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)

kernel_module/anonymous_inode.h

@@ -0,0 +1,9 @@
+#ifndef IOCTL_H
+#define IOCTL_H
+
+#include <linux/ioctl.h>
+
+#define LKMC_ANONYMOUS_INODE_MAGIC 0x33
+#define LKMC_ANONYMOUS_INODE_GET_FD _IOR(LKMC_ANONYMOUS_INODE_MAGIC, 0, int)
+
+#endif

kernel_module/fops.c

@@ -17,13 +17,13 @@ in drivers (syscalls being the other one).
 Here we use debugfs.
 */
 
+#include <asm/uaccess.h> /* copy_from_user, copy_to_user */
 #include <linux/debugfs.h>
 #include <linux/errno.h> /* EFAULT */
 #include <linux/fs.h>
 #include <linux/kernel.h> /* min */
 #include <linux/module.h>
 #include <linux/printk.h> /* printk */
-#include <asm/uaccess.h> /* copy_from_user, copy_to_user */
 
 MODULE_LICENSE("GPL");
 
@@ -95,7 +95,7 @@ static ssize_t write(struct file *filp, const char __user *buf, size_t len, loff
 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)
+static int release(struct inode *inode, struct file *filp)
 {
 	printk(KERN_INFO "release\n");
 	return 0;

kernel_module/ioctl.c

@@ -73,7 +73,10 @@ static const struct file_operations fops = {
 static int myinit(void)
 {
 	dir = debugfs_create_dir("lkmc_ioctl", 0);
-	debugfs_create_file("f", 0666, dir, NULL, &fops);
+	/* ioctl permissions are not automatically restricted by rwx as for read / write,
+	 * but we could of course implement that ourselves:
+	 * https://stackoverflow.com/questions/29891803/user-permission-check-on-ioctl-command */
+	debugfs_create_file("f", 0, dir, NULL, &fops);
 	return 0;
 }
 

kernel_module/user/anonymous_inode.c

@@ -0,0 +1,40 @@
+#define _GNU_SOURCE
+#include <errno.h>
+#include <fcntl.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <sys/ioctl.h>
+#include <sys/stat.h>
+#include <sys/types.h>
+#include <unistd.h> /* sleep */
+
+#include "../anonymous_inode.h"
+
+int main(int argc, char **argv)
+{
+	char buf[1024];
+	int fd_ioctl, fd_ioctl_anon, ret;
+
+	if (argc < 2) {
+		puts("Usage: ./prog <ioctl-file>");
+		return EXIT_FAILURE;
+	}
+	fd_ioctl = open(argv[1], O_RDONLY);
+	if (fd_ioctl == -1) {
+		perror("open");
+		return EXIT_FAILURE;
+	}
+	ret = ioctl(fd_ioctl, LKMC_ANONYMOUS_INODE_GET_FD, &fd_ioctl_anon);
+	if (ret == -1) {
+		perror("ioctl");
+		return EXIT_FAILURE;
+	}
+	ret = read(fd_ioctl_anon, buf, sizeof(buf));
+	printf("%.*s\n", ret, buf);
+	sleep(1);
+	ret = read(fd_ioctl_anon, buf, sizeof(buf));
+	printf("%.*s\n", ret, buf);
+	close(fd_ioctl_anon);
+	close(fd_ioctl);
+	return EXIT_SUCCESS;
+}

kernel_module/user/ioctl.c

@@ -15,8 +15,8 @@ int main(int argc, char **argv)
 	int fd, arg_int, ret;
 	lkmc_ioctl_struct arg_struct;
 
-	if (argc < 4) {
-		puts("Usage: ./prog <ioctl-file> <cmd> <arg>");
+	if (argc < 2) {
+		puts("Usage: ./prog <ioctl-file>");
 		return EXIT_FAILURE;
 	}
 	fd = open(argv[1], O_RDONLY);