to write to it infinitely.
*/
static ssize_t write(struct file *filp, const char __user *buf, size_t len, loff_t *off)
{
ssize_t ret;
pr_info("write len=%zu off=%lld\n", len, (long long)*off);
if (BUFLEN <= *off) {
ret = 0;
} else {
if (BUFLEN - (size_t)*off < len) {
ret = -ENOSPC;
} else {
if (copy_from_user(data + *off, buf, len)) {
ret = -EFAULT;
} else {
ret = len;
pr_info("buf = %.*s\n", (int)len, data + *off);
*off += ret;
}
}
}
pr_info("ret = %lld\n", (long long)ret);
return ret;
}
/* Called on the last close:
* http://stackoverflow.com/questions/11393674/why-is-the-close-function-is-called-release-in-struct-file-operations-in-the-l
*/
static int release(struct inode *inode, struct file *filp)
{
pr_info("release\n");
return 0;
}
static loff_t llseek(struct file *filp, loff_t off, int whence)
{
loff_t newpos;
pr_info("llseek off=%lld whence=%lld\n", (long long)off, (long long)whence);
switch(whence) {
case SEEK_SET:
newpos = off;
break;
case SEEK_CUR:
newpos = filp->f_pos + off;
break;
case SEEK_END:
newpos = BUFLEN + off;
break;
default:
return -EINVAL;
}
if (newpos < 0) return -EINVAL;
filp->f_pos = newpos;
pr_info("newpos = %lld\n", (long long)newpos);
return newpos;
}
static const struct file_operations fops = {
/* Prevents rmmod while fops are running.
* Try removing this for poll, which waits a lot. */
.owner = THIS_MODULE,
.llseek = llseek,
.open = open,
.read = read,
.release = release,
.write = write,
};
static int myinit(void)
{
#if STATIC == 0
data = kmalloc(BUFLEN, GFP_KERNEL);
if (!data)
return -ENOMEM;
data[0] = 'a';
data[1] = 'b';
data[2] = 'c';
data[3] = 'd';
#endif
debugfs_file = debugfs_create_file("lkmc_fops", S_IRUSR | S_IWUSR, NULL, NULL, &fops);
return 0;
}
static void myexit(void)
{
#if STATIC == 0
kfree(data);
#endif
debugfs_remove_recursive(debugfs_file);
}
module_init(myinit)
module_exit(myexit)
MODULE_LICENSE("GPL");