linux-imx

/*

 * Overlayfs NFS export support.

 *

 * Amir Goldstein <amir73il@gmail.com>

 *

 * Copyright (C) 2017-2018 CTERA Networks. All Rights Reserved.

 *

 * This program is free software; you can redistribute it and/or modify it

 * under the terms of the GNU General Public License version 2 as published by

 * the Free Software Foundation.

 */

​

#include <linux/fs.h>

#include <linux/cred.h>

#include <linux/mount.h>

#include <linux/namei.h>

#include <linux/xattr.h>

#include <linux/exportfs.h>

#include <linux/ratelimit.h>

#include "overlayfs.h"

​

static int ovl_encode_maybe_copy_up(struct dentry *dentry)

{

    int err;

​

    if (ovl_dentry_upper(dentry))

        return 0;

​

    err = ovl_want_write(dentry);

    if (!err) {

        err = ovl_copy_up(dentry);

        ovl_drop_write(dentry);

    }

​

    if (err) {

        pr_warn_ratelimited("overlayfs: 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 oe->numlower;

​

    /*

     * If it's an unindexed merge dir, then it's not connectable with any

     * lower layer

     */

    if (ovl_dentry_upper(dentry) &&