Linux 6.14-rc1

[linux.git] / fs / overlayfs / super.c

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

#include <uapi/linux/magic.h>
#include <linux/fs.h>
#include <linux/namei.h>
#include <linux/xattr.h>
#include <linux/mount.h>
#include <linux/parser.h>
#include <linux/module.h>
#include <linux/statfs.h>
#include <linux/seq_file.h>
#include <linux/posix_acl_xattr.h>
#include <linux/exportfs.h>
#include <linux/file.h>
#include <linux/fs_context.h>
#include <linux/fs_parser.h>
#include "overlayfs.h"
#include "params.h"

MODULE_AUTHOR("Miklos Szeredi <miklos@szeredi.hu>");
MODULE_DESCRIPTION("Overlay filesystem");
MODULE_LICENSE("GPL");


struct ovl_dir_cache;

static struct dentry *ovl_d_real(struct dentry *dentry, enum d_real_type type)
{
        struct dentry *upper, *lower;
        int err;

        switch (type) {
        case D_REAL_DATA:
        case D_REAL_METADATA:
                break;
        default:
                goto bug;
        }

        if (!d_is_reg(dentry)) {
                /* d_real_inode() is only relevant for regular files */
                return dentry;
        }

        upper = ovl_dentry_upper(dentry);
        if (upper && (type == D_REAL_METADATA ||
                      ovl_has_upperdata(d_inode(dentry))))
                return upper;

        if (type == D_REAL_METADATA) {
                lower = ovl_dentry_lower(dentry);
                goto real_lower;
        }

        /*
         * Best effort lazy lookup of lowerdata for D_REAL_DATA case to return
         * the real lowerdata dentry.  The only current caller of d_real() with
         * D_REAL_DATA is d_real_inode() from trace_uprobe and this caller is
         * likely going to be followed reading from the file, before placing
         * uprobes on offset within the file, so lowerdata should be available
         * when setting the uprobe.
         */
        err = ovl_verify_lowerdata(dentry);
        if (err)
                goto bug;
        lower = ovl_dentry_lowerdata(dentry);
        if (!lower)
                goto bug;

real_lower:
        /* Handle recursion into stacked lower fs */
        return d_real(lower, type);

bug:
        WARN(1, "%s(%pd4, %d): real dentry not found\n", __func__, dentry, type);
        return dentry;
}

static int ovl_revalidate_real(struct dentry *d, unsigned int flags, bool weak)
{
        int ret = 1;

        if (!d)
                return 1;

        if (weak) {
                if (d->d_flags & DCACHE_OP_WEAK_REVALIDATE)
                        ret =  d->d_op->d_weak_revalidate(d, flags);
        } else if (d->d_flags & DCACHE_OP_REVALIDATE) {
                struct dentry *parent;
                struct inode *dir;
                struct name_snapshot n;

                if (flags & LOOKUP_RCU) {
                        parent = READ_ONCE(d->d_parent);
                        dir = d_inode_rcu(parent);
                        if (!dir)
                                return -ECHILD;
                } else {
                        parent = dget_parent(d);
                        dir = d_inode(parent);
                }
                take_dentry_name_snapshot(&n, d);
                ret = d->d_op->d_revalidate(dir, &n.name, d, flags);
                release_dentry_name_snapshot(&n);
                if (!(flags & LOOKUP_RCU))
                        dput(parent);
                if (!ret) {
                        if (!(flags & LOOKUP_RCU))
                                d_invalidate(d);
                        ret = -ESTALE;
                }
        }
        return ret;
}

static int ovl_dentry_revalidate_common(struct dentry *dentry,
                                        unsigned int flags, bool weak)
{
        struct ovl_entry *oe;
        struct ovl_path *lowerstack;
        struct inode *inode = d_inode_rcu(dentry);
        struct dentry *upper;
        unsigned int i;
        int ret = 1;

        /* Careful in RCU mode */
        if (!inode)
                return -ECHILD;

        oe = OVL_I_E(inode);
        lowerstack = ovl_lowerstack(oe);
        upper = ovl_i_dentry_upper(inode);
        if (upper)
                ret = ovl_revalidate_real(upper, flags, weak);

        for (i = 0; ret > 0 && i < ovl_numlower(oe); i++)
                ret = ovl_revalidate_real(lowerstack[i].dentry, flags, weak);

        return ret;
}

static int ovl_dentry_revalidate(struct inode *dir, const struct qstr *name,
                                 struct dentry *dentry, unsigned int flags)
{
        return ovl_dentry_revalidate_common(dentry, flags, false);
}

static int ovl_dentry_weak_revalidate(struct dentry *dentry, unsigned int flags)
{
        return ovl_dentry_revalidate_common(dentry, flags, true);
}

static const struct dentry_operations ovl_dentry_operations = {
        .d_real = ovl_d_real,
        .d_revalidate = ovl_dentry_revalidate,
        .d_weak_revalidate = ovl_dentry_weak_revalidate,
};

static struct kmem_cache *ovl_inode_cachep;

static struct inode *ovl_alloc_inode(struct super_block *sb)
{
        struct ovl_inode *oi = alloc_inode_sb(sb, ovl_inode_cachep, GFP_KERNEL);

        if (!oi)
                return NULL;

        oi->cache = NULL;
        oi->redirect = NULL;
        oi->version = 0;
        oi->flags = 0;
        oi->__upperdentry = NULL;
        oi->lowerdata_redirect = NULL;
        oi->oe = NULL;
        mutex_init(&oi->lock);

        return &oi->vfs_inode;
}

static void ovl_free_inode(struct inode *inode)
{
        struct ovl_inode *oi = OVL_I(inode);

        kfree(oi->redirect);
        kfree(oi->oe);
        mutex_destroy(&oi->lock);
        kmem_cache_free(ovl_inode_cachep, oi);
}

static void ovl_destroy_inode(struct inode *inode)
{
        struct ovl_inode *oi = OVL_I(inode);

        dput(oi->__upperdentry);
        ovl_stack_put(ovl_lowerstack(oi->oe), ovl_numlower(oi->oe));
        if (S_ISDIR(inode->i_mode))
                ovl_dir_cache_free(inode);
        else
                kfree(oi->lowerdata_redirect);
}

static void ovl_put_super(struct super_block *sb)
{
        struct ovl_fs *ofs = OVL_FS(sb);

        if (ofs)
                ovl_free_fs(ofs);
}

/* Sync real dirty inodes in upper filesystem (if it exists) */
static int ovl_sync_fs(struct super_block *sb, int wait)
{
        struct ovl_fs *ofs = OVL_FS(sb);
        struct super_block *upper_sb;
        int ret;

        ret = ovl_sync_status(ofs);

        if (ret < 0)
                return -EIO;

        if (!ret)
                return ret;

        /*
         * Not called for sync(2) call or an emergency sync (SB_I_SKIP_SYNC).
         * All the super blocks will be iterated, including upper_sb.
         *
         * If this is a syncfs(2) call, then we do need to call
         * sync_filesystem() on upper_sb, but enough if we do it when being
         * called with wait == 1.
         */
        if (!wait)
                return 0;

        upper_sb = ovl_upper_mnt(ofs)->mnt_sb;

        down_read(&upper_sb->s_umount);
        ret = sync_filesystem(upper_sb);
        up_read(&upper_sb->s_umount);

        return ret;
}

/**
 * ovl_statfs
 * @dentry: The dentry to query
 * @buf: The struct kstatfs to fill in with stats
 *
 * Get the filesystem statistics.  As writes always target the upper layer
 * filesystem pass the statfs to the upper filesystem (if it exists)
 */
static int ovl_statfs(struct dentry *dentry, struct kstatfs *buf)
{
        struct super_block *sb = dentry->d_sb;
        struct ovl_fs *ofs = OVL_FS(sb);
        struct dentry *root_dentry = sb->s_root;
        struct path path;
        int err;

        ovl_path_real(root_dentry, &path);

        err = vfs_statfs(&path, buf);
        if (!err) {
                buf->f_namelen = ofs->namelen;
                buf->f_type = OVERLAYFS_SUPER_MAGIC;
                if (ovl_has_fsid(ofs))
                        buf->f_fsid = uuid_to_fsid(sb->s_uuid.b);
        }

        return err;
}

static const struct super_operations ovl_super_operations = {
        .alloc_inode    = ovl_alloc_inode,
        .free_inode     = ovl_free_inode,
        .destroy_inode  = ovl_destroy_inode,
        .drop_inode     = generic_delete_inode,
        .put_super      = ovl_put_super,
        .sync_fs        = ovl_sync_fs,
        .statfs         = ovl_statfs,
        .show_options   = ovl_show_options,
};

#define OVL_WORKDIR_NAME "work"
#define OVL_INDEXDIR_NAME "index"

static struct dentry *ovl_workdir_create(struct ovl_fs *ofs,
                                         const char *name, bool persist)
{
        struct inode *dir =  ofs->workbasedir->d_inode;
        struct vfsmount *mnt = ovl_upper_mnt(ofs);
        struct dentry *work;
        int err;
        bool retried = false;

        inode_lock_nested(dir, I_MUTEX_PARENT);
retry:
        work = ovl_lookup_upper(ofs, name, ofs->workbasedir, strlen(name));

        if (!IS_ERR(work)) {
                struct iattr attr = {
                        .ia_valid = ATTR_MODE,
                        .ia_mode = S_IFDIR | 0,
                };

                if (work->d_inode) {
                        err = -EEXIST;
                        if (retried)
                                goto out_dput;

                        if (persist)
                                goto out_unlock;

                        retried = true;
                        err = ovl_workdir_cleanup(ofs, dir, mnt, work, 0);
                        dput(work);
                        if (err == -EINVAL) {
                                work = ERR_PTR(err);
                                goto out_unlock;
                        }
                        goto retry;
                }

                err = ovl_mkdir_real(ofs, dir, &work, attr.ia_mode);
                if (err)
                        goto out_dput;

                /* Weird filesystem returning with hashed negative (kernfs)? */
                err = -EINVAL;
                if (d_really_is_negative(work))
                        goto out_dput;

                /*
                 * Try to remove POSIX ACL xattrs from workdir.  We are good if:
                 *
                 * a) success (there was a POSIX ACL xattr and was removed)
                 * b) -ENODATA (there was no POSIX ACL xattr)
                 * c) -EOPNOTSUPP (POSIX ACL xattrs are not supported)
                 *
                 * There are various other error values that could effectively
                 * mean that the xattr doesn't exist (e.g. -ERANGE is returned
                 * if the xattr name is too long), but the set of filesystems
                 * allowed as upper are limited to "normal" ones, where checking
                 * for the above two errors is sufficient.
                 */
                err = ovl_do_remove_acl(ofs, work, XATTR_NAME_POSIX_ACL_DEFAULT);
                if (err && err != -ENODATA && err != -EOPNOTSUPP)
                        goto out_dput;

                err = ovl_do_remove_acl(ofs, work, XATTR_NAME_POSIX_ACL_ACCESS);
                if (err && err != -ENODATA && err != -EOPNOTSUPP)
                        goto out_dput;

                /* Clear any inherited mode bits */
                inode_lock(work->d_inode);
                err = ovl_do_notify_change(ofs, work, &attr);
                inode_unlock(work->d_inode);
                if (err)
                        goto out_dput;
        } else {
                err = PTR_ERR(work);
                goto out_err;
        }
out_unlock:
        inode_unlock(dir);
        return work;

out_dput:
        dput(work);
out_err:
        pr_warn("failed to create directory %s/%s (errno: %i); mounting read-only\n",
                ofs->config.workdir, name, -err);
        work = NULL;
        goto out_unlock;
}

static int ovl_check_namelen(const struct path *path, struct ovl_fs *ofs,
                             const char *name)
{
        struct kstatfs statfs;
        int err = vfs_statfs(path, &statfs);

        if (err)
                pr_err("statfs failed on '%s'\n", name);
        else
                ofs->namelen = max(ofs->namelen, statfs.f_namelen);

        return err;
}

static int ovl_lower_dir(const char *name, struct path *path,
                         struct ovl_fs *ofs, int *stack_depth)
{
        int fh_type;
        int err;

        err = ovl_check_namelen(path, ofs, name);
        if (err)
                return err;

        *stack_depth = max(*stack_depth, path->mnt->mnt_sb->s_stack_depth);

        /*
         * The inodes index feature and NFS export need to encode and decode
         * file handles, so they require that all layers support them.
         */
        fh_type = ovl_can_decode_fh(path->dentry->d_sb);
        if ((ofs->config.nfs_export ||
             (ofs->config.index && ofs->config.upperdir)) && !fh_type) {
                ofs->config.index = false;
                ofs->config.nfs_export = false;
                pr_warn("fs on '%s' does not support file handles, falling back to index=off,nfs_export=off.\n",
                        name);
        }
        ofs->nofh |= !fh_type;
        /*
         * Decoding origin file handle is required for persistent st_ino.
         * Without persistent st_ino, xino=auto falls back to xino=off.
         */
        if (ofs->config.xino == OVL_XINO_AUTO &&
            ofs->config.upperdir && !fh_type) {
                ofs->config.xino = OVL_XINO_OFF;
                pr_warn("fs on '%s' does not support file handles, falling back to xino=off.\n",
                        name);
        }

        /* Check if lower fs has 32bit inode numbers */
        if (fh_type != FILEID_INO32_GEN)
                ofs->xino_mode = -1;

        return 0;
}

/* Workdir should not be subdir of upperdir and vice versa */
static bool ovl_workdir_ok(struct dentry *workdir, struct dentry *upperdir)
{
        bool ok = false;

        if (workdir != upperdir) {
                struct dentry *trap = lock_rename(workdir, upperdir);
                if (!IS_ERR(trap))
                        unlock_rename(workdir, upperdir);
                ok = (trap == NULL);
        }
        return ok;
}

static int ovl_setup_trap(struct super_block *sb, struct dentry *dir,
                          struct inode **ptrap, const char *name)
{
        struct inode *trap;
        int err;

        trap = ovl_get_trap_inode(sb, dir);
        err = PTR_ERR_OR_ZERO(trap);
        if (err) {
                if (err == -ELOOP)
                        pr_err("conflicting %s path\n", name);
                return err;
        }

        *ptrap = trap;
        return 0;
}

/*
 * Determine how we treat concurrent use of upperdir/workdir based on the
 * index feature. This is papering over mount leaks of container runtimes,
 * for example, an old overlay mount is leaked and now its upperdir is
 * attempted to be used as a lower layer in a new overlay mount.
 */
static int ovl_report_in_use(struct ovl_fs *ofs, const char *name)
{
        if (ofs->config.index) {
                pr_err("%s is in-use as upperdir/workdir of another mount, mount with '-o index=off' to override exclusive upperdir protection.\n",
                       name);
                return -EBUSY;
        } else {
                pr_warn("%s is in-use as upperdir/workdir of another mount, accessing files from both mounts will result in undefined behavior.\n",
                        name);
                return 0;
        }
}

static int ovl_get_upper(struct super_block *sb, struct ovl_fs *ofs,
                         struct ovl_layer *upper_layer,
                         const struct path *upperpath)
{
        struct vfsmount *upper_mnt;
        int err;

        /* Upperdir path should not be r/o */
        if (__mnt_is_readonly(upperpath->mnt)) {
                pr_err("upper fs is r/o, try multi-lower layers mount\n");
                err = -EINVAL;
                goto out;
        }

        err = ovl_check_namelen(upperpath, ofs, ofs->config.upperdir);
        if (err)
                goto out;

        err = ovl_setup_trap(sb, upperpath->dentry, &upper_layer->trap,
                             "upperdir");
        if (err)
                goto out;

        upper_mnt = clone_private_mount(upperpath);
        err = PTR_ERR(upper_mnt);
        if (IS_ERR(upper_mnt)) {
                pr_err("failed to clone upperpath\n");
                goto out;
        }

        /* Don't inherit atime flags */
        upper_mnt->mnt_flags &= ~(MNT_NOATIME | MNT_NODIRATIME | MNT_RELATIME);
        upper_layer->mnt = upper_mnt;
        upper_layer->idx = 0;
        upper_layer->fsid = 0;

        /*
         * Inherit SB_NOSEC flag from upperdir.
         *
         * This optimization changes behavior when a security related attribute
         * (suid/sgid/security.*) is changed on an underlying layer.  This is
         * okay because we don't yet have guarantees in that case, but it will
         * need careful treatment once we want to honour changes to underlying
         * filesystems.
         */
        if (upper_mnt->mnt_sb->s_flags & SB_NOSEC)
                sb->s_flags |= SB_NOSEC;

        if (ovl_inuse_trylock(ovl_upper_mnt(ofs)->mnt_root)) {
                ofs->upperdir_locked = true;
        } else {
                err = ovl_report_in_use(ofs, "upperdir");
                if (err)
                        goto out;
        }

        err = 0;
out:
        return err;
}

/*
 * Returns 1 if RENAME_WHITEOUT is supported, 0 if not supported and
 * negative values if error is encountered.
 */
static int ovl_check_rename_whiteout(struct ovl_fs *ofs)
{
        struct dentry *workdir = ofs->workdir;
        struct inode *dir = d_inode(workdir);
        struct dentry *temp;
        struct dentry *dest;
        struct dentry *whiteout;
        struct name_snapshot name;
        int err;

        inode_lock_nested(dir, I_MUTEX_PARENT);

        temp = ovl_create_temp(ofs, workdir, OVL_CATTR(S_IFREG | 0));
        err = PTR_ERR(temp);
        if (IS_ERR(temp))
                goto out_unlock;

        dest = ovl_lookup_temp(ofs, workdir);
        err = PTR_ERR(dest);
        if (IS_ERR(dest)) {
                dput(temp);
                goto out_unlock;
        }

        /* Name is inline and stable - using snapshot as a copy helper */
        take_dentry_name_snapshot(&name, temp);
        err = ovl_do_rename(ofs, dir, temp, dir, dest, RENAME_WHITEOUT);
        if (err) {
                if (err == -EINVAL)
                        err = 0;
                goto cleanup_temp;
        }

        whiteout = ovl_lookup_upper(ofs, name.name.name, workdir, name.name.len);
        err = PTR_ERR(whiteout);
        if (IS_ERR(whiteout))
                goto cleanup_temp;

        err = ovl_upper_is_whiteout(ofs, whiteout);

        /* Best effort cleanup of whiteout and temp file */
        if (err)
                ovl_cleanup(ofs, dir, whiteout);
        dput(whiteout);

cleanup_temp:
        ovl_cleanup(ofs, dir, temp);
        release_dentry_name_snapshot(&name);
        dput(temp);
        dput(dest);

out_unlock:
        inode_unlock(dir);

        return err;
}

static struct dentry *ovl_lookup_or_create(struct ovl_fs *ofs,
                                           struct dentry *parent,
                                           const char *name, umode_t mode)
{
        size_t len = strlen(name);
        struct dentry *child;

        inode_lock_nested(parent->d_inode, I_MUTEX_PARENT);
        child = ovl_lookup_upper(ofs, name, parent, len);
        if (!IS_ERR(child) && !child->d_inode)
                child = ovl_create_real(ofs, parent->d_inode, child,
                                        OVL_CATTR(mode));
        inode_unlock(parent->d_inode);
        dput(parent);

        return child;
}

/*
 * Creates $workdir/work/incompat/volatile/dirty file if it is not already
 * present.
 */
static int ovl_create_volatile_dirty(struct ovl_fs *ofs)
{
        unsigned int ctr;
        struct dentry *d = dget(ofs->workbasedir);
        static const char *const volatile_path[] = {
                OVL_WORKDIR_NAME, "incompat", "volatile", "dirty"
        };
        const char *const *name = volatile_path;

        for (ctr = ARRAY_SIZE(volatile_path); ctr; ctr--, name++) {
                d = ovl_lookup_or_create(ofs, d, *name, ctr > 1 ? S_IFDIR : S_IFREG);
                if (IS_ERR(d))
                        return PTR_ERR(d);
        }
        dput(d);
        return 0;
}

static int ovl_make_workdir(struct super_block *sb, struct ovl_fs *ofs,
                            const struct path *workpath)
{
        struct vfsmount *mnt = ovl_upper_mnt(ofs);
        struct dentry *workdir;
        struct file *tmpfile;
        bool rename_whiteout;
        bool d_type;
        int fh_type;
        int err;

        err = mnt_want_write(mnt);
        if (err)
                return err;

        workdir = ovl_workdir_create(ofs, OVL_WORKDIR_NAME, false);
        err = PTR_ERR(workdir);
        if (IS_ERR_OR_NULL(workdir))
                goto out;

        ofs->workdir = workdir;

        err = ovl_setup_trap(sb, ofs->workdir, &ofs->workdir_trap, "workdir");
        if (err)
                goto out;

        /*
         * Upper should support d_type, else whiteouts are visible.  Given
         * workdir and upper are on same fs, we can do iterate_dir() on
         * workdir. This check requires successful creation of workdir in
         * previous step.
         */
        err = ovl_check_d_type_supported(workpath);
        if (err < 0)
                goto out;

        d_type = err;
        if (!d_type)
                pr_warn("upper fs needs to support d_type.\n");

        /* Check if upper/work fs supports O_TMPFILE */
        tmpfile = ovl_do_tmpfile(ofs, ofs->workdir, S_IFREG | 0);
        ofs->tmpfile = !IS_ERR(tmpfile);
        if (ofs->tmpfile)
                fput(tmpfile);
        else
                pr_warn("upper fs does not support tmpfile.\n");


        /* Check if upper/work fs supports RENAME_WHITEOUT */
        err = ovl_check_rename_whiteout(ofs);
        if (err < 0)
                goto out;

        rename_whiteout = err;
        if (!rename_whiteout)
                pr_warn("upper fs does not support RENAME_WHITEOUT.\n");

        /*
         * Check if upper/work fs supports (trusted|user).overlay.* xattr
         */
        err = ovl_setxattr(ofs, ofs->workdir, OVL_XATTR_OPAQUE, "0", 1);
        if (err) {
                pr_warn("failed to set xattr on upper\n");
                ofs->noxattr = true;
                if (ovl_redirect_follow(ofs)) {
                        ofs->config.redirect_mode = OVL_REDIRECT_NOFOLLOW;
                        pr_warn("...falling back to redirect_dir=nofollow.\n");
                }
                if (ofs->config.metacopy) {
                        ofs->config.metacopy = false;
                        pr_warn("...falling back to metacopy=off.\n");
                }
                if (ofs->config.index) {
                        ofs->config.index = false;
                        pr_warn("...falling back to index=off.\n");
                }
                if (ovl_has_fsid(ofs)) {
                        ofs->config.uuid = OVL_UUID_NULL;
                        pr_warn("...falling back to uuid=null.\n");
                }
                /*
                 * xattr support is required for persistent st_ino.
                 * Without persistent st_ino, xino=auto falls back to xino=off.
                 */
                if (ofs->config.xino == OVL_XINO_AUTO) {
                        ofs->config.xino = OVL_XINO_OFF;
                        pr_warn("...falling back to xino=off.\n");
                }
                if (err == -EPERM && !ofs->config.userxattr)
                        pr_info("try mounting with 'userxattr' option\n");
                err = 0;
        } else {
                ovl_removexattr(ofs, ofs->workdir, OVL_XATTR_OPAQUE);
        }

        /*
         * We allowed sub-optimal upper fs configuration and don't want to break
         * users over kernel upgrade, but we never allowed remote upper fs, so
         * we can enforce strict requirements for remote upper fs.
         */
        if (ovl_dentry_remote(ofs->workdir) &&
            (!d_type || !rename_whiteout || ofs->noxattr)) {
                pr_err("upper fs missing required features.\n");
                err = -EINVAL;
                goto out;
        }

        /*
         * For volatile mount, create a incompat/volatile/dirty file to keep
         * track of it.
         */
        if (ofs->config.ovl_volatile) {
                err = ovl_create_volatile_dirty(ofs);
                if (err < 0) {
                        pr_err("Failed to create volatile/dirty file.\n");
                        goto out;
                }
        }

        /* Check if upper/work fs supports file handles */
        fh_type = ovl_can_decode_fh(ofs->workdir->d_sb);
        if (ofs->config.index && !fh_type) {
                ofs->config.index = false;
                pr_warn("upper fs does not support file handles, falling back to index=off.\n");
        }
        ofs->nofh |= !fh_type;

        /* Check if upper fs has 32bit inode numbers */
        if (fh_type != FILEID_INO32_GEN)
                ofs->xino_mode = -1;

        /* NFS export of r/w mount depends on index */
        if (ofs->config.nfs_export && !ofs->config.index) {
                pr_warn("NFS export requires \"index=on\", falling back to nfs_export=off.\n");
                ofs->config.nfs_export = false;
        }
out:
        mnt_drop_write(mnt);
        return err;
}

static int ovl_get_workdir(struct super_block *sb, struct ovl_fs *ofs,
                           const struct path *upperpath,
                           const struct path *workpath)
{
        int err;

        err = -EINVAL;
        if (upperpath->mnt != workpath->mnt) {
                pr_err("workdir and upperdir must reside under the same mount\n");
                return err;
        }
        if (!ovl_workdir_ok(workpath->dentry, upperpath->dentry)) {
                pr_err("workdir and upperdir must be separate subtrees\n");
                return err;
        }

        ofs->workbasedir = dget(workpath->dentry);

        if (ovl_inuse_trylock(ofs->workbasedir)) {
                ofs->workdir_locked = true;
        } else {
                err = ovl_report_in_use(ofs, "workdir");
                if (err)
                        return err;
        }

        err = ovl_setup_trap(sb, ofs->workbasedir, &ofs->workbasedir_trap,
                             "workdir");
        if (err)
                return err;

        return ovl_make_workdir(sb, ofs, workpath);
}

static int ovl_get_indexdir(struct super_block *sb, struct ovl_fs *ofs,
                            struct ovl_entry *oe, const struct path *upperpath)
{
        struct vfsmount *mnt = ovl_upper_mnt(ofs);
        struct dentry *indexdir;
        struct dentry *origin = ovl_lowerstack(oe)->dentry;
        const struct ovl_fh *fh;
        int err;

        fh = ovl_get_origin_fh(ofs, origin);
        if (IS_ERR(fh))
                return PTR_ERR(fh);

        err = mnt_want_write(mnt);
        if (err)
                goto out_free_fh;

        /* Verify lower root is upper root origin */
        err = ovl_verify_origin_fh(ofs, upperpath->dentry, fh, true);
        if (err) {
                pr_err("failed to verify upper root origin\n");
                goto out;
        }

        /* index dir will act also as workdir */
        iput(ofs->workdir_trap);
        ofs->workdir_trap = NULL;
        dput(ofs->workdir);
        ofs->workdir = NULL;
        indexdir = ovl_workdir_create(ofs, OVL_INDEXDIR_NAME, true);
        if (IS_ERR(indexdir)) {
                err = PTR_ERR(indexdir);
        } else if (indexdir) {
                ofs->workdir = indexdir;
                err = ovl_setup_trap(sb, indexdir, &ofs->workdir_trap,
                                     "indexdir");
                if (err)
                        goto out;

                /*
                 * Verify upper root is exclusively associated with index dir.
                 * Older kernels stored upper fh in ".overlay.origin"
                 * xattr. If that xattr exists, verify that it is a match to
                 * upper dir file handle. In any case, verify or set xattr
                 * ".overlay.upper" to indicate that index may have
                 * directory entries.
                 */
                if (ovl_check_origin_xattr(ofs, indexdir)) {
                        err = ovl_verify_origin_xattr(ofs, indexdir,
                                                      OVL_XATTR_ORIGIN,
                                                      upperpath->dentry, true,
                                                      false);
                        if (err)
                                pr_err("failed to verify index dir 'origin' xattr\n");
                }
                err = ovl_verify_upper(ofs, indexdir, upperpath->dentry, true);
                if (err)
                        pr_err("failed to verify index dir 'upper' xattr\n");

                /* Cleanup bad/stale/orphan index entries */
                if (!err)
                        err = ovl_indexdir_cleanup(ofs);
        }
        if (err || !indexdir)
                pr_warn("try deleting index dir or mounting with '-o index=off' to disable inodes index.\n");

out:
        mnt_drop_write(mnt);
out_free_fh:
        kfree(fh);
        return err;
}

static bool ovl_lower_uuid_ok(struct ovl_fs *ofs, const uuid_t *uuid)
{
        unsigned int i;

        if (!ofs->config.nfs_export && !ovl_upper_mnt(ofs))
                return true;

        /*
         * We allow using single lower with null uuid for index and nfs_export
         * for example to support those features with single lower squashfs.
         * To avoid regressions in setups of overlay with re-formatted lower
         * squashfs, do not allow decoding origin with lower null uuid unless
         * user opted-in to one of the new features that require following the
         * lower inode of non-dir upper.
         */
        if (ovl_allow_offline_changes(ofs) && uuid_is_null(uuid))
                return false;

        for (i = 0; i < ofs->numfs; i++) {
                /*
                 * We use uuid to associate an overlay lower file handle with a
                 * lower layer, so we can accept lower fs with null uuid as long
                 * as all lower layers with null uuid are on the same fs.
                 * if we detect multiple lower fs with the same uuid, we
                 * disable lower file handle decoding on all of them.
                 */
                if (ofs->fs[i].is_lower &&
                    uuid_equal(&ofs->fs[i].sb->s_uuid, uuid)) {
                        ofs->fs[i].bad_uuid = true;
                        return false;
                }
        }
        return true;
}

/* Get a unique fsid for the layer */
static int ovl_get_fsid(struct ovl_fs *ofs, const struct path *path)
{
        struct super_block *sb = path->mnt->mnt_sb;
        unsigned int i;
        dev_t dev;
        int err;
        bool bad_uuid = false;
        bool warn = false;

        for (i = 0; i < ofs->numfs; i++) {
                if (ofs->fs[i].sb == sb)
                        return i;
        }

        if (!ovl_lower_uuid_ok(ofs, &sb->s_uuid)) {
                bad_uuid = true;
                if (ofs->config.xino == OVL_XINO_AUTO) {
                        ofs->config.xino = OVL_XINO_OFF;
                        warn = true;
                }
                if (ofs->config.index || ofs->config.nfs_export) {
                        ofs->config.index = false;
                        ofs->config.nfs_export = false;
                        warn = true;
                }
                if (warn) {
                        pr_warn("%s uuid detected in lower fs '%pd2', falling back to xino=%s,index=off,nfs_export=off.\n",
                                uuid_is_null(&sb->s_uuid) ? "null" :
                                                            "conflicting",
                                path->dentry, ovl_xino_mode(&ofs->config));
                }
        }

        err = get_anon_bdev(&dev);
        if (err) {
                pr_err("failed to get anonymous bdev for lowerpath\n");
                return err;
        }

        ofs->fs[ofs->numfs].sb = sb;
        ofs->fs[ofs->numfs].pseudo_dev = dev;
        ofs->fs[ofs->numfs].bad_uuid = bad_uuid;

        return ofs->numfs++;
}

/*
 * The fsid after the last lower fsid is used for the data layers.
 * It is a "null fs" with a null sb, null uuid, and no pseudo dev.
 */
static int ovl_get_data_fsid(struct ovl_fs *ofs)
{
        return ofs->numfs;
}


static int ovl_get_layers(struct super_block *sb, struct ovl_fs *ofs,
                          struct ovl_fs_context *ctx, struct ovl_layer *layers)
{
        int err;
        unsigned int i;
        size_t nr_merged_lower;

        ofs->fs = kcalloc(ctx->nr + 2, sizeof(struct ovl_sb), GFP_KERNEL);
        if (ofs->fs == NULL)
                return -ENOMEM;

        /*
         * idx/fsid 0 are reserved for upper fs even with lower only overlay
         * and the last fsid is reserved for "null fs" of the data layers.
         */
        ofs->numfs++;

        /*
         * All lower layers that share the same fs as upper layer, use the same
         * pseudo_dev as upper layer.  Allocate fs[0].pseudo_dev even for lower
         * only overlay to simplify ovl_fs_free().
         * is_lower will be set if upper fs is shared with a lower layer.
         */
        err = get_anon_bdev(&ofs->fs[0].pseudo_dev);
        if (err) {
                pr_err("failed to get anonymous bdev for upper fs\n");
                return err;
        }

        if (ovl_upper_mnt(ofs)) {
                ofs->fs[0].sb = ovl_upper_mnt(ofs)->mnt_sb;
                ofs->fs[0].is_lower = false;
        }

        nr_merged_lower = ctx->nr - ctx->nr_data;
        for (i = 0; i < ctx->nr; i++) {
                struct ovl_fs_context_layer *l = &ctx->lower[i];
                struct vfsmount *mnt;
                struct inode *trap;
                int fsid;

                if (i < nr_merged_lower)
                        fsid = ovl_get_fsid(ofs, &l->path);
                else
                        fsid = ovl_get_data_fsid(ofs);
                if (fsid < 0)
                        return fsid;

                /*
                 * Check if lower root conflicts with this overlay layers before
                 * checking if it is in-use as upperdir/workdir of "another"
                 * mount, because we do not bother to check in ovl_is_inuse() if
                 * the upperdir/workdir is in fact in-use by our
                 * upperdir/workdir.
                 */
                err = ovl_setup_trap(sb, l->path.dentry, &trap, "lowerdir");
                if (err)
                        return err;

                if (ovl_is_inuse(l->path.dentry)) {
                        err = ovl_report_in_use(ofs, "lowerdir");
                        if (err) {
                                iput(trap);
                                return err;
                        }
                }

                mnt = clone_private_mount(&l->path);
                err = PTR_ERR(mnt);
                if (IS_ERR(mnt)) {
                        pr_err("failed to clone lowerpath\n");
                        iput(trap);
                        return err;
                }

                /*
                 * Make lower layers R/O.  That way fchmod/fchown on lower file
                 * will fail instead of modifying lower fs.
                 */
                mnt->mnt_flags |= MNT_READONLY | MNT_NOATIME;

                layers[ofs->numlayer].trap = trap;
                layers[ofs->numlayer].mnt = mnt;
                layers[ofs->numlayer].idx = ofs->numlayer;
                layers[ofs->numlayer].fsid = fsid;
                layers[ofs->numlayer].fs = &ofs->fs[fsid];
                /* Store for printing lowerdir=... in ovl_show_options() */
                ofs->config.lowerdirs[ofs->numlayer] = l->name;
                l->name = NULL;
                ofs->numlayer++;
                ofs->fs[fsid].is_lower = true;
        }

        /*
         * When all layers on same fs, overlay can use real inode numbers.
         * With mount option "xino=<on|auto>", mounter declares that there are
         * enough free high bits in underlying fs to hold the unique fsid.
         * If overlayfs does encounter underlying inodes using the high xino
         * bits reserved for fsid, it emits a warning and uses the original
         * inode number or a non persistent inode number allocated from a
         * dedicated range.
         */
        if (ofs->numfs - !ovl_upper_mnt(ofs) == 1) {
                if (ofs->config.xino == OVL_XINO_ON)
                        pr_info("\"xino=on\" is useless with all layers on same fs, ignore.\n");
                ofs->xino_mode = 0;
        } else if (ofs->config.xino == OVL_XINO_OFF) {
                ofs->xino_mode = -1;
        } else if (ofs->xino_mode < 0) {
                /*
                 * This is a roundup of number of bits needed for encoding
                 * fsid, where fsid 0 is reserved for upper fs (even with
                 * lower only overlay) +1 extra bit is reserved for the non
                 * persistent inode number range that is used for resolving
                 * xino lower bits overflow.
                 */
                BUILD_BUG_ON(ilog2(OVL_MAX_STACK) > 30);
                ofs->xino_mode = ilog2(ofs->numfs - 1) + 2;
        }

        if (ofs->xino_mode > 0) {
                pr_info("\"xino\" feature enabled using %d upper inode bits.\n",
                        ofs->xino_mode);
        }

        return 0;
}

static struct ovl_entry *ovl_get_lowerstack(struct super_block *sb,
                                            struct ovl_fs_context *ctx,
                                            struct ovl_fs *ofs,
                                            struct ovl_layer *layers)
{
        int err;
        unsigned int i;
        size_t nr_merged_lower;
        struct ovl_entry *oe;
        struct ovl_path *lowerstack;

        struct ovl_fs_context_layer *l;

        if (!ofs->config.upperdir && ctx->nr == 1) {
                pr_err("at least 2 lowerdir are needed while upperdir nonexistent\n");
                return ERR_PTR(-EINVAL);
        }

        err = -EINVAL;
        for (i = 0; i < ctx->nr; i++) {
                l = &ctx->lower[i];

                err = ovl_lower_dir(l->name, &l->path, ofs, &sb->s_stack_depth);
                if (err)
                        return ERR_PTR(err);
        }

        err = -EINVAL;
        sb->s_stack_depth++;
        if (sb->s_stack_depth > FILESYSTEM_MAX_STACK_DEPTH) {
                pr_err("maximum fs stacking depth exceeded\n");
                return ERR_PTR(err);
        }

        err = ovl_get_layers(sb, ofs, ctx, layers);
        if (err)
                return ERR_PTR(err);

        err = -ENOMEM;
        /* Data-only layers are not merged in root directory */
        nr_merged_lower = ctx->nr - ctx->nr_data;
        oe = ovl_alloc_entry(nr_merged_lower);
        if (!oe)
                return ERR_PTR(err);

        lowerstack = ovl_lowerstack(oe);
        for (i = 0; i < nr_merged_lower; i++) {
                l = &ctx->lower[i];
                lowerstack[i].dentry = dget(l->path.dentry);
                lowerstack[i].layer = &ofs->layers[i + 1];
        }
        ofs->numdatalayer = ctx->nr_data;

        return oe;
}

/*
 * Check if this layer root is a descendant of:
 * - another layer of this overlayfs instance
 * - upper/work dir of any overlayfs instance
 */
static int ovl_check_layer(struct super_block *sb, struct ovl_fs *ofs,
                           struct dentry *dentry, const char *name,
                           bool is_lower)
{
        struct dentry *next = dentry, *parent;
        int err = 0;

        if (!dentry)
                return 0;

        parent = dget_parent(next);

        /* Walk back ancestors to root (inclusive) looking for traps */
        while (!err && parent != next) {
                if (is_lower && ovl_lookup_trap_inode(sb, parent)) {
                        err = -ELOOP;
                        pr_err("overlapping %s path\n", name);
                } else if (ovl_is_inuse(parent)) {
                        err = ovl_report_in_use(ofs, name);
                }
                next = parent;
                parent = dget_parent(next);
                dput(next);
        }

        dput(parent);

        return err;
}

/*
 * Check if any of the layers or work dirs overlap.
 */
static int ovl_check_overlapping_layers(struct super_block *sb,
                                        struct ovl_fs *ofs)
{
        int i, err;

        if (ovl_upper_mnt(ofs)) {
                err = ovl_check_layer(sb, ofs, ovl_upper_mnt(ofs)->mnt_root,
                                      "upperdir", false);
                if (err)
                        return err;

                /*
                 * Checking workbasedir avoids hitting ovl_is_inuse(parent) of
                 * this instance and covers overlapping work and index dirs,
                 * unless work or index dir have been moved since created inside
                 * workbasedir.  In that case, we already have their traps in
                 * inode cache and we will catch that case on lookup.
                 */
                err = ovl_check_layer(sb, ofs, ofs->workbasedir, "workdir",
                                      false);
                if (err)
                        return err;
        }

        for (i = 1; i < ofs->numlayer; i++) {
                err = ovl_check_layer(sb, ofs,
                                      ofs->layers[i].mnt->mnt_root,
                                      "lowerdir", true);
                if (err)
                        return err;
        }

        return 0;
}

static struct dentry *ovl_get_root(struct super_block *sb,
                                   struct dentry *upperdentry,
                                   struct ovl_entry *oe)
{
        struct dentry *root;
        struct ovl_fs *ofs = OVL_FS(sb);
        struct ovl_path *lowerpath = ovl_lowerstack(oe);
        unsigned long ino = d_inode(lowerpath->dentry)->i_ino;
        int fsid = lowerpath->layer->fsid;
        struct ovl_inode_params oip = {
                .upperdentry = upperdentry,
                .oe = oe,
        };

        root = d_make_root(ovl_new_inode(sb, S_IFDIR, 0));
        if (!root)
                return NULL;

        if (upperdentry) {
                /* Root inode uses upper st_ino/i_ino */
                ino = d_inode(upperdentry)->i_ino;
                fsid = 0;
                ovl_dentry_set_upper_alias(root);
                if (ovl_is_impuredir(sb, upperdentry))
                        ovl_set_flag(OVL_IMPURE, d_inode(root));
        }

        /* Look for xwhiteouts marker except in the lowermost layer */
        for (int i = 0; i < ovl_numlower(oe) - 1; i++, lowerpath++) {
                struct path path = {
                        .mnt = lowerpath->layer->mnt,
                        .dentry = lowerpath->dentry,
                };

                /* overlay.opaque=x means xwhiteouts directory */
                if (ovl_get_opaquedir_val(ofs, &path) == 'x') {
                        ovl_layer_set_xwhiteouts(ofs, lowerpath->layer);
                        ovl_dentry_set_xwhiteouts(root);
                }
        }

        /* Root is always merge -> can have whiteouts */
        ovl_set_flag(OVL_WHITEOUTS, d_inode(root));
        ovl_dentry_set_flag(OVL_E_CONNECTED, root);
        ovl_set_upperdata(d_inode(root));
        ovl_inode_init(d_inode(root), &oip, ino, fsid);
        ovl_dentry_init_flags(root, upperdentry, oe, DCACHE_OP_WEAK_REVALIDATE);
        /* root keeps a reference of upperdentry */
        dget(upperdentry);

        return root;
}

int ovl_fill_super(struct super_block *sb, struct fs_context *fc)
{
        struct ovl_fs *ofs = sb->s_fs_info;
        struct ovl_fs_context *ctx = fc->fs_private;
        struct dentry *root_dentry;
        struct ovl_entry *oe;
        struct ovl_layer *layers;
        struct cred *cred;
        int err;

        err = -EIO;
        if (WARN_ON(fc->user_ns != current_user_ns()))
                goto out_err;

        sb->s_d_op = &ovl_dentry_operations;

        err = -ENOMEM;
        ofs->creator_cred = cred = prepare_creds();
        if (!cred)
                goto out_err;

        err = ovl_fs_params_verify(ctx, &ofs->config);
        if (err)
                goto out_err;

        err = -EINVAL;
        if (ctx->nr == 0) {
                if (!(fc->sb_flags & SB_SILENT))
                        pr_err("missing 'lowerdir'\n");
                goto out_err;
        }

        err = -ENOMEM;
        layers = kcalloc(ctx->nr + 1, sizeof(struct ovl_layer), GFP_KERNEL);
        if (!layers)
                goto out_err;

        ofs->config.lowerdirs = kcalloc(ctx->nr + 1, sizeof(char *), GFP_KERNEL);
        if (!ofs->config.lowerdirs) {
                kfree(layers);
                goto out_err;
        }
        ofs->layers = layers;
        /*
         * Layer 0 is reserved for upper even if there's no upper.
         * config.lowerdirs[0] is used for storing the user provided colon
         * separated lowerdir string.
         */
        ofs->config.lowerdirs[0] = ctx->lowerdir_all;
        ctx->lowerdir_all = NULL;
        ofs->numlayer = 1;

        sb->s_stack_depth = 0;
        sb->s_maxbytes = MAX_LFS_FILESIZE;
        atomic_long_set(&ofs->last_ino, 1);
        /* Assume underlying fs uses 32bit inodes unless proven otherwise */
        if (ofs->config.xino != OVL_XINO_OFF) {
                ofs->xino_mode = BITS_PER_LONG - 32;
                if (!ofs->xino_mode) {
                        pr_warn("xino not supported on 32bit kernel, falling back to xino=off.\n");
                        ofs->config.xino = OVL_XINO_OFF;
                }
        }

        /* alloc/destroy_inode needed for setting up traps in inode cache */
        sb->s_op = &ovl_super_operations;

        if (ofs->config.upperdir) {
                struct super_block *upper_sb;

                err = -EINVAL;
                if (!ofs->config.workdir) {
                        pr_err("missing 'workdir'\n");
                        goto out_err;
                }

                err = ovl_get_upper(sb, ofs, &layers[0], &ctx->upper);
                if (err)
                        goto out_err;

                upper_sb = ovl_upper_mnt(ofs)->mnt_sb;
                if (!ovl_should_sync(ofs)) {
                        ofs->errseq = errseq_sample(&upper_sb->s_wb_err);
                        if (errseq_check(&upper_sb->s_wb_err, ofs->errseq)) {
                                err = -EIO;
                                pr_err("Cannot mount volatile when upperdir has an unseen error. Sync upperdir fs to clear state.\n");
                                goto out_err;
                        }
                }

                err = ovl_get_workdir(sb, ofs, &ctx->upper, &ctx->work);
                if (err)
                        goto out_err;

                if (!ofs->workdir)
                        sb->s_flags |= SB_RDONLY;

                sb->s_stack_depth = upper_sb->s_stack_depth;
                sb->s_time_gran = upper_sb->s_time_gran;
        }
        oe = ovl_get_lowerstack(sb, ctx, ofs, layers);
        err = PTR_ERR(oe);
        if (IS_ERR(oe))
                goto out_err;

        /* If the upper fs is nonexistent, we mark overlayfs r/o too */
        if (!ovl_upper_mnt(ofs))
                sb->s_flags |= SB_RDONLY;

        if (!ovl_origin_uuid(ofs) && ofs->numfs > 1) {
                pr_warn("The uuid=off requires a single fs for lower and upper, falling back to uuid=null.\n");
                ofs->config.uuid = OVL_UUID_NULL;
        } else if (ovl_has_fsid(ofs) && ovl_upper_mnt(ofs)) {
                /* Use per instance persistent uuid/fsid */
                ovl_init_uuid_xattr(sb, ofs, &ctx->upper);
        }

        if (!ovl_force_readonly(ofs) && ofs->config.index) {
                err = ovl_get_indexdir(sb, ofs, oe, &ctx->upper);
                if (err)
                        goto out_free_oe;

                /* Force r/o mount with no index dir */
                if (!ofs->workdir)
                        sb->s_flags |= SB_RDONLY;
        }

        err = ovl_check_overlapping_layers(sb, ofs);
        if (err)
                goto out_free_oe;

        /* Show index=off in /proc/mounts for forced r/o mount */
        if (!ofs->workdir) {
                ofs->config.index = false;
                if (ovl_upper_mnt(ofs) && ofs->config.nfs_export) {
                        pr_warn("NFS export requires an index dir, falling back to nfs_export=off.\n");
                        ofs->config.nfs_export = false;
                }
        }

        if (ofs->config.metacopy && ofs->config.nfs_export) {
                pr_warn("NFS export is not supported with metadata only copy up, falling back to nfs_export=off.\n");
                ofs->config.nfs_export = false;
        }

        /*
         * Support encoding decodable file handles with nfs_export=on
         * and encoding non-decodable file handles with nfs_export=off
         * if all layers support file handles.
         */
        if (ofs->config.nfs_export)
                sb->s_export_op = &ovl_export_operations;
        else if (!ofs->nofh)
                sb->s_export_op = &ovl_export_fid_operations;

        /* Never override disk quota limits or use reserved space */
        cap_lower(cred->cap_effective, CAP_SYS_RESOURCE);

        sb->s_magic = OVERLAYFS_SUPER_MAGIC;
        sb->s_xattr = ovl_xattr_handlers(ofs);
        sb->s_fs_info = ofs;
#ifdef CONFIG_FS_POSIX_ACL
        sb->s_flags |= SB_POSIXACL;
#endif
        sb->s_iflags |= SB_I_SKIP_SYNC;
        /*
         * Ensure that umask handling is done by the filesystems used
         * for the the upper layer instead of overlayfs as that would
         * lead to unexpected results.
         */
        sb->s_iflags |= SB_I_NOUMASK;
        sb->s_iflags |= SB_I_EVM_HMAC_UNSUPPORTED;

        err = -ENOMEM;
        root_dentry = ovl_get_root(sb, ctx->upper.dentry, oe);
        if (!root_dentry)
                goto out_free_oe;

        sb->s_root = root_dentry;

        return 0;

out_free_oe:
        ovl_free_entry(oe);
out_err:
        ovl_free_fs(ofs);
        sb->s_fs_info = NULL;
        return err;
}

struct file_system_type ovl_fs_type = {
        .owner                  = THIS_MODULE,
        .name                   = "overlay",
        .init_fs_context        = ovl_init_fs_context,
        .parameters             = ovl_parameter_spec,
        .fs_flags               = FS_USERNS_MOUNT,
        .kill_sb                = kill_anon_super,
};
MODULE_ALIAS_FS("overlay");

static void ovl_inode_init_once(void *foo)
{
        struct ovl_inode *oi = foo;

        inode_init_once(&oi->vfs_inode);
}

static int __init ovl_init(void)
{
        int err;

        ovl_inode_cachep = kmem_cache_create("ovl_inode",
                                             sizeof(struct ovl_inode), 0,
                                             (SLAB_RECLAIM_ACCOUNT|
                                              SLAB_ACCOUNT),
                                             ovl_inode_init_once);
        if (ovl_inode_cachep == NULL)
                return -ENOMEM;

        err = register_filesystem(&ovl_fs_type);
        if (!err)
                return 0;

        kmem_cache_destroy(ovl_inode_cachep);

        return err;
}

static void __exit ovl_exit(void)
{
        unregister_filesystem(&ovl_fs_type);

        /*
         * Make sure all delayed rcu free inodes are flushed before we
         * destroy cache.
         */
        rcu_barrier();
        kmem_cache_destroy(ovl_inode_cachep);
}

module_init(ovl_init);
module_exit(ovl_exit);