fs/overlayfs/export.c - third_party/kernel - Git at Google

 /*
  * Overlayfs NFS export support.
  *
  * Amir Goldstein <amir73il@gmail.com>
  *
  * Copyright (C) 2017-2018 CTERA Networks. All Rights Reserved.
  *
  * 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.
  */

 #include <linux/fs.h>
 #include <linux/cred.h>
 #include <linux/mount.h>
 #include <linux/namei.h>
 #include <linux/xattr.h>
 #include <linux/exportfs.h>
 #include <linux/ratelimit.h>
 #include "overlayfs.h"

 /*
  * We only need to encode origin if there is a chance that the same object was
  * encoded pre copy up and then we need to stay consistent with the same
  * encoding also after copy up. If non-pure upper is not indexed, then it was
  * copied up before NFS export was enabled. In that case we don't need to worry
  * about staying consistent with pre copy up encoding and we encode an upper
  * file handle. Overlay root dentry is a private case of non-indexed upper.
  *
  * The following table summarizes the different file handle encodings used for
  * different overlay object types:
  *
  *  Object type		| Encoding
  * --------------------------------
  *  Pure upper		| U
  *  Non-indexed upper	| U
  *  Indexed upper	| L (*)
  *  Non-upper		| L (*)
  *
  * U = upper file handle
  * L = lower file handle
  *
  * (*) Connecting an overlay dir from real lower dentry is not always
  * possible when there are redirects in lower layers. To mitigate this case,
  * we copy up the lower dir first and then encode an upper dir file handle.
  */
 static bool ovl_should_encode_origin(struct dentry *dentry)
 {
 	struct ovl_fs *ofs = dentry->d_sb->s_fs_info;

 	if (!ovl_dentry_lower(dentry))
 		return false;

 	/*
 	 * Decoding a merge dir, whose origin's parent is under a redirected
 	 * lower dir is not always possible. As a simple aproximation, we do
 	 * not encode lower dir file handles when overlay has multiple lower
 	 * layers and origin is below the topmost lower layer.
 	 *
 	 * TODO: copy up only the parent that is under redirected lower.
 	 */
 	if (d_is_dir(dentry) && ofs->upper_mnt &&
 	    OVL_E(dentry)->lowerstack[0].layer->idx > 1)
 		return false;

 	/* Decoding a non-indexed upper from origin is not implemented */
 	if (ovl_dentry_upper(dentry) &&
 	    !ovl_test_flag(OVL_INDEX, d_inode(dentry)))
 		return false;

 	return true;
 }

 static int ovl_encode_maybe_copy_up(struct dentry *dentry)
 {
 	int err;

 	if (ovl_dentry_upper(dentry))
 		return 0;

 	err = ovl_want_write(dentry);
 	if (err)
 		return err;

 	err = ovl_copy_up(dentry);

 	ovl_drop_write(dentry);
 	return err;
 }

 static int ovl_d_to_fh(struct dentry *dentry, char *buf, int buflen)
 {
 	struct dentry *origin = ovl_dentry_lower(dentry);
 	struct ovl_fh *fh = NULL;
 	int err;

 	/*
 	 * If we should not encode a lower dir file handle, copy up and encode
 	 * an upper dir file handle.
 	 */
 	if (!ovl_should_encode_origin(dentry)) {
 		err = ovl_encode_maybe_copy_up(dentry);
 		if (err)
 			goto fail;

 		origin = NULL;
 	}

 	/* Encode an upper or origin file handle */
 	fh = ovl_encode_fh(origin ?: ovl_dentry_upper(dentry), !origin);
 	err = PTR_ERR(fh);
 	if (IS_ERR(fh))
 		goto fail;

 	err = -EOVERFLOW;
 	if (fh->len > buflen)
 		goto fail;

 	memcpy(buf, (char *)fh, fh->len);
 	err = fh->len;

 out:
 	kfree(fh);
 	return err;

 fail:
 	pr_warn_ratelimited("overlayfs: failed to encode file handle (%pd2, err=%i, buflen=%d, len=%d, type=%d)\n",
 			    dentry, err, buflen, fh ? (int)fh->len : 0,
 			    fh ? fh->type : 0);
 	goto out;
 }

 static int ovl_dentry_to_fh(struct dentry *dentry, u32 *fid, int *max_len)
 {
 	int res, len = *max_len << 2;

 	res = ovl_d_to_fh(dentry, (char *)fid, len);
 	if (res <= 0)
 		return FILEID_INVALID;

 	len = res;

 	/* Round up to dwords */
 	*max_len = (len + 3) >> 2;
 	return OVL_FILEID;
 }

 static int ovl_encode_inode_fh(struct inode *inode, u32 *fid, int *max_len,
 			       struct inode *parent)
 {
 	struct dentry *dentry;
 	int type;

 	/* TODO: encode connectable file handles */
 	if (parent)
 		return FILEID_INVALID;

 	dentry = d_find_any_alias(inode);
 	if (WARN_ON(!dentry))
 		return FILEID_INVALID;

 	type = ovl_dentry_to_fh(dentry, fid, max_len);

 	dput(dentry);
 	return type;
 }

 /*
  * Find or instantiate an overlay dentry from real dentries and index.
  */
 static struct dentry *ovl_obtain_alias(struct super_block *sb,
 				       struct dentry *upper_alias,
 				       struct ovl_path *lowerpath,
 				       struct dentry *index)
 {
 	struct dentry *lower = lowerpath ? lowerpath->dentry : NULL;
 	struct dentry *upper = upper_alias ?: index;
 	struct dentry *dentry;
 	struct inode *inode;
 	struct ovl_entry *oe;

 	/* We get overlay directory dentries with ovl_lookup_real() */
 	if (d_is_dir(upper ?: lower))
 		return ERR_PTR(-EIO);

 	inode = ovl_get_inode(sb, dget(upper), lower, index, !!lower);
 	if (IS_ERR(inode)) {
 		dput(upper);
 		return ERR_CAST(inode);
 	}

 	if (index)
 		ovl_set_flag(OVL_INDEX, inode);

 	dentry = d_find_any_alias(inode);
 	if (!dentry) {
 		dentry = d_alloc_anon(inode->i_sb);
 		if (!dentry)
 			goto nomem;
 		oe = ovl_alloc_entry(lower ? 1 : 0);
 		if (!oe)
 			goto nomem;

 		if (lower) {
 			oe->lowerstack->dentry = dget(lower);
 			oe->lowerstack->layer = lowerpath->layer;
 		}
 		dentry->d_fsdata = oe;
 		if (upper_alias)
 			ovl_dentry_set_upper_alias(dentry);
 	}

 	return d_instantiate_anon(dentry, inode);

 nomem:
 	iput(inode);
 	dput(dentry);
 	return ERR_PTR(-ENOMEM);
 }

 /* Get the upper or lower dentry in stach whose on layer @idx */
 static struct dentry *ovl_dentry_real_at(struct dentry *dentry, int idx)
 {
 	struct ovl_entry *oe = dentry->d_fsdata;
 	int i;

 	if (!idx)
 		return ovl_dentry_upper(dentry);

 	for (i = 0; i < oe->numlower; i++) {
 		if (oe->lowerstack[i].layer->idx == idx)
 			return oe->lowerstack[i].dentry;
 	}

 	return NULL;
 }

 /*
  * Lookup a child overlay dentry to get a connected overlay dentry whose real
  * dentry is @real. If @real is on upper layer, we lookup a child overlay
  * dentry with the same name as the real dentry. Otherwise, we need to consult
  * index for lookup.
  */
 static struct dentry *ovl_lookup_real_one(struct dentry *connected,
 					  struct dentry *real,
 					  struct ovl_layer *layer)
 {
 	struct inode *dir = d_inode(connected);
 	struct dentry *this, *parent = NULL;
 	struct name_snapshot name;
 	int err;

 	/*
 	 * Lookup child overlay dentry by real name. The dir mutex protects us
 	 * from racing with overlay rename. If the overlay dentry that is above
 	 * real has already been moved to a parent that is not under the
 	 * connected overlay dir, we return -ECHILD and restart the lookup of
 	 * connected real path from the top.
 	 */
 	inode_lock_nested(dir, I_MUTEX_PARENT);
 	err = -ECHILD;
 	parent = dget_parent(real);
 	if (ovl_dentry_real_at(connected, layer->idx) != parent)
 		goto fail;

 	/*
 	 * We also need to take a snapshot of real dentry name to protect us
 	 * from racing with underlying layer rename. In this case, we don't
 	 * care about returning ESTALE, only from dereferencing a free name
 	 * pointer because we hold no lock on the real dentry.
 	 */
 	take_dentry_name_snapshot(&name, real);
 	this = lookup_one_len(name.name, connected, strlen(name.name));
 	err = PTR_ERR(this);
 	if (IS_ERR(this)) {
 		goto fail;
 	} else if (!this || !this->d_inode) {
 		dput(this);
 		err = -ENOENT;
 		goto fail;
 	} else if (ovl_dentry_real_at(this, layer->idx) != real) {
 		dput(this);
 		err = -ESTALE;
 		goto fail;
 	}

 out:
 	release_dentry_name_snapshot(&name);
 	dput(parent);
 	inode_unlock(dir);
 	return this;

 fail:
 	pr_warn_ratelimited("overlayfs: failed to lookup one by real (%pd2, layer=%d, connected=%pd2, err=%i)\n",
 			    real, layer->idx, connected, err);
 	this = ERR_PTR(err);
 	goto out;
 }

 static struct dentry *ovl_lookup_real(struct super_block *sb,
 				      struct dentry *real,
 				      struct ovl_layer *layer);

 /*
  * Lookup an indexed or hashed overlay dentry by real inode.
  */
 static struct dentry *ovl_lookup_real_inode(struct super_block *sb,
 					    struct dentry *real,
 					    struct ovl_layer *layer)
 {
 	struct ovl_fs *ofs = sb->s_fs_info;
 	struct ovl_layer upper_layer = { .mnt = ofs->upper_mnt };
 	struct dentry *index = NULL;
 	struct dentry *this = NULL;
 	struct inode *inode;

 	/*
 	 * Decoding upper dir from index is expensive, so first try to lookup
 	 * overlay dentry in inode/dcache.
 	 */
 	inode = ovl_lookup_inode(sb, real, !layer->idx);
 	if (IS_ERR(inode))
 		return ERR_CAST(inode);
 	if (inode) {
 		this = d_find_any_alias(inode);
 		iput(inode);
 	}

 	/*
 	 * For decoded lower dir file handle, lookup index by origin to check
 	 * if lower dir was copied up and and/or removed.
 	 */
 	if (!this && layer->idx && ofs->indexdir && !WARN_ON(!d_is_dir(real))) {
 		index = ovl_lookup_index(ofs, NULL, real, false);
 		if (IS_ERR(index))
 			return index;
 	}

 	/* Get connected upper overlay dir from index */
 	if (index) {
 		struct dentry *upper = ovl_index_upper(ofs, index);

 		dput(index);
 		if (IS_ERR_OR_NULL(upper))
 			return upper;

 		/*
 		 * ovl_lookup_real() in lower layer may call recursively once to
 		 * ovl_lookup_real() in upper layer. The first level call walks
 		 * back lower parents to the topmost indexed parent. The second
 		 * recursive call walks back from indexed upper to the topmost
 		 * connected/hashed upper parent (or up to root).
 		 */
 		this = ovl_lookup_real(sb, upper, &upper_layer);
 		dput(upper);
 	}

 	if (!this)
 		return NULL;

 	if (WARN_ON(ovl_dentry_real_at(this, layer->idx) != real)) {
 		dput(this);
 		this = ERR_PTR(-EIO);
 	}

 	return this;
 }

 /*
  * Lookup an indexed or hashed overlay dentry, whose real dentry is an
  * ancestor of @real.
  */
 static struct dentry *ovl_lookup_real_ancestor(struct super_block *sb,
 					       struct dentry *real,
 					       struct ovl_layer *layer)
 {
 	struct dentry *next, *parent = NULL;
 	struct dentry *ancestor = ERR_PTR(-EIO);

 	if (real == layer->mnt->mnt_root)
 		return dget(sb->s_root);

 	/* Find the topmost indexed or hashed ancestor */
 	next = dget(real);
 	for (;;) {
 		parent = dget_parent(next);

 		/*
 		 * Lookup a matching overlay dentry in inode/dentry
 		 * cache or in index by real inode.
 		 */
 		ancestor = ovl_lookup_real_inode(sb, next, layer);
 		if (ancestor)
 			break;

 		if (parent == layer->mnt->mnt_root) {
 			ancestor = dget(sb->s_root);
 			break;
 		}

 		/*
 		 * If @real has been moved out of the layer root directory,
 		 * we will eventully hit the real fs root. This cannot happen
 		 * by legit overlay rename, so we return error in that case.
 		 */
 		if (parent == next) {
 			ancestor = ERR_PTR(-EXDEV);
 			break;
 		}

 		dput(next);
 		next = parent;
 	}

 	dput(parent);
 	dput(next);

 	return ancestor;
 }

 /*
  * Lookup a connected overlay dentry whose real dentry is @real.
  * If @real is on upper layer, we lookup a child overlay dentry with the same
  * path the real dentry. Otherwise, we need to consult index for lookup.
  */
 static struct dentry *ovl_lookup_real(struct super_block *sb,
 				      struct dentry *real,
 				      struct ovl_layer *layer)
 {
 	struct dentry *connected;
 	int err = 0;

 	connected = ovl_lookup_real_ancestor(sb, real, layer);
 	if (IS_ERR(connected))
 		return connected;

 	while (!err) {
 		struct dentry *next, *this;
 		struct dentry *parent = NULL;
 		struct dentry *real_connected = ovl_dentry_real_at(connected,
 								   layer->idx);

 		if (real_connected == real)
 			break;

 		/* Find the topmost dentry not yet connected */
 		next = dget(real);
 		for (;;) {
 			parent = dget_parent(next);

 			if (parent == real_connected)
 				break;

 			/*
 			 * If real has been moved out of 'real_connected',
 			 * we will not find 'real_connected' and hit the layer
 			 * root. In that case, we need to restart connecting.
 			 * This game can go on forever in the worst case. We
 			 * may want to consider taking s_vfs_rename_mutex if
 			 * this happens more than once.
 			 */
 			if (parent == layer->mnt->mnt_root) {
 				dput(connected);
 				connected = dget(sb->s_root);
 				break;
 			}

 			/*
 			 * If real file has been moved out of the layer root
 			 * directory, we will eventully hit the real fs root.
 			 * This cannot happen by legit overlay rename, so we
 			 * return error in that case.
 			 */
 			if (parent == next) {
 				err = -EXDEV;
 				break;
 			}

 			dput(next);
 			next = parent;
 		}

 		if (!err) {
 			this = ovl_lookup_real_one(connected, next, layer);
 			if (IS_ERR(this))
 				err = PTR_ERR(this);

 			/*
 			 * Lookup of child in overlay can fail when racing with
 			 * overlay rename of child away from 'connected' parent.
 			 * In this case, we need to restart the lookup from the
 			 * top, because we cannot trust that 'real_connected' is
 			 * still an ancestor of 'real'. There is a good chance
 			 * that the renamed overlay ancestor is now in cache, so
 			 * ovl_lookup_real_ancestor() will find it and we can
 			 * continue to connect exactly from where lookup failed.
 			 */
 			if (err == -ECHILD) {
 				this = ovl_lookup_real_ancestor(sb, real,
 								layer);
 				err = IS_ERR(this) ? PTR_ERR(this) : 0;
 			}
 			if (!err) {
 				dput(connected);
 				connected = this;
 			}
 		}

 		dput(parent);
 		dput(next);
 	}

 	if (err)
 		goto fail;

 	return connected;

 fail:
 	pr_warn_ratelimited("overlayfs: failed to lookup by real (%pd2, layer=%d, connected=%pd2, err=%i)\n",
 			    real, layer->idx, connected, err);
 	dput(connected);
 	return ERR_PTR(err);
 }

 /*
  * Get an overlay dentry from upper/lower real dentries and index.
  */
 static struct dentry *ovl_get_dentry(struct super_block *sb,
 				     struct dentry *upper,
 				     struct ovl_path *lowerpath,
 				     struct dentry *index)
 {
 	struct ovl_fs *ofs = sb->s_fs_info;
 	struct ovl_layer upper_layer = { .mnt = ofs->upper_mnt };
 	struct ovl_layer *layer = upper ? &upper_layer : lowerpath->layer;
 	struct dentry *real = upper ?: (index ?: lowerpath->dentry);

 	/*
 	 * Obtain a disconnected overlay dentry from a non-dir real dentry
 	 * and index.
 	 */
 	if (!d_is_dir(real))
 		return ovl_obtain_alias(sb, upper, lowerpath, index);

 	/* Removed empty directory? */
 	if ((real->d_flags & DCACHE_DISCONNECTED) || d_unhashed(real))
 		return ERR_PTR(-ENOENT);

 	/*
 	 * If real dentry is connected and hashed, get a connected overlay
 	 * dentry whose real dentry is @real.
 	 */
 	return ovl_lookup_real(sb, real, layer);
 }

 static struct dentry *ovl_upper_fh_to_d(struct super_block *sb,
 					struct ovl_fh *fh)
 {
 	struct ovl_fs *ofs = sb->s_fs_info;
 	struct dentry *dentry;
 	struct dentry *upper;

 	if (!ofs->upper_mnt)
 		return ERR_PTR(-EACCES);

 	upper = ovl_decode_fh(fh, ofs->upper_mnt);
 	if (IS_ERR_OR_NULL(upper))
 		return upper;

 	dentry = ovl_get_dentry(sb, upper, NULL, NULL);
 	dput(upper);

 	return dentry;
 }

 static struct dentry *ovl_lower_fh_to_d(struct super_block *sb,
 					struct ovl_fh *fh)
 {
 	struct ovl_fs *ofs = sb->s_fs_info;
 	struct ovl_path origin = { };
 	struct ovl_path *stack = &origin;
 	struct dentry *dentry = NULL;
 	struct dentry *index = NULL;
 	struct inode *inode = NULL;
 	bool is_deleted = false;
 	int err;

 	/* First lookup indexed upper by fh */
 	if (ofs->indexdir) {
 		index = ovl_get_index_fh(ofs, fh);
 		err = PTR_ERR(index);
 		if (IS_ERR(index)) {
 			if (err != -ESTALE)
 				return ERR_PTR(err);

 			/* Found a whiteout index - treat as deleted inode */
 			is_deleted = true;
 			index = NULL;
 		}
 	}

 	/* Then try to get upper dir by index */
 	if (index && d_is_dir(index)) {
 		struct dentry *upper = ovl_index_upper(ofs, index);

 		err = PTR_ERR(upper);
 		if (IS_ERR_OR_NULL(upper))
 			goto out_err;

 		dentry = ovl_get_dentry(sb, upper, NULL, NULL);
 		dput(upper);
 		goto out;
 	}

 	/* Then lookup origin by fh */
 	err = ovl_check_origin_fh(ofs, fh, NULL, &stack);
 	if (err) {
 		goto out_err;
 	} else if (index) {
 		err = ovl_verify_origin(index, origin.dentry, false);
 		if (err)
 			goto out_err;
 	} else if (is_deleted) {
 		/* Lookup deleted non-dir by origin inode */
 		if (!d_is_dir(origin.dentry))
 			inode = ovl_lookup_inode(sb, origin.dentry, false);
 		err = -ESTALE;
 		if (!inode || atomic_read(&inode->i_count) == 1)
 			goto out_err;

 		/* Deleted but still open? */
 		index = dget(ovl_i_dentry_upper(inode));
 	}

 	dentry = ovl_get_dentry(sb, NULL, &origin, index);

 out:
 	dput(origin.dentry);
 	dput(index);
 	iput(inode);
 	return dentry;

 out_err:
 	dentry = ERR_PTR(err);
 	goto out;
 }

 static struct dentry *ovl_fh_to_dentry(struct super_block *sb, struct fid *fid,
 				       int fh_len, int fh_type)
 {
 	struct dentry *dentry = NULL;
 	struct ovl_fh *fh = (struct ovl_fh *) fid;
 	int len = fh_len << 2;
 	unsigned int flags = 0;
 	int err;

 	err = -EINVAL;
 	if (fh_type != OVL_FILEID)
 		goto out_err;

 	err = ovl_check_fh_len(fh, len);
 	if (err)
 		goto out_err;

 	flags = fh->flags;
 	dentry = (flags & OVL_FH_FLAG_PATH_UPPER) ?
 		 ovl_upper_fh_to_d(sb, fh) :
 		 ovl_lower_fh_to_d(sb, fh);
 	err = PTR_ERR(dentry);
 	if (IS_ERR(dentry) && err != -ESTALE)
 		goto out_err;

 	return dentry;

 out_err:
 	pr_warn_ratelimited("overlayfs: failed to decode file handle (len=%d, type=%d, flags=%x, err=%i)\n",
 			    len, fh_type, flags, err);
 	return ERR_PTR(err);
 }

 static struct dentry *ovl_fh_to_parent(struct super_block *sb, struct fid *fid,
 				       int fh_len, int fh_type)
 {
 	pr_warn_ratelimited("overlayfs: connectable file handles not supported; use 'no_subtree_check' exportfs option.\n");
 	return ERR_PTR(-EACCES);
 }

 static int ovl_get_name(struct dentry *parent, char *name,
 			struct dentry *child)
 {
 	/*
 	 * ovl_fh_to_dentry() returns connected dir overlay dentries and
 	 * ovl_fh_to_parent() is not implemented, so we should not get here.
 	 */
 	WARN_ON_ONCE(1);
 	return -EIO;
 }

 static struct dentry *ovl_get_parent(struct dentry *dentry)
 {
 	/*
 	 * ovl_fh_to_dentry() returns connected dir overlay dentries, so we
 	 * should not get here.
 	 */
 	WARN_ON_ONCE(1);
 	return ERR_PTR(-EIO);
 }

 const struct export_operations ovl_export_operations = {
 	.encode_fh	= ovl_encode_inode_fh,
 	.fh_to_dentry	= ovl_fh_to_dentry,
 	.fh_to_parent	= ovl_fh_to_parent,
 	.get_name	= ovl_get_name,
 	.get_parent	= ovl_get_parent,
 };
	/*
	* Overlayfs NFS export support.
	*
	* Amir Goldstein <amir73il@gmail.com>
	*
	* Copyright (C) 2017-2018 CTERA Networks. All Rights Reserved.
	*
	* 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.
	*/

	#include <linux/fs.h>
	#include <linux/cred.h>
	#include <linux/mount.h>
	#include <linux/namei.h>
	#include <linux/xattr.h>
	#include <linux/exportfs.h>
	#include <linux/ratelimit.h>
	#include "overlayfs.h"

	/*
	* We only need to encode origin if there is a chance that the same object was
	* encoded pre copy up and then we need to stay consistent with the same
	* encoding also after copy up. If non-pure upper is not indexed, then it was
	* copied up before NFS export was enabled. In that case we don't need to worry
	* about staying consistent with pre copy up encoding and we encode an upper
	* file handle. Overlay root dentry is a private case of non-indexed upper.
	*
	* The following table summarizes the different file handle encodings used for
	* different overlay object types:
	*
	* Object type \| Encoding
	* --------------------------------
	* Pure upper \| U
	* Non-indexed upper \| U
	* Indexed upper \| L (*)
	* Non-upper \| L (*)
	*
	* U = upper file handle
	* L = lower file handle
	*
	* (*) Connecting an overlay dir from real lower dentry is not always
	* possible when there are redirects in lower layers. To mitigate this case,
	* we copy up the lower dir first and then encode an upper dir file handle.
	*/
	static bool ovl_should_encode_origin(struct dentry *dentry)
	{
	struct ovl_fs *ofs = dentry->d_sb->s_fs_info;

	if (!ovl_dentry_lower(dentry))
	return false;

	/*
	* Decoding a merge dir, whose origin's parent is under a redirected
	* lower dir is not always possible. As a simple aproximation, we do
	* not encode lower dir file handles when overlay has multiple lower
	* layers and origin is below the topmost lower layer.
	*
	* TODO: copy up only the parent that is under redirected lower.
	*/
	if (d_is_dir(dentry) && ofs->upper_mnt &&
	OVL_E(dentry)->lowerstack[0].layer->idx > 1)
	return false;

	/* Decoding a non-indexed upper from origin is not implemented */
	if (ovl_dentry_upper(dentry) &&
	!ovl_test_flag(OVL_INDEX, d_inode(dentry)))
	return false;

	return true;
	}

	static int ovl_encode_maybe_copy_up(struct dentry *dentry)
	{
	int err;

	if (ovl_dentry_upper(dentry))
	return 0;

	err = ovl_want_write(dentry);
	if (err)
	return err;

	err = ovl_copy_up(dentry);

	ovl_drop_write(dentry);
	return err;
	}

	static int ovl_d_to_fh(struct dentry dentry, char buf, int buflen)
	{
	struct dentry *origin = ovl_dentry_lower(dentry);
	struct ovl_fh *fh = NULL;
	int err;

	/*
	* If we should not encode a lower dir file handle, copy up and encode
	* an upper dir file handle.
	*/
	if (!ovl_should_encode_origin(dentry)) {
	err = ovl_encode_maybe_copy_up(dentry);
	if (err)
	goto fail;

	origin = NULL;
	}

	/* Encode an upper or origin file handle */
	fh = ovl_encode_fh(origin ?: ovl_dentry_upper(dentry), !origin);
	err = PTR_ERR(fh);
	if (IS_ERR(fh))
	goto fail;

	err = -EOVERFLOW;
	if (fh->len > buflen)
	goto fail;

	memcpy(buf, (char *)fh, fh->len);
	err = fh->len;

	out:
	kfree(fh);
	return err;

	fail:
	pr_warn_ratelimited("overlayfs: failed to encode file handle (%pd2, err=%i, buflen=%d, len=%d, type=%d)\n",
	dentry, err, buflen, fh ? (int)fh->len : 0,
	fh ? fh->type : 0);
	goto out;
	}

	static int ovl_dentry_to_fh(struct dentry dentry, u32 fid, int *max_len)
	{
	int res, len = *max_len << 2;

	res = ovl_d_to_fh(dentry, (char *)fid, len);
	if (res <= 0)
	return FILEID_INVALID;

	len = res;

	/* Round up to dwords */
	*max_len = (len + 3) >> 2;
	return OVL_FILEID;
	}

	static int ovl_encode_inode_fh(struct inode inode, u32 fid, int *max_len,
	struct inode *parent)
	{
	struct dentry *dentry;
	int type;

	/* TODO: encode connectable file handles */
	if (parent)
	return FILEID_INVALID;

	dentry = d_find_any_alias(inode);
	if (WARN_ON(!dentry))
	return FILEID_INVALID;

	type = ovl_dentry_to_fh(dentry, fid, max_len);

	dput(dentry);
	return type;
	}

	/*
	* Find or instantiate an overlay dentry from real dentries and index.
	*/
	static struct dentry ovl_obtain_alias(struct super_block sb,
	struct dentry *upper_alias,
	struct ovl_path *lowerpath,
	struct dentry *index)
	{
	struct dentry *lower = lowerpath ? lowerpath->dentry : NULL;
	struct dentry *upper = upper_alias ?: index;
	struct dentry *dentry;
	struct inode *inode;
	struct ovl_entry *oe;

	/* We get overlay directory dentries with ovl_lookup_real() */
	if (d_is_dir(upper ?: lower))
	return ERR_PTR(-EIO);

	inode = ovl_get_inode(sb, dget(upper), lower, index, !!lower);
	if (IS_ERR(inode)) {
	dput(upper);
	return ERR_CAST(inode);
	}

	if (index)
	ovl_set_flag(OVL_INDEX, inode);

	dentry = d_find_any_alias(inode);
	if (!dentry) {
	dentry = d_alloc_anon(inode->i_sb);
	if (!dentry)
	goto nomem;
	oe = ovl_alloc_entry(lower ? 1 : 0);
	if (!oe)
	goto nomem;

	if (lower) {
	oe->lowerstack->dentry = dget(lower);
	oe->lowerstack->layer = lowerpath->layer;
	}
	dentry->d_fsdata = oe;
	if (upper_alias)
	ovl_dentry_set_upper_alias(dentry);
	}

	return d_instantiate_anon(dentry, inode);

	nomem:
	iput(inode);
	dput(dentry);
	return ERR_PTR(-ENOMEM);
	}

	/* Get the upper or lower dentry in stach whose on layer @idx */
	static struct dentry ovl_dentry_real_at(struct dentry dentry, int idx)
	{
	struct ovl_entry *oe = dentry->d_fsdata;
	int i;

	if (!idx)
	return ovl_dentry_upper(dentry);

	for (i = 0; i < oe->numlower; i++) {
	if (oe->lowerstack[i].layer->idx == idx)
	return oe->lowerstack[i].dentry;
	}

	return NULL;
	}

	/*
	* Lookup a child overlay dentry to get a connected overlay dentry whose real
	* dentry is @real. If @real is on upper layer, we lookup a child overlay
	* dentry with the same name as the real dentry. Otherwise, we need to consult
	* index for lookup.
	*/
	static struct dentry ovl_lookup_real_one(struct dentry connected,
	struct dentry *real,
	struct ovl_layer *layer)
	{
	struct inode *dir = d_inode(connected);
	struct dentry this, parent = NULL;
	struct name_snapshot name;
	int err;

	/*
	* Lookup child overlay dentry by real name. The dir mutex protects us
	* from racing with overlay rename. If the overlay dentry that is above
	* real has already been moved to a parent that is not under the
	* connected overlay dir, we return -ECHILD and restart the lookup of
	* connected real path from the top.
	*/
	inode_lock_nested(dir, I_MUTEX_PARENT);
	err = -ECHILD;
	parent = dget_parent(real);
	if (ovl_dentry_real_at(connected, layer->idx) != parent)
	goto fail;

	/*
	* We also need to take a snapshot of real dentry name to protect us
	* from racing with underlying layer rename. In this case, we don't
	* care about returning ESTALE, only from dereferencing a free name
	* pointer because we hold no lock on the real dentry.
	*/
	take_dentry_name_snapshot(&name, real);
	this = lookup_one_len(name.name, connected, strlen(name.name));
	err = PTR_ERR(this);
	if (IS_ERR(this)) {
	goto fail;
	} else if (!this \|\| !this->d_inode) {
	dput(this);
	err = -ENOENT;
	goto fail;
	} else if (ovl_dentry_real_at(this, layer->idx) != real) {
	dput(this);
	err = -ESTALE;
	goto fail;
	}

	out:
	release_dentry_name_snapshot(&name);
	dput(parent);
	inode_unlock(dir);
	return this;

	fail:
	pr_warn_ratelimited("overlayfs: failed to lookup one by real (%pd2, layer=%d, connected=%pd2, err=%i)\n",
	real, layer->idx, connected, err);
	this = ERR_PTR(err);
	goto out;
	}

	static struct dentry ovl_lookup_real(struct super_block sb,
	struct dentry *real,
	struct ovl_layer *layer);

	/*
	* Lookup an indexed or hashed overlay dentry by real inode.
	*/
	static struct dentry ovl_lookup_real_inode(struct super_block sb,
	struct dentry *real,
	struct ovl_layer *layer)
	{
	struct ovl_fs *ofs = sb->s_fs_info;
	struct ovl_layer upper_layer = { .mnt = ofs->upper_mnt };
	struct dentry *index = NULL;
	struct dentry *this = NULL;
	struct inode *inode;

	/*
	* Decoding upper dir from index is expensive, so first try to lookup
	* overlay dentry in inode/dcache.
	*/
	inode = ovl_lookup_inode(sb, real, !layer->idx);
	if (IS_ERR(inode))
	return ERR_CAST(inode);
	if (inode) {
	this = d_find_any_alias(inode);
	iput(inode);
	}

	/*
	* For decoded lower dir file handle, lookup index by origin to check
	* if lower dir was copied up and and/or removed.
	*/
	if (!this && layer->idx && ofs->indexdir && !WARN_ON(!d_is_dir(real))) {
	index = ovl_lookup_index(ofs, NULL, real, false);
	if (IS_ERR(index))
	return index;
	}

	/* Get connected upper overlay dir from index */
	if (index) {
	struct dentry *upper = ovl_index_upper(ofs, index);

	dput(index);
	if (IS_ERR_OR_NULL(upper))
	return upper;

	/*
	* ovl_lookup_real() in lower layer may call recursively once to
	* ovl_lookup_real() in upper layer. The first level call walks
	* back lower parents to the topmost indexed parent. The second
	* recursive call walks back from indexed upper to the topmost
	* connected/hashed upper parent (or up to root).
	*/
	this = ovl_lookup_real(sb, upper, &upper_layer);
	dput(upper);
	}

	if (!this)
	return NULL;

	if (WARN_ON(ovl_dentry_real_at(this, layer->idx) != real)) {
	dput(this);
	this = ERR_PTR(-EIO);
	}

	return this;
	}

	/*
	* Lookup an indexed or hashed overlay dentry, whose real dentry is an
	* ancestor of @real.
	*/
	static struct dentry ovl_lookup_real_ancestor(struct super_block sb,
	struct dentry *real,
	struct ovl_layer *layer)
	{
	struct dentry next, parent = NULL;
	struct dentry *ancestor = ERR_PTR(-EIO);

	if (real == layer->mnt->mnt_root)
	return dget(sb->s_root);

	/* Find the topmost indexed or hashed ancestor */
	next = dget(real);
	for (;;) {
	parent = dget_parent(next);

	/*
	* Lookup a matching overlay dentry in inode/dentry
	* cache or in index by real inode.
	*/
	ancestor = ovl_lookup_real_inode(sb, next, layer);
	if (ancestor)
	break;

	if (parent == layer->mnt->mnt_root) {
	ancestor = dget(sb->s_root);
	break;
	}

	/*
	* If @real has been moved out of the layer root directory,
	* we will eventully hit the real fs root. This cannot happen
	* by legit overlay rename, so we return error in that case.
	*/
	if (parent == next) {
	ancestor = ERR_PTR(-EXDEV);
	break;
	}

	dput(next);
	next = parent;
	}

	dput(parent);
	dput(next);

	return ancestor;
	}

	/*
	* Lookup a connected overlay dentry whose real dentry is @real.
	* If @real is on upper layer, we lookup a child overlay dentry with the same
	* path the real dentry. Otherwise, we need to consult index for lookup.
	*/
	static struct dentry ovl_lookup_real(struct super_block sb,
	struct dentry *real,
	struct ovl_layer *layer)
	{
	struct dentry *connected;
	int err = 0;

	connected = ovl_lookup_real_ancestor(sb, real, layer);
	if (IS_ERR(connected))
	return connected;

	while (!err) {
	struct dentry next, this;
	struct dentry *parent = NULL;
	struct dentry *real_connected = ovl_dentry_real_at(connected,
	layer->idx);

	if (real_connected == real)
	break;

	/* Find the topmost dentry not yet connected */
	next = dget(real);
	for (;;) {
	parent = dget_parent(next);

	if (parent == real_connected)
	break;

	/*
	* If real has been moved out of 'real_connected',
	* we will not find 'real_connected' and hit the layer
	* root. In that case, we need to restart connecting.
	* This game can go on forever in the worst case. We
	* may want to consider taking s_vfs_rename_mutex if
	* this happens more than once.
	*/
	if (parent == layer->mnt->mnt_root) {
	dput(connected);
	connected = dget(sb->s_root);
	break;
	}

	/*
	* If real file has been moved out of the layer root
	* directory, we will eventully hit the real fs root.
	* This cannot happen by legit overlay rename, so we
	* return error in that case.
	*/
	if (parent == next) {
	err = -EXDEV;
	break;
	}

	dput(next);
	next = parent;
	}

	if (!err) {
	this = ovl_lookup_real_one(connected, next, layer);
	if (IS_ERR(this))
	err = PTR_ERR(this);

	/*
	* Lookup of child in overlay can fail when racing with
	* overlay rename of child away from 'connected' parent.
	* In this case, we need to restart the lookup from the
	* top, because we cannot trust that 'real_connected' is
	* still an ancestor of 'real'. There is a good chance
	* that the renamed overlay ancestor is now in cache, so
	* ovl_lookup_real_ancestor() will find it and we can
	* continue to connect exactly from where lookup failed.
	*/
	if (err == -ECHILD) {
	this = ovl_lookup_real_ancestor(sb, real,
	layer);
	err = IS_ERR(this) ? PTR_ERR(this) : 0;
	}
	if (!err) {
	dput(connected);
	connected = this;
	}
	}

	dput(parent);
	dput(next);
	}

	if (err)
	goto fail;

	return connected;

	fail:
	pr_warn_ratelimited("overlayfs: failed to lookup by real (%pd2, layer=%d, connected=%pd2, err=%i)\n",
	real, layer->idx, connected, err);
	dput(connected);
	return ERR_PTR(err);
	}

	/*
	* Get an overlay dentry from upper/lower real dentries and index.
	*/
	static struct dentry ovl_get_dentry(struct super_block sb,
	struct dentry *upper,
	struct ovl_path *lowerpath,
	struct dentry *index)
	{
	struct ovl_fs *ofs = sb->s_fs_info;
	struct ovl_layer upper_layer = { .mnt = ofs->upper_mnt };
	struct ovl_layer *layer = upper ? &upper_layer : lowerpath->layer;
	struct dentry *real = upper ?: (index ?: lowerpath->dentry);

	/*
	* Obtain a disconnected overlay dentry from a non-dir real dentry
	* and index.
	*/
	if (!d_is_dir(real))
	return ovl_obtain_alias(sb, upper, lowerpath, index);

	/* Removed empty directory? */
	if ((real->d_flags & DCACHE_DISCONNECTED) \|\| d_unhashed(real))
	return ERR_PTR(-ENOENT);

	/*
	* If real dentry is connected and hashed, get a connected overlay
	* dentry whose real dentry is @real.
	*/
	return ovl_lookup_real(sb, real, layer);
	}

	static struct dentry ovl_upper_fh_to_d(struct super_block sb,
	struct ovl_fh *fh)
	{
	struct ovl_fs *ofs = sb->s_fs_info;
	struct dentry *dentry;
	struct dentry *upper;

	if (!ofs->upper_mnt)
	return ERR_PTR(-EACCES);

	upper = ovl_decode_fh(fh, ofs->upper_mnt);
	if (IS_ERR_OR_NULL(upper))
	return upper;

	dentry = ovl_get_dentry(sb, upper, NULL, NULL);
	dput(upper);

	return dentry;
	}

	static struct dentry ovl_lower_fh_to_d(struct super_block sb,
	struct ovl_fh *fh)
	{
	struct ovl_fs *ofs = sb->s_fs_info;
	struct ovl_path origin = { };
	struct ovl_path *stack = &origin;
	struct dentry *dentry = NULL;
	struct dentry *index = NULL;
	struct inode *inode = NULL;
	bool is_deleted = false;
	int err;

	/* First lookup indexed upper by fh */
	if (ofs->indexdir) {
	index = ovl_get_index_fh(ofs, fh);
	err = PTR_ERR(index);
	if (IS_ERR(index)) {
	if (err != -ESTALE)
	return ERR_PTR(err);

	/* Found a whiteout index - treat as deleted inode */
	is_deleted = true;
	index = NULL;
	}
	}

	/* Then try to get upper dir by index */
	if (index && d_is_dir(index)) {
	struct dentry *upper = ovl_index_upper(ofs, index);

	err = PTR_ERR(upper);
	if (IS_ERR_OR_NULL(upper))
	goto out_err;

	dentry = ovl_get_dentry(sb, upper, NULL, NULL);
	dput(upper);
	goto out;
	}

	/* Then lookup origin by fh */
	err = ovl_check_origin_fh(ofs, fh, NULL, &stack);
	if (err) {
	goto out_err;
	} else if (index) {
	err = ovl_verify_origin(index, origin.dentry, false);
	if (err)
	goto out_err;
	} else if (is_deleted) {
	/* Lookup deleted non-dir by origin inode */
	if (!d_is_dir(origin.dentry))
	inode = ovl_lookup_inode(sb, origin.dentry, false);
	err = -ESTALE;
	if (!inode \|\| atomic_read(&inode->i_count) == 1)
	goto out_err;

	/* Deleted but still open? */
	index = dget(ovl_i_dentry_upper(inode));
	}

	dentry = ovl_get_dentry(sb, NULL, &origin, index);

	out:
	dput(origin.dentry);
	dput(index);
	iput(inode);
	return dentry;

	out_err:
	dentry = ERR_PTR(err);
	goto out;
	}

	static struct dentry ovl_fh_to_dentry(struct super_block sb, struct fid *fid,
	int fh_len, int fh_type)
	{
	struct dentry *dentry = NULL;
	struct ovl_fh fh = (struct ovl_fh ) fid;
	int len = fh_len << 2;
	unsigned int flags = 0;
	int err;

	err = -EINVAL;
	if (fh_type != OVL_FILEID)
	goto out_err;

	err = ovl_check_fh_len(fh, len);
	if (err)
	goto out_err;

	flags = fh->flags;
	dentry = (flags & OVL_FH_FLAG_PATH_UPPER) ?
	ovl_upper_fh_to_d(sb, fh) :
	ovl_lower_fh_to_d(sb, fh);
	err = PTR_ERR(dentry);
	if (IS_ERR(dentry) && err != -ESTALE)
	goto out_err;

	return dentry;

	out_err:
	pr_warn_ratelimited("overlayfs: failed to decode file handle (len=%d, type=%d, flags=%x, err=%i)\n",
	len, fh_type, flags, err);
	return ERR_PTR(err);
	}

	static struct dentry ovl_fh_to_parent(struct super_block sb, struct fid *fid,
	int fh_len, int fh_type)
	{
	pr_warn_ratelimited("overlayfs: connectable file handles not supported; use 'no_subtree_check' exportfs option.\n");
	return ERR_PTR(-EACCES);
	}

	static int ovl_get_name(struct dentry parent, char name,
	struct dentry *child)
	{
	/*
	* ovl_fh_to_dentry() returns connected dir overlay dentries and
	* ovl_fh_to_parent() is not implemented, so we should not get here.
	*/
	WARN_ON_ONCE(1);
	return -EIO;
	}

	static struct dentry ovl_get_parent(struct dentry dentry)
	{
	/*
	* ovl_fh_to_dentry() returns connected dir overlay dentries, so we
	* should not get here.
	*/
	WARN_ON_ONCE(1);
	return ERR_PTR(-EIO);
	}

	const struct export_operations ovl_export_operations = {
	.encode_fh = ovl_encode_inode_fh,
	.fh_to_dentry = ovl_fh_to_dentry,
	.fh_to_parent = ovl_fh_to_parent,
	.get_name = ovl_get_name,
	.get_parent = ovl_get_parent,
	};