eCryptfs: introduce device handle for userspace daemon communications

A regular device file was my real preference from the get-go, but I went with
netlink at the time because I thought it would be less complex for managing
send queues (i.e., just do a unicast and move on).  It turns out that we do
not really get that much complexity reduction with netlink, and netlink is
more heavyweight than a device handle.

In addition, the netlink interface to eCryptfs has been broken since 2.6.24.
I am assuming this is a bug in how eCryptfs uses netlink, since the other
in-kernel users of netlink do not seem to be having any problems.  I have had
one report of a user successfully using eCryptfs with netlink on 2.6.24, but
for my own systems, when starting the userspace daemon, the initial helo
message sent to the eCryptfs kernel module results in an oops right off the
bat.  I spent some time looking at it, but I have not yet found the cause.
The netlink interface breaking gave me the motivation to just finish my patch
to migrate to a regular device handle.  If I cannot find out soon why the
netlink interface in eCryptfs broke, I am likely to just send a patch to
disable it in 2.6.24 and 2.6.25.  I would like the device handle to be the
preferred means of communicating with the userspace daemon from 2.6.26 on
forward.

This patch:

Functions to facilitate reading and writing to the eCryptfs miscellaneous
device handle.  This will replace the netlink interface as the preferred
mechanism for communicating with the userspace eCryptfs daemon.

Each user has his own daemon, which registers itself by opening the eCryptfs
device handle.  Only one daemon per euid may be registered at any given time.
The eCryptfs module sends a message to a daemon by adding its message to the
daemon's outgoing message queue.  The daemon reads the device handle to get
the oldest message off the queue.

Incoming messages from the userspace daemon are immediately handled.  If the
message is a response, then the corresponding process that is blocked waiting
for the response is awakened.
Signed-off-by: Michael Halcrow <mhalcrow@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/ecryptfs/miscdev.c

+/**
+ * eCryptfs: Linux filesystem encryption layer
+ *
+ * Copyright (C) 2008 International Business Machines Corp.
+ *   Author(s): Michael A. Halcrow <mhalcrow@us.ibm.com>
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License version
+ * 2 as published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA
+ * 02111-1307, USA.
+ */
+
+#include <linux/fs.h>
+#include <linux/hash.h>
+#include <linux/random.h>
+#include <linux/miscdevice.h>
+#include <linux/poll.h>
+#include <linux/wait.h>
+#include <linux/module.h>
+#include "ecryptfs_kernel.h"
+
+static atomic_t ecryptfs_num_miscdev_opens;
+
+/**
+ * ecryptfs_miscdev_poll
+ * @file: dev file (ignored)
+ * @pt: dev poll table (ignored)
+ *
+ * Returns the poll mask
+ */
+static unsigned int
+ecryptfs_miscdev_poll(struct file *file, poll_table *pt)
+{
+	struct ecryptfs_daemon *daemon;
+	unsigned int mask = 0;
+	int rc;
+
+	mutex_lock(&ecryptfs_daemon_hash_mux);
+	/* TODO: Just use file->private_data? */
+	rc = ecryptfs_find_daemon_by_euid(&daemon, current->euid);
+	BUG_ON(rc || !daemon);
+	mutex_lock(&daemon->mux);
+	mutex_unlock(&ecryptfs_daemon_hash_mux);
+	if (daemon->flags & ECRYPTFS_DAEMON_ZOMBIE) {
+		printk(KERN_WARNING "%s: Attempt to poll on zombified "
+		       "daemon\n", __func__);
+		goto out_unlock_daemon;
+	}
+	if (daemon->flags & ECRYPTFS_DAEMON_IN_READ)
+		goto out_unlock_daemon;
+	if (daemon->flags & ECRYPTFS_DAEMON_IN_POLL)
+		goto out_unlock_daemon;
+	daemon->flags |= ECRYPTFS_DAEMON_IN_POLL;
+	mutex_unlock(&daemon->mux);
+	poll_wait(file, &daemon->wait, pt);
+	mutex_lock(&daemon->mux);
+	if (!list_empty(&daemon->msg_ctx_out_queue))
+		mask |= POLLIN | POLLRDNORM;
+out_unlock_daemon:
+	daemon->flags &= ~ECRYPTFS_DAEMON_IN_POLL;
+	mutex_unlock(&daemon->mux);
+	return mask;
+}
+
+/**
+ * ecryptfs_miscdev_open
+ * @inode: inode of miscdev handle (ignored)
+ * @file: file for miscdev handle (ignored)
+ *
+ * Returns zero on success; non-zero otherwise
+ */
+static int
+ecryptfs_miscdev_open(struct inode *inode, struct file *file)
+{
+	struct ecryptfs_daemon *daemon = NULL;
+	int rc;
+
+	mutex_lock(&ecryptfs_daemon_hash_mux);
+	rc = try_module_get(THIS_MODULE);
+	if (rc == 0) {
+		rc = -EIO;
+		printk(KERN_ERR "%s: Error attempting to increment module use "
+		       "count; rc = [%d]\n", __func__, rc);
+		goto out_unlock_daemon_list;
+	}
+	rc = ecryptfs_find_daemon_by_euid(&daemon, current->euid);
+	if (rc || !daemon) {
+		rc = ecryptfs_spawn_daemon(&daemon, current->euid,
+					   current->pid);
+		if (rc) {
+			printk(KERN_ERR "%s: Error attempting to spawn daemon; "
+			       "rc = [%d]\n", __func__, rc);
+			goto out_module_put_unlock_daemon_list;
+		}
+	}
+	mutex_lock(&daemon->mux);
+	if (daemon->pid != current->pid) {
+		rc = -EINVAL;
+		printk(KERN_ERR "%s: pid [%d] has registered with euid [%d], "
+		       "but pid [%d] has attempted to open the handle "
+		       "instead\n", __func__, daemon->pid, daemon->euid,
+		       current->pid);
+		goto out_unlock_daemon;
+	}
+	if (daemon->flags & ECRYPTFS_DAEMON_MISCDEV_OPEN) {
+		rc = -EBUSY;
+		printk(KERN_ERR "%s: Miscellaneous device handle may only be "
+		       "opened once per daemon; pid [%d] already has this "
+		       "handle open\n", __func__, daemon->pid);
+		goto out_unlock_daemon;
+	}
+	daemon->flags |= ECRYPTFS_DAEMON_MISCDEV_OPEN;
+	atomic_inc(&ecryptfs_num_miscdev_opens);
+out_unlock_daemon:
+	mutex_unlock(&daemon->mux);
+out_module_put_unlock_daemon_list:
+	if (rc)
+		module_put(THIS_MODULE);
+out_unlock_daemon_list:
+	mutex_unlock(&ecryptfs_daemon_hash_mux);
+	return rc;
+}
+
+/**
+ * ecryptfs_miscdev_release
+ * @inode: inode of fs/ecryptfs/euid handle (ignored)
+ * @file: file for fs/ecryptfs/euid handle (ignored)
+ *
+ * This keeps the daemon registered until the daemon sends another
+ * ioctl to fs/ecryptfs/ctl or until the kernel module unregisters.
+ *
+ * Returns zero on success; non-zero otherwise
+ */
+static int
+ecryptfs_miscdev_release(struct inode *inode, struct file *file)
+{
+	struct ecryptfs_daemon *daemon = NULL;
+	int rc;
+
+	mutex_lock(&ecryptfs_daemon_hash_mux);
+	rc = ecryptfs_find_daemon_by_euid(&daemon, current->euid);
+	BUG_ON(rc || !daemon);
+	mutex_lock(&daemon->mux);
+	BUG_ON(daemon->pid != current->pid);
+	BUG_ON(!(daemon->flags & ECRYPTFS_DAEMON_MISCDEV_OPEN));
+	daemon->flags &= ~ECRYPTFS_DAEMON_MISCDEV_OPEN;
+	atomic_dec(&ecryptfs_num_miscdev_opens);
+	mutex_unlock(&daemon->mux);
+	rc = ecryptfs_exorcise_daemon(daemon);
+	if (rc) {
+		printk(KERN_CRIT "%s: Fatal error whilst attempting to "
+		       "shut down daemon; rc = [%d]. Please report this "
+		       "bug.\n", __func__, rc);
+		BUG();
+	}
+	module_put(THIS_MODULE);
+	mutex_unlock(&ecryptfs_daemon_hash_mux);
+	return rc;
+}
+
+/**
+ * ecryptfs_send_miscdev
+ * @data: Data to send to daemon; may be NULL
+ * @data_size: Amount of data to send to daemon
+ * @msg_ctx: Message context, which is used to handle the reply. If
+ *           this is NULL, then we do not expect a reply.
+ * @msg_type: Type of message
+ * @msg_flags: Flags for message
+ * @daemon: eCryptfs daemon object
+ *
+ * Add msg_ctx to queue and then, if it exists, notify the blocked
+ * miscdevess about the data being available. Must be called with
+ * ecryptfs_daemon_hash_mux held.
+ *
+ * Returns zero on success; non-zero otherwise
+ */
+int ecryptfs_send_miscdev(char *data, size_t data_size,
+			  struct ecryptfs_msg_ctx *msg_ctx, u8 msg_type,
+			  u16 msg_flags, struct ecryptfs_daemon *daemon)
+{
+	int rc = 0;
+
+	mutex_lock(&msg_ctx->mux);
+	if (data) {
+		msg_ctx->msg = kmalloc((sizeof(*msg_ctx->msg) + data_size),
+				       GFP_KERNEL);
+		if (!msg_ctx->msg) {
+			rc = -ENOMEM;
+			printk(KERN_ERR "%s: Out of memory whilst attempting "
+			       "to kmalloc(%d, GFP_KERNEL)\n", __func__,
+			       (sizeof(*msg_ctx->msg) + data_size));
+			goto out_unlock;
+		}
+	} else
+		msg_ctx->msg = NULL;
+	msg_ctx->msg->index = msg_ctx->index;
+	msg_ctx->msg->data_len = data_size;
+	msg_ctx->type = msg_type;
+	if (data) {
+		memcpy(msg_ctx->msg->data, data, data_size);
+		msg_ctx->msg_size = (sizeof(*msg_ctx->msg) + data_size);
+	} else
+		msg_ctx->msg_size = 0;
+	mutex_lock(&daemon->mux);
+	list_add_tail(&msg_ctx->daemon_out_list, &daemon->msg_ctx_out_queue);
+	daemon->num_queued_msg_ctx++;
+	wake_up_interruptible(&daemon->wait);
+	mutex_unlock(&daemon->mux);
+out_unlock:
+	mutex_unlock(&msg_ctx->mux);
+	return rc;
+}
+
+/**
+ * ecryptfs_miscdev_read - format and send message from queue
+ * @file: fs/ecryptfs/euid miscdevfs handle (ignored)
+ * @buf: User buffer into which to copy the next message on the daemon queue
+ * @count: Amount of space available in @buf
+ * @ppos: Offset in file (ignored)
+ *
+ * Pulls the most recent message from the daemon queue, formats it for
+ * being sent via a miscdevfs handle, and copies it into @buf
+ *
+ * Returns the number of bytes copied into the user buffer
+ */
+static int
+ecryptfs_miscdev_read(struct file *file, char __user *buf, size_t count,
+		      loff_t *ppos)
+{
+	struct ecryptfs_daemon *daemon;
+	struct ecryptfs_msg_ctx *msg_ctx;
+	size_t packet_length_size;
+	u32 counter_nbo;
+	char packet_length[3];
+	size_t i;
+	size_t total_length;
+	int rc;
+
+	mutex_lock(&ecryptfs_daemon_hash_mux);
+	/* TODO: Just use file->private_data? */
+	rc = ecryptfs_find_daemon_by_euid(&daemon, current->euid);
+	BUG_ON(rc || !daemon);
+	mutex_lock(&daemon->mux);
+	if (daemon->flags & ECRYPTFS_DAEMON_ZOMBIE) {
+		rc = 0;
+		printk(KERN_WARNING "%s: Attempt to read from zombified "
+		       "daemon\n", __func__);
+		goto out_unlock_daemon;
+	}
+	if (daemon->flags & ECRYPTFS_DAEMON_IN_READ) {
+		rc = 0;
+		goto out_unlock_daemon;
+	}
+	/* This daemon will not go away so long as this flag is set */
+	daemon->flags |= ECRYPTFS_DAEMON_IN_READ;
+	mutex_unlock(&ecryptfs_daemon_hash_mux);
+check_list:
+	if (list_empty(&daemon->msg_ctx_out_queue)) {
+		mutex_unlock(&daemon->mux);
+		rc = wait_event_interruptible(
+			daemon->wait, !list_empty(&daemon->msg_ctx_out_queue));
+		mutex_lock(&daemon->mux);
+		if (rc < 0) {
+			rc = 0;
+			goto out_unlock_daemon;
+		}
+	}
+	if (daemon->flags & ECRYPTFS_DAEMON_ZOMBIE) {
+		rc = 0;
+		goto out_unlock_daemon;
+	}
+	if (list_empty(&daemon->msg_ctx_out_queue)) {
+		/* Something else jumped in since the
+		 * wait_event_interruptable() and removed the
+		 * message from the queue; try again */
+		goto check_list;
+	}
+	BUG_ON(current->euid != daemon->euid);
+	BUG_ON(current->pid != daemon->pid);
+	msg_ctx = list_first_entry(&daemon->msg_ctx_out_queue,
+				   struct ecryptfs_msg_ctx, daemon_out_list);
+	BUG_ON(!msg_ctx);
+	mutex_lock(&msg_ctx->mux);
+	if (msg_ctx->msg) {
+		rc = ecryptfs_write_packet_length(packet_length,
+						  msg_ctx->msg_size,
+						  &packet_length_size);
+		if (rc) {
+			rc = 0;
+			printk(KERN_WARNING "%s: Error writing packet length; "
+			       "rc = [%d]\n", __func__, rc);
+			goto out_unlock_msg_ctx;
+		}
+	} else {
+		packet_length_size = 0;
+		msg_ctx->msg_size = 0;
+	}
+	/* miscdevfs packet format:
+	 *  Octet 0: Type
+	 *  Octets 1-4: network byte order msg_ctx->counter
+	 *  Octets 5-N0: Size of struct ecryptfs_message to follow
+	 *  Octets N0-N1: struct ecryptfs_message (including data)
+	 *
+	 *  Octets 5-N1 not written if the packet type does not
+	 *  include a message */
+	total_length = (1 + 4 + packet_length_size + msg_ctx->msg_size);
+	if (count < total_length) {
+		rc = 0;
+		printk(KERN_WARNING "%s: Only given user buffer of "
+		       "size [%Zd], but we need [%Zd] to read the "
+		       "pending message\n", __func__, count, total_length);
+		goto out_unlock_msg_ctx;
+	}
+	i = 0;
+	buf[i++] = msg_ctx->type;
+	counter_nbo = cpu_to_be32(msg_ctx->counter);
+	memcpy(&buf[i], (char *)&counter_nbo, 4);
+	i += 4;
+	if (msg_ctx->msg) {
+		memcpy(&buf[i], packet_length, packet_length_size);
+		i += packet_length_size;
+		rc = copy_to_user(&buf[i], msg_ctx->msg, msg_ctx->msg_size);
+		if (rc) {
+			printk(KERN_ERR "%s: copy_to_user returned error "
+			       "[%d]\n", __func__, rc);
+			goto out_unlock_msg_ctx;
+		}
+		i += msg_ctx->msg_size;
+	}
+	rc = i;
+	list_del(&msg_ctx->daemon_out_list);
+	kfree(msg_ctx->msg);
+	msg_ctx->msg = NULL;
+	/* We do not expect a reply from the userspace daemon for any
+	 * message type other than ECRYPTFS_MSG_REQUEST */
+	if (msg_ctx->type != ECRYPTFS_MSG_REQUEST)
+		ecryptfs_msg_ctx_alloc_to_free(msg_ctx);
+out_unlock_msg_ctx:
+	mutex_unlock(&msg_ctx->mux);
+out_unlock_daemon:
+	daemon->flags &= ~ECRYPTFS_DAEMON_IN_READ;
+	mutex_unlock(&daemon->mux);
+	return rc;
+}
+
+/**
+ * ecryptfs_miscdev_helo
+ * @euid: effective user id of miscdevess sending helo packet
+ * @pid: miscdevess id of miscdevess sending helo packet
+ *
+ * Returns zero on success; non-zero otherwise
+ */
+static int ecryptfs_miscdev_helo(uid_t uid, pid_t pid)
+{
+	int rc;
+
+	rc = ecryptfs_process_helo(ECRYPTFS_TRANSPORT_MISCDEV, uid, pid);
+	if (rc)
+		printk(KERN_WARNING "Error processing HELO; rc = [%d]\n", rc);
+	return rc;
+}
+
+/**
+ * ecryptfs_miscdev_quit
+ * @euid: effective user id of miscdevess sending quit packet
+ * @pid: miscdevess id of miscdevess sending quit packet
+ *
+ * Returns zero on success; non-zero otherwise
+ */
+static int ecryptfs_miscdev_quit(uid_t euid, pid_t pid)
+{
+	int rc;
+
+	rc = ecryptfs_process_quit(euid, pid);
+	if (rc)
+		printk(KERN_WARNING
+		       "Error processing QUIT message; rc = [%d]\n", rc);
+	return rc;
+}
+
+/**
+ * ecryptfs_miscdev_response - miscdevess response to message previously sent to daemon
+ * @data: Bytes comprising struct ecryptfs_message
+ * @data_size: sizeof(struct ecryptfs_message) + data len
+ * @euid: Effective user id of miscdevess sending the miscdev response
+ * @pid: Miscdevess id of miscdevess sending the miscdev response
+ * @seq: Sequence number for miscdev response packet
+ *
+ * Returns zero on success; non-zero otherwise
+ */
+static int ecryptfs_miscdev_response(char *data, size_t data_size,
+					  uid_t euid, pid_t pid, u32 seq)
+{
+	struct ecryptfs_message *msg = (struct ecryptfs_message *)data;
+	int rc;
+
+	if ((sizeof(*msg) + msg->data_len) != data_size) {
+		printk(KERN_WARNING "%s: (sizeof(*msg) + msg->data_len) = "
+		       "[%Zd]; data_size = [%Zd]. Invalid packet.\n", __func__,
+		       (sizeof(*msg) + msg->data_len), data_size);
+		rc = -EINVAL;
+		goto out;
+	}
+	rc = ecryptfs_process_response(msg, euid, pid, seq);
+	if (rc)
+		printk(KERN_ERR
+		       "Error processing response message; rc = [%d]\n", rc);
+out:
+	return rc;
+}
+
+/**
+ * ecryptfs_miscdev_write - handle write to daemon miscdev handle
+ * @file: File for misc dev handle (ignored)
+ * @buf: Buffer containing user data
+ * @count: Amount of data in @buf
+ * @ppos: Pointer to offset in file (ignored)
+ *
+ * miscdevfs packet format:
+ *  Octet 0: Type
+ *  Octets 1-4: network byte order msg_ctx->counter (0's for non-response)
+ *  Octets 5-N0: Size of struct ecryptfs_message to follow
+ *  Octets N0-N1: struct ecryptfs_message (including data)
+ *
+ * Returns the number of bytes read from @buf
+ */
+static ssize_t
+ecryptfs_miscdev_write(struct file *file, const char __user *buf,
+		       size_t count, loff_t *ppos)
+{
+	u32 counter_nbo, seq;
+	size_t packet_size, packet_size_length, i;
+	ssize_t sz = 0;
+	char *data;
+	int rc;
+
+	if (count == 0)
+		goto out;
+	data = kmalloc(count, GFP_KERNEL);
+	if (!data) {
+		printk(KERN_ERR "%s: Out of memory whilst attempting to "
+		       "kmalloc([%Zd], GFP_KERNEL)\n", __func__, count);
+		goto out;
+	}
+	rc = copy_from_user(data, buf, count);
+	if (rc) {
+		printk(KERN_ERR "%s: copy_from_user returned error [%d]\n",
+		       __func__, rc);
+		goto out_free;
+	}
+	sz = count;
+	i = 0;
+	switch (data[i++]) {
+	case ECRYPTFS_MSG_RESPONSE:
+		if (count < (1 + 4 + 1 + sizeof(struct ecryptfs_message))) {
+			printk(KERN_WARNING "%s: Minimum acceptable packet "
+			       "size is [%Zd], but amount of data written is "
+			       "only [%Zd]. Discarding response packet.\n",
+			       __func__,
+			       (1 + 4 + 1 + sizeof(struct ecryptfs_message)),
+			       count);
+			goto out_free;
+		}
+		memcpy((char *)&counter_nbo, &data[i], 4);
+		seq = be32_to_cpu(counter_nbo);
+		i += 4;
+		rc = ecryptfs_parse_packet_length(&data[i], &packet_size,
+						  &packet_size_length);
+		if (rc) {
+			printk(KERN_WARNING "%s: Error parsing packet length; "
+			       "rc = [%d]\n", __func__, rc);
+			goto out_free;
+		}
+		i += packet_size_length;
+		if ((1 + 4 + packet_size_length + packet_size) != count) {
+			printk(KERN_WARNING "%s: (1 + packet_size_length([%Zd])"
+			       " + packet_size([%Zd]))([%Zd]) != "
+			       "count([%Zd]). Invalid packet format.\n",
+			       __func__, packet_size_length, packet_size,
+			       (1 + packet_size_length + packet_size), count);
+			goto out_free;
+		}
+		rc = ecryptfs_miscdev_response(&data[i], packet_size,
+					       current->euid,
+					       current->pid, seq);
+		if (rc)
+			printk(KERN_WARNING "%s: Failed to deliver miscdev "
+			       "response to requesting operation; rc = [%d]\n",
+			       __func__, rc);
+		break;
+	case ECRYPTFS_MSG_HELO:
+		rc = ecryptfs_miscdev_helo(current->euid, current->pid);
+		if (rc) {
+			printk(KERN_ERR "%s: Error attempting to process "
+			       "helo from pid [%d]; rc = [%d]\n", __func__,
+			       current->pid, rc);
+			goto out_free;
+		}
+		break;
+	case ECRYPTFS_MSG_QUIT:
+		rc = ecryptfs_miscdev_quit(current->euid, current->pid);
+		if (rc) {
+			printk(KERN_ERR "%s: Error attempting to process "
+			       "quit from pid [%d]; rc = [%d]\n", __func__,
+			       current->pid, rc);
+			goto out_free;
+		}
+		break;
+	default:
+		ecryptfs_printk(KERN_WARNING, "Dropping miscdev "
+				"message of unrecognized type [%d]\n",
+				data[0]);
+		break;
+	}
+out_free:
+	kfree(data);
+out:
+	return sz;
+}
+
+
+static const struct file_operations ecryptfs_miscdev_fops = {
+	.open    = ecryptfs_miscdev_open,
+	.poll    = ecryptfs_miscdev_poll,
+	.read    = ecryptfs_miscdev_read,
+	.write   = ecryptfs_miscdev_write,
+	.release = ecryptfs_miscdev_release,
+};
+
+static struct miscdevice ecryptfs_miscdev = {
+	.minor = MISC_DYNAMIC_MINOR,
+	.name  = "ecryptfs",
+	.fops  = &ecryptfs_miscdev_fops
+};
+
+/**
+ * ecryptfs_init_ecryptfs_miscdev
+ *
+ * Messages sent to the userspace daemon from the kernel are placed on
+ * a queue associated with the daemon. The next read against the
+ * miscdev handle by that daemon will return the oldest message placed
+ * on the message queue for the daemon.
+ *
+ * Returns zero on success; non-zero otherwise
+ */
+int ecryptfs_init_ecryptfs_miscdev(void)
+{
+	int rc;
+
+	atomic_set(&ecryptfs_num_miscdev_opens, 0);
+	mutex_lock(&ecryptfs_daemon_hash_mux);
+	rc = misc_register(&ecryptfs_miscdev);
+	if (rc)
+		printk(KERN_ERR "%s: Failed to register miscellaneous device "
+		       "for communications with userspace daemons; rc = [%d]\n",
+		       __func__, rc);
+	mutex_unlock(&ecryptfs_daemon_hash_mux);
+	return rc;
+}
+
+/**
+ * ecryptfs_destroy_ecryptfs_miscdev
+ *
+ * All of the daemons must be exorcised prior to calling this
+ * function.
+ */
+void ecryptfs_destroy_ecryptfs_miscdev(void)
+{
+	BUG_ON(atomic_read(&ecryptfs_num_miscdev_opens) != 0);
+	misc_deregister(&ecryptfs_miscdev);
+}