[PATCH] FUSE - device functions

This adds the FUSE device handling functions.

This contains the following files:

 o dev.c
    - fuse device operations (read, write, release, poll)
    - registers misc device
    - support for sending requests to userspace

Signed-off-by: Miklos Szeredi <miklos@szeredi.hu>
Signed-off-by: Adrian Bunk <bunk@stusta.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

Documentation/filesystems/fuse.txt

@@ -0,0 +1,341 @@
+Definitions
+~~~~~~~~~~~
+
+Userspace filesystem:
+
+  A filesystem in which data and metadata are provided by an ordinary
+  userspace process.  The filesystem can be accessed normally through
+  the kernel interface.
+
+Filesystem daemon:
+
+  The process(es) providing the data and metadata of the filesystem.
+
+Non-privileged mount (or user mount):
+
+  A userspace filesystem mounted by a non-privileged (non-root) user.
+  The filesystem daemon is running with the privileges of the mounting
+  user.  NOTE: this is not the same as mounts allowed with the "user"
+  option in /etc/fstab, which is not discussed here.
+
+Mount owner:
+
+  The user who does the mounting.
+
+User:
+
+  The user who is performing filesystem operations.
+
+What is FUSE?
+~~~~~~~~~~~~~
+
+FUSE is a userspace filesystem framework.  It consists of a kernel
+module (fuse.ko), a userspace library (libfuse.*) and a mount utility
+(fusermount).
+
+One of the most important features of FUSE is allowing secure,
+non-privileged mounts.  This opens up new possibilities for the use of
+filesystems.  A good example is sshfs: a secure network filesystem
+using the sftp protocol.
+
+The userspace library and utilities are available from the FUSE
+homepage:
+
+  http://fuse.sourceforge.net/
+
+Mount options
+~~~~~~~~~~~~~
+
+'fd=N'
+
+  The file descriptor to use for communication between the userspace
+  filesystem and the kernel.  The file descriptor must have been
+  obtained by opening the FUSE device ('/dev/fuse').
+
+'rootmode=M'
+
+  The file mode of the filesystem's root in octal representation.
+
+'user_id=N'
+
+  The numeric user id of the mount owner.
+
+'group_id=N'
+
+  The numeric group id of the mount owner.
+
+'default_permissions'
+
+  By default FUSE doesn't check file access permissions, the
+  filesystem is free to implement it's access policy or leave it to
+  the underlying file access mechanism (e.g. in case of network
+  filesystems).  This option enables permission checking, restricting
+  access based on file mode.  This is option is usually useful
+  together with the 'allow_other' mount option.
+
+'allow_other'
+
+  This option overrides the security measure restricting file access
+  to the user mounting the filesystem.  This option is by default only
+  allowed to root, but this restriction can be removed with a
+  (userspace) configuration option.
+
+'kernel_cache'
+
+  This option disables flushing the cache of the file contents on
+  every open().  This should only be enabled on filesystems, where the
+  file data is never changed externally (not through the mounted FUSE
+  filesystem).  Thus it is not suitable for network filesystems and
+  other "intermediate" filesystems.
+
+  NOTE: if this option is not specified (and neither 'direct_io') data
+  is still cached after the open(), so a read() system call will not
+  always initiate a read operation.
+
+'direct_io'
+
+  This option disables the use of page cache (file content cache) in
+  the kernel for this filesystem.  This has several affects:
+
+     - Each read() or write() system call will initiate one or more
+       read or write operations, data will not be cached in the
+       kernel.
+
+     - The return value of the read() and write() system calls will
+       correspond to the return values of the read and write
+       operations.  This is useful for example if the file size is not
+       known in advance (before reading it).
+
+'max_read=N'
+
+  With this option the maximum size of read operations can be set.
+  The default is infinite.  Note that the size of read requests is
+  limited anyway to 32 pages (which is 128kbyte on i386).
+
+How do non-privileged mounts work?
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Since the mount() system call is a privileged operation, a helper
+program (fusermount) is needed, which is installed setuid root.
+
+The implication of providing non-privileged mounts is that the mount
+owner must not be able to use this capability to compromise the
+system.  Obvious requirements arising from this are:
+
+ A) mount owner should not be able to get elevated privileges with the
+    help of the mounted filesystem
+
+ B) mount owner should not get illegitimate access to information from
+    other users' and the super user's processes
+
+ C) mount owner should not be able to induce undesired behavior in
+    other users' or the super user's processes
+
+How are requirements fulfilled?
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+ A) The mount owner could gain elevated privileges by either:
+
+     1) creating a filesystem containing a device file, then opening
+	this device
+
+     2) creating a filesystem containing a suid or sgid application,
+	then executing this application
+
+    The solution is not to allow opening device files and ignore
+    setuid and setgid bits when executing programs.  To ensure this
+    fusermount always adds "nosuid" and "nodev" to the mount options
+    for non-privileged mounts.
+
+ B) If another user is accessing files or directories in the
+    filesystem, the filesystem daemon serving requests can record the
+    exact sequence and timing of operations performed.  This
+    information is otherwise inaccessible to the mount owner, so this
+    counts as an information leak.
+
+    The solution to this problem will be presented in point 2) of C).
+
+ C) There are several ways in which the mount owner can induce
+    undesired behavior in other users' processes, such as:
+
+     1) mounting a filesystem over a file or directory which the mount
+        owner could otherwise not be able to modify (or could only
+        make limited modifications).
+
+        This is solved in fusermount, by checking the access
+        permissions on the mountpoint and only allowing the mount if
+        the mount owner can do unlimited modification (has write
+        access to the mountpoint, and mountpoint is not a "sticky"
+        directory)
+
+     2) Even if 1) is solved the mount owner can change the behavior
+        of other users' processes.
+
+         i) It can slow down or indefinitely delay the execution of a
+           filesystem operation creating a DoS against the user or the
+           whole system.  For example a suid application locking a
+           system file, and then accessing a file on the mount owner's
+           filesystem could be stopped, and thus causing the system
+           file to be locked forever.
+
+         ii) It can present files or directories of unlimited length, or
+           directory structures of unlimited depth, possibly causing a
+           system process to eat up diskspace, memory or other
+           resources, again causing DoS.
+
+	The solution to this as well as B) is not to allow processes
+	to access the filesystem, which could otherwise not be
+	monitored or manipulated by the mount owner.  Since if the
+	mount owner can ptrace a process, it can do all of the above
+	without using a FUSE mount, the same criteria as used in
+	ptrace can be used to check if a process is allowed to access
+	the filesystem or not.
+
+	Note that the ptrace check is not strictly necessary to
+	prevent B/2/i, it is enough to check if mount owner has enough
+	privilege to send signal to the process accessing the
+	filesystem, since SIGSTOP can be used to get a similar effect.
+
+I think these limitations are unacceptable?
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+If a sysadmin trusts the users enough, or can ensure through other
+measures, that system processes will never enter non-privileged
+mounts, it can relax the last limitation with a "user_allow_other"
+config option.  If this config option is set, the mounting user can
+add the "allow_other" mount option which disables the check for other
+users' processes.
+
+Kernel - userspace interface
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+The following diagram shows how a filesystem operation (in this
+example unlink) is performed in FUSE.
+
+NOTE: everything in this description is greatly simplified
+
+ |  "rm /mnt/fuse/file"               |  FUSE filesystem daemon
+ |                                    |
+ |                                    |  >sys_read()
+ |                                    |    >fuse_dev_read()
+ |                                    |      >request_wait()
+ |                                    |        [sleep on fc->waitq]
+ |                                    |
+ |  >sys_unlink()                     |
+ |    >fuse_unlink()                  |
+ |      [get request from             |
+ |       fc->unused_list]             |
+ |      >request_send()               |
+ |        [queue req on fc->pending]  |
+ |        [wake up fc->waitq]         |        [woken up]
+ |        >request_wait_answer()      |
+ |          [sleep on req->waitq]     |
+ |                                    |      <request_wait()
+ |                                    |      [remove req from fc->pending]
+ |                                    |      [copy req to read buffer]
+ |                                    |      [add req to fc->processing]
+ |                                    |    <fuse_dev_read()
+ |                                    |  <sys_read()
+ |                                    |
+ |                                    |  [perform unlink]
+ |                                    |
+ |                                    |  >sys_write()
+ |                                    |    >fuse_dev_write()
+ |                                    |      [look up req in fc->processing]
+ |                                    |      [remove from fc->processing]
+ |                                    |      [copy write buffer to req]
+ |          [woken up]                |      [wake up req->waitq]
+ |                                    |    <fuse_dev_write()
+ |                                    |  <sys_write()
+ |        <request_wait_answer()      |
+ |      <request_send()               |
+ |      [add request to               |
+ |       fc->unused_list]             |
+ |    <fuse_unlink()                  |
+ |  <sys_unlink()                     |
+
+There are a couple of ways in which to deadlock a FUSE filesystem.
+Since we are talking about unprivileged userspace programs,
+something must be done about these.
+
+Scenario 1 -  Simple deadlock
+-----------------------------
+
+ |  "rm /mnt/fuse/file"               |  FUSE filesystem daemon
+ |                                    |
+ |  >sys_unlink("/mnt/fuse/file")     |
+ |    [acquire inode semaphore        |
+ |     for "file"]                    |
+ |    >fuse_unlink()                  |
+ |      [sleep on req->waitq]         |
+ |                                    |  <sys_read()
+ |                                    |  >sys_unlink("/mnt/fuse/file")
+ |                                    |    [acquire inode semaphore
+ |                                    |     for "file"]
+ |                                    |    *DEADLOCK*
+
+The solution for this is to allow requests to be interrupted while
+they are in userspace:
+
+ |      [interrupted by signal]       |
+ |    <fuse_unlink()                  |
+ |    [release semaphore]             |    [semaphore acquired]
+ |  <sys_unlink()                     |
+ |                                    |    >fuse_unlink()
+ |                                    |      [queue req on fc->pending]
+ |                                    |      [wake up fc->waitq]
+ |                                    |      [sleep on req->waitq]
+
+If the filesystem daemon was single threaded, this will stop here,
+since there's no other thread to dequeue and execute the request.
+In this case the solution is to kill the FUSE daemon as well.  If
+there are multiple serving threads, you just have to kill them as
+long as any remain.
+
+Moral: a filesystem which deadlocks, can soon find itself dead.
+
+Scenario 2 - Tricky deadlock
+----------------------------
+
+This one needs a carefully crafted filesystem.  It's a variation on
+the above, only the call back to the filesystem is not explicit,
+but is caused by a pagefault.
+
+ |  Kamikaze filesystem thread 1      |  Kamikaze filesystem thread 2
+ |                                    |
+ |  [fd = open("/mnt/fuse/file")]     |  [request served normally]
+ |  [mmap fd to 'addr']               |
+ |  [close fd]                        |  [FLUSH triggers 'magic' flag]
+ |  [read a byte from addr]           |
+ |    >do_page_fault()                |
+ |      [find or create page]         |
+ |      [lock page]                   |
+ |      >fuse_readpage()              |
+ |         [queue READ request]       |
+ |         [sleep on req->waitq]      |
+ |                                    |  [read request to buffer]
+ |                                    |  [create reply header before addr]
+ |                                    |  >sys_write(addr - headerlength)
+ |                                    |    >fuse_dev_write()
+ |                                    |      [look up req in fc->processing]
+ |                                    |      [remove from fc->processing]
+ |                                    |      [copy write buffer to req]
+ |                                    |        >do_page_fault()
+ |                                    |           [find or create page]
+ |                                    |           [lock page]
+ |                                    |           * DEADLOCK *
+
+Solution is again to let the the request be interrupted (not
+elaborated further).
+
+An additional problem is that while the write buffer is being
+copied to the request, the request must not be interrupted.  This
+is because the destination address of the copy may not be valid
+after the request is interrupted.
+
+This is solved with doing the copy atomically, and allowing
+interruption while the page(s) belonging to the write buffer are
+faulted with get_user_pages().  The 'req->locked' flag indicates
+when the copy is taking place, and interruption is delayed until
+this flag is unset.
+

fs/fuse/Makefile

@@ -4,4 +4,4 @@
 
 obj-$(CONFIG_FUSE_FS) += fuse.o
 
-fuse-objs := inode.o
+fuse-objs := dev.o inode.o

fs/fuse/dev.c

@@ -0,0 +1,884 @@
+/*
+  FUSE: Filesystem in Userspace
+  Copyright (C) 2001-2005  Miklos Szeredi <miklos@szeredi.hu>
+
+  This program can be distributed under the terms of the GNU GPL.
+  See the file COPYING.
+*/
+
+#include "fuse_i.h"
+
+#include <linux/init.h>
+#include <linux/module.h>
+#include <linux/poll.h>
+#include <linux/uio.h>
+#include <linux/miscdevice.h>
+#include <linux/pagemap.h>
+#include <linux/file.h>
+#include <linux/slab.h>
+
+MODULE_ALIAS_MISCDEV(FUSE_MINOR);
+
+static kmem_cache_t *fuse_req_cachep;
+
+static inline struct fuse_conn *fuse_get_conn(struct file *file)
+{
+	struct fuse_conn *fc;
+	spin_lock(&fuse_lock);
+	fc = file->private_data;
+	if (fc && !fc->sb)
+		fc = NULL;
+	spin_unlock(&fuse_lock);
+	return fc;
+}
+
+static inline void fuse_request_init(struct fuse_req *req)
+{
+	memset(req, 0, sizeof(*req));
+	INIT_LIST_HEAD(&req->list);
+	init_waitqueue_head(&req->waitq);
+	atomic_set(&req->count, 1);
+}
+
+struct fuse_req *fuse_request_alloc(void)
+{
+	struct fuse_req *req = kmem_cache_alloc(fuse_req_cachep, SLAB_KERNEL);
+	if (req)
+		fuse_request_init(req);
+	return req;
+}
+
+void fuse_request_free(struct fuse_req *req)
+{
+	kmem_cache_free(fuse_req_cachep, req);
+}
+
+static inline void block_sigs(sigset_t *oldset)
+{
+	sigset_t mask;
+
+	siginitsetinv(&mask, sigmask(SIGKILL));
+	sigprocmask(SIG_BLOCK, &mask, oldset);
+}
+
+static inline void restore_sigs(sigset_t *oldset)
+{
+	sigprocmask(SIG_SETMASK, oldset, NULL);
+}
+
+void fuse_reset_request(struct fuse_req *req)
+{
+	int preallocated = req->preallocated;
+	BUG_ON(atomic_read(&req->count) != 1);
+	fuse_request_init(req);
+	req->preallocated = preallocated;
+}
+
+static void __fuse_get_request(struct fuse_req *req)
+{
+	atomic_inc(&req->count);
+}
+
+/* Must be called with > 1 refcount */
+static void __fuse_put_request(struct fuse_req *req)
+{
+	BUG_ON(atomic_read(&req->count) < 2);
+	atomic_dec(&req->count);
+}
+
+static struct fuse_req *do_get_request(struct fuse_conn *fc)
+{
+	struct fuse_req *req;
+
+	spin_lock(&fuse_lock);
+	BUG_ON(list_empty(&fc->unused_list));
+	req = list_entry(fc->unused_list.next, struct fuse_req, list);
+	list_del_init(&req->list);
+	spin_unlock(&fuse_lock);
+	fuse_request_init(req);
+	req->preallocated = 1;
+	req->in.h.uid = current->fsuid;
+	req->in.h.gid = current->fsgid;
+	req->in.h.pid = current->pid;
+	return req;
+}
+
+struct fuse_req *fuse_get_request(struct fuse_conn *fc)
+{
+	if (down_interruptible(&fc->outstanding_sem))
+		return NULL;
+	return do_get_request(fc);
+}
+
+/*
+ * Non-interruptible version of the above function is for operations
+ * which can't legally return -ERESTART{SYS,NOINTR}.  This can still
+ * return NULL, but only in case the signal is SIGKILL.
+ */
+struct fuse_req *fuse_get_request_nonint(struct fuse_conn *fc)
+{
+	int intr;
+	sigset_t oldset;
+
+	block_sigs(&oldset);
+	intr = down_interruptible(&fc->outstanding_sem);
+	restore_sigs(&oldset);
+	return intr ? NULL : do_get_request(fc);
+}
+
+static void fuse_putback_request(struct fuse_conn *fc, struct fuse_req *req)
+{
+	spin_lock(&fuse_lock);
+	if (req->preallocated)
+		list_add(&req->list, &fc->unused_list);
+	else
+		fuse_request_free(req);
+
+	/* If we are in debt decrease that first */
+	if (fc->outstanding_debt)
+		fc->outstanding_debt--;
+	else
+		up(&fc->outstanding_sem);
+	spin_unlock(&fuse_lock);
+}
+
+void fuse_put_request(struct fuse_conn *fc, struct fuse_req *req)
+{
+	if (atomic_dec_and_test(&req->count))
+		fuse_putback_request(fc, req);
+}
+
+/*
+ * This function is called when a request is finished.  Either a reply
+ * has arrived or it was interrupted (and not yet sent) or some error
+ * occured during communication with userspace, or the device file was
+ * closed.  It decreases the referece count for the request.  In case
+ * of a background request the referece to the stored objects are
+ * released.  The requester thread is woken up (if still waiting), and
+ * finally the request is either freed or put on the unused_list
+ *
+ * Called with fuse_lock, unlocks it
+ */
+static void request_end(struct fuse_conn *fc, struct fuse_req *req)
+{
+	int putback;
+	req->finished = 1;
+	putback = atomic_dec_and_test(&req->count);
+	spin_unlock(&fuse_lock);
+	if (req->background) {
+		if (req->inode)
+			iput(req->inode);
+		if (req->inode2)
+			iput(req->inode2);
+		if (req->file)
+			fput(req->file);
+	}
+	wake_up(&req->waitq);
+	if (req->in.h.opcode == FUSE_INIT) {
+		int i;
+
+		if (req->misc.init_in_out.major != FUSE_KERNEL_VERSION)
+			fc->conn_error = 1;
+
+		/* After INIT reply is received other requests can go
+		   out.  So do (FUSE_MAX_OUTSTANDING - 1) number of
+		   up()s on outstanding_sem.  The last up() is done in
+		   fuse_putback_request() */
+		for (i = 1; i < FUSE_MAX_OUTSTANDING; i++)
+			up(&fc->outstanding_sem);
+	}
+	if (putback)
+		fuse_putback_request(fc, req);
+}
+
+static void background_request(struct fuse_req *req)
+{
+	/* Need to get hold of the inode(s) and/or file used in the
+	   request, so FORGET and RELEASE are not sent too early */
+	req->background = 1;
+	if (req->inode)
+		req->inode = igrab(req->inode);
+	if (req->inode2)
+		req->inode2 = igrab(req->inode2);
+	if (req->file)
+		get_file(req->file);
+}
+
+static int request_wait_answer_nonint(struct fuse_req *req)
+{
+	int err;
+	sigset_t oldset;
+	block_sigs(&oldset);
+	err = wait_event_interruptible(req->waitq, req->finished);
+	restore_sigs(&oldset);
+	return err;
+}
+
+/* Called with fuse_lock held.  Releases, and then reacquires it. */
+static void request_wait_answer(struct fuse_req *req, int interruptible)
+{
+	int intr;
+
+	spin_unlock(&fuse_lock);
+	if (interruptible)
+		intr = wait_event_interruptible(req->waitq, req->finished);
+	else
+		intr = request_wait_answer_nonint(req);
+	spin_lock(&fuse_lock);
+	if (intr && interruptible && req->sent) {
+		/* If request is already in userspace, only allow KILL
+		   signal to interrupt */
+		spin_unlock(&fuse_lock);
+		intr = request_wait_answer_nonint(req);
+		spin_lock(&fuse_lock);
+	}
+	if (!intr)
+		return;
+
+	if (!interruptible || req->sent)
+		req->out.h.error = -EINTR;
+	else
+		req->out.h.error = -ERESTARTNOINTR;
+
+	req->interrupted = 1;
+	if (req->locked) {
+		/* This is uninterruptible sleep, because data is
+		   being copied to/from the buffers of req.  During
+		   locked state, there mustn't be any filesystem
+		   operation (e.g. page fault), since that could lead
+		   to deadlock */
+		spin_unlock(&fuse_lock);
+		wait_event(req->waitq, !req->locked);
+		spin_lock(&fuse_lock);
+	}
+	if (!req->sent && !list_empty(&req->list)) {
+		list_del(&req->list);
+		__fuse_put_request(req);
+	} else if (!req->finished && req->sent)
+		background_request(req);
+}
+
+static unsigned len_args(unsigned numargs, struct fuse_arg *args)
+{
+	unsigned nbytes = 0;
+	unsigned i;
+
+	for (i = 0; i < numargs; i++)
+		nbytes += args[i].size;
+
+	return nbytes;
+}
+
+static void queue_request(struct fuse_conn *fc, struct fuse_req *req)
+{
+	fc->reqctr++;
+	/* zero is special */
+	if (fc->reqctr == 0)
+		fc->reqctr = 1;
+	req->in.h.unique = fc->reqctr;
+	req->in.h.len = sizeof(struct fuse_in_header) +
+		len_args(req->in.numargs, (struct fuse_arg *) req->in.args);
+	if (!req->preallocated) {
+		/* If request is not preallocated (either FORGET or
+		   RELEASE), then still decrease outstanding_sem, so
+		   user can't open infinite number of files while not
+		   processing the RELEASE requests.  However for
+		   efficiency do it without blocking, so if down()
+		   would block, just increase the debt instead */
+		if (down_trylock(&fc->outstanding_sem))
+			fc->outstanding_debt++;
+	}
+	list_add_tail(&req->list, &fc->pending);
+	wake_up(&fc->waitq);
+}
+
+static void request_send_wait(struct fuse_conn *fc, struct fuse_req *req,
+			      int interruptible)
+{
+	req->isreply = 1;
+	spin_lock(&fuse_lock);
+	if (!fc->file)
+		req->out.h.error = -ENOTCONN;
+	else if (fc->conn_error)
+		req->out.h.error = -ECONNREFUSED;
+	else {
+		queue_request(fc, req);
+		/* acquire extra reference, since request is still needed
+		   after request_end() */
+		__fuse_get_request(req);
+
+		request_wait_answer(req, interruptible);
+	}
+	spin_unlock(&fuse_lock);
+}
+
+void request_send(struct fuse_conn *fc, struct fuse_req *req)
+{
+	request_send_wait(fc, req, 1);
+}
+
+/*
+ * Non-interruptible version of the above function is for operations
+ * which can't legally return -ERESTART{SYS,NOINTR}.  This can still
+ * be interrupted but only with SIGKILL.
+ */
+void request_send_nonint(struct fuse_conn *fc, struct fuse_req *req)
+{
+	request_send_wait(fc, req, 0);
+}
+
+static void request_send_nowait(struct fuse_conn *fc, struct fuse_req *req)
+{
+	spin_lock(&fuse_lock);
+	if (fc->file) {
+		queue_request(fc, req);
+		spin_unlock(&fuse_lock);
+	} else {
+		req->out.h.error = -ENOTCONN;
+		request_end(fc, req);
+	}
+}
+
+void request_send_noreply(struct fuse_conn *fc, struct fuse_req *req)
+{
+	req->isreply = 0;
+	request_send_nowait(fc, req);
+}
+
+void request_send_background(struct fuse_conn *fc, struct fuse_req *req)
+{
+	req->isreply = 1;
+	background_request(req);
+	request_send_nowait(fc, req);
+}
+
+void fuse_send_init(struct fuse_conn *fc)
+{
+	/* This is called from fuse_read_super() so there's guaranteed
+	   to be a request available */
+	struct fuse_req *req = do_get_request(fc);
+	struct fuse_init_in_out *arg = &req->misc.init_in_out;
+	arg->major = FUSE_KERNEL_VERSION;
+	arg->minor = FUSE_KERNEL_MINOR_VERSION;
+	req->in.h.opcode = FUSE_INIT;
+	req->in.numargs = 1;
+	req->in.args[0].size = sizeof(*arg);
+	req->in.args[0].value = arg;
+	req->out.numargs = 1;
+	req->out.args[0].size = sizeof(*arg);
+	req->out.args[0].value = arg;
+	request_send_background(fc, req);
+}
+
+/*
+ * Lock the request.  Up to the next unlock_request() there mustn't be
+ * anything that could cause a page-fault.  If the request was already
+ * interrupted bail out.
+ */
+static inline int lock_request(struct fuse_req *req)
+{
+	int err = 0;
+	if (req) {
+		spin_lock(&fuse_lock);
+		if (req->interrupted)
+			err = -ENOENT;
+		else
+			req->locked = 1;
+		spin_unlock(&fuse_lock);
+	}
+	return err;
+}
+
+/*
+ * Unlock request.  If it was interrupted during being locked, the
+ * requester thread is currently waiting for it to be unlocked, so
+ * wake it up.
+ */
+static inline void unlock_request(struct fuse_req *req)
+{
+	if (req) {
+		spin_lock(&fuse_lock);
+		req->locked = 0;
+		if (req->interrupted)
+			wake_up(&req->waitq);
+		spin_unlock(&fuse_lock);
+	}
+}
+
+struct fuse_copy_state {
+	int write;
+	struct fuse_req *req;
+	const struct iovec *iov;
+	unsigned long nr_segs;
+	unsigned long seglen;
+	unsigned long addr;
+	struct page *pg;
+	void *mapaddr;
+	void *buf;
+	unsigned len;
+};
+
+static void fuse_copy_init(struct fuse_copy_state *cs, int write,
+			   struct fuse_req *req, const struct iovec *iov,
+			   unsigned long nr_segs)
+{
+	memset(cs, 0, sizeof(*cs));
+	cs->write = write;
+	cs->req = req;
+	cs->iov = iov;
+	cs->nr_segs = nr_segs;
+}
+
+/* Unmap and put previous page of userspace buffer */
+static inline void fuse_copy_finish(struct fuse_copy_state *cs)
+{
+	if (cs->mapaddr) {
+		kunmap_atomic(cs->mapaddr, KM_USER0);
+		if (cs->write) {
+			flush_dcache_page(cs->pg);
+			set_page_dirty_lock(cs->pg);
+		}
+		put_page(cs->pg);
+		cs->mapaddr = NULL;
+	}
+}
+
+/*
+ * Get another pagefull of userspace buffer, and map it to kernel
+ * address space, and lock request
+ */
+static int fuse_copy_fill(struct fuse_copy_state *cs)
+{
+	unsigned long offset;
+	int err;
+
+	unlock_request(cs->req);
+	fuse_copy_finish(cs);
+	if (!cs->seglen) {
+		BUG_ON(!cs->nr_segs);
+		cs->seglen = cs->iov[0].iov_len;
+		cs->addr = (unsigned long) cs->iov[0].iov_base;
+		cs->iov ++;
+		cs->nr_segs --;
+	}
+	down_read(&current->mm->mmap_sem);
+	err = get_user_pages(current, current->mm, cs->addr, 1, cs->write, 0,
+			     &cs->pg, NULL);
+	up_read(&current->mm->mmap_sem);
+	if (err < 0)
+		return err;
+	BUG_ON(err != 1);
+	offset = cs->addr % PAGE_SIZE;
+	cs->mapaddr = kmap_atomic(cs->pg, KM_USER0);
+	cs->buf = cs->mapaddr + offset;
+	cs->len = min(PAGE_SIZE - offset, cs->seglen);
+	cs->seglen -= cs->len;
+	cs->addr += cs->len;
+
+	return lock_request(cs->req);
+}
+
+/* Do as much copy to/from userspace buffer as we can */
+static inline int fuse_copy_do(struct fuse_copy_state *cs, void **val,
+			       unsigned *size)
+{
+	unsigned ncpy = min(*size, cs->len);
+	if (val) {
+		if (cs->write)
+			memcpy(cs->buf, *val, ncpy);
+		else
+			memcpy(*val, cs->buf, ncpy);
+		*val += ncpy;
+	}
+	*size -= ncpy;
+	cs->len -= ncpy;
+	cs->buf += ncpy;
+	return ncpy;
+}
+
+/*
+ * Copy a page in the request to/from the userspace buffer.  Must be
+ * done atomically
+ */
+static inline int fuse_copy_page(struct fuse_copy_state *cs, struct page *page,
+				 unsigned offset, unsigned count, int zeroing)
+{
+	if (page && zeroing && count < PAGE_SIZE) {
+		void *mapaddr = kmap_atomic(page, KM_USER1);
+		memset(mapaddr, 0, PAGE_SIZE);
+		kunmap_atomic(mapaddr, KM_USER1);
+	}
+	while (count) {
+		int err;
+		if (!cs->len && (err = fuse_copy_fill(cs)))
+			return err;
+		if (page) {
+			void *mapaddr = kmap_atomic(page, KM_USER1);
+			void *buf = mapaddr + offset;
+			offset += fuse_copy_do(cs, &buf, &count);
+			kunmap_atomic(mapaddr, KM_USER1);
+		} else
+			offset += fuse_copy_do(cs, NULL, &count);
+	}
+	if (page && !cs->write)
+		flush_dcache_page(page);
+	return 0;
+}
+
+/* Copy pages in the request to/from userspace buffer */
+static int fuse_copy_pages(struct fuse_copy_state *cs, unsigned nbytes,
+			   int zeroing)
+{
+	unsigned i;
+	struct fuse_req *req = cs->req;
+	unsigned offset = req->page_offset;
+	unsigned count = min(nbytes, (unsigned) PAGE_SIZE - offset);
+
+	for (i = 0; i < req->num_pages && (nbytes || zeroing); i++) {
+		struct page *page = req->pages[i];
+		int err = fuse_copy_page(cs, page, offset, count, zeroing);
+		if (err)
+			return err;
+
+		nbytes -= count;
+		count = min(nbytes, (unsigned) PAGE_SIZE);
+		offset = 0;
+	}
+	return 0;
+}
+
+/* Copy a single argument in the request to/from userspace buffer */
+static int fuse_copy_one(struct fuse_copy_state *cs, void *val, unsigned size)
+{
+	while (size) {
+		int err;
+		if (!cs->len && (err = fuse_copy_fill(cs)))
+			return err;
+		fuse_copy_do(cs, &val, &size);
+	}
+	return 0;
+}
+
+/* Copy request arguments to/from userspace buffer */
+static int fuse_copy_args(struct fuse_copy_state *cs, unsigned numargs,
+			  unsigned argpages, struct fuse_arg *args,
+			  int zeroing)
+{
+	int err = 0;
+	unsigned i;
+
+	for (i = 0; !err && i < numargs; i++)  {
+		struct fuse_arg *arg = &args[i];
+		if (i == numargs - 1 && argpages)
+			err = fuse_copy_pages(cs, arg->size, zeroing);
+		else
+			err = fuse_copy_one(cs, arg->value, arg->size);
+	}
+	return err;
+}
+
+/* Wait until a request is available on the pending list */
+static void request_wait(struct fuse_conn *fc)
+{
+	DECLARE_WAITQUEUE(wait, current);
+
+	add_wait_queue_exclusive(&fc->waitq, &wait);
+	while (fc->sb && list_empty(&fc->pending)) {
+		set_current_state(TASK_INTERRUPTIBLE);
+		if (signal_pending(current))
+			break;
+
+		spin_unlock(&fuse_lock);
+		schedule();
+		spin_lock(&fuse_lock);
+	}
+	set_current_state(TASK_RUNNING);
+	remove_wait_queue(&fc->waitq, &wait);
+}
+
+/*
+ * Read a single request into the userspace filesystem's buffer.  This
+ * function waits until a request is available, then removes it from
+ * the pending list and copies request data to userspace buffer.  If
+ * no reply is needed (FORGET) or request has been interrupted or
+ * there was an error during the copying then it's finished by calling
+ * request_end().  Otherwise add it to the processing list, and set
+ * the 'sent' flag.
+ */
+static ssize_t fuse_dev_readv(struct file *file, const struct iovec *iov,
+			      unsigned long nr_segs, loff_t *off)
+{
+	int err;
+	struct fuse_conn *fc;
+	struct fuse_req *req;
+	struct fuse_in *in;
+	struct fuse_copy_state cs;
+	unsigned reqsize;
+
+	spin_lock(&fuse_lock);
+	fc = file->private_data;
+	err = -EPERM;
+	if (!fc)
+		goto err_unlock;
+	request_wait(fc);
+	err = -ENODEV;
+	if (!fc->sb)
+		goto err_unlock;
+	err = -ERESTARTSYS;
+	if (list_empty(&fc->pending))
+		goto err_unlock;
+
+	req = list_entry(fc->pending.next, struct fuse_req, list);
+	list_del_init(&req->list);
+	spin_unlock(&fuse_lock);
+
+	in = &req->in;
+	reqsize = req->in.h.len;
+	fuse_copy_init(&cs, 1, req, iov, nr_segs);
+	err = -EINVAL;
+	if (iov_length(iov, nr_segs) >= reqsize) {
+		err = fuse_copy_one(&cs, &in->h, sizeof(in->h));
+		if (!err)
+			err = fuse_copy_args(&cs, in->numargs, in->argpages,
+					     (struct fuse_arg *) in->args, 0);
+	}
+	fuse_copy_finish(&cs);
+
+	spin_lock(&fuse_lock);
+	req->locked = 0;
+	if (!err && req->interrupted)
+		err = -ENOENT;
+	if (err) {
+		if (!req->interrupted)
+			req->out.h.error = -EIO;
+		request_end(fc, req);
+		return err;
+	}
+	if (!req->isreply)
+		request_end(fc, req);
+	else {
+		req->sent = 1;
+		list_add_tail(&req->list, &fc->processing);
+		spin_unlock(&fuse_lock);
+	}
+	return reqsize;
+
+ err_unlock:
+	spin_unlock(&fuse_lock);
+	return err;
+}
+
+static ssize_t fuse_dev_read(struct file *file, char __user *buf,
+			     size_t nbytes, loff_t *off)
+{
+	struct iovec iov;
+	iov.iov_len = nbytes;
+	iov.iov_base = buf;
+	return fuse_dev_readv(file, &iov, 1, off);
+}
+
+/* Look up request on processing list by unique ID */
+static struct fuse_req *request_find(struct fuse_conn *fc, u64 unique)
+{
+	struct list_head *entry;
+
+	list_for_each(entry, &fc->processing) {
+		struct fuse_req *req;
+		req = list_entry(entry, struct fuse_req, list);
+		if (req->in.h.unique == unique)
+			return req;
+	}
+	return NULL;
+}
+
+static int copy_out_args(struct fuse_copy_state *cs, struct fuse_out *out,
+			 unsigned nbytes)
+{
+	unsigned reqsize = sizeof(struct fuse_out_header);
+
+	if (out->h.error)
+		return nbytes != reqsize ? -EINVAL : 0;
+
+	reqsize += len_args(out->numargs, out->args);
+
+	if (reqsize < nbytes || (reqsize > nbytes && !out->argvar))
+		return -EINVAL;
+	else if (reqsize > nbytes) {
+		struct fuse_arg *lastarg = &out->args[out->numargs-1];
+		unsigned diffsize = reqsize - nbytes;
+		if (diffsize > lastarg->size)
+			return -EINVAL;
+		lastarg->size -= diffsize;
+	}
+	return fuse_copy_args(cs, out->numargs, out->argpages, out->args,
+			      out->page_zeroing);
+}
+
+/*
+ * Write a single reply to a request.  First the header is copied from
+ * the write buffer.  The request is then searched on the processing
+ * list by the unique ID found in the header.  If found, then remove
+ * it from the list and copy the rest of the buffer to the request.
+ * The request is finished by calling request_end()
+ */
+static ssize_t fuse_dev_writev(struct file *file, const struct iovec *iov,
+			       unsigned long nr_segs, loff_t *off)
+{
+	int err;
+	unsigned nbytes = iov_length(iov, nr_segs);
+	struct fuse_req *req;
+	struct fuse_out_header oh;
+	struct fuse_copy_state cs;
+	struct fuse_conn *fc = fuse_get_conn(file);
+	if (!fc)
+		return -ENODEV;
+
+	fuse_copy_init(&cs, 0, NULL, iov, nr_segs);
+	if (nbytes < sizeof(struct fuse_out_header))
+		return -EINVAL;
+
+	err = fuse_copy_one(&cs, &oh, sizeof(oh));
+	if (err)
+		goto err_finish;
+	err = -EINVAL;
+	if (!oh.unique || oh.error <= -1000 || oh.error > 0 ||
+	    oh.len != nbytes)
+		goto err_finish;
+
+	spin_lock(&fuse_lock);
+	req = request_find(fc, oh.unique);
+	err = -EINVAL;
+	if (!req)
+		goto err_unlock;
+
+	list_del_init(&req->list);
+	if (req->interrupted) {
+		request_end(fc, req);
+		fuse_copy_finish(&cs);
+		return -ENOENT;
+	}
+	req->out.h = oh;
+	req->locked = 1;
+	cs.req = req;
+	spin_unlock(&fuse_lock);
+
+	err = copy_out_args(&cs, &req->out, nbytes);
+	fuse_copy_finish(&cs);
+
+	spin_lock(&fuse_lock);
+	req->locked = 0;
+	if (!err) {
+		if (req->interrupted)
+			err = -ENOENT;
+	} else if (!req->interrupted)
+		req->out.h.error = -EIO;
+	request_end(fc, req);
+
+	return err ? err : nbytes;
+
+ err_unlock:
+	spin_unlock(&fuse_lock);
+ err_finish:
+	fuse_copy_finish(&cs);
+	return err;
+}
+
+static ssize_t fuse_dev_write(struct file *file, const char __user *buf,
+			      size_t nbytes, loff_t *off)
+{
+	struct iovec iov;
+	iov.iov_len = nbytes;
+	iov.iov_base = (char __user *) buf;
+	return fuse_dev_writev(file, &iov, 1, off);
+}
+
+static unsigned fuse_dev_poll(struct file *file, poll_table *wait)
+{
+	struct fuse_conn *fc = fuse_get_conn(file);
+	unsigned mask = POLLOUT | POLLWRNORM;
+
+	if (!fc)
+		return -ENODEV;
+
+	poll_wait(file, &fc->waitq, wait);
+
+	spin_lock(&fuse_lock);
+	if (!list_empty(&fc->pending))
+                mask |= POLLIN | POLLRDNORM;
+	spin_unlock(&fuse_lock);
+
+	return mask;
+}
+
+/* Abort all requests on the given list (pending or processing) */
+static void end_requests(struct fuse_conn *fc, struct list_head *head)
+{
+	while (!list_empty(head)) {
+		struct fuse_req *req;
+		req = list_entry(head->next, struct fuse_req, list);
+		list_del_init(&req->list);
+		req->out.h.error = -ECONNABORTED;
+		request_end(fc, req);
+		spin_lock(&fuse_lock);
+	}
+}
+
+static int fuse_dev_release(struct inode *inode, struct file *file)
+{
+	struct fuse_conn *fc;
+
+	spin_lock(&fuse_lock);
+	fc = file->private_data;
+	if (fc) {
+		fc->file = NULL;
+		end_requests(fc, &fc->pending);
+		end_requests(fc, &fc->processing);
+		fuse_release_conn(fc);
+	}
+	spin_unlock(&fuse_lock);
+	return 0;
+}
+
+struct file_operations fuse_dev_operations = {
+	.owner		= THIS_MODULE,
+	.llseek		= no_llseek,
+	.read		= fuse_dev_read,
+	.readv		= fuse_dev_readv,
+	.write		= fuse_dev_write,
+	.writev		= fuse_dev_writev,
+	.poll		= fuse_dev_poll,
+	.release	= fuse_dev_release,
+};
+
+static struct miscdevice fuse_miscdevice = {
+	.minor = FUSE_MINOR,
+	.name  = "fuse",
+	.fops = &fuse_dev_operations,
+};
+
+int __init fuse_dev_init(void)
+{
+	int err = -ENOMEM;
+	fuse_req_cachep = kmem_cache_create("fuse_request",
+					    sizeof(struct fuse_req),
+					    0, 0, NULL, NULL);
+	if (!fuse_req_cachep)
+		goto out;
+
+	err = misc_register(&fuse_miscdevice);
+	if (err)
+		goto out_cache_clean;
+
+	return 0;
+
+ out_cache_clean:
+	kmem_cache_destroy(fuse_req_cachep);
+ out:
+	return err;
+}
+
+void fuse_dev_cleanup(void)
+{
+	misc_deregister(&fuse_miscdevice);
+	kmem_cache_destroy(fuse_req_cachep);
+}

fs/fuse/fuse_i.h

@@ -15,6 +15,12 @@
 #include <linux/backing-dev.h>
 #include <asm/semaphore.h>
 
+/** Max number of pages that can be used in a single read request */
+#define FUSE_MAX_PAGES_PER_REQ 32
+
+/** If more requests are outstanding, then the operation will block */
+#define FUSE_MAX_OUTSTANDING 10
+
 /** FUSE inode */
 struct fuse_inode {
 	/** Inode data */
@@ -28,6 +34,123 @@ struct fuse_inode {
 	unsigned long i_time;
 };
 
+/** One input argument of a request */
+struct fuse_in_arg {
+	unsigned size;
+	const void *value;
+};
+
+/** The request input */
+struct fuse_in {
+	/** The request header */
+	struct fuse_in_header h;
+
+	/** True if the data for the last argument is in req->pages */
+	unsigned argpages:1;
+
+	/** Number of arguments */
+	unsigned numargs;
+
+	/** Array of arguments */
+	struct fuse_in_arg args[3];
+};
+
+/** One output argument of a request */
+struct fuse_arg {
+	unsigned size;
+	void *value;
+};
+
+/** The request output */
+struct fuse_out {
+	/** Header returned from userspace */
+	struct fuse_out_header h;
+
+	/** Last argument is variable length (can be shorter than
+	    arg->size) */
+	unsigned argvar:1;
+
+	/** Last argument is a list of pages to copy data to */
+	unsigned argpages:1;
+
+	/** Zero partially or not copied pages */
+	unsigned page_zeroing:1;
+
+	/** Number or arguments */
+	unsigned numargs;
+
+	/** Array of arguments */
+	struct fuse_arg args[3];
+};
+
+struct fuse_req;
+struct fuse_conn;
+
+/**
+ * A request to the client
+ */
+struct fuse_req {
+	/** This can be on either unused_list, pending or processing
+	    lists in fuse_conn */
+	struct list_head list;
+
+	/** refcount */
+	atomic_t count;
+
+	/** True if the request has reply */
+	unsigned isreply:1;
+
+	/** The request is preallocated */
+	unsigned preallocated:1;
+
+	/** The request was interrupted */
+	unsigned interrupted:1;
+
+	/** Request is sent in the background */
+	unsigned background:1;
+
+	/** Data is being copied to/from the request */
+	unsigned locked:1;
+
+	/** Request has been sent to userspace */
+	unsigned sent:1;
+
+	/** The request is finished */
+	unsigned finished:1;
+
+	/** The request input */
+	struct fuse_in in;
+
+	/** The request output */
+	struct fuse_out out;
+
+	/** Used to wake up the task waiting for completion of request*/
+	wait_queue_head_t waitq;
+
+	/** Data for asynchronous requests */
+	union {
+		struct fuse_init_in_out init_in_out;
+	} misc;
+
+	/** page vector */
+	struct page *pages[FUSE_MAX_PAGES_PER_REQ];
+
+	/** number of pages in vector */
+	unsigned num_pages;
+
+	/** offset of data on first page */
+	unsigned page_offset;
+
+	/** Inode used in the request */
+	struct inode *inode;
+
+	/** Second inode used in the request (or NULL) */
+	struct inode *inode2;
+
+	/** File used in the request (or NULL) */
+	struct file *file;
+};
+
 /**
  * A Fuse connection.
  *
@@ -39,9 +162,37 @@ struct fuse_conn {
 	/** The superblock of the mounted filesystem */
 	struct super_block *sb;
 
+	/** The opened client device */
+	struct file *file;
+
 	/** The user id for this mount */
 	uid_t user_id;
 
+	/** Readers of the connection are waiting on this */
+	wait_queue_head_t waitq;
+
+	/** The list of pending requests */
+	struct list_head pending;
+
+	/** The list of requests being processed */
+	struct list_head processing;
+
+	/** Controls the maximum number of outstanding requests */
+	struct semaphore outstanding_sem;
+
+	/** This counts the number of outstanding requests if
+	    outstanding_sem would go negative */
+	unsigned outstanding_debt;
+
+	/** The list of unused requests */
+	struct list_head unused_list;
+
+	/** The next unique request id */
+	u64 reqctr;
+
+	/** Connection failed (version mismatch) */
+	unsigned conn_error : 1;
+
 	/** Backing dev info */
 	struct backing_dev_info bdi;
 };
@@ -71,13 +222,20 @@ static inline u64 get_node_id(struct inode *inode)
 	return get_fuse_inode(inode)->nodeid;
 }
 
+/** Device operations */
+extern struct file_operations fuse_dev_operations;
+
 /**
  * This is the single global spinlock which protects FUSE's structures
  *
  * The following data is protected by this lock:
  *
+ *  - the private_data field of the device file
  *  - the s_fs_info field of the super block
+ *  - unused_list, pending, processing lists in fuse_conn
+ *  - the unique request ID counter reqctr in fuse_conn
  *  - the sb (super_block) field in fuse_conn
+ *  - the file (device file) field in fuse_conn
  */
 extern spinlock_t fuse_lock;
 
@@ -87,3 +245,68 @@ extern spinlock_t fuse_lock;
  */
 void fuse_release_conn(struct fuse_conn *fc);
 
+/**
+ * Initialize the client device
+ */
+int fuse_dev_init(void);
+
+/**
+ * Cleanup the client device
+ */
+void fuse_dev_cleanup(void);
+
+/**
+ * Allocate a request
+ */
+struct fuse_req *fuse_request_alloc(void);
+
+/**
+ * Free a request
+ */
+void fuse_request_free(struct fuse_req *req);
+
+/**
+ * Reinitialize a request, the preallocated flag is left unmodified
+ */
+void fuse_reset_request(struct fuse_req *req);
+
+/**
+ * Reserve a preallocated request
+ */
+struct fuse_req *fuse_get_request(struct fuse_conn *fc);
+
+/**
+ * Reserve a preallocated request, only interruptible by SIGKILL
+ */
+struct fuse_req *fuse_get_request_nonint(struct fuse_conn *fc);
+
+/**
+ * Decrement reference count of a request.  If count goes to zero put
+ * on unused list (preallocated) or free reqest (not preallocated).
+ */
+void fuse_put_request(struct fuse_conn *fc, struct fuse_req *req);
+
+/**
+ * Send a request (synchronous, interruptible)
+ */
+void request_send(struct fuse_conn *fc, struct fuse_req *req);
+
+/**
+ * Send a request (synchronous, non-interruptible except by SIGKILL)
+ */
+void request_send_nonint(struct fuse_conn *fc, struct fuse_req *req);
+
+/**
+ * Send a request with no reply
+ */
+void request_send_noreply(struct fuse_conn *fc, struct fuse_req *req);
+
+/**
+ * Send a request in the background
+ */
+void request_send_background(struct fuse_conn *fc, struct fuse_req *req);
+
+/**
+ * Send the INIT message
+ */
+void fuse_send_init(struct fuse_conn *fc);

fs/fuse/inode.c

@@ -151,6 +151,8 @@ static void fuse_put_super(struct super_block *sb)
 	mount_count --;
 	fc->sb = NULL;
 	fc->user_id = 0;
+	/* Flush all readers on this fs */
+	wake_up_all(&fc->waitq);
 	fuse_release_conn(fc);
 	*get_fuse_conn_super_p(sb) = NULL;
 	spin_unlock(&fuse_lock);
@@ -229,22 +231,51 @@ static int fuse_show_options(struct seq_file *m, struct vfsmount *mnt)
 	return 0;
 }
 
-void fuse_release_conn(struct fuse_conn *fc)
+static void free_conn(struct fuse_conn *fc)
 {
+	while (!list_empty(&fc->unused_list)) {
+		struct fuse_req *req;
+		req = list_entry(fc->unused_list.next, struct fuse_req, list);
+		list_del(&req->list);
+		fuse_request_free(req);
+	}
 	kfree(fc);
 }
 
+/* Must be called with the fuse lock held */
+void fuse_release_conn(struct fuse_conn *fc)
+{
+	if (!fc->sb && !fc->file)
+		free_conn(fc);
+}
+
 static struct fuse_conn *new_conn(void)
 {
 	struct fuse_conn *fc;
 
 	fc = kmalloc(sizeof(*fc), GFP_KERNEL);
 	if (fc != NULL) {
+		int i;
 		memset(fc, 0, sizeof(*fc));
 		fc->sb = NULL;
+		fc->file = NULL;
 		fc->user_id = 0;
+		init_waitqueue_head(&fc->waitq);
+		INIT_LIST_HEAD(&fc->pending);
+		INIT_LIST_HEAD(&fc->processing);
+		INIT_LIST_HEAD(&fc->unused_list);
+		sema_init(&fc->outstanding_sem, 0);
+		for (i = 0; i < FUSE_MAX_OUTSTANDING; i++) {
+			struct fuse_req *req = fuse_request_alloc();
+			if (!req) {
+				free_conn(fc);
+				return NULL;
+			}
+			list_add(&req->list, &fc->unused_list);
+		}
 		fc->bdi.ra_pages = (VM_MAX_READAHEAD * 1024) / PAGE_CACHE_SIZE;
 		fc->bdi.unplug_io_fn = default_unplug_io_fn;
+		fc->reqctr = 0;
 	}
 	return fc;
 }
@@ -253,11 +284,20 @@ static struct fuse_conn *get_conn(struct file *file, struct super_block *sb)
 {
 	struct fuse_conn *fc;
 
+	if (file->f_op != &fuse_dev_operations)
+		return ERR_PTR(-EINVAL);
 	fc = new_conn();
 	if (fc == NULL)
-		return NULL;
+		return ERR_PTR(-ENOMEM);
 	spin_lock(&fuse_lock);
-	fc->sb = sb;
+	if (file->private_data) {
+		free_conn(fc);
+		fc = ERR_PTR(-EINVAL);
+	} else {
+		file->private_data = fc;
+		fc->sb = sb;
+		fc->file = file;
+	}
 	spin_unlock(&fuse_lock);
 	return fc;
 }
@@ -315,8 +355,8 @@ static int fuse_fill_super(struct super_block *sb, void *data, int silent)
 
 	fc = get_conn(file, sb);
 	fput(file);
-	if (fc == NULL)
-		return -EINVAL;
+	if (IS_ERR(fc))
+		return PTR_ERR(fc);
 
 	fc->user_id = d.user_id;
 
@@ -336,6 +376,7 @@ static int fuse_fill_super(struct super_block *sb, void *data, int silent)
 		iput(root);
 		goto err;
 	}
+	fuse_send_init(fc);
 	return 0;
 
  err:
@@ -411,8 +452,14 @@ static int __init fuse_init(void)
 	if (res)
 		goto err;
 
+	res = fuse_dev_init();
+	if (res)
+		goto err_fs_cleanup;
+
 	return 0;
 
+ err_fs_cleanup:
+	fuse_fs_cleanup();
  err:
 	return res;
 }
@@ -422,6 +469,7 @@ static void __exit fuse_exit(void)
 	printk(KERN_DEBUG "fuse exit\n");
 
 	fuse_fs_cleanup();
+	fuse_dev_cleanup();
 }
 
 module_init(fuse_init);

include/linux/fuse.h

@@ -11,7 +11,7 @@
 #include <asm/types.h>
 
 /** Version number of this interface */
-#define FUSE_KERNEL_VERSION 5
+#define FUSE_KERNEL_VERSION 6
 
 /** Minor version number of this interface */
 #define FUSE_KERNEL_MINOR_VERSION 1
@@ -19,6 +19,12 @@
 /** The node ID of the root inode */
 #define FUSE_ROOT_ID 1
 
+/** The major number of the fuse character device */
+#define FUSE_MAJOR 10
+
+/** The minor number of the fuse character device */
+#define FUSE_MINOR 229
+
 struct fuse_attr {
 	__u64	ino;
 	__u64	size;
@@ -36,3 +42,31 @@ struct fuse_attr {
 	__u32	rdev;
 };
 
+enum fuse_opcode {
+	FUSE_INIT          = 26
+};
+
+/* Conservative buffer size for the client */
+#define FUSE_MAX_IN 8192
+
+struct fuse_init_in_out {
+	__u32	major;
+	__u32	minor;
+};
+
+struct fuse_in_header {
+	__u32	len;
+	__u32	opcode;
+	__u64	unique;
+	__u64	nodeid;
+	__u32	uid;
+	__u32	gid;
+	__u32	pid;
+};
+
+struct fuse_out_header {
+	__u32	len;
+	__s32	error;
+	__u64	unique;
+};
+