fs/vboxsf/file.c - linux - Git at Google

 // SPDX-License-Identifier: MIT
 /*
  * VirtualBox Guest Shared Folders support: Regular file inode and file ops.
  *
  * Copyright (C) 2006-2018 Oracle Corporation
  */

 #include <linux/mm.h>
 #include <linux/page-flags.h>
 #include <linux/pagemap.h>
 #include <linux/highmem.h>
 #include <linux/sizes.h>
 #include "vfsmod.h"

 struct vboxsf_handle {
 	u64 handle;
 	u32 root;
 	u32 access_flags;
 	struct kref refcount;
 	struct list_head head;
 };

 struct vboxsf_handle *vboxsf_create_sf_handle(struct inode *inode,
 					      u64 handle, u32 access_flags)
 {
 	struct vboxsf_inode *sf_i = VBOXSF_I(inode);
 	struct vboxsf_handle *sf_handle;

 	sf_handle = kmalloc(sizeof(*sf_handle), GFP_KERNEL);
 	if (!sf_handle)
 		return ERR_PTR(-ENOMEM);

 	/* the host may have given us different attr then requested */
 	sf_i->force_restat = 1;

 	/* init our handle struct and add it to the inode's handles list */
 	sf_handle->handle = handle;
 	sf_handle->root = VBOXSF_SBI(inode->i_sb)->root;
 	sf_handle->access_flags = access_flags;
 	kref_init(&sf_handle->refcount);

 	mutex_lock(&sf_i->handle_list_mutex);
 	list_add(&sf_handle->head, &sf_i->handle_list);
 	mutex_unlock(&sf_i->handle_list_mutex);

 	return sf_handle;
 }

 static int vboxsf_file_open(struct inode *inode, struct file *file)
 {
 	struct vboxsf_sbi *sbi = VBOXSF_SBI(inode->i_sb);
 	struct shfl_createparms params = {};
 	struct vboxsf_handle *sf_handle;
 	u32 access_flags = 0;
 	int err;

 	/*
 	 * We check the value of params.handle afterwards to find out if
 	 * the call succeeded or failed, as the API does not seem to cleanly
 	 * distinguish error and informational messages.
 	 *
 	 * Furthermore, we must set params.handle to SHFL_HANDLE_NIL to
 	 * make the shared folders host service use our mode parameter.
 	 */
 	params.handle = SHFL_HANDLE_NIL;
 	if (file->f_flags & O_CREAT) {
 		params.create_flags |= SHFL_CF_ACT_CREATE_IF_NEW;
 		/*
 		 * We ignore O_EXCL, as the Linux kernel seems to call create
 		 * beforehand itself, so O_EXCL should always fail.
 		 */
 		if (file->f_flags & O_TRUNC)
 			params.create_flags |= SHFL_CF_ACT_OVERWRITE_IF_EXISTS;
 		else
 			params.create_flags |= SHFL_CF_ACT_OPEN_IF_EXISTS;
 	} else {
 		params.create_flags |= SHFL_CF_ACT_FAIL_IF_NEW;
 		if (file->f_flags & O_TRUNC)
 			params.create_flags |= SHFL_CF_ACT_OVERWRITE_IF_EXISTS;
 	}

 	switch (file->f_flags & O_ACCMODE) {
 	case O_RDONLY:
 		access_flags |= SHFL_CF_ACCESS_READ;
 		break;

 	case O_WRONLY:
 		access_flags |= SHFL_CF_ACCESS_WRITE;
 		break;

 	case O_RDWR:
 		access_flags |= SHFL_CF_ACCESS_READWRITE;
 		break;

 	default:
 		WARN_ON(1);
 	}

 	if (file->f_flags & O_APPEND)
 		access_flags |= SHFL_CF_ACCESS_APPEND;

 	params.create_flags |= access_flags;
 	params.info.attr.mode = inode->i_mode;

 	err = vboxsf_create_at_dentry(file_dentry(file), &params);
 	if (err == 0 && params.handle == SHFL_HANDLE_NIL)
 		err = (params.result == SHFL_FILE_EXISTS) ? -EEXIST : -ENOENT;
 	if (err)
 		return err;

 	sf_handle = vboxsf_create_sf_handle(inode, params.handle, access_flags);
 	if (IS_ERR(sf_handle)) {
 		vboxsf_close(sbi->root, params.handle);
 		return PTR_ERR(sf_handle);
 	}

 	file->private_data = sf_handle;
 	return 0;
 }

 static void vboxsf_handle_release(struct kref *refcount)
 {
 	struct vboxsf_handle *sf_handle =
 		container_of(refcount, struct vboxsf_handle, refcount);

 	vboxsf_close(sf_handle->root, sf_handle->handle);
 	kfree(sf_handle);
 }

 void vboxsf_release_sf_handle(struct inode *inode, struct vboxsf_handle *sf_handle)
 {
 	struct vboxsf_inode *sf_i = VBOXSF_I(inode);

 	mutex_lock(&sf_i->handle_list_mutex);
 	list_del(&sf_handle->head);
 	mutex_unlock(&sf_i->handle_list_mutex);

 	kref_put(&sf_handle->refcount, vboxsf_handle_release);
 }

 static int vboxsf_file_release(struct inode *inode, struct file *file)
 {
 	/*
 	 * When a file is closed on our (the guest) side, we want any subsequent
 	 * accesses done on the host side to see all changes done from our side.
 	 */
 	filemap_write_and_wait(inode->i_mapping);

 	vboxsf_release_sf_handle(inode, file->private_data);
 	return 0;
 }

 /*
  * Write back dirty pages now, because there may not be any suitable
  * open files later
  */
 static void vboxsf_vma_close(struct vm_area_struct *vma)
 {
 	filemap_write_and_wait(vma->vm_file->f_mapping);
 }

 static const struct vm_operations_struct vboxsf_file_vm_ops = {
 	.close		= vboxsf_vma_close,
 	.fault		= filemap_fault,
 	.map_pages	= filemap_map_pages,
 };

 static int vboxsf_file_mmap(struct file *file, struct vm_area_struct *vma)
 {
 	int err;

 	err = generic_file_mmap(file, vma);
 	if (!err)
 		vma->vm_ops = &vboxsf_file_vm_ops;

 	return err;
 }

 /*
  * Note that since we are accessing files on the host's filesystem, files
  * may always be changed underneath us by the host!
  *
  * The vboxsf API between the guest and the host does not offer any functions
  * to deal with this. There is no inode-generation to check for changes, no
  * events / callback on changes and no way to lock files.
  *
  * To avoid returning stale data when a file gets *opened* on our (the guest)
  * side, we do a "stat" on the host side, then compare the mtime with the
  * last known mtime and invalidate the page-cache if they differ.
  * This is done from vboxsf_inode_revalidate().
  *
  * When reads are done through the read_iter fop, it is possible to do
  * further cache revalidation then, there are 3 options to deal with this:
  *
  * 1)  Rely solely on the revalidation done at open time
  * 2)  Do another "stat" and compare mtime again. Unfortunately the vboxsf
  *     host API does not allow stat on handles, so we would need to use
  *     file->f_path.dentry and the stat will then fail if the file was unlinked
  *     or renamed (and there is no thing like NFS' silly-rename). So we get:
  * 2a) "stat" and compare mtime, on stat failure invalidate the cache
  * 2b) "stat" and compare mtime, on stat failure do nothing
  * 3)  Simply always call invalidate_inode_pages2_range on the range of the read
  *
  * Currently we are keeping things KISS and using option 1. this allows
  * directly using generic_file_read_iter without wrapping it.
  *
  * This means that only data written on the host side before open() on
  * the guest side is guaranteed to be seen by the guest. If necessary
  * we may provide other read-cache strategies in the future and make this
  * configurable through a mount option.
  */
 const struct file_operations vboxsf_reg_fops = {
 	.llseek = generic_file_llseek,
 	.read_iter = generic_file_read_iter,
 	.write_iter = generic_file_write_iter,
 	.mmap = vboxsf_file_mmap,
 	.open = vboxsf_file_open,
 	.release = vboxsf_file_release,
 	.fsync = noop_fsync,
 	.splice_read = filemap_splice_read,
 };

 const struct inode_operations vboxsf_reg_iops = {
 	.getattr = vboxsf_getattr,
 	.setattr = vboxsf_setattr
 };

 static int vboxsf_read_folio(struct file *file, struct folio *folio)
 {
 	struct page *page = &folio->page;
 	struct vboxsf_handle *sf_handle = file->private_data;
 	loff_t off = page_offset(page);
 	u32 nread = PAGE_SIZE;
 	u8 *buf;
 	int err;

 	buf = kmap(page);

 	err = vboxsf_read(sf_handle->root, sf_handle->handle, off, &nread, buf);
 	if (err == 0) {
 		memset(&buf[nread], 0, PAGE_SIZE - nread);
 		flush_dcache_page(page);
 		SetPageUptodate(page);
 	} else {
 		SetPageError(page);
 	}

 	kunmap(page);
 	unlock_page(page);
 	return err;
 }

 static struct vboxsf_handle *vboxsf_get_write_handle(struct vboxsf_inode *sf_i)
 {
 	struct vboxsf_handle *h, *sf_handle = NULL;

 	mutex_lock(&sf_i->handle_list_mutex);
 	list_for_each_entry(h, &sf_i->handle_list, head) {
 		if (h->access_flags == SHFL_CF_ACCESS_WRITE ||
 		    h->access_flags == SHFL_CF_ACCESS_READWRITE) {
 			kref_get(&h->refcount);
 			sf_handle = h;
 			break;
 		}
 	}
 	mutex_unlock(&sf_i->handle_list_mutex);

 	return sf_handle;
 }

 static int vboxsf_writepage(struct page *page, struct writeback_control *wbc)
 {
 	struct inode *inode = page->mapping->host;
 	struct vboxsf_inode *sf_i = VBOXSF_I(inode);
 	struct vboxsf_handle *sf_handle;
 	loff_t off = page_offset(page);
 	loff_t size = i_size_read(inode);
 	u32 nwrite = PAGE_SIZE;
 	u8 *buf;
 	int err;

 	if (off + PAGE_SIZE > size)
 		nwrite = size & ~PAGE_MASK;

 	sf_handle = vboxsf_get_write_handle(sf_i);
 	if (!sf_handle)
 		return -EBADF;

 	buf = kmap(page);
 	err = vboxsf_write(sf_handle->root, sf_handle->handle,
 			   off, &nwrite, buf);
 	kunmap(page);

 	kref_put(&sf_handle->refcount, vboxsf_handle_release);

 	if (err == 0) {
 		ClearPageError(page);
 		/* mtime changed */
 		sf_i->force_restat = 1;
 	} else {
 		ClearPageUptodate(page);
 	}

 	unlock_page(page);
 	return err;
 }

 static int vboxsf_write_end(struct file *file, struct address_space *mapping,
 			    loff_t pos, unsigned int len, unsigned int copied,
 			    struct page *page, void *fsdata)
 {
 	struct inode *inode = mapping->host;
 	struct vboxsf_handle *sf_handle = file->private_data;
 	unsigned int from = pos & ~PAGE_MASK;
 	u32 nwritten = len;
 	u8 *buf;
 	int err;

 	/* zero the stale part of the page if we did a short copy */
 	if (!PageUptodate(page) && copied < len)
 		zero_user(page, from + copied, len - copied);

 	buf = kmap(page);
 	err = vboxsf_write(sf_handle->root, sf_handle->handle,
 			   pos, &nwritten, buf + from);
 	kunmap(page);

 	if (err) {
 		nwritten = 0;
 		goto out;
 	}

 	/* mtime changed */
 	VBOXSF_I(inode)->force_restat = 1;

 	if (!PageUptodate(page) && nwritten == PAGE_SIZE)
 		SetPageUptodate(page);

 	pos += nwritten;
 	if (pos > inode->i_size)
 		i_size_write(inode, pos);

 out:
 	unlock_page(page);
 	put_page(page);

 	return nwritten;
 }

 /*
  * Note simple_write_begin does not read the page from disk on partial writes
  * this is ok since vboxsf_write_end only writes the written parts of the
  * page and it does not call SetPageUptodate for partial writes.
  */
 const struct address_space_operations vboxsf_reg_aops = {
 	.read_folio = vboxsf_read_folio,
 	.writepage = vboxsf_writepage,
 	.dirty_folio = filemap_dirty_folio,
 	.write_begin = simple_write_begin,
 	.write_end = vboxsf_write_end,
 };

 static const char *vboxsf_get_link(struct dentry *dentry, struct inode *inode,
 				   struct delayed_call *done)
 {
 	struct vboxsf_sbi *sbi = VBOXSF_SBI(inode->i_sb);
 	struct shfl_string *path;
 	char *link;
 	int err;

 	if (!dentry)
 		return ERR_PTR(-ECHILD);

 	path = vboxsf_path_from_dentry(sbi, dentry);
 	if (IS_ERR(path))
 		return ERR_CAST(path);

 	link = kzalloc(PATH_MAX, GFP_KERNEL);
 	if (!link) {
 		__putname(path);
 		return ERR_PTR(-ENOMEM);
 	}

 	err = vboxsf_readlink(sbi->root, path, PATH_MAX, link);
 	__putname(path);
 	if (err) {
 		kfree(link);
 		return ERR_PTR(err);
 	}

 	set_delayed_call(done, kfree_link, link);
 	return link;
 }

 const struct inode_operations vboxsf_lnk_iops = {
 	.get_link = vboxsf_get_link
 };
	// SPDX-License-Identifier: MIT
	/*
	* VirtualBox Guest Shared Folders support: Regular file inode and file ops.
	*
	* Copyright (C) 2006-2018 Oracle Corporation
	*/

	#include <linux/mm.h>
	#include <linux/page-flags.h>
	#include <linux/pagemap.h>
	#include <linux/highmem.h>
	#include <linux/sizes.h>
	#include "vfsmod.h"

	struct vboxsf_handle {
	u64 handle;
	u32 root;
	u32 access_flags;
	struct kref refcount;
	struct list_head head;
	};

	struct vboxsf_handle vboxsf_create_sf_handle(struct inode inode,
	u64 handle, u32 access_flags)
	{
	struct vboxsf_inode *sf_i = VBOXSF_I(inode);
	struct vboxsf_handle *sf_handle;

	sf_handle = kmalloc(sizeof(*sf_handle), GFP_KERNEL);
	if (!sf_handle)
	return ERR_PTR(-ENOMEM);

	/* the host may have given us different attr then requested */
	sf_i->force_restat = 1;

	/* init our handle struct and add it to the inode's handles list */
	sf_handle->handle = handle;
	sf_handle->root = VBOXSF_SBI(inode->i_sb)->root;
	sf_handle->access_flags = access_flags;
	kref_init(&sf_handle->refcount);

	mutex_lock(&sf_i->handle_list_mutex);
	list_add(&sf_handle->head, &sf_i->handle_list);
	mutex_unlock(&sf_i->handle_list_mutex);

	return sf_handle;
	}

	static int vboxsf_file_open(struct inode inode, struct file file)
	{
	struct vboxsf_sbi *sbi = VBOXSF_SBI(inode->i_sb);
	struct shfl_createparms params = {};
	struct vboxsf_handle *sf_handle;
	u32 access_flags = 0;
	int err;

	/*
	* We check the value of params.handle afterwards to find out if
	* the call succeeded or failed, as the API does not seem to cleanly
	* distinguish error and informational messages.
	*
	* Furthermore, we must set params.handle to SHFL_HANDLE_NIL to
	* make the shared folders host service use our mode parameter.
	*/
	params.handle = SHFL_HANDLE_NIL;
	if (file->f_flags & O_CREAT) {
	params.create_flags \|= SHFL_CF_ACT_CREATE_IF_NEW;
	/*
	* We ignore O_EXCL, as the Linux kernel seems to call create
	* beforehand itself, so O_EXCL should always fail.
	*/
	if (file->f_flags & O_TRUNC)
	params.create_flags \|= SHFL_CF_ACT_OVERWRITE_IF_EXISTS;
	else
	params.create_flags \|= SHFL_CF_ACT_OPEN_IF_EXISTS;
	} else {
	params.create_flags \|= SHFL_CF_ACT_FAIL_IF_NEW;
	if (file->f_flags & O_TRUNC)
	params.create_flags \|= SHFL_CF_ACT_OVERWRITE_IF_EXISTS;
	}

	switch (file->f_flags & O_ACCMODE) {
	case O_RDONLY:
	access_flags \|= SHFL_CF_ACCESS_READ;
	break;

	case O_WRONLY:
	access_flags \|= SHFL_CF_ACCESS_WRITE;
	break;

	case O_RDWR:
	access_flags \|= SHFL_CF_ACCESS_READWRITE;
	break;

	default:
	WARN_ON(1);
	}

	if (file->f_flags & O_APPEND)
	access_flags \|= SHFL_CF_ACCESS_APPEND;

	params.create_flags \|= access_flags;
	params.info.attr.mode = inode->i_mode;

	err = vboxsf_create_at_dentry(file_dentry(file), &params);
	if (err == 0 && params.handle == SHFL_HANDLE_NIL)
	err = (params.result == SHFL_FILE_EXISTS) ? -EEXIST : -ENOENT;
	if (err)
	return err;

	sf_handle = vboxsf_create_sf_handle(inode, params.handle, access_flags);
	if (IS_ERR(sf_handle)) {
	vboxsf_close(sbi->root, params.handle);
	return PTR_ERR(sf_handle);
	}

	file->private_data = sf_handle;
	return 0;
	}

	static void vboxsf_handle_release(struct kref *refcount)
	{
	struct vboxsf_handle *sf_handle =
	container_of(refcount, struct vboxsf_handle, refcount);

	vboxsf_close(sf_handle->root, sf_handle->handle);
	kfree(sf_handle);
	}

	void vboxsf_release_sf_handle(struct inode inode, struct vboxsf_handle sf_handle)
	{
	struct vboxsf_inode *sf_i = VBOXSF_I(inode);

	mutex_lock(&sf_i->handle_list_mutex);
	list_del(&sf_handle->head);
	mutex_unlock(&sf_i->handle_list_mutex);

	kref_put(&sf_handle->refcount, vboxsf_handle_release);
	}

	static int vboxsf_file_release(struct inode inode, struct file file)
	{
	/*
	* When a file is closed on our (the guest) side, we want any subsequent
	* accesses done on the host side to see all changes done from our side.
	*/
	filemap_write_and_wait(inode->i_mapping);

	vboxsf_release_sf_handle(inode, file->private_data);
	return 0;
	}

	/*
	* Write back dirty pages now, because there may not be any suitable
	* open files later
	*/
	static void vboxsf_vma_close(struct vm_area_struct *vma)
	{
	filemap_write_and_wait(vma->vm_file->f_mapping);
	}

	static const struct vm_operations_struct vboxsf_file_vm_ops = {
	.close = vboxsf_vma_close,
	.fault = filemap_fault,
	.map_pages = filemap_map_pages,
	};

	static int vboxsf_file_mmap(struct file file, struct vm_area_struct vma)
	{
	int err;

	err = generic_file_mmap(file, vma);
	if (!err)
	vma->vm_ops = &vboxsf_file_vm_ops;

	return err;
	}

	/*
	* Note that since we are accessing files on the host's filesystem, files
	* may always be changed underneath us by the host!
	*
	* The vboxsf API between the guest and the host does not offer any functions
	* to deal with this. There is no inode-generation to check for changes, no
	* events / callback on changes and no way to lock files.
	*
	* To avoid returning stale data when a file gets opened on our (the guest)
	* side, we do a "stat" on the host side, then compare the mtime with the
	* last known mtime and invalidate the page-cache if they differ.
	* This is done from vboxsf_inode_revalidate().
	*
	* When reads are done through the read_iter fop, it is possible to do
	* further cache revalidation then, there are 3 options to deal with this:
	*
	* 1) Rely solely on the revalidation done at open time
	* 2) Do another "stat" and compare mtime again. Unfortunately the vboxsf
	* host API does not allow stat on handles, so we would need to use
	* file->f_path.dentry and the stat will then fail if the file was unlinked
	* or renamed (and there is no thing like NFS' silly-rename). So we get:
	* 2a) "stat" and compare mtime, on stat failure invalidate the cache
	* 2b) "stat" and compare mtime, on stat failure do nothing
	* 3) Simply always call invalidate_inode_pages2_range on the range of the read
	*
	* Currently we are keeping things KISS and using option 1. this allows
	* directly using generic_file_read_iter without wrapping it.
	*
	* This means that only data written on the host side before open() on
	* the guest side is guaranteed to be seen by the guest. If necessary
	* we may provide other read-cache strategies in the future and make this
	* configurable through a mount option.
	*/
	const struct file_operations vboxsf_reg_fops = {
	.llseek = generic_file_llseek,
	.read_iter = generic_file_read_iter,
	.write_iter = generic_file_write_iter,
	.mmap = vboxsf_file_mmap,
	.open = vboxsf_file_open,
	.release = vboxsf_file_release,
	.fsync = noop_fsync,
	.splice_read = filemap_splice_read,
	};

	const struct inode_operations vboxsf_reg_iops = {
	.getattr = vboxsf_getattr,
	.setattr = vboxsf_setattr
	};

	static int vboxsf_read_folio(struct file file, struct folio folio)
	{
	struct page *page = &folio->page;
	struct vboxsf_handle *sf_handle = file->private_data;
	loff_t off = page_offset(page);
	u32 nread = PAGE_SIZE;
	u8 *buf;
	int err;

	buf = kmap(page);

	err = vboxsf_read(sf_handle->root, sf_handle->handle, off, &nread, buf);
	if (err == 0) {
	memset(&buf[nread], 0, PAGE_SIZE - nread);
	flush_dcache_page(page);
	SetPageUptodate(page);
	} else {
	SetPageError(page);
	}

	kunmap(page);
	unlock_page(page);
	return err;
	}

	static struct vboxsf_handle vboxsf_get_write_handle(struct vboxsf_inode sf_i)
	{
	struct vboxsf_handle h, sf_handle = NULL;

	mutex_lock(&sf_i->handle_list_mutex);
	list_for_each_entry(h, &sf_i->handle_list, head) {
	if (h->access_flags == SHFL_CF_ACCESS_WRITE \|\|
	h->access_flags == SHFL_CF_ACCESS_READWRITE) {
	kref_get(&h->refcount);
	sf_handle = h;
	break;
	}
	}
	mutex_unlock(&sf_i->handle_list_mutex);

	return sf_handle;
	}

	static int vboxsf_writepage(struct page page, struct writeback_control wbc)
	{
	struct inode *inode = page->mapping->host;
	struct vboxsf_inode *sf_i = VBOXSF_I(inode);
	struct vboxsf_handle *sf_handle;
	loff_t off = page_offset(page);
	loff_t size = i_size_read(inode);
	u32 nwrite = PAGE_SIZE;
	u8 *buf;
	int err;

	if (off + PAGE_SIZE > size)
	nwrite = size & ~PAGE_MASK;

	sf_handle = vboxsf_get_write_handle(sf_i);
	if (!sf_handle)
	return -EBADF;

	buf = kmap(page);
	err = vboxsf_write(sf_handle->root, sf_handle->handle,
	off, &nwrite, buf);
	kunmap(page);

	kref_put(&sf_handle->refcount, vboxsf_handle_release);

	if (err == 0) {
	ClearPageError(page);
	/* mtime changed */
	sf_i->force_restat = 1;
	} else {
	ClearPageUptodate(page);
	}

	unlock_page(page);
	return err;
	}

	static int vboxsf_write_end(struct file file, struct address_space mapping,
	loff_t pos, unsigned int len, unsigned int copied,
	struct page page, void fsdata)
	{
	struct inode *inode = mapping->host;
	struct vboxsf_handle *sf_handle = file->private_data;
	unsigned int from = pos & ~PAGE_MASK;
	u32 nwritten = len;
	u8 *buf;
	int err;

	/* zero the stale part of the page if we did a short copy */
	if (!PageUptodate(page) && copied < len)
	zero_user(page, from + copied, len - copied);

	buf = kmap(page);
	err = vboxsf_write(sf_handle->root, sf_handle->handle,
	pos, &nwritten, buf + from);
	kunmap(page);

	if (err) {
	nwritten = 0;
	goto out;
	}

	/* mtime changed */
	VBOXSF_I(inode)->force_restat = 1;

	if (!PageUptodate(page) && nwritten == PAGE_SIZE)
	SetPageUptodate(page);

	pos += nwritten;
	if (pos > inode->i_size)
	i_size_write(inode, pos);

	out:
	unlock_page(page);
	put_page(page);

	return nwritten;
	}

	/*
	* Note simple_write_begin does not read the page from disk on partial writes
	* this is ok since vboxsf_write_end only writes the written parts of the
	* page and it does not call SetPageUptodate for partial writes.
	*/
	const struct address_space_operations vboxsf_reg_aops = {
	.read_folio = vboxsf_read_folio,
	.writepage = vboxsf_writepage,
	.dirty_folio = filemap_dirty_folio,
	.write_begin = simple_write_begin,
	.write_end = vboxsf_write_end,
	};

	static const char vboxsf_get_link(struct dentry dentry, struct inode *inode,
	struct delayed_call *done)
	{
	struct vboxsf_sbi *sbi = VBOXSF_SBI(inode->i_sb);
	struct shfl_string *path;
	char *link;
	int err;

	if (!dentry)
	return ERR_PTR(-ECHILD);

	path = vboxsf_path_from_dentry(sbi, dentry);
	if (IS_ERR(path))
	return ERR_CAST(path);

	link = kzalloc(PATH_MAX, GFP_KERNEL);
	if (!link) {
	__putname(path);
	return ERR_PTR(-ENOMEM);
	}

	err = vboxsf_readlink(sbi->root, path, PATH_MAX, link);
	__putname(path);
	if (err) {
	kfree(link);
	return ERR_PTR(err);
	}

	set_delayed_call(done, kfree_link, link);
	return link;
	}

	const struct inode_operations vboxsf_lnk_iops = {
	.get_link = vboxsf_get_link
	};