fs/ecryptfs/read_write.c - maze/linux - Git at Google

 /**
  * eCryptfs: Linux filesystem encryption layer
  *
  * Copyright (C) 2007 International Business Machines Corp.
  *   Author(s): Michael A. Halcrow <mahalcro@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 as
  * published by the Free Software Foundation; either version 2 of the
  * License, or (at your option) any later version.
  *
  * 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/pagemap.h>
 #include "ecryptfs_kernel.h"

 /**
  * ecryptfs_write_lower
  * @ecryptfs_inode: The eCryptfs inode
  * @data: Data to write
  * @offset: Byte offset in the lower file to which to write the data
  * @size: Number of bytes from @data to write at @offset in the lower
  *        file
  *
  * Write data to the lower file.
  *
  * Returns zero on success; non-zero on error
  */
 int ecryptfs_write_lower(struct inode *ecryptfs_inode, char *data,
 			 loff_t offset, size_t size)
 {
 	struct ecryptfs_inode_info *inode_info;
 	ssize_t octets_written;
 	mm_segment_t fs_save;
 	int rc = 0;

 	inode_info = ecryptfs_inode_to_private(ecryptfs_inode);
 	mutex_lock(&inode_info->lower_file_mutex);
 	BUG_ON(!inode_info->lower_file);
 	inode_info->lower_file->f_pos = offset;
 	fs_save = get_fs();
 	set_fs(get_ds());
 	octets_written = vfs_write(inode_info->lower_file, data, size,
 				   &inode_info->lower_file->f_pos);
 	set_fs(fs_save);
 	if (octets_written < 0) {
 		printk(KERN_ERR "%s: octets_written = [%td]; "
 		       "expected [%td]\n", __FUNCTION__, octets_written, size);
 		rc = -EINVAL;
 	}
 	mutex_unlock(&inode_info->lower_file_mutex);
 	mark_inode_dirty_sync(ecryptfs_inode);
 	return rc;
 }

 /**
  * ecryptfs_write_lower_page_segment
  * @ecryptfs_inode: The eCryptfs inode
  * @page_for_lower: The page containing the data to be written to the
  *                  lower file
  * @offset_in_page: The offset in the @page_for_lower from which to
  *                  start writing the data
  * @size: The amount of data from @page_for_lower to write to the
  *        lower file
  *
  * Determines the byte offset in the file for the given page and
  * offset within the page, maps the page, and makes the call to write
  * the contents of @page_for_lower to the lower inode.
  *
  * Returns zero on success; non-zero otherwise
  */
 int ecryptfs_write_lower_page_segment(struct inode *ecryptfs_inode,
 				      struct page *page_for_lower,
 				      size_t offset_in_page, size_t size)
 {
 	char *virt;
 	loff_t offset;
 	int rc;

 	offset = (page_for_lower->index << PAGE_CACHE_SHIFT) + offset_in_page;
 	virt = kmap(page_for_lower);
 	rc = ecryptfs_write_lower(ecryptfs_inode, virt, offset, size);
 	kunmap(page_for_lower);
 	return rc;
 }

 /**
  * ecryptfs_write
  * @ecryptfs_file: The eCryptfs file into which to write
  * @data: Virtual address where data to write is located
  * @offset: Offset in the eCryptfs file at which to begin writing the
  *          data from @data
  * @size: The number of bytes to write from @data
  *
  * Write an arbitrary amount of data to an arbitrary location in the
  * eCryptfs inode page cache. This is done on a page-by-page, and then
  * by an extent-by-extent, basis; individual extents are encrypted and
  * written to the lower page cache (via VFS writes). This function
  * takes care of all the address translation to locations in the lower
  * filesystem; it also handles truncate events, writing out zeros
  * where necessary.
  *
  * Returns zero on success; non-zero otherwise
  */
 int ecryptfs_write(struct file *ecryptfs_file, char *data, loff_t offset,
 		   size_t size)
 {
 	struct page *ecryptfs_page;
 	char *ecryptfs_page_virt;
 	u64 ecryptfs_file_size = i_size_read(ecryptfs_file->f_dentry->d_inode);
 	loff_t data_offset = 0;
 	loff_t pos;
 	int rc = 0;

 	if (offset > ecryptfs_file_size)
 		pos = ecryptfs_file_size;
 	else
 		pos = offset;
 	while (pos < (offset + size)) {
 		pgoff_t ecryptfs_page_idx = (pos >> PAGE_CACHE_SHIFT);
 		size_t start_offset_in_page = (pos & ~PAGE_CACHE_MASK);
 		size_t num_bytes = (PAGE_CACHE_SIZE - start_offset_in_page);
 		size_t total_remaining_bytes = ((offset + size) - pos);

 		if (num_bytes > total_remaining_bytes)
 			num_bytes = total_remaining_bytes;
 		if (pos < offset) {
 			size_t total_remaining_zeros = (offset - pos);

 			if (num_bytes > total_remaining_zeros)
 				num_bytes = total_remaining_zeros;
 		}
 		ecryptfs_page = ecryptfs_get1page(ecryptfs_file,
 						  ecryptfs_page_idx);
 		if (IS_ERR(ecryptfs_page)) {
 			rc = PTR_ERR(ecryptfs_page);
 			printk(KERN_ERR "%s: Error getting page at "
 			       "index [%ld] from eCryptfs inode "
 			       "mapping; rc = [%d]\n", __FUNCTION__,
 			       ecryptfs_page_idx, rc);
 			goto out;
 		}
 		if (start_offset_in_page) {
 			/* Read in the page from the lower
 			 * into the eCryptfs inode page cache,
 			 * decrypting */
 			rc = ecryptfs_decrypt_page(ecryptfs_page);
 			if (rc) {
 				printk(KERN_ERR "%s: Error decrypting "
 				       "page; rc = [%d]\n",
 				       __FUNCTION__, rc);
 				page_cache_release(ecryptfs_page);
 				goto out;
 			}
 		}
 		ecryptfs_page_virt = kmap_atomic(ecryptfs_page, KM_USER0);
 		if (pos >= offset) {
 			memcpy(((char *)ecryptfs_page_virt
 				+ start_offset_in_page),
 			       (data + data_offset), num_bytes);
 			data_offset += num_bytes;
 		} else {
 			/* We are extending past the previous end of the file.
 			 * Fill in zero values up to the start of where we
 			 * will be writing data. */
 			memset(((char *)ecryptfs_page_virt
 				+ start_offset_in_page), 0, num_bytes);
 		}
 		kunmap_atomic(ecryptfs_page_virt, KM_USER0);
 		flush_dcache_page(ecryptfs_page);
 		rc = ecryptfs_encrypt_page(ecryptfs_page);
 		if (rc) {
 			printk(KERN_ERR "%s: Error encrypting "
 			       "page; rc = [%d]\n", __FUNCTION__, rc);
 			page_cache_release(ecryptfs_page);
 			goto out;
 		}
 		page_cache_release(ecryptfs_page);
 		pos += num_bytes;
 	}
 	if ((offset + size) > ecryptfs_file_size) {
 		i_size_write(ecryptfs_file->f_dentry->d_inode, (offset + size));
 		rc = ecryptfs_write_inode_size_to_metadata(
 			ecryptfs_file->f_dentry->d_inode);
 		if (rc) {
 			printk(KERN_ERR	"Problem with "
 			       "ecryptfs_write_inode_size_to_metadata; "
 			       "rc = [%d]\n", rc);
 			goto out;
 		}
 	}
 out:
 	return rc;
 }

 /**
  * ecryptfs_read_lower
  * @data: The read data is stored here by this function
  * @offset: Byte offset in the lower file from which to read the data
  * @size: Number of bytes to read from @offset of the lower file and
  *        store into @data
  * @ecryptfs_inode: The eCryptfs inode
  *
  * Read @size bytes of data at byte offset @offset from the lower
  * inode into memory location @data.
  *
  * Returns zero on success; non-zero on error
  */
 int ecryptfs_read_lower(char *data, loff_t offset, size_t size,
 			struct inode *ecryptfs_inode)
 {
 	struct ecryptfs_inode_info *inode_info =
 		ecryptfs_inode_to_private(ecryptfs_inode);
 	ssize_t octets_read;
 	mm_segment_t fs_save;
 	size_t i;
 	int rc = 0;

 	mutex_lock(&inode_info->lower_file_mutex);
 	BUG_ON(!inode_info->lower_file);
 	inode_info->lower_file->f_pos = offset;
 	fs_save = get_fs();
 	set_fs(get_ds());
 	octets_read = vfs_read(inode_info->lower_file, data, size,
 			       &inode_info->lower_file->f_pos);
 	set_fs(fs_save);
 	if (octets_read < 0) {
 		printk(KERN_ERR "%s: octets_read = [%td]; "
 		       "expected [%td]\n", __FUNCTION__, octets_read, size);
 		rc = -EINVAL;
 	}
 	mutex_unlock(&inode_info->lower_file_mutex);
 	for (i = 0; i < size; i += PAGE_CACHE_SIZE) {
 		struct page *data_page;

 		data_page = virt_to_page(data + i);
 		flush_dcache_page(data_page);
 		if (rc)
 			ClearPageUptodate(data_page);
 		else
 			SetPageUptodate(data_page);
 	}
 	return rc;
 }

 /**
  * ecryptfs_read_lower_page_segment
  * @page_for_ecryptfs: The page into which data for eCryptfs will be
  *                     written
  * @offset_in_page: Offset in @page_for_ecryptfs from which to start
  *                  writing
  * @size: The number of bytes to write into @page_for_ecryptfs
  * @ecryptfs_inode: The eCryptfs inode
  *
  * Determines the byte offset in the file for the given page and
  * offset within the page, maps the page, and makes the call to read
  * the contents of @page_for_ecryptfs from the lower inode.
  *
  * Returns zero on success; non-zero otherwise
  */
 int ecryptfs_read_lower_page_segment(struct page *page_for_ecryptfs,
 				     pgoff_t page_index,
 				     size_t offset_in_page, size_t size,
 				     struct inode *ecryptfs_inode)
 {
 	char *virt;
 	loff_t offset;
 	int rc;

 	offset = ((page_index << PAGE_CACHE_SHIFT) + offset_in_page);
 	virt = kmap(page_for_ecryptfs);
 	rc = ecryptfs_read_lower(virt, offset, size, ecryptfs_inode);
 	kunmap(page_for_ecryptfs);
 	return rc;
 }

 /**
  * ecryptfs_read
  * @data: The virtual address into which to write the data read (and
  *        possibly decrypted) from the lower file
  * @offset: The offset in the decrypted view of the file from which to
  *          read into @data
  * @size: The number of bytes to read into @data
  * @ecryptfs_file: The eCryptfs file from which to read
  *
  * Read an arbitrary amount of data from an arbitrary location in the
  * eCryptfs page cache. This is done on an extent-by-extent basis;
  * individual extents are decrypted and read from the lower page
  * cache (via VFS reads). This function takes care of all the
  * address translation to locations in the lower filesystem.
  *
  * Returns zero on success; non-zero otherwise
  */
 int ecryptfs_read(char *data, loff_t offset, size_t size,
 		  struct file *ecryptfs_file)
 {
 	struct page *ecryptfs_page;
 	char *ecryptfs_page_virt;
 	u64 ecryptfs_file_size = i_size_read(ecryptfs_file->f_dentry->d_inode);
 	loff_t data_offset = 0;
 	loff_t pos;
 	int rc = 0;

 	if ((offset + size) > ecryptfs_file_size) {
 		rc = -EINVAL;
 		printk(KERN_ERR "%s: Attempt to read data past the end of the "
 			"file; offset = [%lld]; size = [%td]; "
 		       "ecryptfs_file_size = [%lld]\n",
 		       __FUNCTION__, offset, size, ecryptfs_file_size);
 		goto out;
 	}
 	pos = offset;
 	while (pos < (offset + size)) {
 		pgoff_t ecryptfs_page_idx = (pos >> PAGE_CACHE_SHIFT);
 		size_t start_offset_in_page = (pos & ~PAGE_CACHE_MASK);
 		size_t num_bytes = (PAGE_CACHE_SIZE - start_offset_in_page);
 		size_t total_remaining_bytes = ((offset + size) - pos);

 		if (num_bytes > total_remaining_bytes)
 			num_bytes = total_remaining_bytes;
 		ecryptfs_page = ecryptfs_get1page(ecryptfs_file,
 						  ecryptfs_page_idx);
 		if (IS_ERR(ecryptfs_page)) {
 			rc = PTR_ERR(ecryptfs_page);
 			printk(KERN_ERR "%s: Error getting page at "
 			       "index [%ld] from eCryptfs inode "
 			       "mapping; rc = [%d]\n", __FUNCTION__,
 			       ecryptfs_page_idx, rc);
 			goto out;
 		}
 		rc = ecryptfs_decrypt_page(ecryptfs_page);
 		if (rc) {
 			printk(KERN_ERR "%s: Error decrypting "
 			       "page; rc = [%d]\n", __FUNCTION__, rc);
 			page_cache_release(ecryptfs_page);
 			goto out;
 		}
 		ecryptfs_page_virt = kmap_atomic(ecryptfs_page, KM_USER0);
 		memcpy((data + data_offset),
 		       ((char *)ecryptfs_page_virt + start_offset_in_page),
 		       num_bytes);
 		kunmap_atomic(ecryptfs_page_virt, KM_USER0);
 		page_cache_release(ecryptfs_page);
 		pos += num_bytes;
 		data_offset += num_bytes;
 	}
 out:
 	return rc;
 }
	/**
	* eCryptfs: Linux filesystem encryption layer
	*
	* Copyright (C) 2007 International Business Machines Corp.
	* Author(s): Michael A. Halcrow <mahalcro@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 as
	* published by the Free Software Foundation; either version 2 of the
	* License, or (at your option) any later version.
	*
	* 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/pagemap.h>
	#include "ecryptfs_kernel.h"

	/**
	* ecryptfs_write_lower
	* @ecryptfs_inode: The eCryptfs inode
	* @data: Data to write
	* @offset: Byte offset in the lower file to which to write the data
	* @size: Number of bytes from @data to write at @offset in the lower
	* file
	*
	* Write data to the lower file.
	*
	* Returns zero on success; non-zero on error
	*/
	int ecryptfs_write_lower(struct inode ecryptfs_inode, char data,
	loff_t offset, size_t size)
	{
	struct ecryptfs_inode_info *inode_info;
	ssize_t octets_written;
	mm_segment_t fs_save;
	int rc = 0;

	inode_info = ecryptfs_inode_to_private(ecryptfs_inode);
	mutex_lock(&inode_info->lower_file_mutex);
	BUG_ON(!inode_info->lower_file);
	inode_info->lower_file->f_pos = offset;
	fs_save = get_fs();
	set_fs(get_ds());
	octets_written = vfs_write(inode_info->lower_file, data, size,
	&inode_info->lower_file->f_pos);
	set_fs(fs_save);
	if (octets_written < 0) {
	printk(KERN_ERR "%s: octets_written = [%td]; "
	"expected [%td]\n", __FUNCTION__, octets_written, size);
	rc = -EINVAL;
	}
	mutex_unlock(&inode_info->lower_file_mutex);
	mark_inode_dirty_sync(ecryptfs_inode);
	return rc;
	}

	/**
	* ecryptfs_write_lower_page_segment
	* @ecryptfs_inode: The eCryptfs inode
	* @page_for_lower: The page containing the data to be written to the
	* lower file
	* @offset_in_page: The offset in the @page_for_lower from which to
	* start writing the data
	* @size: The amount of data from @page_for_lower to write to the
	* lower file
	*
	* Determines the byte offset in the file for the given page and
	* offset within the page, maps the page, and makes the call to write
	* the contents of @page_for_lower to the lower inode.
	*
	* Returns zero on success; non-zero otherwise
	*/
	int ecryptfs_write_lower_page_segment(struct inode *ecryptfs_inode,
	struct page *page_for_lower,
	size_t offset_in_page, size_t size)
	{
	char *virt;
	loff_t offset;
	int rc;

	offset = (page_for_lower->index << PAGE_CACHE_SHIFT) + offset_in_page;
	virt = kmap(page_for_lower);
	rc = ecryptfs_write_lower(ecryptfs_inode, virt, offset, size);
	kunmap(page_for_lower);
	return rc;
	}

	/**
	* ecryptfs_write
	* @ecryptfs_file: The eCryptfs file into which to write
	* @data: Virtual address where data to write is located
	* @offset: Offset in the eCryptfs file at which to begin writing the
	* data from @data
	* @size: The number of bytes to write from @data
	*
	* Write an arbitrary amount of data to an arbitrary location in the
	* eCryptfs inode page cache. This is done on a page-by-page, and then
	* by an extent-by-extent, basis; individual extents are encrypted and
	* written to the lower page cache (via VFS writes). This function
	* takes care of all the address translation to locations in the lower
	* filesystem; it also handles truncate events, writing out zeros
	* where necessary.
	*
	* Returns zero on success; non-zero otherwise
	*/
	int ecryptfs_write(struct file ecryptfs_file, char data, loff_t offset,
	size_t size)
	{
	struct page *ecryptfs_page;
	char *ecryptfs_page_virt;
	u64 ecryptfs_file_size = i_size_read(ecryptfs_file->f_dentry->d_inode);
	loff_t data_offset = 0;
	loff_t pos;
	int rc = 0;

	if (offset > ecryptfs_file_size)
	pos = ecryptfs_file_size;
	else
	pos = offset;
	while (pos < (offset + size)) {
	pgoff_t ecryptfs_page_idx = (pos >> PAGE_CACHE_SHIFT);
	size_t start_offset_in_page = (pos & ~PAGE_CACHE_MASK);
	size_t num_bytes = (PAGE_CACHE_SIZE - start_offset_in_page);
	size_t total_remaining_bytes = ((offset + size) - pos);

	if (num_bytes > total_remaining_bytes)
	num_bytes = total_remaining_bytes;
	if (pos < offset) {
	size_t total_remaining_zeros = (offset - pos);

	if (num_bytes > total_remaining_zeros)
	num_bytes = total_remaining_zeros;
	}
	ecryptfs_page = ecryptfs_get1page(ecryptfs_file,
	ecryptfs_page_idx);
	if (IS_ERR(ecryptfs_page)) {
	rc = PTR_ERR(ecryptfs_page);
	printk(KERN_ERR "%s: Error getting page at "
	"index [%ld] from eCryptfs inode "
	"mapping; rc = [%d]\n", __FUNCTION__,
	ecryptfs_page_idx, rc);
	goto out;
	}
	if (start_offset_in_page) {
	/* Read in the page from the lower
	* into the eCryptfs inode page cache,
	* decrypting */
	rc = ecryptfs_decrypt_page(ecryptfs_page);
	if (rc) {
	printk(KERN_ERR "%s: Error decrypting "
	"page; rc = [%d]\n",
	__FUNCTION__, rc);
	page_cache_release(ecryptfs_page);
	goto out;
	}
	}
	ecryptfs_page_virt = kmap_atomic(ecryptfs_page, KM_USER0);
	if (pos >= offset) {
	memcpy(((char *)ecryptfs_page_virt
	+ start_offset_in_page),
	(data + data_offset), num_bytes);
	data_offset += num_bytes;
	} else {
	/* We are extending past the previous end of the file.
	* Fill in zero values up to the start of where we
	* will be writing data. */
	memset(((char *)ecryptfs_page_virt
	+ start_offset_in_page), 0, num_bytes);
	}
	kunmap_atomic(ecryptfs_page_virt, KM_USER0);
	flush_dcache_page(ecryptfs_page);
	rc = ecryptfs_encrypt_page(ecryptfs_page);
	if (rc) {
	printk(KERN_ERR "%s: Error encrypting "
	"page; rc = [%d]\n", __FUNCTION__, rc);
	page_cache_release(ecryptfs_page);
	goto out;
	}
	page_cache_release(ecryptfs_page);
	pos += num_bytes;
	}
	if ((offset + size) > ecryptfs_file_size) {
	i_size_write(ecryptfs_file->f_dentry->d_inode, (offset + size));
	rc = ecryptfs_write_inode_size_to_metadata(
	ecryptfs_file->f_dentry->d_inode);
	if (rc) {
	printk(KERN_ERR "Problem with "
	"ecryptfs_write_inode_size_to_metadata; "
	"rc = [%d]\n", rc);
	goto out;
	}
	}
	out:
	return rc;
	}

	/**
	* ecryptfs_read_lower
	* @data: The read data is stored here by this function
	* @offset: Byte offset in the lower file from which to read the data
	* @size: Number of bytes to read from @offset of the lower file and
	* store into @data
	* @ecryptfs_inode: The eCryptfs inode
	*
	* Read @size bytes of data at byte offset @offset from the lower
	* inode into memory location @data.
	*
	* Returns zero on success; non-zero on error
	*/
	int ecryptfs_read_lower(char *data, loff_t offset, size_t size,
	struct inode *ecryptfs_inode)
	{
	struct ecryptfs_inode_info *inode_info =
	ecryptfs_inode_to_private(ecryptfs_inode);
	ssize_t octets_read;
	mm_segment_t fs_save;
	size_t i;
	int rc = 0;

	mutex_lock(&inode_info->lower_file_mutex);
	BUG_ON(!inode_info->lower_file);
	inode_info->lower_file->f_pos = offset;
	fs_save = get_fs();
	set_fs(get_ds());
	octets_read = vfs_read(inode_info->lower_file, data, size,
	&inode_info->lower_file->f_pos);
	set_fs(fs_save);
	if (octets_read < 0) {
	printk(KERN_ERR "%s: octets_read = [%td]; "
	"expected [%td]\n", __FUNCTION__, octets_read, size);
	rc = -EINVAL;
	}
	mutex_unlock(&inode_info->lower_file_mutex);
	for (i = 0; i < size; i += PAGE_CACHE_SIZE) {
	struct page *data_page;

	data_page = virt_to_page(data + i);
	flush_dcache_page(data_page);
	if (rc)
	ClearPageUptodate(data_page);
	else
	SetPageUptodate(data_page);
	}
	return rc;
	}

	/**
	* ecryptfs_read_lower_page_segment
	* @page_for_ecryptfs: The page into which data for eCryptfs will be
	* written
	* @offset_in_page: Offset in @page_for_ecryptfs from which to start
	* writing
	* @size: The number of bytes to write into @page_for_ecryptfs
	* @ecryptfs_inode: The eCryptfs inode
	*
	* Determines the byte offset in the file for the given page and
	* offset within the page, maps the page, and makes the call to read
	* the contents of @page_for_ecryptfs from the lower inode.
	*
	* Returns zero on success; non-zero otherwise
	*/
	int ecryptfs_read_lower_page_segment(struct page *page_for_ecryptfs,
	pgoff_t page_index,
	size_t offset_in_page, size_t size,
	struct inode *ecryptfs_inode)
	{
	char *virt;
	loff_t offset;
	int rc;

	offset = ((page_index << PAGE_CACHE_SHIFT) + offset_in_page);
	virt = kmap(page_for_ecryptfs);
	rc = ecryptfs_read_lower(virt, offset, size, ecryptfs_inode);
	kunmap(page_for_ecryptfs);
	return rc;
	}

	/**
	* ecryptfs_read
	* @data: The virtual address into which to write the data read (and
	* possibly decrypted) from the lower file
	* @offset: The offset in the decrypted view of the file from which to
	* read into @data
	* @size: The number of bytes to read into @data
	* @ecryptfs_file: The eCryptfs file from which to read
	*
	* Read an arbitrary amount of data from an arbitrary location in the
	* eCryptfs page cache. This is done on an extent-by-extent basis;
	* individual extents are decrypted and read from the lower page
	* cache (via VFS reads). This function takes care of all the
	* address translation to locations in the lower filesystem.
	*
	* Returns zero on success; non-zero otherwise
	*/
	int ecryptfs_read(char *data, loff_t offset, size_t size,
	struct file *ecryptfs_file)
	{
	struct page *ecryptfs_page;
	char *ecryptfs_page_virt;
	u64 ecryptfs_file_size = i_size_read(ecryptfs_file->f_dentry->d_inode);
	loff_t data_offset = 0;
	loff_t pos;
	int rc = 0;

	if ((offset + size) > ecryptfs_file_size) {
	rc = -EINVAL;
	printk(KERN_ERR "%s: Attempt to read data past the end of the "
	"file; offset = [%lld]; size = [%td]; "
	"ecryptfs_file_size = [%lld]\n",
	__FUNCTION__, offset, size, ecryptfs_file_size);
	goto out;
	}
	pos = offset;
	while (pos < (offset + size)) {
	pgoff_t ecryptfs_page_idx = (pos >> PAGE_CACHE_SHIFT);
	size_t start_offset_in_page = (pos & ~PAGE_CACHE_MASK);
	size_t num_bytes = (PAGE_CACHE_SIZE - start_offset_in_page);
	size_t total_remaining_bytes = ((offset + size) - pos);

	if (num_bytes > total_remaining_bytes)
	num_bytes = total_remaining_bytes;
	ecryptfs_page = ecryptfs_get1page(ecryptfs_file,
	ecryptfs_page_idx);
	if (IS_ERR(ecryptfs_page)) {
	rc = PTR_ERR(ecryptfs_page);
	printk(KERN_ERR "%s: Error getting page at "
	"index [%ld] from eCryptfs inode "
	"mapping; rc = [%d]\n", __FUNCTION__,
	ecryptfs_page_idx, rc);
	goto out;
	}
	rc = ecryptfs_decrypt_page(ecryptfs_page);
	if (rc) {
	printk(KERN_ERR "%s: Error decrypting "
	"page; rc = [%d]\n", __FUNCTION__, rc);
	page_cache_release(ecryptfs_page);
	goto out;
	}
	ecryptfs_page_virt = kmap_atomic(ecryptfs_page, KM_USER0);
	memcpy((data + data_offset),
	((char *)ecryptfs_page_virt + start_offset_in_page),
	num_bytes);
	kunmap_atomic(ecryptfs_page_virt, KM_USER0);
	page_cache_release(ecryptfs_page);
	pos += num_bytes;
	data_offset += num_bytes;
	}
	out:
	return rc;
	}