// SPDX-License-Identifier: GPL-2.0
/*
 *  inode.c - part of debugfs, a tiny little debug file system
 *
 *  Copyright (C) 2004 Greg Kroah-Hartman <greg@kroah.com>
 *  Copyright (C) 2004 IBM Inc.
 *
 *  debugfs is for people to use instead of /proc or /sys.
 *  See ./Documentation/core-api/kernel-api.rst for more details.
 */

#include <linux/module.h>
#include <linux/fs.h>
#include <linux/mount.h>
#include <linux/pagemap.h>
#include <linux/init.h>
#include <linux/kobject.h>
#include <linux/namei.h>
#include <linux/debugfs.h>
#include <linux/fsnotify.h>
#include <linux/string.h>
#include <linux/seq_file.h>
#include <linux/parser.h>
#include <linux/magic.h>
#include <linux/slab.h>

#include "internal.h"

#define DEBUGFS_DEFAULT_MODE    0700

static struct vfsmount *debugfs_mount;
static int debugfs_mount_count;
static bool debugfs_registered;

static struct inode *debugfs_get_inode(struct super_block *sb)
{
        struct inode *inode = new_inode(sb);
        if (inode) {
                inode->i_ino = get_next_ino();
                inode->i_atime = inode->i_mtime =
                        inode->i_ctime = current_time(inode);
        }
        return inode;
}

struct debugfs_mount_opts {
        kuid_t uid;
        kgid_t gid;
        umode_t mode;
};

enum {
        Opt_uid,
        Opt_gid,
        Opt_mode,
        Opt_err
};

static const match_table_t tokens = {
        {Opt_uid, "uid=%u"},
        {Opt_gid, "gid=%u"},
        {Opt_mode, "mode=%o"},
        {Opt_err, NULL}
};

struct debugfs_fs_info {
        struct debugfs_mount_opts mount_opts;
};

static int debugfs_parse_options(char *data, struct debugfs_mount_opts *opts)
{
        substring_t args[MAX_OPT_ARGS];
        int option;
        int token;
        kuid_t uid;
        kgid_t gid;
        char *p;

        opts->mode = DEBUGFS_DEFAULT_MODE;

        while ((p = strsep(&data, ",")) != NULL) {
                if (!*p)
                        continue;

                token = match_token(p, tokens, args);
                switch (token) {
                case Opt_uid:
                        if (match_int(&args[0], &option))
                                return -EINVAL;
                        uid = make_kuid(current_user_ns(), option);
                        if (!uid_valid(uid))
                                return -EINVAL;
                        opts->uid = uid;
                        break;
                case Opt_gid:
                        if (match_int(&args[0], &option))
                                return -EINVAL;
                        gid = make_kgid(current_user_ns(), option);
                        if (!gid_valid(gid))
                                return -EINVAL;
                        opts->gid = gid;
                        break;
                case Opt_mode:
                        if (match_octal(&args[0], &option))
                                return -EINVAL;
                        opts->mode = option & S_IALLUGO;
                        break;
                /*
                 * We might like to report bad mount options here;
                 * but traditionally debugfs has ignored all mount options
                 */
                }
        }

        return 0;
}

static int debugfs_apply_options(struct super_block *sb)
{
        struct debugfs_fs_info *fsi = sb->s_fs_info;
        struct inode *inode = d_inode(sb->s_root);
        struct debugfs_mount_opts *opts = &fsi->mount_opts;

        inode->i_mode &= ~S_IALLUGO;
        inode->i_mode |= opts->mode;

        inode->i_uid = opts->uid;
        inode->i_gid = opts->gid;

        return 0;
}

static int debugfs_remount(struct super_block *sb, int *flags, char *data)
{
        int err;
        struct debugfs_fs_info *fsi = sb->s_fs_info;

        sync_filesystem(sb);
        err = debugfs_parse_options(data, &fsi->mount_opts);
        if (err)
                goto fail;

        debugfs_apply_options(sb);

fail:
        return err;
}

static int debugfs_show_options(struct seq_file *m, struct dentry *root)
{
        struct debugfs_fs_info *fsi = root->d_sb->s_fs_info;
        struct debugfs_mount_opts *opts = &fsi->mount_opts;

        if (!uid_eq(opts->uid, GLOBAL_ROOT_UID))
                seq_printf(m, ",uid=%u",
                           from_kuid_munged(&init_user_ns, opts->uid));
        if (!gid_eq(opts->gid, GLOBAL_ROOT_GID))
                seq_printf(m, ",gid=%u",
                           from_kgid_munged(&init_user_ns, opts->gid));
        if (opts->mode != DEBUGFS_DEFAULT_MODE)
                seq_printf(m, ",mode=%o", opts->mode);

        return 0;
}

static void debugfs_evict_inode(struct inode *inode)
{
        truncate_inode_pages_final(&inode->i_data);
        clear_inode(inode);
        if (S_ISLNK(inode->i_mode))
                kfree(inode->i_link);
}

static const struct super_operations debugfs_super_operations = {
        .statfs         = simple_statfs,
        .remount_fs     = debugfs_remount,
        .show_options   = debugfs_show_options,
        .evict_inode    = debugfs_evict_inode,
};

static void debugfs_release_dentry(struct dentry *dentry)
{
        void *fsd = dentry->d_fsdata;

        if (!((unsigned long)fsd & DEBUGFS_FSDATA_IS_REAL_FOPS_BIT))
                kfree(dentry->d_fsdata);
}

static struct vfsmount *debugfs_automount(struct path *path)
{
        debugfs_automount_t f;
        f = (debugfs_automount_t)path->dentry->d_fsdata;
        return f(path->dentry, d_inode(path->dentry)->i_private);
}

static const struct dentry_operations debugfs_dops = {
        .d_delete = always_delete_dentry,
        .d_release = debugfs_release_dentry,
        .d_automount = debugfs_automount,
};

static int debug_fill_super(struct super_block *sb, void *data, int silent)
{
        static const struct tree_descr debug_files[] = {{""}};
        struct debugfs_fs_info *fsi;
        int err;

        fsi = kzalloc(sizeof(struct debugfs_fs_info), GFP_KERNEL);
        sb->s_fs_info = fsi;
        if (!fsi) {
                err = -ENOMEM;
                goto fail;
        }

        err = debugfs_parse_options(data, &fsi->mount_opts);
        if (err)
                goto fail;

        err  =  simple_fill_super(sb, DEBUGFS_MAGIC, debug_files);
        if (err)
                goto fail;

        sb->s_op = &debugfs_super_operations;
        sb->s_d_op = &debugfs_dops;

        debugfs_apply_options(sb);

        return 0;

fail:
        kfree(fsi);
        sb->s_fs_info = NULL;
        return err;
}

static struct dentry *debug_mount(struct file_system_type *fs_type,
                        int flags, const char *dev_name,
                        void *data)
{
        return mount_single(fs_type, flags, data, debug_fill_super);
}

static struct file_system_type debug_fs_type = {
        .owner =        THIS_MODULE,
        .name =         "debugfs",
        .mount =        debug_mount,
        .kill_sb =      kill_litter_super,
};
MODULE_ALIAS_FS("debugfs");

/**
 * debugfs_lookup() - look up an existing debugfs file
 * @name: a pointer to a string containing the name of the file to look up.
 * @parent: a pointer to the parent dentry of the file.
 *
 * This function will return a pointer to a dentry if it succeeds.  If the file
 * doesn't exist or an error occurs, %NULL will be returned.  The returned
 * dentry must be passed to dput() when it is no longer needed.
 *
 * If debugfs is not enabled in the kernel, the value -%ENODEV will be
 * returned.
 */
struct dentry *debugfs_lookup(const char *name, struct dentry *parent)
{
        struct dentry *dentry;

        if (IS_ERR(parent))
                return NULL;

        if (!parent)
                parent = debugfs_mount->mnt_root;

        inode_lock(d_inode(parent));
        dentry = lookup_one_len(name, parent, strlen(name));
        inode_unlock(d_inode(parent));

        if (IS_ERR(dentry))
                return NULL;
        if (!d_really_is_positive(dentry)) {
                dput(dentry);
                return NULL;
        }
        return dentry;
}
EXPORT_SYMBOL_GPL(debugfs_lookup);

static struct dentry *start_creating(const char *name, struct dentry *parent)
{
        struct dentry *dentry;
        int error;

        pr_debug("debugfs: creating file '%s'\n",name);

        if (IS_ERR(parent))
                return parent;

        error = simple_pin_fs(&debug_fs_type, &debugfs_mount,
                              &debugfs_mount_count);
        if (error)
                return ERR_PTR(error);

        /* If the parent is not specified, we create it in the root.
         * We need the root dentry to do this, which is in the super
         * block. A pointer to that is in the struct vfsmount that we
         * have around.
         */
        if (!parent)
                parent = debugfs_mount->mnt_root;

        inode_lock(d_inode(parent));
        dentry = lookup_one_len(name, parent, strlen(name));
        if (!IS_ERR(dentry) && d_really_is_positive(dentry)) {
                dput(dentry);
                dentry = ERR_PTR(-EEXIST);
        }

        if (IS_ERR(dentry)) {
                inode_unlock(d_inode(parent));
                simple_release_fs(&debugfs_mount, &debugfs_mount_count);
        }

        return dentry;
}

static struct dentry *failed_creating(struct dentry *dentry)
{
        inode_unlock(d_inode(dentry->d_parent));
        dput(dentry);
        simple_release_fs(&debugfs_mount, &debugfs_mount_count);
        return NULL;
}

static struct dentry *end_creating(struct dentry *dentry)
{
        inode_unlock(d_inode(dentry->d_parent));
        return dentry;
}

static struct dentry *__debugfs_create_file(const char *name, umode_t mode,
                                struct dentry *parent, void *data,
                                const struct file_operations *proxy_fops,
                                const struct file_operations *real_fops)
{
        struct dentry *dentry;
        struct inode *inode;

        if (!(mode & S_IFMT))
                mode |= S_IFREG;
        BUG_ON(!S_ISREG(mode));
        dentry = start_creating(name, parent);

        if (IS_ERR(dentry))
                return NULL;

        inode = debugfs_get_inode(dentry->d_sb);
        if (unlikely(!inode))
                return failed_creating(dentry);

        inode->i_mode = mode;
        inode->i_private = data;

        inode->i_fop = proxy_fops;
        dentry->d_fsdata = (void *)((unsigned long)real_fops |
                                DEBUGFS_FSDATA_IS_REAL_FOPS_BIT);

        d_instantiate(dentry, inode);
        fsnotify_create(d_inode(dentry->d_parent), dentry);
        return end_creating(dentry);
}

/**
 * debugfs_create_file - create a file in the debugfs filesystem
 * @name: a pointer to a string containing the name of the file to create.
 * @mode: the permission that the file should have.
 * @parent: a pointer to the parent dentry for this file.  This should be a
 *          directory dentry if set.  If this parameter is NULL, then the
 *          file will be created in the root of the debugfs filesystem.
 * @data: a pointer to something that the caller will want to get to later
 *        on.  The inode.i_private pointer will point to this value on
 *        the open() call.
 * @fops: a pointer to a struct file_operations that should be used for
 *        this file.
 *
 * This is the basic "create a file" function for debugfs.  It allows for a
 * wide range of flexibility in creating a file, or a directory (if you want
 * to create a directory, the debugfs_create_dir() function is
 * recommended to be used instead.)
 *
 * This function will return a pointer to a dentry if it succeeds.  This
 * pointer must be passed to the debugfs_remove() function when the file is
 * to be removed (no automatic cleanup happens if your module is unloaded,
 * you are responsible here.)  If an error occurs, %NULL will be returned.
 *
 * If debugfs is not enabled in the kernel, the value -%ENODEV will be
 * returned.
 */
struct dentry *debugfs_create_file(const char *name, umode_t mode,
                                   struct dentry *parent, void *data,
                                   const struct file_operations *fops)
{

        return __debugfs_create_file(name, mode, parent, data,
                                fops ? &debugfs_full_proxy_file_operations :
                                        &debugfs_noop_file_operations,
                                fops);
}
EXPORT_SYMBOL_GPL(debugfs_create_file);

/**
 * debugfs_create_file_unsafe - create a file in the debugfs filesystem
 * @name: a pointer to a string containing the name of the file to create.
 * @mode: the permission that the file should have.
 * @parent: a pointer to the parent dentry for this file.  This should be a
 *          directory dentry if set.  If this parameter is NULL, then the
 *          file will be created in the root of the debugfs filesystem.
 * @data: a pointer to something that the caller will want to get to later
 *        on.  The inode.i_private pointer will point to this value on
 *        the open() call.
 * @fops: a pointer to a struct file_operations that should be used for
 *        this file.
 *
 * debugfs_create_file_unsafe() is completely analogous to
 * debugfs_create_file(), the only difference being that the fops
 * handed it will not get protected against file removals by the
 * debugfs core.
 *
 * It is your responsibility to protect your struct file_operation
 * methods against file removals by means of debugfs_use_file_start()
 * and debugfs_use_file_finish(). ->open() is still protected by
 * debugfs though.
 *
 * Any struct file_operations defined by means of
 * DEFINE_DEBUGFS_ATTRIBUTE() is protected against file removals and
 * thus, may be used here.
 */
struct dentry *debugfs_create_file_unsafe(const char *name, umode_t mode,
                                   struct dentry *parent, void *data,
                                   const struct file_operations *fops)
{

        return __debugfs_create_file(name, mode, parent, data,
                                fops ? &debugfs_open_proxy_file_operations :
                                        &debugfs_noop_file_operations,
                                fops);
}
EXPORT_SYMBOL_GPL(debugfs_create_file_unsafe);

/**
 * debugfs_create_file_size - create a file in the debugfs filesystem
 * @name: a pointer to a string containing the name of the file to create.
 * @mode: the permission that the file should have.
 * @parent: a pointer to the parent dentry for this file.  This should be a
 *          directory dentry if set.  If this parameter is NULL, then the
 *          file will be created in the root of the debugfs filesystem.
 * @data: a pointer to something that the caller will want to get to later
 *        on.  The inode.i_private pointer will point to this value on
 *        the open() call.
 * @fops: a pointer to a struct file_operations that should be used for
 *        this file.
 * @file_size: initial file size
 *
 * This is the basic "create a file" function for debugfs.  It allows for a
 * wide range of flexibility in creating a file, or a directory (if you want
 * to create a directory, the debugfs_create_dir() function is
 * recommended to be used instead.)
 *
 * This function will return a pointer to a dentry if it succeeds.  This
 * pointer must be passed to the debugfs_remove() function when the file is
 * to be removed (no automatic cleanup happens if your module is unloaded,
 * you are responsible here.)  If an error occurs, %NULL will be returned.
 *
 * If debugfs is not enabled in the kernel, the value -%ENODEV will be
 * returned.
 */
struct dentry *debugfs_create_file_size(const char *name, umode_t mode,
                                        struct dentry *parent, void *data,
                                        const struct file_operations *fops,
                                        loff_t file_size)
{
        struct dentry *de = debugfs_create_file(name, mode, parent, data, fops);

        if (de)
                d_inode(de)->i_size = file_size;
        return de;
}
EXPORT_SYMBOL_GPL(debugfs_create_file_size);

/**
 * debugfs_create_dir - create a directory in the debugfs filesystem
 * @name: a pointer to a string containing the name of the directory to
 *        create.
 * @parent: a pointer to the parent dentry for this file.  This should be a
 *          directory dentry if set.  If this parameter is NULL, then the
 *          directory will be created in the root of the debugfs filesystem.
 *
 * This function creates a directory in debugfs with the given name.
 *
 * This function will return a pointer to a dentry if it succeeds.  This
 * pointer must be passed to the debugfs_remove() function when the file is
 * to be removed (no automatic cleanup happens if your module is unloaded,
 * you are responsible here.)  If an error occurs, %NULL will be returned.
 *
 * If debugfs is not enabled in the kernel, the value -%ENODEV will be
 * returned.
 */
struct dentry *debugfs_create_dir(const char *name, struct dentry *parent)
{
        struct dentry *dentry = start_creating(name, parent);
        struct inode *inode;

        if (IS_ERR(dentry))
                return NULL;

        inode = debugfs_get_inode(dentry->d_sb);
        if (unlikely(!inode))
                return failed_creating(dentry);

        inode->i_mode = S_IFDIR | S_IRWXU | S_IRUGO | S_IXUGO;
        inode->i_op = &simple_dir_inode_operations;
        inode->i_fop = &simple_dir_operations;

        /* directory inodes start off with i_nlink == 2 (for "." entry) */
        inc_nlink(inode);
        d_instantiate(dentry, inode);
        inc_nlink(d_inode(dentry->d_parent));
        fsnotify_mkdir(d_inode(dentry->d_parent), dentry);
        return end_creating(dentry);
}
EXPORT_SYMBOL_GPL(debugfs_create_dir);

/**
 * debugfs_create_automount - create automount point in the debugfs filesystem
 * @name: a pointer to a string containing the name of the file to create.
 * @parent: a pointer to the parent dentry for this file.  This should be a
 *          directory dentry if set.  If this parameter is NULL, then the
 *          file will be created in the root of the debugfs filesystem.
 * @f: function to be called when pathname resolution steps on that one.
 * @data: opaque argument to pass to f().
 *
 * @f should return what ->d_automount() would.
 */
struct dentry *debugfs_create_automount(const char *name,
                                        struct dentry *parent,
                                        debugfs_automount_t f,
                                        void *data)
{
        struct dentry *dentry = start_creating(name, parent);
        struct inode *inode;

        if (IS_ERR(dentry))
                return NULL;

        inode = debugfs_get_inode(dentry->d_sb);
        if (unlikely(!inode))
                return failed_creating(dentry);

        make_empty_dir_inode(inode);
        inode->i_flags |= S_AUTOMOUNT;
        inode->i_private = data;
        dentry->d_fsdata = (void *)f;
        /* directory inodes start off with i_nlink == 2 (for "." entry) */
        inc_nlink(inode);
        d_instantiate(dentry, inode);
        inc_nlink(d_inode(dentry->d_parent));
        fsnotify_mkdir(d_inode(dentry->d_parent), dentry);
        return end_creating(dentry);
}
EXPORT_SYMBOL(debugfs_create_automount);

/**
 * debugfs_create_symlink- create a symbolic link in the debugfs filesystem
 * @name: a pointer to a string containing the name of the symbolic link to
 *        create.
 * @parent: a pointer to the parent dentry for this symbolic link.  This
 *          should be a directory dentry if set.  If this parameter is NULL,
 *          then the symbolic link will be created in the root of the debugfs
 *          filesystem.
 * @target: a pointer to a string containing the path to the target of the
 *          symbolic link.
 *
 * This function creates a symbolic link with the given name in debugfs that
 * links to the given target path.
 *
 * This function will return a pointer to a dentry if it succeeds.  This
 * pointer must be passed to the debugfs_remove() function when the symbolic
 * link is to be removed (no automatic cleanup happens if your module is
 * unloaded, you are responsible here.)  If an error occurs, %NULL will be
 * returned.
 *
 * If debugfs is not enabled in the kernel, the value -%ENODEV will be
 * returned.
 */
struct dentry *debugfs_create_symlink(const char *name, struct dentry *parent,
                                      const char *target)
{
        struct dentry *dentry;
        struct inode *inode;
        char *link = kstrdup(target, GFP_KERNEL);
        if (!link)
                return NULL;

        dentry = start_creating(name, parent);
        if (IS_ERR(dentry)) {
                kfree(link);
                return NULL;
        }

        inode = debugfs_get_inode(dentry->d_sb);
        if (unlikely(!inode)) {
                kfree(link);
                return failed_creating(dentry);
        }
        inode->i_mode = S_IFLNK | S_IRWXUGO;
        inode->i_op = &simple_symlink_inode_operations;
        inode->i_link = link;
        d_instantiate(dentry, inode);
        return end_creating(dentry);
}
EXPORT_SYMBOL_GPL(debugfs_create_symlink);

static void __debugfs_remove_file(struct dentry *dentry, struct dentry *parent)
{
        struct debugfs_fsdata *fsd;

        simple_unlink(d_inode(parent), dentry);
        d_delete(dentry);

        /*
         * Paired with the closing smp_mb() implied by a successful
         * cmpxchg() in debugfs_file_get(): either
         * debugfs_file_get() must see a dead dentry or we must see a
         * debugfs_fsdata instance at ->d_fsdata here (or both).
         */
        smp_mb();
        fsd = READ_ONCE(dentry->d_fsdata);
        if ((unsigned long)fsd & DEBUGFS_FSDATA_IS_REAL_FOPS_BIT)
                return;
        if (!refcount_dec_and_test(&fsd->active_users))
                wait_for_completion(&fsd->active_users_drained);
}

static int __debugfs_remove(struct dentry *dentry, struct dentry *parent)
{
        int ret = 0;

        if (simple_positive(dentry)) {
                dget(dentry);
                if (!d_is_reg(dentry)) {
                        if (d_is_dir(dentry))
                                ret = simple_rmdir(d_inode(parent), dentry);
                        else
                                simple_unlink(d_inode(parent), dentry);
                        if (!ret)
                                d_delete(dentry);
                } else {
                        __debugfs_remove_file(dentry, parent);
                }
                dput(dentry);
        }
        return ret;
}

/**
 * debugfs_remove - removes a file or directory from the debugfs filesystem
 * @dentry: a pointer to a the dentry of the file or directory to be
 *          removed.  If this parameter is NULL or an error value, nothing
 *          will be done.
 *
 * This function removes a file or directory in debugfs that was previously
 * created with a call to another debugfs function (like
 * debugfs_create_file() or variants thereof.)
 *
 * This function is required to be called in order for the file to be
 * removed, no automatic cleanup of files will happen when a module is
 * removed, you are responsible here.
 */
void debugfs_remove(struct dentry *dentry)
{
        struct dentry *parent;
        int ret;

        if (IS_ERR_OR_NULL(dentry))
                return;

        parent = dentry->d_parent;
        inode_lock(d_inode(parent));
        ret = __debugfs_remove(dentry, parent);
        inode_unlock(d_inode(parent));
        if (!ret)
                simple_release_fs(&debugfs_mount, &debugfs_mount_count);
}
EXPORT_SYMBOL_GPL(debugfs_remove);

/**
 * debugfs_remove_recursive - recursively removes a directory
 * @dentry: a pointer to a the dentry of the directory to be removed.  If this
 *          parameter is NULL or an error value, nothing will be done.
 *
 * This function recursively removes a directory tree in debugfs that
 * was previously created with a call to another debugfs function
 * (like debugfs_create_file() or variants thereof.)
 *
 * This function is required to be called in order for the file to be
 * removed, no automatic cleanup of files will happen when a module is
 * removed, you are responsible here.
 */
void debugfs_remove_recursive(struct dentry *dentry)
{
        struct dentry *child, *parent;

        if (IS_ERR_OR_NULL(dentry))
                return;

        parent = dentry;
 down:
        inode_lock(d_inode(parent));
 loop:
        /*
         * The parent->d_subdirs is protected by the d_lock. Outside that
         * lock, the child can be unlinked and set to be freed which can
         * use the d_u.d_child as the rcu head and corrupt this list.
         */
        spin_lock(&parent->d_lock);
        list_for_each_entry(child, &parent->d_subdirs, d_child) {
                if (!simple_positive(child))
                        continue;

                /* perhaps simple_empty(child) makes more sense */
                if (!list_empty(&child->d_subdirs)) {
                        spin_unlock(&parent->d_lock);
                        inode_unlock(d_inode(parent));
                        parent = child;
                        goto down;
                }

                spin_unlock(&parent->d_lock);

                if (!__debugfs_remove(child, parent))
                        simple_release_fs(&debugfs_mount, &debugfs_mount_count);

                /*
                 * The parent->d_lock protects agaist child from unlinking
                 * from d_subdirs. When releasing the parent->d_lock we can
                 * no longer trust that the next pointer is valid.
                 * Restart the loop. We'll skip this one with the
                 * simple_positive() check.
                 */
                goto loop;
        }
        spin_unlock(&parent->d_lock);

        inode_unlock(d_inode(parent));
        child = parent;
        parent = parent->d_parent;
        inode_lock(d_inode(parent));

        if (child != dentry)
                /* go up */
                goto loop;

        if (!__debugfs_remove(child, parent))
                simple_release_fs(&debugfs_mount, &debugfs_mount_count);
        inode_unlock(d_inode(parent));
}
EXPORT_SYMBOL_GPL(debugfs_remove_recursive);

/**
 * debugfs_rename - rename a file/directory in the debugfs filesystem
 * @old_dir: a pointer to the parent dentry for the renamed object. This
 *          should be a directory dentry.
 * @old_dentry: dentry of an object to be renamed.
 * @new_dir: a pointer to the parent dentry where the object should be
 *          moved. This should be a directory dentry.
 * @new_name: a pointer to a string containing the target name.
 *
 * This function renames a file/directory in debugfs.  The target must not
 * exist for rename to succeed.
 *
 * This function will return a pointer to old_dentry (which is updated to
 * reflect renaming) if it succeeds. If an error occurs, %NULL will be
 * returned.
 *
 * If debugfs is not enabled in the kernel, the value -%ENODEV will be
 * returned.
 */
struct dentry *debugfs_rename(struct dentry *old_dir, struct dentry *old_dentry,
                struct dentry *new_dir, const char *new_name)
{
        int error;
        struct dentry *dentry = NULL, *trap;
        struct name_snapshot old_name;

        trap = lock_rename(new_dir, old_dir);
        /* Source or destination directories don't exist? */
        if (d_really_is_negative(old_dir) || d_really_is_negative(new_dir))
                goto exit;
        /* Source does not exist, cyclic rename, or mountpoint? */
        if (d_really_is_negative(old_dentry) || old_dentry == trap ||
            d_mountpoint(old_dentry))
                goto exit;
        dentry = lookup_one_len(new_name, new_dir, strlen(new_name));
        /* Lookup failed, cyclic rename or target exists? */
        if (IS_ERR(dentry) || dentry == trap || d_really_is_positive(dentry))
                goto exit;

        take_dentry_name_snapshot(&old_name, old_dentry);

        error = simple_rename(d_inode(old_dir), old_dentry, d_inode(new_dir),
                              dentry, 0);
        if (error) {
                release_dentry_name_snapshot(&old_name);
                goto exit;
        }
        d_move(old_dentry, dentry);
        fsnotify_move(d_inode(old_dir), d_inode(new_dir), old_name.name,
                d_is_dir(old_dentry),
                NULL, old_dentry);
        release_dentry_name_snapshot(&old_name);
        unlock_rename(new_dir, old_dir);
        dput(dentry);
        return old_dentry;
exit:
        if (dentry && !IS_ERR(dentry))
                dput(dentry);
        unlock_rename(new_dir, old_dir);
        return NULL;
}
EXPORT_SYMBOL_GPL(debugfs_rename);

/**
 * debugfs_initialized - Tells whether debugfs has been registered
 */
bool debugfs_initialized(void)
{
        return debugfs_registered;
}
EXPORT_SYMBOL_GPL(debugfs_initialized);

static int __init debugfs_init(void)
{
        int retval;

        retval = sysfs_create_mount_point(kernel_kobj, "debug");
        if (retval)
                return retval;

        retval = register_filesystem(&debug_fs_type);
        if (retval)
                sysfs_remove_mount_point(kernel_kobj, "debug");
        else
                debugfs_registered = true;

        return retval;
}
core_initcall(debugfs_init);