linux/fs/overlayfs/namei.c

// SPDX-License-Identifier: GPL-2.0-only
/*
 * Copyright (C) 2011 Novell Inc.
 * Copyright (C) 2016 Red Hat, Inc.
 */

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

struct ovl_lookup_data {
	struct super_block *sb;
	struct qstr name;
	bool is_dir;
	bool opaque;
	bool stop;
	bool last;
	char *redirect;
	bool metacopy;
};

static int ovl_check_redirect(struct dentry *dentry, struct ovl_lookup_data *d,
			      size_t prelen, const char *post)
{
	int res;
	char *buf;
	struct ovl_fs *ofs = OVL_FS(d->sb);

	buf = ovl_get_redirect_xattr(ofs, dentry, prelen + strlen(post));
	if (IS_ERR_OR_NULL(buf))
		return PTR_ERR(buf);

	if (buf[0] == '/') {
		/*
		 * One of the ancestor path elements in an absolute path
		 * lookup in ovl_lookup_layer() could have been opaque and
		 * that will stop further lookup in lower layers (d->stop=true)
		 * But we have found an absolute redirect in decendant path
		 * element and that should force continue lookup in lower
		 * layers (reset d->stop).
		 */
		d->stop = false;
	} else {
		res = strlen(buf) + 1;
		memmove(buf + prelen, buf, res);
		memcpy(buf, d->name.name, prelen);
	}

	strcat(buf, post);
	kfree(d->redirect);
	d->redirect = buf;
	d->name.name = d->redirect;
	d->name.len = strlen(d->redirect);

	return 0;
}

static int ovl_acceptable(void *ctx, struct dentry *dentry)
{
	/*
	 * A non-dir origin may be disconnected, which is fine, because
	 * we only need it for its unique inode number.
	 */
	if (!d_is_dir(dentry))
		return 1;

	/* Don't decode a deleted empty directory */
	if (d_unhashed(dentry))
		return 0;

	/* Check if directory belongs to the layer we are decoding from */
	return is_subdir(dentry, ((struct vfsmount *)ctx)->mnt_root);
}

/*
 * Check validity of an overlay file handle buffer.
 *
 * Return 0 for a valid file handle.
 * Return -ENODATA for "origin unknown".
 * Return <0 for an invalid file handle.
 */
int ovl_check_fb_len(struct ovl_fb *fb, int fb_len)
{
	if (fb_len < sizeof(struct ovl_fb) || fb_len < fb->len)
		return -EINVAL;

	if (fb->magic != OVL_FH_MAGIC)
		return -EINVAL;

	/* Treat larger version and unknown flags as "origin unknown" */
	if (fb->version > OVL_FH_VERSION || fb->flags & ~OVL_FH_FLAG_ALL)
		return -ENODATA;

	/* Treat endianness mismatch as "origin unknown" */
	if (!(fb->flags & OVL_FH_FLAG_ANY_ENDIAN) &&
	    (fb->flags & OVL_FH_FLAG_BIG_ENDIAN) != OVL_FH_FLAG_CPU_ENDIAN)
		return -ENODATA;

	return 0;
}

static struct ovl_fh *ovl_get_fh(struct ovl_fs *ofs, struct dentry *dentry,
				 enum ovl_xattr ox)
{
	int res, err;
	struct ovl_fh *fh = NULL;

	res = ovl_do_getxattr(ofs, dentry, ox, NULL, 0);
	if (res < 0) {
		if (res == -ENODATA || res == -EOPNOTSUPP)
			return NULL;
		goto fail;
	}
	/* Zero size value means "copied up but origin unknown" */
	if (res == 0)
		return NULL;

	fh = kzalloc(res + OVL_FH_WIRE_OFFSET, GFP_KERNEL);
	if (!fh)
		return ERR_PTR(-ENOMEM);

	res = ovl_do_getxattr(ofs, dentry, ox, fh->buf, res);
	if (res < 0)
		goto fail;

	err = ovl_check_fb_len(&fh->fb, res);
	if (err < 0) {
		if (err == -ENODATA)
			goto out;
		goto invalid;
	}

	return fh;

out:
	kfree(fh);
	return NULL;

fail:
	pr_warn_ratelimited("failed to get origin (%i)\n", res);
	goto out;
invalid:
	pr_warn_ratelimited("invalid origin (%*phN)\n", res, fh);
	goto out;
}

struct dentry *ovl_decode_real_fh(struct ovl_fh *fh, struct vfsmount *mnt,
				  bool connected)
{
	struct dentry *real;
	int bytes;

	/*
	 * Make sure that the stored uuid matches the uuid of the lower
	 * layer where file handle will be decoded.
	 */
	if (!uuid_equal(&fh->fb.uuid, &mnt->mnt_sb->s_uuid))
		return NULL;

	bytes = (fh->fb.len - offsetof(struct ovl_fb, fid));
	real = exportfs_decode_fh(mnt, (struct fid *)fh->fb.fid,
				  bytes >> 2, (int)fh->fb.type,
				  connected ? ovl_acceptable : NULL, mnt);
	if (IS_ERR(real)) {
		/*
		 * Treat stale file handle to lower file as "origin unknown".
		 * upper file handle could become stale when upper file is
		 * unlinked and this information is needed to handle stale
		 * index entries correctly.
		 */
		if (real == ERR_PTR(-ESTALE) &&
		    !(fh->fb.flags & OVL_FH_FLAG_PATH_UPPER))
			real = NULL;
		return real;
	}

	if (ovl_dentry_weird(real)) {
		dput(real);
		return NULL;
	}

	return real;
}

static bool ovl_is_opaquedir(struct super_block *sb, struct dentry *dentry)
{
	return ovl_check_dir_xattr(sb, dentry, OVL_XATTR_OPAQUE);
}

static struct dentry *ovl_lookup_positive_unlocked(const char *name,
						   struct dentry *base, int len,
						   bool drop_negative)
{
	struct dentry *ret = lookup_one_len_unlocked(name, base, len);

	if (!IS_ERR(ret) && d_flags_negative(smp_load_acquire(&ret->d_flags))) {
		if (drop_negative && ret->d_lockref.count == 1) {
			spin_lock(&ret->d_lock);
			/* Recheck condition under lock */
			if (d_is_negative(ret) && ret->d_lockref.count == 1)
				__d_drop(ret);
			spin_unlock(&ret->d_lock);
		}
		dput(ret);
		ret = ERR_PTR(-ENOENT);
	}
	return ret;
}

static int ovl_lookup_single(struct dentry *base, struct ovl_lookup_data *d,
			     const char *name, unsigned int namelen,
			     size_t prelen, const char *post,
			     struct dentry **ret, bool drop_negative)
{
	struct dentry *this;
	int err;
	bool last_element = !post[0];

	this = ovl_lookup_positive_unlocked(name, base, namelen, drop_negative);
	if (IS_ERR(this)) {
		err = PTR_ERR(this);
		this = NULL;
		if (err == -ENOENT || err == -ENAMETOOLONG)
			goto out;
		goto out_err;
	}

	if (ovl_dentry_weird(this)) {
		/* Don't support traversing automounts and other weirdness */
		err = -EREMOTE;
		goto out_err;
	}
	if (ovl_is_whiteout(this)) {
		d->stop = d->opaque = true;
		goto put_and_out;
	}
	/*
	 * This dentry should be a regular file if previous layer lookup
	 * found a metacopy dentry.
	 */
	if (last_element && d->metacopy && !d_is_reg(this)) {
		d->stop = true;
		goto put_and_out;
	}
	if (!d_can_lookup(this)) {
		if (d->is_dir || !last_element) {
			d->stop = true;
			goto put_and_out;
		}
		err = ovl_check_metacopy_xattr(OVL_FS(d->sb), this);
		if (err < 0)
			goto out_err;

		d->metacopy = err;
		d->stop = !d->metacopy;
		if (!d->metacopy || d->last)
			goto out;
	} else {
		if (ovl_lookup_trap_inode(d->sb, this)) {
			/* Caught in a trap of overlapping layers */
			err = -ELOOP;
			goto out_err;
		}

		if (last_element)
			d->is_dir = true;
		if (d->last)
			goto out;

		if (ovl_is_opaquedir(d->sb, this)) {
			d->stop = true;
			if (last_element)
				d->opaque = true;
			goto out;
		}
	}
	err = ovl_check_redirect(this, d, prelen, post);
	if (err)
		goto out_err;
out:
	*ret = this;
	return 0;

put_and_out:
	dput(this);
	this = NULL;
	goto out;

out_err:
	dput(this);
	return err;
}

static int ovl_lookup_layer(struct dentry *base, struct ovl_lookup_data *d,
			    struct dentry **ret, bool drop_negative)
{
	/* Counting down from the end, since the prefix can change */
	size_t rem = d->name.len - 1;
	struct dentry *dentry = NULL;
	int err;

	if (d->name.name[0] != '/')
		return ovl_lookup_single(base, d, d->name.name, d->name.len,
					 0, "", ret, drop_negative);

	while (!IS_ERR_OR_NULL(base) && d_can_lookup(base)) {
		const char *s = d->name.name + d->name.len - rem;
		const char *next = strchrnul(s, '/');
		size_t thislen = next - s;
		bool end = !next[0];

		/* Verify we did not go off the rails */
		if (WARN_ON(s[-1] != '/'))
			return -EIO;

		err = ovl_lookup_single(base, d, s, thislen,
					d->name.len - rem, next, &base,
					drop_negative);
		dput(dentry);
		if (err)
			return err;
		dentry = base;
		if (end)
			break;

		rem -= thislen + 1;

		if (WARN_ON(rem >= d->name.len))
			return -EIO;
	}
	*ret = dentry;
	return 0;
}


int ovl_check_origin_fh(struct ovl_fs *ofs, struct ovl_fh *fh, bool connected,
			struct dentry *upperdentry, struct ovl_path **stackp)
{
	struct dentry *origin = NULL;
	int i;

	for (i = 1; i < ofs->numlayer; i++) {
		/*
		 * If lower fs uuid is not unique among lower fs we cannot match
		 * fh->uuid to layer.
		 */
		if (ofs->layers[i].fsid &&
		    ofs->layers[i].fs->bad_uuid)
			continue;

		origin = ovl_decode_real_fh(fh, ofs->layers[i].mnt,
					    connected);
		if (origin)
			break;
	}

	if (!origin)
		return -ESTALE;
	else if (IS_ERR(origin))
		return PTR_ERR(origin);

	if (upperdentry && !ovl_is_whiteout(upperdentry) &&
	    ((d_inode(origin)->i_mode ^ d_inode(upperdentry)->i_mode) & S_IFMT))
		goto invalid;

	if (!*stackp)
		*stackp = kmalloc(sizeof(struct ovl_path), GFP_KERNEL);
	if (!*stackp) {
		dput(origin);
		return -ENOMEM;
	}
	**stackp = (struct ovl_path){
		.dentry = origin,
		.layer = &ofs->layers[i]
	};

	return 0;

invalid:
	pr_warn_ratelimited("invalid origin (%pd2, ftype=%x, origin ftype=%x).\n",
			    upperdentry, d_inode(upperdentry)->i_mode & S_IFMT,
			    d_inode(origin)->i_mode & S_IFMT);
	dput(origin);
	return -EIO;
}

static int ovl_check_origin(struct ovl_fs *ofs, struct dentry *upperdentry,
			    struct ovl_path **stackp)
{
	struct ovl_fh *fh = ovl_get_fh(ofs, upperdentry, OVL_XATTR_ORIGIN);
	int err;

	if (IS_ERR_OR_NULL(fh))
		return PTR_ERR(fh);

	err = ovl_check_origin_fh(ofs, fh, false, upperdentry, stackp);
	kfree(fh);

	if (err) {
		if (err == -ESTALE)
			return 0;
		return err;
	}

	return 0;
}

/*
 * Verify that @fh matches the file handle stored in xattr @name.
 * Return 0 on match, -ESTALE on mismatch, < 0 on error.
 */
static int ovl_verify_fh(struct ovl_fs *ofs, struct dentry *dentry,
			 enum ovl_xattr ox, const struct ovl_fh *fh)
{
	struct ovl_fh *ofh = ovl_get_fh(ofs, dentry, ox);
	int err = 0;

	if (!ofh)
		return -ENODATA;

	if (IS_ERR(ofh))
		return PTR_ERR(ofh);

	if (fh->fb.len != ofh->fb.len || memcmp(&fh->fb, &ofh->fb, fh->fb.len))
		err = -ESTALE;

	kfree(ofh);
	return err;
}

/*
 * Verify that @real dentry matches the file handle stored in xattr @name.
 *
 * If @set is true and there is no stored file handle, encode @real and store
 * file handle in xattr @name.
 *
 * Return 0 on match, -ESTALE on mismatch, -ENODATA on no xattr, < 0 on error.
 */
int ovl_verify_set_fh(struct ovl_fs *ofs, struct dentry *dentry,
		      enum ovl_xattr ox, struct dentry *real, bool is_upper,
		      bool set)
{
	struct inode *inode;
	struct ovl_fh *fh;
	int err;

	fh = ovl_encode_real_fh(real, is_upper);
	err = PTR_ERR(fh);
	if (IS_ERR(fh)) {
		fh = NULL;
		goto fail;
	}

	err = ovl_verify_fh(ofs, dentry, ox, fh);
	if (set && err == -ENODATA)
		err = ovl_do_setxattr(ofs, dentry, ox, fh->buf, fh->fb.len);
	if (err)
		goto fail;

out:
	kfree(fh);
	return err;

fail:
	inode = d_inode(real);
	pr_warn_ratelimited("failed to verify %s (%pd2, ino=%lu, err=%i)\n",
			    is_upper ? "upper" : "origin", real,
			    inode ? inode->i_ino : 0, err);
	goto out;
}

/* Get upper dentry from index */
struct dentry *ovl_index_upper(struct ovl_fs *ofs, struct dentry *index)
{
	struct ovl_fh *fh;
	struct dentry *upper;

	if (!d_is_dir(index))
		return dget(index);

	fh = ovl_get_fh(ofs, index, OVL_XATTR_UPPER);
	if (IS_ERR_OR_NULL(fh))
		return ERR_CAST(fh);

	upper = ovl_decode_real_fh(fh, ovl_upper_mnt(ofs), true);
	kfree(fh);

	if (IS_ERR_OR_NULL(upper))
		return upper ?: ERR_PTR(-ESTALE);

	if (!d_is_dir(upper)) {
		pr_warn_ratelimited("invalid index upper (%pd2, upper=%pd2).\n",
				    index, upper);
		dput(upper);
		return ERR_PTR(-EIO);
	}

	return upper;
}

/*
 * Verify that an index entry name matches the origin file handle stored in
 * OVL_XATTR_ORIGIN and that origin file handle can be decoded to lower path.
 * Return 0 on match, -ESTALE on mismatch or stale origin, < 0 on error.
 */
int ovl_verify_index(struct ovl_fs *ofs, struct dentry *index)
{
	struct ovl_fh *fh = NULL;
	size_t len;
	struct ovl_path origin = { };
	struct ovl_path *stack = &origin;
	struct dentry *upper = NULL;
	int err;

	if (!d_inode(index))
		return 0;

	err = -EINVAL;
	if (index->d_name.len < sizeof(struct ovl_fb)*2)
		goto fail;

	err = -ENOMEM;
	len = index->d_name.len / 2;
	fh = kzalloc(len + OVL_FH_WIRE_OFFSET, GFP_KERNEL);
	if (!fh)
		goto fail;

	err = -EINVAL;
	if (hex2bin(fh->buf, index->d_name.name, len))
		goto fail;

	err = ovl_check_fb_len(&fh->fb, len);
	if (err)
		goto fail;

	/*
	 * Whiteout index entries are used as an indication that an exported
	 * overlay file handle should be treated as stale (i.e. after unlink
	 * of the overlay inode). These entries contain no origin xattr.
	 */
	if (ovl_is_whiteout(index))
		goto out;

	/*
	 * Verifying directory index entries are not stale is expensive, so
	 * only verify stale dir index if NFS export is enabled.
	 */
	if (d_is_dir(index) && !ofs->config.nfs_export)
		goto out;

	/*
	 * Directory index entries should have 'upper' xattr pointing to the
	 * real upper dir. Non-dir index entries are hardlinks to the upper
	 * real inode. For non-dir index, we can read the copy up origin xattr
	 * directly from the index dentry, but for dir index we first need to
	 * decode the upper directory.
	 */
	upper = ovl_index_upper(ofs, index);
	if (IS_ERR_OR_NULL(upper)) {
		err = PTR_ERR(upper);
		/*
		 * Directory index entries with no 'upper' xattr need to be
		 * removed. When dir index entry has a stale 'upper' xattr,
		 * we assume that upper dir was removed and we treat the dir
		 * index as orphan entry that needs to be whited out.
		 */
		if (err == -ESTALE)
			goto orphan;
		else if (!err)
			err = -ESTALE;
		goto fail;
	}

	err = ovl_verify_fh(ofs, upper, OVL_XATTR_ORIGIN, fh);
	dput(upper);
	if (err)
		goto fail;

	/* Check if non-dir index is orphan and don't warn before cleaning it */
	if (!d_is_dir(index) && d_inode(index)->i_nlink == 1) {
		err = ovl_check_origin_fh(ofs, fh, false, index, &stack);
		if (err)
			goto fail;

		if (ovl_get_nlink(ofs, origin.dentry, index, 0) == 0)
			goto orphan;
	}

out:
	dput(origin.dentry);
	kfree(fh);
	return err;

fail:
	pr_warn_ratelimited("failed to verify index (%pd2, ftype=%x, err=%i)\n",
			    index, d_inode(index)->i_mode & S_IFMT, err);
	goto out;

orphan:
	pr_warn_ratelimited("orphan index entry (%pd2, ftype=%x, nlink=%u)\n",
			    index, d_inode(index)->i_mode & S_IFMT,
			    d_inode(index)->i_nlink);
	err = -ENOENT;
	goto out;
}

static int ovl_get_index_name_fh(struct ovl_fh *fh, struct qstr *name)
{
	char *n, *s;

	n = kcalloc(fh->fb.len, 2, GFP_KERNEL);
	if (!n)
		return -ENOMEM;

	s  = bin2hex(n, fh->buf, fh->fb.len);
	*name = (struct qstr) QSTR_INIT(n, s - n);

	return 0;

}

/*
 * Lookup in indexdir for the index entry of a lower real inode or a copy up
 * origin inode. The index entry name is the hex representation of the lower
 * inode file handle.
 *
 * If the index dentry in negative, then either no lower aliases have been
 * copied up yet, or aliases have been copied up in older kernels and are
 * not indexed.
 *
 * If the index dentry for a copy up origin inode is positive, but points
 * to an inode different than the upper inode, then either the upper inode
 * has been copied up and not indexed or it was indexed, but since then
 * index dir was cleared. Either way, that index cannot be used to indentify
 * the overlay inode.
 */
int ovl_get_index_name(struct dentry *origin, struct qstr *name)
{
	struct ovl_fh *fh;
	int err;

	fh = ovl_encode_real_fh(origin, false);
	if (IS_ERR(fh))
		return PTR_ERR(fh);

	err = ovl_get_index_name_fh(fh, name);

	kfree(fh);
	return err;
}

/* Lookup index by file handle for NFS export */
struct dentry *ovl_get_index_fh(struct ovl_fs *ofs, struct ovl_fh *fh)
{
	struct dentry *index;
	struct qstr name;
	int err;

	err = ovl_get_index_name_fh(fh, &name);
	if (err)
		return ERR_PTR(err);

	index = lookup_positive_unlocked(name.name, ofs->indexdir, name.len);
	kfree(name.name);
	if (IS_ERR(index)) {
		if (PTR_ERR(index) == -ENOENT)
			index = NULL;
		return index;
	}

	if (ovl_is_whiteout(index))
		err = -ESTALE;
	else if (ovl_dentry_weird(index))
		err = -EIO;
	else
		return index;

	dput(index);
	return ERR_PTR(err);
}

struct dentry *ovl_lookup_index(struct ovl_fs *ofs, struct dentry *upper,
				struct dentry *origin, bool verify)
{
	struct dentry *index;
	struct inode *inode;
	struct qstr name;
	bool is_dir = d_is_dir(origin);
	int err;

	err = ovl_get_index_name(origin, &name);
	if (err)
		return ERR_PTR(err);

	index = lookup_positive_unlocked(name.name, ofs->indexdir, name.len);
	if (IS_ERR(index)) {
		err = PTR_ERR(index);
		if (err == -ENOENT) {
			index = NULL;
			goto out;
		}
		pr_warn_ratelimited("failed inode index lookup (ino=%lu, key=%.*s, err=%i);\n"
				    "overlayfs: mount with '-o index=off' to disable inodes index.\n",
				    d_inode(origin)->i_ino, name.len, name.name,
				    err);
		goto out;
	}

	inode = d_inode(index);
	if (ovl_is_whiteout(index) && !verify) {
		/*
		 * When index lookup is called with !verify for decoding an
		 * overlay file handle, a whiteout index implies that decode
		 * should treat file handle as stale and no need to print a
		 * warning about it.
		 */
		dput(index);
		index = ERR_PTR(-ESTALE);
		goto out;
	} else if (ovl_dentry_weird(index) || ovl_is_whiteout(index) ||
		   ((inode->i_mode ^ d_inode(origin)->i_mode) & S_IFMT)) {
		/*
		 * Index should always be of the same file type as origin
		 * except for the case of a whiteout index. A whiteout
		 * index should only exist if all lower aliases have been
		 * unlinked, which means that finding a lower origin on lookup
		 * whose index is a whiteout should be treated as an error.
		 */
		pr_warn_ratelimited("bad index found (index=%pd2, ftype=%x, origin ftype=%x).\n",
				    index, d_inode(index)->i_mode & S_IFMT,
				    d_inode(origin)->i_mode & S_IFMT);
		goto fail;
	} else if (is_dir && verify) {
		if (!upper) {
			pr_warn_ratelimited("suspected uncovered redirected dir found (origin=%pd2, index=%pd2).\n",
					    origin, index);
			goto fail;
		}

		/* Verify that dir index 'upper' xattr points to upper dir */
		err = ovl_verify_upper(ofs, index, upper, false);
		if (err) {
			if (err == -ESTALE) {
				pr_warn_ratelimited("suspected multiply redirected dir found (upper=%pd2, origin=%pd2, index=%pd2).\n",
						    upper, origin, index);
			}
			goto fail;
		}
	} else if (upper && d_inode(upper) != inode) {
		goto out_dput;
	}
out:
	kfree(name.name);
	return index;

out_dput:
	dput(index);
	index = NULL;
	goto out;

fail:
	dput(index);
	index = ERR_PTR(-EIO);
	goto out;
}

/*
 * Returns next layer in stack starting from top.
 * Returns -1 if this is the last layer.
 */
int ovl_path_next(int idx, struct dentry *dentry, struct path *path)
{
	struct ovl_entry *oe = dentry->d_fsdata;

	BUG_ON(idx < 0);
	if (idx == 0) {
		ovl_path_upper(dentry, path);
		if (path->dentry)
			return oe->numlower ? 1 : -1;
		idx++;
	}
	BUG_ON(idx > oe->numlower);
	path->dentry = oe->lowerstack[idx - 1].dentry;
	path->mnt = oe->lowerstack[idx - 1].layer->mnt;

	return (idx < oe->numlower) ? idx + 1 : -1;
}

/* Fix missing 'origin' xattr */
static int ovl_fix_origin(struct ovl_fs *ofs, struct dentry *dentry,
			  struct dentry *lower, struct dentry *upper)
{
	int err;

	if (ovl_check_origin_xattr(ofs, upper))
		return 0;

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

	err = ovl_set_origin(dentry, lower, upper);
	if (!err)
		err = ovl_set_impure(dentry->d_parent, upper->d_parent);

	ovl_drop_write(dentry);
	return err;
}

struct dentry *ovl_lookup(struct inode *dir, struct dentry *dentry,
			  unsigned int flags)
{
	struct ovl_entry *oe;
	const struct cred *old_cred;
	struct ovl_fs *ofs = dentry->d_sb->s_fs_info;
	struct ovl_entry *poe = dentry->d_parent->d_fsdata;
	struct ovl_entry *roe = dentry->d_sb->s_root->d_fsdata;
	struct ovl_path *stack = NULL, *origin_path = NULL;
	struct dentry *upperdir, *upperdentry = NULL;
	struct dentry *origin = NULL;
	struct dentry *index = NULL;
	unsigned int ctr = 0;
	struct inode *inode = NULL;
	bool upperopaque = false;
	char *upperredirect = NULL;
	struct dentry *this;
	unsigned int i;
	int err;
	bool uppermetacopy = false;
	struct ovl_lookup_data d = {
		.sb = dentry->d_sb,
		.name = dentry->d_name,
		.is_dir = false,
		.opaque = false,
		.stop = false,
		.last = ofs->config.redirect_follow ? false : !poe->numlower,
		.redirect = NULL,
		.metacopy = false,
	};

	if (dentry->d_name.len > ofs->namelen)
		return ERR_PTR(-ENAMETOOLONG);

	old_cred = ovl_override_creds(dentry->d_sb);
	upperdir = ovl_dentry_upper(dentry->d_parent);
	if (upperdir) {
		err = ovl_lookup_layer(upperdir, &d, &upperdentry, true);
		if (err)
			goto out;

		if (upperdentry && upperdentry->d_flags & DCACHE_OP_REAL) {
			dput(upperdentry);
			err = -EREMOTE;
			goto out;
		}
		if (upperdentry && !d.is_dir) {
			/*
			 * Lookup copy up origin by decoding origin file handle.
			 * We may get a disconnected dentry, which is fine,
			 * because we only need to hold the origin inode in
			 * cache and use its inode number.  We may even get a
			 * connected dentry, that is not under any of the lower
			 * layers root.  That is also fine for using it's inode
			 * number - it's the same as if we held a reference
			 * to a dentry in lower layer that was moved under us.
			 */
			err = ovl_check_origin(ofs, upperdentry, &origin_path);
			if (err)
				goto out_put_upper;

			if (d.metacopy)
				uppermetacopy = true;
		}

		if (d.redirect) {
			err = -ENOMEM;
			upperredirect = kstrdup(d.redirect, GFP_KERNEL);
			if (!upperredirect)
				goto out_put_upper;
			if (d.redirect[0] == '/')
				poe = roe;
		}
		upperopaque = d.opaque;
	}

	if (!d.stop && poe->numlower) {
		err = -ENOMEM;
		stack = kcalloc(ofs->numlayer - 1, sizeof(struct ovl_path),
				GFP_KERNEL);
		if (!stack)
			goto out_put_upper;
	}

	for (i = 0; !d.stop && i < poe->numlower; i++) {
		struct ovl_path lower = poe->lowerstack[i];

		if (!ofs->config.redirect_follow)
			d.last = i == poe->numlower - 1;
		else
			d.last = lower.layer->idx == roe->numlower;

		err = ovl_lookup_layer(lower.dentry, &d, &this, false);
		if (err)
			goto out_put;

		if (!this)
			continue;

		if ((uppermetacopy || d.metacopy) && !ofs->config.metacopy) {
			err = -EPERM;
			pr_warn_ratelimited("refusing to follow metacopy origin for (%pd2)\n", dentry);
			goto out_put;
		}

		/*
		 * If no origin fh is stored in upper of a merge dir, store fh
		 * of lower dir and set upper parent "impure".
		 */
		if (upperdentry && !ctr && !ofs->noxattr && d.is_dir) {
			err = ovl_fix_origin(ofs, dentry, this, upperdentry);
			if (err) {
				dput(this);
				goto out_put;
			}
		}

		/*
		 * When "verify_lower" feature is enabled, do not merge with a
		 * lower dir that does not match a stored origin xattr. In any
		 * case, only verified origin is used for index lookup.
		 *
		 * For non-dir dentry, if index=on, then ensure origin
		 * matches the dentry found using path based lookup,
		 * otherwise error out.
		 */
		if (upperdentry && !ctr &&
		    ((d.is_dir && ovl_verify_lower(dentry->d_sb)) ||
		     (!d.is_dir && ofs->config.index && origin_path))) {
			err = ovl_verify_origin(ofs, upperdentry, this, false);
			if (err) {
				dput(this);
				if (d.is_dir)
					break;
				goto out_put;
			}
			origin = this;
		}

		if (d.metacopy && ctr) {
			/*
			 * Do not store intermediate metacopy dentries in
			 * lower chain, except top most lower metacopy dentry.
			 * Continue the loop so that if there is an absolute
			 * redirect on this dentry, poe can be reset to roe.
			 */
			dput(this);
			this = NULL;
		} else {
			stack[ctr].dentry = this;
			stack[ctr].layer = lower.layer;
			ctr++;
		}

		/*
		 * Following redirects can have security consequences: it's like
		 * a symlink into the lower layer without the permission checks.
		 * This is only a problem if the upper layer is untrusted (e.g
		 * comes from an USB drive).  This can allow a non-readable file
		 * or directory to become readable.
		 *
		 * Only following redirects when redirects are enabled disables
		 * this attack vector when not necessary.
		 */
		err = -EPERM;
		if (d.redirect && !ofs->config.redirect_follow) {
			pr_warn_ratelimited("refusing to follow redirect for (%pd2)\n",
					    dentry);
			goto out_put;
		}

		if (d.stop)
			break;

		if (d.redirect && d.redirect[0] == '/' && poe != roe) {
			poe = roe;
			/* Find the current layer on the root dentry */
			i = lower.layer->idx - 1;
		}
	}

	/*
	 * For regular non-metacopy upper dentries, there is no lower
	 * path based lookup, hence ctr will be zero. If a dentry is found
	 * using ORIGIN xattr on upper, install it in stack.
	 *
	 * For metacopy dentry, path based lookup will find lower dentries.
	 * Just make sure a corresponding data dentry has been found.
	 */
	if (d.metacopy || (uppermetacopy && !ctr)) {
		err = -EIO;
		goto out_put;
	} else if (!d.is_dir && upperdentry && !ctr && origin_path) {
		if (WARN_ON(stack != NULL)) {
			err = -EIO;
			goto out_put;
		}
		stack = origin_path;
		ctr = 1;
		origin = origin_path->dentry;
		origin_path = NULL;
	}

	/*
	 * Always lookup index if there is no-upperdentry.
	 *
	 * For the case of upperdentry, we have set origin by now if it
	 * needed to be set. There are basically three cases.
	 *
	 * For directories, lookup index by lower inode and verify it matches
	 * upper inode. We only trust dir index if we verified that lower dir
	 * matches origin, otherwise dir index entries may be inconsistent
	 * and we ignore them.
	 *
	 * For regular upper, we already set origin if upper had ORIGIN
	 * xattr. There is no verification though as there is no path
	 * based dentry lookup in lower in this case.
	 *
	 * For metacopy upper, we set a verified origin already if index
	 * is enabled and if upper had an ORIGIN xattr.
	 *
	 */
	if (!upperdentry && ctr)
		origin = stack[0].dentry;

	if (origin && ovl_indexdir(dentry->d_sb) &&
	    (!d.is_dir || ovl_index_all(dentry->d_sb))) {
		index = ovl_lookup_index(ofs, upperdentry, origin, true);
		if (IS_ERR(index)) {
			err = PTR_ERR(index);
			index = NULL;
			goto out_put;
		}
	}

	oe = ovl_alloc_entry(ctr);
	err = -ENOMEM;
	if (!oe)
		goto out_put;

	memcpy(oe->lowerstack, stack, sizeof(struct ovl_path) * ctr);
	dentry->d_fsdata = oe;

	if (upperopaque)
		ovl_dentry_set_opaque(dentry);

	if (upperdentry)
		ovl_dentry_set_upper_alias(dentry);
	else if (index) {
		upperdentry = dget(index);
		upperredirect = ovl_get_redirect_xattr(ofs, upperdentry, 0);
		if (IS_ERR(upperredirect)) {
			err = PTR_ERR(upperredirect);
			upperredirect = NULL;
			goto out_free_oe;
		}
		err = ovl_check_metacopy_xattr(ofs, upperdentry);
		if (err < 0)
			goto out_free_oe;
		uppermetacopy = err;
	}

	if (upperdentry || ctr) {
		struct ovl_inode_params oip = {
			.upperdentry = upperdentry,
			.lowerpath = stack,
			.index = index,
			.numlower = ctr,
			.redirect = upperredirect,
			.lowerdata = (ctr > 1 && !d.is_dir) ?
				      stack[ctr - 1].dentry : NULL,
		};

		inode = ovl_get_inode(dentry->d_sb, &oip);
		err = PTR_ERR(inode);
		if (IS_ERR(inode))
			goto out_free_oe;
		if (upperdentry && !uppermetacopy)
			ovl_set_flag(OVL_UPPERDATA, inode);
	}

	ovl_dentry_update_reval(dentry, upperdentry,
			DCACHE_OP_REVALIDATE | DCACHE_OP_WEAK_REVALIDATE);

	revert_creds(old_cred);
	if (origin_path) {
		dput(origin_path->dentry);
		kfree(origin_path);
	}
	dput(index);
	kfree(stack);
	kfree(d.redirect);
	return d_splice_alias(inode, dentry);

out_free_oe:
	dentry->d_fsdata = NULL;
	kfree(oe);
out_put:
	dput(index);
	for (i = 0; i < ctr; i++)
		dput(stack[i].dentry);
	kfree(stack);
out_put_upper:
	if (origin_path) {
		dput(origin_path->dentry);
		kfree(origin_path);
	}
	dput(upperdentry);
	kfree(upperredirect);
out:
	kfree(d.redirect);
	revert_creds(old_cred);
	return ERR_PTR(err);
}

bool ovl_lower_positive(struct dentry *dentry)
{
	struct ovl_entry *poe = dentry->d_parent->d_fsdata;
	const struct qstr *name = &dentry->d_name;
	const struct cred *old_cred;
	unsigned int i;
	bool positive = false;
	bool done = false;

	/*
	 * If dentry is negative, then lower is positive iff this is a
	 * whiteout.
	 */
	if (!dentry->d_inode)
		return ovl_dentry_is_opaque(dentry);

	/* Negative upper -> positive lower */
	if (!ovl_dentry_upper(dentry))
		return true;

	old_cred = ovl_override_creds(dentry->d_sb);
	/* Positive upper -> have to look up lower to see whether it exists */
	for (i = 0; !done && !positive && i < poe->numlower; i++) {
		struct dentry *this;
		struct dentry *lowerdir = poe->lowerstack[i].dentry;

		this = lookup_positive_unlocked(name->name, lowerdir,
					       name->len);
		if (IS_ERR(this)) {
			switch (PTR_ERR(this)) {
			case -ENOENT:
			case -ENAMETOOLONG:
				break;

			default:
				/*
				 * Assume something is there, we just couldn't
				 * access it.
				 */
				positive = true;
				break;
			}
		} else {
			positive = !ovl_is_whiteout(this);
			done = true;
			dput(this);
		}
	}
	revert_creds(old_cred);

	return positive;
}