drivers/xen/gntalloc.c - maze/linux - Git at Google

 /******************************************************************************
  * gntalloc.c
  *
  * Device for creating grant references (in user-space) that may be shared
  * with other domains.
  *
  * 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
  */

 /*
  * This driver exists to allow userspace programs in Linux to allocate kernel
  * memory that will later be shared with another domain.  Without this device,
  * Linux userspace programs cannot create grant references.
  *
  * How this stuff works:
  *   X -> granting a page to Y
  *   Y -> mapping the grant from X
  *
  *   1. X uses the gntalloc device to allocate a page of kernel memory, P.
  *   2. X creates an entry in the grant table that says domid(Y) can access P.
  *      This is done without a hypercall unless the grant table needs expansion.
  *   3. X gives the grant reference identifier, GREF, to Y.
  *   4. Y maps the page, either directly into kernel memory for use in a backend
  *      driver, or via a the gntdev device to map into the address space of an
  *      application running in Y. This is the first point at which Xen does any
  *      tracking of the page.
  *   5. A program in X mmap()s a segment of the gntalloc device that corresponds
  *      to the shared page, and can now communicate with Y over the shared page.
  *
  *
  * NOTE TO USERSPACE LIBRARIES:
  *   The grant allocation and mmap()ing are, naturally, two separate operations.
  *   You set up the sharing by calling the create ioctl() and then the mmap().
  *   Teardown requires munmap() and either close() or ioctl().
  *
  * WARNING: Since Xen does not allow a guest to forcibly end the use of a grant
  * reference, this device can be used to consume kernel memory by leaving grant
  * references mapped by another domain when an application exits. Therefore,
  * there is a global limit on the number of pages that can be allocated. When
  * all references to the page are unmapped, it will be freed during the next
  * grant operation.
  */

 #include <linux/atomic.h>
 #include <linux/module.h>
 #include <linux/miscdevice.h>
 #include <linux/kernel.h>
 #include <linux/init.h>
 #include <linux/slab.h>
 #include <linux/fs.h>
 #include <linux/device.h>
 #include <linux/mm.h>
 #include <linux/uaccess.h>
 #include <linux/types.h>
 #include <linux/list.h>

 #include <xen/xen.h>
 #include <xen/page.h>
 #include <xen/grant_table.h>
 #include <xen/gntalloc.h>

 static int limit = 1024;
 module_param(limit, int, 0644);
 MODULE_PARM_DESC(limit, "Maximum number of grants that may be allocated by "
 		"the gntalloc device");

 static LIST_HEAD(gref_list);
 static DEFINE_SPINLOCK(gref_lock);
 static int gref_size;

 /* Metadata on a grant reference. */
 struct gntalloc_gref {
 	struct list_head next_gref;  /* list entry gref_list */
 	struct list_head next_file;  /* list entry file->list, if open */
 	struct page *page;	     /* The shared page */
 	uint64_t file_index;         /* File offset for mmap() */
 	unsigned int users;          /* Use count - when zero, waiting on Xen */
 	grant_ref_t gref_id;         /* The grant reference number */
 };

 struct gntalloc_file_private_data {
 	struct list_head list;
 	uint64_t index;
 };

 static void __del_gref(struct gntalloc_gref *gref);

 static void do_cleanup(void)
 {
 	struct gntalloc_gref *gref, *n;
 	list_for_each_entry_safe(gref, n, &gref_list, next_gref) {
 		if (!gref->users)
 			__del_gref(gref);
 	}
 }

 static int add_grefs(struct ioctl_gntalloc_alloc_gref *op,
 	uint32_t *gref_ids, struct gntalloc_file_private_data *priv)
 {
 	int i, rc, readonly;
 	LIST_HEAD(queue_gref);
 	LIST_HEAD(queue_file);
 	struct gntalloc_gref *gref;

 	readonly = !(op->flags & GNTALLOC_FLAG_WRITABLE);
 	rc = -ENOMEM;
 	for (i = 0; i < op->count; i++) {
 		gref = kzalloc(sizeof(*gref), GFP_KERNEL);
 		if (!gref)
 			goto undo;
 		list_add_tail(&gref->next_gref, &queue_gref);
 		list_add_tail(&gref->next_file, &queue_file);
 		gref->users = 1;
 		gref->file_index = op->index + i * PAGE_SIZE;
 		gref->page = alloc_page(GFP_KERNEL|__GFP_ZERO);
 		if (!gref->page)
 			goto undo;

 		/* Grant foreign access to the page. */
 		gref->gref_id = gnttab_grant_foreign_access(op->domid,
 			pfn_to_mfn(page_to_pfn(gref->page)), readonly);
 		if (gref->gref_id < 0) {
 			rc = gref->gref_id;
 			goto undo;
 		}
 		gref_ids[i] = gref->gref_id;
 	}

 	/* Add to gref lists. */
 	spin_lock(&gref_lock);
 	list_splice_tail(&queue_gref, &gref_list);
 	list_splice_tail(&queue_file, &priv->list);
 	spin_unlock(&gref_lock);

 	return 0;

 undo:
 	spin_lock(&gref_lock);
 	gref_size -= (op->count - i);

 	list_for_each_entry(gref, &queue_file, next_file) {
 		/* __del_gref does not remove from queue_file */
 		__del_gref(gref);
 	}

 	/* It's possible for the target domain to map the just-allocated grant
 	 * references by blindly guessing their IDs; if this is done, then
 	 * __del_gref will leave them in the queue_gref list. They need to be
 	 * added to the global list so that we can free them when they are no
 	 * longer referenced.
 	 */
 	if (unlikely(!list_empty(&queue_gref)))
 		list_splice_tail(&queue_gref, &gref_list);
 	spin_unlock(&gref_lock);
 	return rc;
 }

 static void __del_gref(struct gntalloc_gref *gref)
 {
 	if (gref->gref_id > 0) {
 		if (gnttab_query_foreign_access(gref->gref_id))
 			return;

 		if (!gnttab_end_foreign_access_ref(gref->gref_id, 0))
 			return;
 	}

 	gref_size--;
 	list_del(&gref->next_gref);

 	if (gref->page)
 		__free_page(gref->page);

 	kfree(gref);
 }

 /* finds contiguous grant references in a file, returns the first */
 static struct gntalloc_gref *find_grefs(struct gntalloc_file_private_data *priv,
 		uint64_t index, uint32_t count)
 {
 	struct gntalloc_gref *rv = NULL, *gref;
 	list_for_each_entry(gref, &priv->list, next_file) {
 		if (gref->file_index == index && !rv)
 			rv = gref;
 		if (rv) {
 			if (gref->file_index != index)
 				return NULL;
 			index += PAGE_SIZE;
 			count--;
 			if (count == 0)
 				return rv;
 		}
 	}
 	return NULL;
 }

 /*
  * -------------------------------------
  *  File operations.
  * -------------------------------------
  */
 static int gntalloc_open(struct inode *inode, struct file *filp)
 {
 	struct gntalloc_file_private_data *priv;

 	priv = kzalloc(sizeof(*priv), GFP_KERNEL);
 	if (!priv)
 		goto out_nomem;
 	INIT_LIST_HEAD(&priv->list);

 	filp->private_data = priv;

 	pr_debug("%s: priv %p\n", __func__, priv);

 	return 0;

 out_nomem:
 	return -ENOMEM;
 }

 static int gntalloc_release(struct inode *inode, struct file *filp)
 {
 	struct gntalloc_file_private_data *priv = filp->private_data;
 	struct gntalloc_gref *gref;

 	pr_debug("%s: priv %p\n", __func__, priv);

 	spin_lock(&gref_lock);
 	while (!list_empty(&priv->list)) {
 		gref = list_entry(priv->list.next,
 			struct gntalloc_gref, next_file);
 		list_del(&gref->next_file);
 		gref->users--;
 		if (gref->users == 0)
 			__del_gref(gref);
 	}
 	kfree(priv);
 	spin_unlock(&gref_lock);

 	return 0;
 }

 static long gntalloc_ioctl_alloc(struct gntalloc_file_private_data *priv,
 		struct ioctl_gntalloc_alloc_gref __user *arg)
 {
 	int rc = 0;
 	struct ioctl_gntalloc_alloc_gref op;
 	uint32_t *gref_ids;

 	pr_debug("%s: priv %p\n", __func__, priv);

 	if (copy_from_user(&op, arg, sizeof(op))) {
 		rc = -EFAULT;
 		goto out;
 	}

 	gref_ids = kzalloc(sizeof(gref_ids[0]) * op.count, GFP_TEMPORARY);
 	if (!gref_ids) {
 		rc = -ENOMEM;
 		goto out;
 	}

 	spin_lock(&gref_lock);
 	/* Clean up pages that were at zero (local) users but were still mapped
 	 * by remote domains. Since those pages count towards the limit that we
 	 * are about to enforce, removing them here is a good idea.
 	 */
 	do_cleanup();
 	if (gref_size + op.count > limit) {
 		spin_unlock(&gref_lock);
 		rc = -ENOSPC;
 		goto out_free;
 	}
 	gref_size += op.count;
 	op.index = priv->index;
 	priv->index += op.count * PAGE_SIZE;
 	spin_unlock(&gref_lock);

 	rc = add_grefs(&op, gref_ids, priv);
 	if (rc < 0)
 		goto out_free;

 	/* Once we finish add_grefs, it is unsafe to touch the new reference,
 	 * since it is possible for a concurrent ioctl to remove it (by guessing
 	 * its index). If the userspace application doesn't provide valid memory
 	 * to write the IDs to, then it will need to close the file in order to
 	 * release - which it will do by segfaulting when it tries to access the
 	 * IDs to close them.
 	 */
 	if (copy_to_user(arg, &op, sizeof(op))) {
 		rc = -EFAULT;
 		goto out_free;
 	}
 	if (copy_to_user(arg->gref_ids, gref_ids,
 			sizeof(gref_ids[0]) * op.count)) {
 		rc = -EFAULT;
 		goto out_free;
 	}

 out_free:
 	kfree(gref_ids);
 out:
 	return rc;
 }

 static long gntalloc_ioctl_dealloc(struct gntalloc_file_private_data *priv,
 		void __user *arg)
 {
 	int i, rc = 0;
 	struct ioctl_gntalloc_dealloc_gref op;
 	struct gntalloc_gref *gref, *n;

 	pr_debug("%s: priv %p\n", __func__, priv);

 	if (copy_from_user(&op, arg, sizeof(op))) {
 		rc = -EFAULT;
 		goto dealloc_grant_out;
 	}

 	spin_lock(&gref_lock);
 	gref = find_grefs(priv, op.index, op.count);
 	if (gref) {
 		/* Remove from the file list only, and decrease reference count.
 		 * The later call to do_cleanup() will remove from gref_list and
 		 * free the memory if the pages aren't mapped anywhere.
 		 */
 		for (i = 0; i < op.count; i++) {
 			n = list_entry(gref->next_file.next,
 				struct gntalloc_gref, next_file);
 			list_del(&gref->next_file);
 			gref->users--;
 			gref = n;
 		}
 	} else {
 		rc = -EINVAL;
 	}

 	do_cleanup();

 	spin_unlock(&gref_lock);
 dealloc_grant_out:
 	return rc;
 }

 static long gntalloc_ioctl(struct file *filp, unsigned int cmd,
 		unsigned long arg)
 {
 	struct gntalloc_file_private_data *priv = filp->private_data;

 	switch (cmd) {
 	case IOCTL_GNTALLOC_ALLOC_GREF:
 		return gntalloc_ioctl_alloc(priv, (void __user *)arg);

 	case IOCTL_GNTALLOC_DEALLOC_GREF:
 		return gntalloc_ioctl_dealloc(priv, (void __user *)arg);

 	default:
 		return -ENOIOCTLCMD;
 	}

 	return 0;
 }

 static void gntalloc_vma_close(struct vm_area_struct *vma)
 {
 	struct gntalloc_gref *gref = vma->vm_private_data;
 	if (!gref)
 		return;

 	spin_lock(&gref_lock);
 	gref->users--;
 	if (gref->users == 0)
 		__del_gref(gref);
 	spin_unlock(&gref_lock);
 }

 static struct vm_operations_struct gntalloc_vmops = {
 	.close = gntalloc_vma_close,
 };

 static int gntalloc_mmap(struct file *filp, struct vm_area_struct *vma)
 {
 	struct gntalloc_file_private_data *priv = filp->private_data;
 	struct gntalloc_gref *gref;
 	int count = (vma->vm_end - vma->vm_start) >> PAGE_SHIFT;
 	int rv, i;

 	pr_debug("%s: priv %p, page %lu+%d\n", __func__,
 		       priv, vma->vm_pgoff, count);

 	if (!(vma->vm_flags & VM_SHARED)) {
 		printk(KERN_ERR "%s: Mapping must be shared.\n", __func__);
 		return -EINVAL;
 	}

 	spin_lock(&gref_lock);
 	gref = find_grefs(priv, vma->vm_pgoff << PAGE_SHIFT, count);
 	if (gref == NULL) {
 		rv = -ENOENT;
 		pr_debug("%s: Could not find grant reference",
 				__func__);
 		goto out_unlock;
 	}

 	vma->vm_private_data = gref;

 	vma->vm_flags |= VM_RESERVED;
 	vma->vm_flags |= VM_DONTCOPY;
 	vma->vm_flags |= VM_PFNMAP | VM_PFN_AT_MMAP;

 	vma->vm_ops = &gntalloc_vmops;

 	for (i = 0; i < count; i++) {
 		gref->users++;
 		rv = vm_insert_page(vma, vma->vm_start + i * PAGE_SIZE,
 				gref->page);
 		if (rv)
 			goto out_unlock;

 		gref = list_entry(gref->next_file.next,
 				struct gntalloc_gref, next_file);
 	}
 	rv = 0;

 out_unlock:
 	spin_unlock(&gref_lock);
 	return rv;
 }

 static const struct file_operations gntalloc_fops = {
 	.owner = THIS_MODULE,
 	.open = gntalloc_open,
 	.release = gntalloc_release,
 	.unlocked_ioctl = gntalloc_ioctl,
 	.mmap = gntalloc_mmap
 };

 /*
  * -------------------------------------
  * Module creation/destruction.
  * -------------------------------------
  */
 static struct miscdevice gntalloc_miscdev = {
 	.minor	= MISC_DYNAMIC_MINOR,
 	.name	= "xen/gntalloc",
 	.fops	= &gntalloc_fops,
 };

 static int __init gntalloc_init(void)
 {
 	int err;

 	if (!xen_domain())
 		return -ENODEV;

 	err = misc_register(&gntalloc_miscdev);
 	if (err != 0) {
 		printk(KERN_ERR "Could not register misc gntalloc device\n");
 		return err;
 	}

 	pr_debug("Created grant allocation device at %d,%d\n",
 			MISC_MAJOR, gntalloc_miscdev.minor);

 	return 0;
 }

 static void __exit gntalloc_exit(void)
 {
 	misc_deregister(&gntalloc_miscdev);
 }

 module_init(gntalloc_init);
 module_exit(gntalloc_exit);

 MODULE_LICENSE("GPL");
 MODULE_AUTHOR("Carter Weatherly <carter.weatherly@jhuapl.edu>, "
 		"Daniel De Graaf <dgdegra@tycho.nsa.gov>");
 MODULE_DESCRIPTION("User-space grant reference allocator driver");
	/******************************************************************************
	* gntalloc.c
	*
	* Device for creating grant references (in user-space) that may be shared
	* with other domains.
	*
	* 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
	*/

	/*
	* This driver exists to allow userspace programs in Linux to allocate kernel
	* memory that will later be shared with another domain. Without this device,
	* Linux userspace programs cannot create grant references.
	*
	* How this stuff works:
	* X -> granting a page to Y
	* Y -> mapping the grant from X
	*
	* 1. X uses the gntalloc device to allocate a page of kernel memory, P.
	* 2. X creates an entry in the grant table that says domid(Y) can access P.
	* This is done without a hypercall unless the grant table needs expansion.
	* 3. X gives the grant reference identifier, GREF, to Y.
	* 4. Y maps the page, either directly into kernel memory for use in a backend
	* driver, or via a the gntdev device to map into the address space of an
	* application running in Y. This is the first point at which Xen does any
	* tracking of the page.
	* 5. A program in X mmap()s a segment of the gntalloc device that corresponds
	* to the shared page, and can now communicate with Y over the shared page.
	*
	*
	* NOTE TO USERSPACE LIBRARIES:
	* The grant allocation and mmap()ing are, naturally, two separate operations.
	* You set up the sharing by calling the create ioctl() and then the mmap().
	* Teardown requires munmap() and either close() or ioctl().
	*
	* WARNING: Since Xen does not allow a guest to forcibly end the use of a grant
	* reference, this device can be used to consume kernel memory by leaving grant
	* references mapped by another domain when an application exits. Therefore,
	* there is a global limit on the number of pages that can be allocated. When
	* all references to the page are unmapped, it will be freed during the next
	* grant operation.
	*/

	#include <linux/atomic.h>
	#include <linux/module.h>
	#include <linux/miscdevice.h>
	#include <linux/kernel.h>
	#include <linux/init.h>
	#include <linux/slab.h>
	#include <linux/fs.h>
	#include <linux/device.h>
	#include <linux/mm.h>
	#include <linux/uaccess.h>
	#include <linux/types.h>
	#include <linux/list.h>

	#include <xen/xen.h>
	#include <xen/page.h>
	#include <xen/grant_table.h>
	#include <xen/gntalloc.h>

	static int limit = 1024;
	module_param(limit, int, 0644);
	MODULE_PARM_DESC(limit, "Maximum number of grants that may be allocated by "
	"the gntalloc device");

	static LIST_HEAD(gref_list);
	static DEFINE_SPINLOCK(gref_lock);
	static int gref_size;

	/* Metadata on a grant reference. */
	struct gntalloc_gref {
	struct list_head next_gref; /* list entry gref_list */
	struct list_head next_file; /* list entry file->list, if open */
	struct page page; / The shared page */
	uint64_t file_index; /* File offset for mmap() */
	unsigned int users; /* Use count - when zero, waiting on Xen */
	grant_ref_t gref_id; /* The grant reference number */
	};

	struct gntalloc_file_private_data {
	struct list_head list;
	uint64_t index;
	};

	static void __del_gref(struct gntalloc_gref *gref);

	static void do_cleanup(void)
	{
	struct gntalloc_gref gref, n;
	list_for_each_entry_safe(gref, n, &gref_list, next_gref) {
	if (!gref->users)
	__del_gref(gref);
	}
	}

	static int add_grefs(struct ioctl_gntalloc_alloc_gref *op,
	uint32_t gref_ids, struct gntalloc_file_private_data priv)
	{
	int i, rc, readonly;
	LIST_HEAD(queue_gref);
	LIST_HEAD(queue_file);
	struct gntalloc_gref *gref;

	readonly = !(op->flags & GNTALLOC_FLAG_WRITABLE);
	rc = -ENOMEM;
	for (i = 0; i < op->count; i++) {
	gref = kzalloc(sizeof(*gref), GFP_KERNEL);
	if (!gref)
	goto undo;
	list_add_tail(&gref->next_gref, &queue_gref);
	list_add_tail(&gref->next_file, &queue_file);
	gref->users = 1;
	gref->file_index = op->index + i * PAGE_SIZE;
	gref->page = alloc_page(GFP_KERNEL\|__GFP_ZERO);
	if (!gref->page)
	goto undo;

	/* Grant foreign access to the page. */
	gref->gref_id = gnttab_grant_foreign_access(op->domid,
	pfn_to_mfn(page_to_pfn(gref->page)), readonly);
	if (gref->gref_id < 0) {
	rc = gref->gref_id;
	goto undo;
	}
	gref_ids[i] = gref->gref_id;
	}

	/* Add to gref lists. */
	spin_lock(&gref_lock);
	list_splice_tail(&queue_gref, &gref_list);
	list_splice_tail(&queue_file, &priv->list);
	spin_unlock(&gref_lock);

	return 0;

	undo:
	spin_lock(&gref_lock);
	gref_size -= (op->count - i);

	list_for_each_entry(gref, &queue_file, next_file) {
	/* __del_gref does not remove from queue_file */
	__del_gref(gref);
	}

	/* It's possible for the target domain to map the just-allocated grant
	* references by blindly guessing their IDs; if this is done, then
	* __del_gref will leave them in the queue_gref list. They need to be
	* added to the global list so that we can free them when they are no
	* longer referenced.
	*/
	if (unlikely(!list_empty(&queue_gref)))
	list_splice_tail(&queue_gref, &gref_list);
	spin_unlock(&gref_lock);
	return rc;
	}

	static void __del_gref(struct gntalloc_gref *gref)
	{
	if (gref->gref_id > 0) {
	if (gnttab_query_foreign_access(gref->gref_id))
	return;

	if (!gnttab_end_foreign_access_ref(gref->gref_id, 0))
	return;
	}

	gref_size--;
	list_del(&gref->next_gref);

	if (gref->page)
	__free_page(gref->page);

	kfree(gref);
	}

	/* finds contiguous grant references in a file, returns the first */
	static struct gntalloc_gref find_grefs(struct gntalloc_file_private_data priv,
	uint64_t index, uint32_t count)
	{
	struct gntalloc_gref rv = NULL, gref;
	list_for_each_entry(gref, &priv->list, next_file) {
	if (gref->file_index == index && !rv)
	rv = gref;
	if (rv) {
	if (gref->file_index != index)
	return NULL;
	index += PAGE_SIZE;
	count--;
	if (count == 0)
	return rv;
	}
	}
	return NULL;
	}

	/*
	* -------------------------------------
	* File operations.
	* -------------------------------------
	*/
	static int gntalloc_open(struct inode inode, struct file filp)
	{
	struct gntalloc_file_private_data *priv;

	priv = kzalloc(sizeof(*priv), GFP_KERNEL);
	if (!priv)
	goto out_nomem;
	INIT_LIST_HEAD(&priv->list);

	filp->private_data = priv;

	pr_debug("%s: priv %p\n", __func__, priv);

	return 0;

	out_nomem:
	return -ENOMEM;
	}

	static int gntalloc_release(struct inode inode, struct file filp)
	{
	struct gntalloc_file_private_data *priv = filp->private_data;
	struct gntalloc_gref *gref;

	pr_debug("%s: priv %p\n", __func__, priv);

	spin_lock(&gref_lock);
	while (!list_empty(&priv->list)) {
	gref = list_entry(priv->list.next,
	struct gntalloc_gref, next_file);
	list_del(&gref->next_file);
	gref->users--;
	if (gref->users == 0)
	__del_gref(gref);
	}
	kfree(priv);
	spin_unlock(&gref_lock);

	return 0;
	}

	static long gntalloc_ioctl_alloc(struct gntalloc_file_private_data *priv,
	struct ioctl_gntalloc_alloc_gref __user *arg)
	{
	int rc = 0;
	struct ioctl_gntalloc_alloc_gref op;
	uint32_t *gref_ids;

	pr_debug("%s: priv %p\n", __func__, priv);

	if (copy_from_user(&op, arg, sizeof(op))) {
	rc = -EFAULT;
	goto out;
	}

	gref_ids = kzalloc(sizeof(gref_ids[0]) * op.count, GFP_TEMPORARY);
	if (!gref_ids) {
	rc = -ENOMEM;
	goto out;
	}

	spin_lock(&gref_lock);
	/* Clean up pages that were at zero (local) users but were still mapped
	* by remote domains. Since those pages count towards the limit that we
	* are about to enforce, removing them here is a good idea.
	*/
	do_cleanup();
	if (gref_size + op.count > limit) {
	spin_unlock(&gref_lock);
	rc = -ENOSPC;
	goto out_free;
	}
	gref_size += op.count;
	op.index = priv->index;
	priv->index += op.count * PAGE_SIZE;
	spin_unlock(&gref_lock);

	rc = add_grefs(&op, gref_ids, priv);
	if (rc < 0)
	goto out_free;

	/* Once we finish add_grefs, it is unsafe to touch the new reference,
	* since it is possible for a concurrent ioctl to remove it (by guessing
	* its index). If the userspace application doesn't provide valid memory
	* to write the IDs to, then it will need to close the file in order to
	* release - which it will do by segfaulting when it tries to access the
	* IDs to close them.
	*/
	if (copy_to_user(arg, &op, sizeof(op))) {
	rc = -EFAULT;
	goto out_free;
	}
	if (copy_to_user(arg->gref_ids, gref_ids,
	sizeof(gref_ids[0]) * op.count)) {
	rc = -EFAULT;
	goto out_free;
	}

	out_free:
	kfree(gref_ids);
	out:
	return rc;
	}

	static long gntalloc_ioctl_dealloc(struct gntalloc_file_private_data *priv,
	void __user *arg)
	{
	int i, rc = 0;
	struct ioctl_gntalloc_dealloc_gref op;
	struct gntalloc_gref gref, n;

	pr_debug("%s: priv %p\n", __func__, priv);

	if (copy_from_user(&op, arg, sizeof(op))) {
	rc = -EFAULT;
	goto dealloc_grant_out;
	}

	spin_lock(&gref_lock);
	gref = find_grefs(priv, op.index, op.count);
	if (gref) {
	/* Remove from the file list only, and decrease reference count.
	* The later call to do_cleanup() will remove from gref_list and
	* free the memory if the pages aren't mapped anywhere.
	*/
	for (i = 0; i < op.count; i++) {
	n = list_entry(gref->next_file.next,
	struct gntalloc_gref, next_file);
	list_del(&gref->next_file);
	gref->users--;
	gref = n;
	}
	} else {
	rc = -EINVAL;
	}

	do_cleanup();

	spin_unlock(&gref_lock);
	dealloc_grant_out:
	return rc;
	}

	static long gntalloc_ioctl(struct file *filp, unsigned int cmd,
	unsigned long arg)
	{
	struct gntalloc_file_private_data *priv = filp->private_data;

	switch (cmd) {
	case IOCTL_GNTALLOC_ALLOC_GREF:
	return gntalloc_ioctl_alloc(priv, (void __user *)arg);

	case IOCTL_GNTALLOC_DEALLOC_GREF:
	return gntalloc_ioctl_dealloc(priv, (void __user *)arg);

	default:
	return -ENOIOCTLCMD;
	}

	return 0;
	}

	static void gntalloc_vma_close(struct vm_area_struct *vma)
	{
	struct gntalloc_gref *gref = vma->vm_private_data;
	if (!gref)
	return;

	spin_lock(&gref_lock);
	gref->users--;
	if (gref->users == 0)
	__del_gref(gref);
	spin_unlock(&gref_lock);
	}

	static struct vm_operations_struct gntalloc_vmops = {
	.close = gntalloc_vma_close,
	};

	static int gntalloc_mmap(struct file filp, struct vm_area_struct vma)
	{
	struct gntalloc_file_private_data *priv = filp->private_data;
	struct gntalloc_gref *gref;
	int count = (vma->vm_end - vma->vm_start) >> PAGE_SHIFT;
	int rv, i;

	pr_debug("%s: priv %p, page %lu+%d\n", __func__,
	priv, vma->vm_pgoff, count);

	if (!(vma->vm_flags & VM_SHARED)) {
	printk(KERN_ERR "%s: Mapping must be shared.\n", __func__);
	return -EINVAL;
	}

	spin_lock(&gref_lock);
	gref = find_grefs(priv, vma->vm_pgoff << PAGE_SHIFT, count);
	if (gref == NULL) {
	rv = -ENOENT;
	pr_debug("%s: Could not find grant reference",
	__func__);
	goto out_unlock;
	}

	vma->vm_private_data = gref;

	vma->vm_flags \|= VM_RESERVED;
	vma->vm_flags \|= VM_DONTCOPY;
	vma->vm_flags \|= VM_PFNMAP \| VM_PFN_AT_MMAP;

	vma->vm_ops = &gntalloc_vmops;

	for (i = 0; i < count; i++) {
	gref->users++;
	rv = vm_insert_page(vma, vma->vm_start + i * PAGE_SIZE,
	gref->page);
	if (rv)
	goto out_unlock;

	gref = list_entry(gref->next_file.next,
	struct gntalloc_gref, next_file);
	}
	rv = 0;

	out_unlock:
	spin_unlock(&gref_lock);
	return rv;
	}

	static const struct file_operations gntalloc_fops = {
	.owner = THIS_MODULE,
	.open = gntalloc_open,
	.release = gntalloc_release,
	.unlocked_ioctl = gntalloc_ioctl,
	.mmap = gntalloc_mmap
	};

	/*
	* -------------------------------------
	* Module creation/destruction.
	* -------------------------------------
	*/
	static struct miscdevice gntalloc_miscdev = {
	.minor = MISC_DYNAMIC_MINOR,
	.name = "xen/gntalloc",
	.fops = &gntalloc_fops,
	};

	static int __init gntalloc_init(void)
	{
	int err;

	if (!xen_domain())
	return -ENODEV;

	err = misc_register(&gntalloc_miscdev);
	if (err != 0) {
	printk(KERN_ERR "Could not register misc gntalloc device\n");
	return err;
	}

	pr_debug("Created grant allocation device at %d,%d\n",
	MISC_MAJOR, gntalloc_miscdev.minor);

	return 0;
	}

	static void __exit gntalloc_exit(void)
	{
	misc_deregister(&gntalloc_miscdev);
	}

	module_init(gntalloc_init);
	module_exit(gntalloc_exit);

	MODULE_LICENSE("GPL");
	MODULE_AUTHOR("Carter Weatherly <carter.weatherly@jhuapl.edu>, "
	"Daniel De Graaf <dgdegra@tycho.nsa.gov>");
	MODULE_DESCRIPTION("User-space grant reference allocator driver");