Merge tag 'for-linus' of git://git.kernel.org/pub/scm/virt/kvm/kvm

[linux-stable.git] / fs / overlayfs / export.c

// SPDX-License-Identifier: GPL-2.0-only
/*
 * Overlayfs NFS export support.
 *
 * Amir Goldstein <amir73il@gmail.com>
 *
 * Copyright (C) 2017-2018 CTERA Networks. All Rights Reserved.
 */

#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"

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

        if (ovl_dentry_upper(dentry))
                return 0;

        err = ovl_copy_up(dentry);
        if (err) {
                pr_warn_ratelimited("failed to copy up on encode (%pd2, err=%i)\n",
                                    dentry, err);
        }

        return err;
}

/*
 * Before encoding a non-upper directory file handle from real layer N, we need
 * to check if it will be possible to reconnect an overlay dentry from the real
 * lower decoded dentry. This is done by following the overlay ancestry up to a
 * "layer N connected" ancestor and verifying that all parents along the way are
 * "layer N connectable". If an ancestor that is NOT "layer N connectable" is
 * found, we need to copy up an ancestor, which is "layer N connectable", thus
 * making that ancestor "layer N connected". For example:
 *
 * layer 1: /a
 * layer 2: /a/b/c
 *
 * The overlay dentry /a is NOT "layer 2 connectable", because if dir /a is
 * copied up and renamed, upper dir /a will be indexed by lower dir /a from
 * layer 1. The dir /a from layer 2 will never be indexed, so the algorithm (*)
 * in ovl_lookup_real_ancestor() will not be able to lookup a connected overlay
 * dentry from the connected lower dentry /a/b/c.
 *
 * To avoid this problem on decode time, we need to copy up an ancestor of
 * /a/b/c, which is "layer 2 connectable", on encode time. That ancestor is
 * /a/b. After copy up (and index) of /a/b, it will become "layer 2 connected"
 * and when the time comes to decode the file handle from lower dentry /a/b/c,
 * ovl_lookup_real_ancestor() will find the indexed ancestor /a/b and decoding
 * a connected overlay dentry will be accomplished.
 *
 * (*) the algorithm in ovl_lookup_real_ancestor() can be improved to lookup an
 * entry /a in the lower layers above layer N and find the indexed dir /a from
 * layer 1. If that improvement is made, then the check for "layer N connected"
 * will need to verify there are no redirects in lower layers above N. In the
 * example above, /a will be "layer 2 connectable". However, if layer 2 dir /a
 * is a target of a layer 1 redirect, then /a will NOT be "layer 2 connectable":
 *
 * layer 1: /A (redirect = /a)
 * layer 2: /a/b/c
 */

/* Return the lowest layer for encoding a connectable file handle */
static int ovl_connectable_layer(struct dentry *dentry)
{
        struct ovl_entry *oe = OVL_E(dentry);

        /* We can get overlay root from root of any layer */
        if (dentry == dentry->d_sb->s_root)
                return ovl_numlower(oe);

        /*
         * If it's an unindexed merge dir, then it's not connectable with any
         * lower layer
         */
        if (ovl_dentry_upper(dentry) &&
            !ovl_test_flag(OVL_INDEX, d_inode(dentry)))
                return 0;

        /* We can get upper/overlay path from indexed/lower dentry */
        return ovl_lowerstack(oe)->layer->idx;
}

/*
 * @dentry is "connected" if all ancestors up to root or a "connected" ancestor
 * have the same uppermost lower layer as the origin's layer. We may need to
 * copy up a "connectable" ancestor to make it "connected". A "connected" dentry
 * cannot become non "connected", so cache positive result in dentry flags.
 *
 * Return the connected origin layer or < 0 on error.
 */
static int ovl_connect_layer(struct dentry *dentry)
{
        struct dentry *next, *parent = NULL;
        struct ovl_entry *oe = OVL_E(dentry);
        int origin_layer;
        int err = 0;

        if (WARN_ON(dentry == dentry->d_sb->s_root) ||
            WARN_ON(!ovl_dentry_lower(dentry)))
                return -EIO;

        origin_layer = ovl_lowerstack(oe)->layer->idx;
        if (ovl_dentry_test_flag(OVL_E_CONNECTED, dentry))
                return origin_layer;

        /* Find the topmost origin layer connectable ancestor of @dentry */
        next = dget(dentry);
        for (;;) {
                parent = dget_parent(next);
                if (WARN_ON(parent == next)) {
                        err = -EIO;
                        break;
                }

                /*
                 * If @parent is not origin layer connectable, then copy up
                 * @next which is origin layer connectable and we are done.
                 */
                if (ovl_connectable_layer(parent) < origin_layer) {
                        err = ovl_encode_maybe_copy_up(next);
                        break;
                }

                /* If @parent is connected or indexed we are done */
                if (ovl_dentry_test_flag(OVL_E_CONNECTED, parent) ||
                    ovl_test_flag(OVL_INDEX, d_inode(parent)))
                        break;

                dput(next);
                next = parent;
        }

        dput(parent);
        dput(next);

        if (!err)
                ovl_dentry_set_flag(OVL_E_CONNECTED, dentry);

        return err ?: origin_layer;
}

/*
 * 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
 *
 * (*) Decoding a connected overlay dir from real lower dentry is not always
 * possible when there are redirects in lower layers and non-indexed merge dirs.
 * To mitigate those case, we may copy up the lower dir ancestor before encode
 * of a decodable file handle for non-upper dir.
 *
 * Return 0 for upper file handle, > 0 for lower file handle or < 0 on error.
 */
static int ovl_check_encode_origin(struct dentry *dentry)
{
        struct ovl_fs *ofs = OVL_FS(dentry->d_sb);
        bool decodable = ofs->config.nfs_export;

        /* No upper layer? */
        if (!ovl_upper_mnt(ofs))
                return 1;

        /* Lower file handle for non-upper non-decodable */
        if (!ovl_dentry_upper(dentry) && !decodable)
                return 1;

        /* Upper file handle for pure upper */
        if (!ovl_dentry_lower(dentry))
                return 0;

        /*
         * Root is never indexed, so if there's an upper layer, encode upper for
         * root.
         */
        if (dentry == dentry->d_sb->s_root)
                return 0;

        /*
         * Upper decodable file handle for non-indexed upper.
         */
        if (ovl_dentry_upper(dentry) && decodable &&
            !ovl_test_flag(OVL_INDEX, d_inode(dentry)))
                return 0;

        /*
         * Decoding a merge dir, whose origin's ancestor is under a redirected
         * lower dir or under a non-indexed upper is not always possible.
         * ovl_connect_layer() will try to make origin's layer "connected" by
         * copying up a "connectable" ancestor.
         */
        if (d_is_dir(dentry) && decodable)
                return ovl_connect_layer(dentry);

        /* Lower file handle for indexed and non-upper dir/non-dir */
        return 1;
}

static int ovl_dentry_to_fid(struct ovl_fs *ofs, struct dentry *dentry,
                             u32 *fid, int buflen)
{
        struct ovl_fh *fh = NULL;
        int err, enc_lower;
        int len;

        /*
         * Check if we should encode a lower or upper file handle and maybe
         * copy up an ancestor to make lower file handle connectable.
         */
        err = enc_lower = ovl_check_encode_origin(dentry);
        if (enc_lower < 0)
                goto fail;

        /* Encode an upper or lower file handle */
        fh = ovl_encode_real_fh(ofs, enc_lower ? ovl_dentry_lower(dentry) :
                                ovl_dentry_upper(dentry), !enc_lower);
        if (IS_ERR(fh))
                return PTR_ERR(fh);

        len = OVL_FH_LEN(fh);
        if (len <= buflen)
                memcpy(fid, fh, len);
        err = len;

out:
        kfree(fh);
        return err;

fail:
        pr_warn_ratelimited("failed to encode file handle (%pd2, err=%i)\n",
                            dentry, err);
        goto out;
}

static int ovl_encode_fh(struct inode *inode, u32 *fid, int *max_len,
                         struct inode *parent)
{
        struct ovl_fs *ofs = OVL_FS(inode->i_sb);
        struct dentry *dentry;
        int bytes, buflen = *max_len << 2;

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

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

        bytes = ovl_dentry_to_fid(ofs, dentry, fid, buflen);
        dput(dentry);
        if (bytes <= 0)
                return FILEID_INVALID;

        *max_len = bytes >> 2;
        if (bytes > buflen)
                return FILEID_INVALID;

        return OVL_FILEID_V1;
}

/*
 * 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 inode *inode = NULL;
        struct ovl_entry *oe;
        struct ovl_inode_params oip = {
                .index = index,
        };

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

        oe = ovl_alloc_entry(!!lower);
        if (!oe)
                return ERR_PTR(-ENOMEM);

        oip.upperdentry = dget(upper);
        if (lower) {
                ovl_lowerstack(oe)->dentry = dget(lower);
                ovl_lowerstack(oe)->layer = lowerpath->layer;
        }
        oip.oe = oe;
        inode = ovl_get_inode(sb, &oip);
        if (IS_ERR(inode)) {
                ovl_free_entry(oe);
                dput(upper);
                return ERR_CAST(inode);
        }

        if (upper)
                ovl_set_flag(OVL_UPPERDATA, inode);

        return d_obtain_alias(inode);
}

/* Get the upper or lower dentry in stack whose on layer @idx */
static struct dentry *ovl_dentry_real_at(struct dentry *dentry, int idx)
{
        struct ovl_entry *oe = OVL_E(dentry);
        struct ovl_path *lowerstack = ovl_lowerstack(oe);
        int i;

        if (!idx)
                return ovl_dentry_upper(dentry);

        for (i = 0; i < ovl_numlower(oe); i++) {
                if (lowerstack[i].layer->idx == idx)
                        return 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,
                                          const 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);
        /*
         * No idmap handling here: it's an internal lookup.  Could skip
         * permission checking altogether, but for now just use non-idmap
         * transformed ids.
         */
        this = lookup_one_len(name.name.name, connected, name.name.len);
        release_dentry_name_snapshot(&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:
        dput(parent);
        inode_unlock(dir);
        return this;

fail:
        pr_warn_ratelimited("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,
                                      const 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,
                                            const struct ovl_layer *layer)
{
        struct ovl_fs *ofs = OVL_FS(sb);
        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 && ovl_indexdir(sb) && !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, true);

                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, &ofs->layers[0]);
                dput(upper);
        }

        if (IS_ERR_OR_NULL(this))
                return this;

        if (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,
                                               const 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,
                                      const 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 = PTR_ERR_OR_ZERO(this);
                        }
                        if (!err) {
                                dput(connected);
                                connected = this;
                        }
                }

                dput(parent);
                dput(next);
        }

        if (err)
                goto fail;

        return connected;

fail:
        pr_warn_ratelimited("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 = OVL_FS(sb);
        const struct ovl_layer *layer = upper ? &ofs->layers[0] : 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 = OVL_FS(sb);
        struct dentry *dentry;
        struct dentry *upper;

        if (!ovl_upper_mnt(ofs))
                return ERR_PTR(-EACCES);

        upper = ovl_decode_real_fh(ofs, fh, ovl_upper_mnt(ofs), true);
        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 = OVL_FS(sb);
        struct ovl_path origin = { };
        struct ovl_path *stack = &origin;
        struct dentry *dentry = NULL;
        struct dentry *index = NULL;
        struct inode *inode;
        int err;

        /* First lookup overlay inode in inode cache by origin fh */
        err = ovl_check_origin_fh(ofs, fh, false, NULL, &stack);
        if (err)
                return ERR_PTR(err);

        if (!d_is_dir(origin.dentry) ||
            !(origin.dentry->d_flags & DCACHE_DISCONNECTED)) {
                inode = ovl_lookup_inode(sb, origin.dentry, false);
                err = PTR_ERR(inode);
                if (IS_ERR(inode))
                        goto out_err;
                if (inode) {
                        dentry = d_find_any_alias(inode);
                        iput(inode);
                        if (dentry)
                                goto out;
                }
        }

        /* Then lookup indexed upper/whiteout by origin fh */
        if (ovl_indexdir(sb)) {
                index = ovl_get_index_fh(ofs, fh);
                err = PTR_ERR(index);
                if (IS_ERR(index)) {
                        index = NULL;
                        goto out_err;
                }
        }

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

                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;
        }

        /* Find origin.dentry again with ovl_acceptable() layer check */
        if (d_is_dir(origin.dentry)) {
                dput(origin.dentry);
                origin.dentry = NULL;
                err = ovl_check_origin_fh(ofs, fh, true, NULL, &stack);
                if (err)
                        goto out_err;
        }
        if (index) {
                err = ovl_verify_origin(ofs, index, origin.dentry, false);
                if (err)
                        goto out_err;
        }

        /* Get a connected non-upper dir or disconnected non-dir */
        dentry = ovl_get_dentry(sb, NULL, &origin, index);

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

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

static struct ovl_fh *ovl_fid_to_fh(struct fid *fid, int buflen, int fh_type)
{
        struct ovl_fh *fh;

        /* If on-wire inner fid is aligned - nothing to do */
        if (fh_type == OVL_FILEID_V1)
                return (struct ovl_fh *)fid;

        if (fh_type != OVL_FILEID_V0)
                return ERR_PTR(-EINVAL);

        if (buflen <= OVL_FH_WIRE_OFFSET)
                return ERR_PTR(-EINVAL);

        fh = kzalloc(buflen, GFP_KERNEL);
        if (!fh)
                return ERR_PTR(-ENOMEM);

        /* Copy unaligned inner fh into aligned buffer */
        memcpy(fh->buf, fid, buflen - OVL_FH_WIRE_OFFSET);
        return fh;
}

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 = NULL;
        int len = fh_len << 2;
        unsigned int flags = 0;
        int err;

        fh = ovl_fid_to_fh(fid, len, fh_type);
        err = PTR_ERR(fh);
        if (IS_ERR(fh))
                goto out_err;

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

        flags = fh->fb.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;

out:
        /* We may have needed to re-align OVL_FILEID_V0 */
        if (!IS_ERR_OR_NULL(fh) && fh != (void *)fid)
                kfree(fh);

        return dentry;

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

static struct dentry *ovl_fh_to_parent(struct super_block *sb, struct fid *fid,
                                       int fh_len, int fh_type)
{
        pr_warn_ratelimited("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_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,
};

/* encode_fh() encodes non-decodable file handles with nfs_export=off */
const struct export_operations ovl_export_fid_operations = {
        .encode_fh      = ovl_encode_fh,
};