[PATCH 5/5 UPDATED] CUSE: implement CUSE - Character device in Userspace

[Tejun Heo]

CUSE enables implementing character devices in userspace.  With recent
additions of nonblock, lseek, ioctl and poll support, FUSE already has
most of what's necessary to implement character devices.  All CUSE has
to do is bonding all those components - FUSE, chardev and the driver
model - nicely.

Due to the number of different objects involved and many ways an
instance can fail, object lifetime rules are a tad bit complex.
Please take a look at the comment on top of fs/fuse/cuse.c for
details.

Other than that, it's mostly straight forward.  Client opens
/dev/cuse, kernel starts conversation with CUSE_INIT.  The client
tells CUSE which device it wants to create.  CUSE creates the device
for the client and the rest works the same way as in a direct IO FUSE
session.

Each CUSE device has a corresponding directory /sys/class/cuse/DEVNAME
(which is symlink to /sys/devices/virtual/class/DEVNAME if
SYSFS_DEPRECATED is turned off) which hosts "waiting" and "abort"
among other things.  Those two files have the same meaning as the FUSE
control files.

The only notable lacking feature compared to in-kernel implementation
is mmap support.

Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
---
Bitfield dropped from struct cuse_conn as suggested by Andrew.
Switched back to MISC_DYNAMIC_MINOR as Miklos and Andrew suggested.

Thanks.

 fs/Kconfig            |   10 
 fs/fuse/Makefile      |    1 
 fs/fuse/cuse.c        |  719 ++++++++++++++++++++++++++++++++++++++++++++++++++
 include/linux/cuse.h  |   47 +++
 include/linux/fuse.h  |    2 
 include/linux/magic.h |    5 
 6 files changed, 782 insertions(+), 2 deletions(-)

Index: work/fs/fuse/cuse.c
===================================================================
--- /dev/null
+++ work/fs/fuse/cuse.c
@@ -0,0 +1,719 @@
+/*
+ * CUSE: Character device in Userspace
+ *
+ * Copyright (C) 2008       SUSE Linux Products GmbH
+ * Copyright (C) 2008       Tejun Heo <teheo@xxxxxxx>
+ *
+ * This file is released under the GPLv2.
+ *
+ * CUSE enables character devices to be implemented from userland much
+ * like FUSE allows filesystems.  On initialization /dev/cuse is
+ * created.  By opening the file and replying to the CUSE_INIT request
+ * userland CUSE server can create a character device.  After that the
+ * operation is very similar to FUSE.
+ *
+ * CUSE bridges several objects to implement a character device using
+ * userland backend.  The lifetime rules of the involved objects are a
+ * bit complex.
+ *
+ * cuse_conn	: contains fuse_conn and serves as bonding structure
+ * channel	: file handle connected to the userland CUSE server
+ * cdev		: the implemented character device
+ * mnt		: vfsmount which serves dentry and inode for cdev
+ * dev		: generic device for cdev
+ *
+ * Note that 'channel' is what 'dev' is in FUSE.  As CUSE deals with
+ * devices, it's called 'channel' to reduce confusion.
+ *
+ * channel determines when the character device dies.  When channel is
+ * closed, everything should begin to destruct.  As cuse_conn and mnt
+ * dereference each other unlike FUSE where mnt can outlive fuse_conn,
+ * both should be destructed at the same time.  This is achieved by
+ * giving the base reference of cuse_conn to mnt and never referencing
+ * cuse_conn directly, so both channel and cdev have reference to mnt
+ * which in turn has single reference to cuse_conn.
+ *
+ * On CUSE server disconnect, cuse_channel_release() unregisters dev,
+ * deletes cdev and puts mnt.  When the cdev is released, it puts mnt
+ * which in turn puts the cuse_conn on release.
+ *
+ * cuse_conn_get/put() takes cuse_conn and manipulates the reference
+ * count of mnt for convenience.
+ */
+
+#include <linux/cuse.h>
+
+#include <linux/cdev.h>
+#include <linux/device.h>
+#include <linux/file.h>
+#include <linux/fs.h>
+#include <linux/kdev_t.h>
+#include <linux/kthread.h>
+#include <linux/magic.h>
+#include <linux/miscdevice.h>
+#include <linux/stat.h>
+
+#include "fuse_i.h"
+
+struct cuse_conn {
+	struct fuse_conn	fc;
+	struct cdev		cdev;
+	struct vfsmount		*mnt;
+	struct device		*dev;
+
+	/* init parameters, set once during initialization */
+	bool			unrestricted_ioctl;
+
+	/* the following two bools are protected by cuse_disconnect_lock */
+	bool			cdev_added;
+	bool			disconnected;	/* channel disconnected */
+};
+
+static struct class *cuse_class;
+static DEFINE_SPINLOCK(cuse_disconnect_lock);
+
+static inline struct cuse_conn *fc_to_cc(struct fuse_conn *fc)
+{
+	return container_of(fc, struct cuse_conn, fc);
+}
+
+static inline struct cuse_conn *cdev_to_cc(struct cdev *cdev)
+{
+	return container_of(cdev, struct cuse_conn, cdev);
+}
+
+/*
+ * Connection reference count helpers.  Note that instead of directly
+ * referencing cc->fc, its mnt, which holds single reference to
+ * cc->fc, is referenced.
+ */
+static inline struct cuse_conn *cuse_conn_get(struct cuse_conn *cc)
+{
+	mntget(cc->mnt);
+	return cc;
+}
+
+static inline void cuse_conn_put(struct cuse_conn *cc)
+{
+	mntput(cc->mnt);
+}
+
+
+/**************************************************************************
+ * CUSE frontend operations
+ *
+ * These are file operations for the character device.
+ *
+ * On open, CUSE opens a file from the FUSE mnt and stores it to
+ * private_data of the open file.  All other ops call FUSE ops on the
+ * FUSE file.
+ */
+
+static ssize_t cuse_direct_read(struct file *file, char __user *buf,
+				size_t count, loff_t *ppos)
+{
+	return fuse_direct_io(file->private_data, buf, count, ppos, 0);
+}
+
+static ssize_t cuse_direct_write(struct file *file, const char __user *buf,
+				 size_t count, loff_t *ppos)
+{
+	/*
+	 * No locking or generic_write_checks(), the server is
+	 * responsible for locking and sanity checks.
+	 */
+	return fuse_direct_io(file->private_data, buf, count, ppos, 1);
+}
+
+static int cuse_open(struct inode *inode, struct file *file)
+{
+	struct cuse_conn *cc = cdev_to_cc(inode->i_cdev);
+	struct file *cfile;
+
+	cfile = dentry_open(dget(cc->mnt->mnt_root), mntget(cc->mnt),
+			    file->f_flags);
+	if (IS_ERR(cfile))
+		return PTR_ERR(cfile);
+
+	file->private_data = cfile;
+	return 0;
+}
+
+static int cuse_flush(struct file *file, fl_owner_t id)
+{
+	return fuse_flush(file->private_data, id);
+}
+
+static int cuse_release(struct inode *inode, struct file *file)
+{
+	return filp_close(file->private_data, NULL);
+}
+
+static int cuse_fsync(struct file *file, struct dentry *de, int datasync)
+{
+	return fuse_fsync(file->private_data, de, datasync);
+}
+
+static unsigned cuse_file_poll(struct file *file, poll_table *wait)
+{
+	return fuse_file_poll(file->private_data, wait);
+}
+
+static long cuse_file_ioctl(struct file *file, unsigned int cmd,
+			    unsigned long arg)
+{
+	struct cuse_conn *cc = cdev_to_cc(file->f_dentry->d_inode->i_cdev);
+	unsigned int flags = 0;
+
+	if (cc->unrestricted_ioctl)
+		flags |= FUSE_IOCTL_UNRESTRICTED;
+
+	return fuse_file_do_ioctl(file->private_data, cmd, arg, flags);
+}
+
+static long cuse_file_compat_ioctl(struct file *file, unsigned int cmd,
+				   unsigned long arg)
+{
+	struct cuse_conn *cc = cdev_to_cc(file->f_dentry->d_inode->i_cdev);
+	unsigned int flags = FUSE_IOCTL_COMPAT;
+
+	if (cc->unrestricted_ioctl)
+		flags |= FUSE_IOCTL_UNRESTRICTED;
+
+	return fuse_file_do_ioctl(file->private_data, cmd, arg, flags);
+}
+
+static const struct file_operations cuse_frontend_fops = {
+	.read			= cuse_direct_read,
+	.write			= cuse_direct_write,
+	.open			= cuse_open,
+	.flush			= cuse_flush,
+	.release		= cuse_release,
+	.fsync			= cuse_fsync,
+	.poll			= cuse_file_poll,
+	.unlocked_ioctl		= cuse_file_ioctl,
+	.compat_ioctl		= cuse_file_compat_ioctl,
+};
+
+
+/**************************************************************************
+ * CUSE filesystem operations
+ *
+ * CUSE filesystem type implementation.  Each CUSE device instance is
+ * served by a separate superblock of cuse_fs type.
+ */
+
+static void cuse_fc_release(struct fuse_conn *fc)
+{
+	struct cuse_conn *cc = fc_to_cc(fc);
+
+	kfree(cc);
+}
+
+static int cuse_fill_super(struct super_block *sb, void *data, int silent)
+{
+	struct cuse_conn *cc = NULL;
+	struct dentry *root_dentry = NULL;
+	struct inode *root = NULL;
+	int rc;
+
+	sb->s_magic = CUSE_SUPER_MAGIC;
+	sb->s_op = &fuse_super_operations;
+	sb->s_maxbytes = MAX_LFS_FILESIZE;
+
+	cc = kzalloc(sizeof(*cc), GFP_KERNEL);
+	if (!cc)
+		goto err_nomem;
+	rc = fuse_conn_init(&cc->fc, sb);
+	if (rc)
+		goto err;
+
+	/* cuse isn't accessible to mortal users, give it some latitude */
+	cc->fc.flags = FUSE_ALLOW_OTHER;
+	cc->fc.user_id = current->euid;
+	cc->fc.group_id = current->egid;
+	cc->fc.max_read = FUSE_MAX_PAGES_PER_REQ * PAGE_SIZE;
+	cc->fc.release = cuse_fc_release;
+
+	/* transfer the initial cc refcnt to sb */
+	sb->s_fs_info = &cc->fc;
+	cc = NULL;
+
+	root = fuse_get_root_inode(sb, S_IFREG);
+	if (!root)
+		goto err_nomem;
+
+	root_dentry = d_alloc_root(root);
+	if (!root_dentry)
+		goto err_nomem;
+
+	sb->s_root = root_dentry;
+
+	return 0;
+
+err_nomem:
+	rc = -ENOMEM;
+err:
+	if (root_dentry)
+		dput(root_dentry);
+	else if (root)
+		iput(root);
+	kfree(cc);
+	return rc;
+}
+
+static int cuse_get_sb(struct file_system_type *fs_type, int flags,
+		       const char *dev_name, void *data, struct vfsmount *mnt)
+{
+	return get_sb_nodev(fs_type, flags, data, cuse_fill_super, mnt);
+}
+
+static struct file_system_type cuse_fs = {
+	.name		= "cuse",
+	.get_sb		= cuse_get_sb,
+	.kill_sb	= kill_anon_super,
+};
+
+
+/**************************************************************************
+ * CUSE channel initialization and destruction
+ */
+
+struct cuse_devinfo {
+	const char		*name;
+};
+
+/**
+ * cuse_parse_one - parse one key=value pair
+ * @pp: i/o parameter for the current position
+ * @end: points to one past the end of the packed string
+ * @keyp: out parameter for key
+ * @valp: out parameter for value
+ *
+ * *@pp points to packed strings - "key0=val0\0key1=val1\0" which ends
+ * at @end - 1.  This function parses one pair and set *@keyp to the
+ * start of the key and *@valp to the start of the value.  Note that
+ * the original string is modified such that the key string is
+ * terminated with '\0'.  *@pp is updated to point to the next string.
+ *
+ * RETURNS:
+ * 1 on successful parse, 0 on EOF, -errno on failure.
+ */
+static int cuse_parse_one(char **pp, char *end, char **keyp, char **valp)
+{
+	char *p = *pp;
+	char *key, *val;
+
+	while (p < end && *p == '\0')
+		p++;
+	if (p == end)
+		return 0;
+
+	if (end[-1] != '\0') {
+		printk(KERN_ERR "CUSE: info not properly terminated\n");
+		return -EINVAL;
+	}
+
+	key = val = p;
+	p += strlen(p);
+
+	if (valp) {
+		strsep(&val, "=");
+		if (!val)
+			val = key + strlen(key);
+		key = strstrip(key);
+		val = strstrip(val);
+	} else
+		key = strstrip(key);
+
+	if (!strlen(key)) {
+		printk(KERN_ERR "CUSE: zero length info key specified\n");
+		return -EINVAL;
+	}
+
+	*pp = p;
+	*keyp = key;
+	if (valp)
+		*valp = val;
+
+	return 1;
+}
+
+/**
+ * cuse_parse_dev_info - parse device info
+ * @p: device info string
+ * @len: length of device info string
+ * @devinfo: out parameter for parsed device info
+ *
+ * Parse @p to extract device info and store it into @devinfo.  String
+ * pointed to by @p is modified by parsing and @devinfo points into
+ * them, so @p shouldn't be freed while @devinfo is in use.
+ *
+ * RETURNS:
+ * 0 on success, -errno on failure.
+ */
+static int cuse_parse_devinfo(char *p, size_t len, struct cuse_devinfo *devinfo)
+{
+	char *end = p + len;
+	char *key, *val;
+	int rc;
+
+	while (true) {
+		rc = cuse_parse_one(&p, end, &key, &val);
+		if (rc < 0)
+			return rc;
+		if (!rc)
+			break;
+		if (strcmp(key, "DEVNAME") == 0)
+			devinfo->name = val;
+		else
+			printk(KERN_WARNING "CUSE: unknown device info \"%s\"\n",
+			       key);
+	}
+
+	if (!devinfo->name || !strlen(devinfo->name)) {
+		printk(KERN_ERR "CUSE: DEVNAME unspecified\n");
+		return -EINVAL;
+	}
+
+	return 0;
+}
+
+static void cuse_gendev_release(struct device *dev)
+{
+	kfree(dev);
+}
+
+static void cuse_cdev_release(struct cdev *cdev)
+{
+	cuse_conn_put(cdev_to_cc(cdev));
+}
+
+/**
+ * cuse_init_worker - worker to finish initializing CUSE channel
+ * @data: cuse_conn to initialize
+ *
+ * CUSE channel is created by opening /dev/cuse but the open itself
+ * can't do much of initialization as it must return to the userland
+ * so that CUSE server and kernel can talk via the open file, so
+ * cuse_channel_open() performs minimal initialization and schedules
+ * this function on a kthread to finish up initialization.
+ *
+ * This function queries userland CUSE server which character device
+ * it wants to create and creates the character device and sets up all
+ * the required data structures for it.  As there are quite a few data
+ * structures involved, object lifetime rule is a bit complex.  Please
+ * read the comment at the top of this file for high level overview.
+ *
+ * RETURNS:
+ * 0 on success, -errno on failure.
+ */
+static int cuse_init_worker(void *data)
+{
+	struct cuse_conn *cc = data;
+	struct cuse_init_in iin = { };
+	struct cuse_init_out iout = { };
+	struct cuse_devinfo devinfo = { };
+	struct fuse_req *req;
+	struct page *page = NULL;
+	struct device *dev;
+	bool disconnected;
+	dev_t devt;
+	int rc;
+
+	BUILD_BUG_ON(CUSE_INIT_INFO_MAX > PAGE_SIZE);
+
+	/* identify ourself and query what the CUSE server wants */
+	req = fuse_get_req(&cc->fc);
+	if (IS_ERR(req)) {
+		rc = PTR_ERR(req);
+		goto out;
+	}
+
+	rc = -ENOMEM;
+	page = alloc_pages(GFP_KERNEL | __GFP_ZERO, 1);
+	if (!page)
+		goto out;
+
+	req->pages[0] = nth_page(page, 0);
+	req->pages[1] = nth_page(page, 1);
+	req->num_pages = 2;
+
+	req->in.h.opcode = CUSE_INIT;
+	req->in.h.nodeid = get_node_id(cc->mnt->mnt_sb->s_root->d_inode);
+	req->in.numargs = 1;
+	req->in.args[0].size = sizeof(iin);
+	req->in.args[0].value = &iin;
+
+	iin.ver_major = CUSE_KERNEL_VERSION;
+	iin.ver_minor = CUSE_KERNEL_MINOR_VERSION;
+
+	req->out.numargs = 2;
+	req->out.args[0].size = sizeof(iout);
+	req->out.args[0].value = &iout;
+	req->out.args[1].size = CUSE_INIT_INFO_MAX;
+	req->out.argpages = 1;
+	req->out.argvar = 1;
+
+	fuse_request_send(&cc->fc, req);
+	rc = req->out.h.error;
+	if (rc)
+		goto out;
+
+	/* parse init reply */
+	cc->unrestricted_ioctl = iout.flags & CUSE_UNRESTRICTED_IOCTL;
+
+	rc = cuse_parse_devinfo(page_address(page), req->out.args[1].size,
+				&devinfo);
+	if (rc)
+		goto out;
+
+	/* MAJ, MIN and name available, let's create the device */
+	devt = MKDEV(iout.dev_major, iout.dev_minor);
+	if (!MAJOR(devt))
+		rc = alloc_chrdev_region(&devt, MINOR(devt), 1, devinfo.name);
+	else
+		rc = register_chrdev_region(devt, 1, devinfo.name);
+	if (rc) {
+		printk(KERN_ERR "CUSE: failed to register chrdev region\n");
+		goto out;
+	}
+
+	rc = -ENOMEM;
+	dev = kzalloc(sizeof(*dev), GFP_KERNEL);
+	if (!dev)
+		goto out_unregister_chrdev_region;
+	device_initialize(dev);
+	dev->class = cuse_class;
+	dev->devt = devt;
+	dev->release = cuse_gendev_release;
+	dev_set_drvdata(dev, cc);
+	dev_set_name(dev, "%s", devinfo.name);
+
+	rc = device_add(dev);
+	if (rc)
+		goto out_put_device;
+
+	/* register cdev */
+	cdev_init(&cc->cdev, &cuse_frontend_fops);
+	cc->cdev.owner = THIS_MODULE;
+	cc->cdev.release = cuse_cdev_release;
+
+	rc = cdev_add(&cc->cdev, devt, 1);
+	if (rc)
+		goto out_put_device;
+	cuse_conn_get(cc);	/* will be released on cdev final put */
+
+	/* transfer dev and cdev ownership to channel */
+	spin_lock(&cuse_disconnect_lock);
+	disconnected = cc->disconnected;
+	if (!disconnected) {
+		cc->dev = dev;
+		cc->cdev_added = true;
+	}
+	spin_unlock(&cuse_disconnect_lock);
+
+	if (disconnected)
+		goto out_cdev_del;
+
+	rc = 0;
+	goto out;
+
+out_cdev_del:
+	cdev_del(&cc->cdev);
+out_put_device:
+	put_device(dev);
+out_unregister_chrdev_region:
+	unregister_chrdev_region(devt, 1);
+out:
+	if (!IS_ERR(req))
+		fuse_put_request(&cc->fc, req);
+	if (page)
+		__free_pages(page, 1);
+
+	if (rc)
+		fuse_abort_conn(&cc->fc);
+
+	cuse_conn_put(cc);
+	return rc;
+}
+
+/**
+ * cuse_channel_open - open method for /dev/cuse
+ * @inode: inode for /dev/cuse
+ * @file: file struct being opened
+ *
+ * Userland CUSE server can create a CUSE device by opening /dev/cuse
+ * and replying to initilaization request the kernel sends.  This
+ * function is responsible for handling CUSE device initialization.
+ * Because the fd opened by this function is used during
+ * initialization, only mnt and cuse_conn are created here and the
+ * reset of initialization is delegated to a kthread.
+ *
+ * RETURNS:
+ * 0 on success, -errno on failure.
+ */
+static int cuse_channel_open(struct inode *inode, struct file *file)
+{
+	struct cuse_conn *cc;
+	struct vfsmount *mnt;
+	struct fuse_req *init_req;
+	struct task_struct *worker;
+	int rc;
+
+	/* Set up cuse_conn.  cuse_conn will be created when filling
+	 * in superblock for the following kern_mount().
+	 */
+	mnt = kern_mount(&cuse_fs);
+	if (IS_ERR(mnt))
+		return PTR_ERR(mnt);
+
+	cc = fc_to_cc(get_fuse_conn_super(mnt->mnt_sb));
+	cc->mnt = mnt;
+
+	/* let's send fuse init request */
+	rc = -ENOMEM;
+	init_req = fuse_request_alloc();
+	if (!init_req)
+		goto err_cc_put;
+
+	cc->fc.connected = 1;
+	file->private_data = fuse_conn_get(&cc->fc);
+	fuse_send_init(&cc->fc, init_req);
+
+	/* Okay, FUSE part of initialization is complete.  The rest of
+	 * the initialization is a bit more involved and requires
+	 * conversing with userland.  Start a kthread.
+	 */
+	worker = kthread_run(cuse_init_worker, cuse_conn_get(cc),
+			     "cuse-init-pid%d", current->pid);
+	if (IS_ERR(worker)) {
+		fput(file);
+		rc = PTR_ERR(worker);
+		goto err_cc_put;
+	}
+
+	return 0;
+
+err_cc_put:
+	cuse_conn_put(cc);
+	return rc;
+}
+
+/**
+ * cuse_channel_release - release method for /dev/cuse
+ * @inode: inode for /dev/cuse
+ * @file: file struct being closed
+ *
+ * Disconnect the channel, deregister CUSE device and initiate
+ * destruction by putting the default reference.
+ *
+ * RETURNS:
+ * 0 on success, -errno on failure.
+ */
+static int cuse_channel_release(struct inode *inode, struct file *file)
+{
+	struct cuse_conn *cc = fc_to_cc(file->private_data);
+	int rc;
+
+	spin_lock(&cuse_disconnect_lock);
+	cc->disconnected = true;
+	spin_unlock(&cuse_disconnect_lock);
+
+	rc = fuse_dev_release(inode, file);
+	if (rc)
+		return rc;
+
+	if (cc->dev)
+		device_unregister(cc->dev);
+	if (cc->cdev_added) {
+		unregister_chrdev_region(cc->cdev.dev, 1);
+		cdev_del(&cc->cdev);
+	}
+	cuse_conn_put(cc);
+
+	return 0;
+}
+
+static struct file_operations cuse_channel_fops; /* initialized during init */
+
+
+/**************************************************************************
+ * Misc stuff and module initializatiion
+ *
+ * CUSE exports the same set of attributes to sysfs as fusectl.
+ */
+
+ssize_t cuse_class_waiting_show(struct device *dev,
+				struct device_attribute *attr, char *buf)
+{
+	struct cuse_conn *cc = dev_get_drvdata(dev);
+
+	return sprintf(buf, "%d\n", atomic_read(&cc->fc.num_waiting));
+}
+
+ssize_t cuse_class_abort_store(struct device *dev,
+			       struct device_attribute *attr,
+			       const char *buf, size_t count)
+{
+	struct cuse_conn *cc = dev_get_drvdata(dev);
+
+	fuse_abort_conn(&cc->fc);
+	return count;
+}
+
+static struct device_attribute cuse_class_dev_attrs[] = {
+	__ATTR(waiting, S_IFREG | 0400, cuse_class_waiting_show, NULL),
+	__ATTR(abort, S_IFREG | 0200, NULL, cuse_class_abort_store),
+	{ }
+};
+
+static struct miscdevice cuse_miscdev = {
+	.minor		= MISC_DYNAMIC_MINOR,
+	.name		= "cuse",
+	.fops		= &cuse_channel_fops,
+};
+
+static int __init cuse_init(void)
+{
+	int rc;
+
+	/* inherit and extend fuse_dev_operations */
+	cuse_channel_fops		= fuse_dev_operations;
+	cuse_channel_fops.owner		= THIS_MODULE;
+	cuse_channel_fops.open		= cuse_channel_open;
+	cuse_channel_fops.release	= cuse_channel_release;
+
+	cuse_class = class_create(THIS_MODULE, "cuse");
+	if (IS_ERR(cuse_class))
+		return PTR_ERR(cuse_class);
+	cuse_class->dev_attrs = cuse_class_dev_attrs;
+
+	rc = misc_register(&cuse_miscdev);
+	if (rc)
+		goto destroy_class;
+	rc = register_filesystem(&cuse_fs);
+	if (rc)
+		goto misc_deregister;
+	return 0;
+
+misc_deregister:
+	misc_deregister(&cuse_miscdev);
+destroy_class:
+	class_destroy(cuse_class);
+	return rc;
+}
+
+static void __exit cuse_exit(void)
+{
+	unregister_filesystem(&cuse_fs);
+	misc_deregister(&cuse_miscdev);
+	class_destroy(cuse_class);
+}
+
+module_init(cuse_init);
+module_exit(cuse_exit);
+
+MODULE_AUTHOR("Tejun Heo <tj@xxxxxxxxxx>");
+MODULE_DESCRIPTION("Character device in Userspace");
+MODULE_LICENSE("GPL");
Index: work/include/linux/cuse.h
===================================================================
--- /dev/null
+++ work/include/linux/cuse.h
@@ -0,0 +1,47 @@
+/*
+ * CUSE: Character device in Userspace
+ * Copyright (C) 2008       SUSE Linux Products GmbH
+ * Copyright (C) 2008       Tejun Heo <teheo@xxxxxxx>
+ *
+ * This file is released under the GPL.
+ */
+
+#ifndef _CUSE_H_
+#define _CUSE_H_
+
+#include <linux/major.h>
+#include <linux/miscdevice.h>
+#include <linux/fuse.h>
+
+#define CUSE_KERNEL_VERSION		0
+#define CUSE_KERNEL_MINOR_VERSION	1
+
+#define CUSE_KERNEL_MAJOR		MISC_MAJOR
+#define CUSE_KERNEL_MINOR		MISC_DYNAMIC_MINOR
+
+#define CUSE_INIT_INFO_MAX		4096
+
+/*
+ * CUSE INIT request/reply flags
+ */
+#define CUSE_UNRESTRICTED_IOCTL		(1 << 0) /* use unrestricted ioctl */
+
+enum cuse_opcode {
+	CUSE_INIT	= CUSE_BASE,
+};
+
+struct cuse_init_in {
+	__u32	ver_major;
+	__u32	ver_minor;
+	__u32	flags;
+	__u32	padding;
+};
+
+struct cuse_init_out {
+	__u32	dev_major;		/* chardev major */
+	__u32	dev_minor;		/* chardev minor */
+	__u32	flags;
+	__u32	padding;
+};
+
+#endif /*_CUSE_H_*/
Index: work/include/linux/fuse.h
===================================================================
--- work.orig/include/linux/fuse.h
+++ work/include/linux/fuse.h
@@ -209,6 +209,8 @@ enum fuse_opcode {
 	FUSE_DESTROY       = 38,
 	FUSE_IOCTL         = 39,
 	FUSE_POLL          = 40,
+
+	CUSE_BASE          = 4096,
 };
 
 enum fuse_notify_code {
Index: work/fs/Kconfig
===================================================================
--- work.orig/fs/Kconfig
+++ work/fs/Kconfig
@@ -419,6 +419,16 @@ config FUSE_FS
 	  If you want to develop a userspace FS, or if you want to use
 	  a filesystem based on FUSE, answer Y or M.
 
+config CUSE
+	tristate "Character device in Userpace support"
+	depends on FUSE_FS
+	help
+	  This FUSE extension allows character devices to be
+	  implemented in userspace.
+
+	  If you want to develop or use userspace character device
+	  based on CUSE, answer Y or M.
+
 config GENERIC_ACL
 	bool
 	select FS_POSIX_ACL
Index: work/fs/fuse/Makefile
===================================================================
--- work.orig/fs/fuse/Makefile
+++ work/fs/fuse/Makefile
@@ -3,5 +3,6 @@
 #
 
 obj-$(CONFIG_FUSE_FS) += fuse.o
+obj-$(CONFIG_CUSE) += cuse.o
 
 fuse-objs := dev.o dir.o file.o inode.o control.o
Index: work/include/linux/magic.h
===================================================================
--- work.orig/include/linux/magic.h
+++ work/include/linux/magic.h
@@ -3,10 +3,11 @@
 
 #define ADFS_SUPER_MAGIC	0xadf5
 #define AFFS_SUPER_MAGIC	0xadff
-#define AFS_SUPER_MAGIC                0x5346414F
+#define AFS_SUPER_MAGIC		0x5346414F
 #define AUTOFS_SUPER_MAGIC	0x0187
 #define CODA_SUPER_MAGIC	0x73757245
-#define DEBUGFS_MAGIC          0x64626720
+#define CUSE_SUPER_MAGIC	0x43555345
+#define DEBUGFS_MAGIC		0x64626720
 #define SYSFS_MAGIC		0x62656572
 #define SECURITYFS_MAGIC	0x73636673
 #define TMPFS_MAGIC		0x01021994
--
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