Anonymous inode!

kernel_module/anonymous_inode.c

@@ -0,0 +1,85 @@
+/*
+https://stackoverflow.com/questions/4508998/what-is-anonymous-inode
+
+anon_inode_getfd example:
+
+- get an anonymous inode via ioctl from a debugfs entry
+- read from that inode
+
+This method allows getting multiple file descriptors from a single filesystem,
+which reduces namespace pollution.
+*/
+
+#include <asm/uaccess.h> /* copy_from_user, copy_to_user */
+#include <linux/anon_inodes.h>
+#include <linux/debugfs.h>
+#include <linux/errno.h> /* EFAULT */
+#include <linux/fs.h>
+#include <linux/jiffies.h>
+#include <linux/kernel.h> /* min */
+#include <linux/module.h>
+#include <linux/printk.h> /* printk */
+
+#include "anonymous_inode.h"
+
+MODULE_LICENSE("GPL");
+
+static struct dentry *dir;
+
+static ssize_t read(struct file *filp, char __user *buf, size_t len, loff_t *off)
+{
+	char kbuf[1024];
+	size_t ret;
+
+	ret = snprintf(kbuf, sizeof(kbuf), "%llu", (unsigned long long)jiffies);
+	if (copy_to_user(buf, kbuf, ret)) {
+		ret = -EFAULT;
+	}
+	return ret;
+}
+
+static const struct file_operations fops_anon = {
+	.read = read,
+};
+
+static long unlocked_ioctl(struct file *filp, unsigned int cmd, unsigned long argp)
+{
+	int fd;
+
+	switch (cmd) {
+		case LKMC_ANONYMOUS_INODE_GET_FD:
+			fd = anon_inode_getfd(
+				"random",
+				&fops_anon,
+				NULL,
+				O_RDONLY | O_CLOEXEC
+			);
+			if (copy_to_user((void __user *)argp, &fd, sizeof(fd))) {
+				return -EFAULT;
+			}
+		break;
+		default:
+			return -EINVAL;
+		break;
+	}
+	return 0;
+}
+
+static const struct file_operations fops_ioctl = {
+	.unlocked_ioctl = unlocked_ioctl
+};
+
+static int myinit(void)
+{
+	dir = debugfs_create_dir("lkmc_anonymous_inode", 0);
+	debugfs_create_file("f", 0, dir, NULL, &fops_ioctl);
+	return 0;
+}
+
+static void myexit(void)
+{
+	debugfs_remove_recursive(dir);
+}
+
+module_init(myinit)
+module_exit(myexit)