hostfs_kern.c

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/*
 * Copyright (C) 2000 - 2007 Jeff Dike (jdike@{addtoit,linux.intel}.com)
 * Licensed under the GPL
 *
 * Ported the filesystem routines to 2.5.
 * 2003-02-10 Petr Baudis <[email protected]>
 */

#include <linux/fs.h>
#include <linux/module.h>
#include <linux/mm.h>
#include <linux/pagemap.h>
#include <linux/statfs.h>
#include <linux/slab.h>
#include <linux/seq_file.h>
#include <linux/mount.h>
#include <linux/namei.h>
#include "hostfs.h"
#include <init.h>
#include <kern.h>

struct hostfs_inode_info {
    int fd;
    fmode_t mode;
    struct inode vfs_inode;
};

static inline struct hostfs_inode_info *HOSTFS_I(struct inode *inode)
{
    return list_entry(inode, struct hostfs_inode_info, vfs_inode);
}

#define FILE_HOSTFS_I(file) HOSTFS_I((file)->f_path.dentry->d_inode)

static int hostfs_d_delete(const struct dentry *dentry)
{
    return 1;
}

static const struct dentry_operations hostfs_dentry_ops = {
    .d_delete       = hostfs_d_delete,
};

/* Changed in hostfs_args before the kernel starts running */
static char *root_ino = "";
static int append = 0;

#define HOSTFS_SUPER_MAGIC 0x00c0ffee

static const struct inode_operations hostfs_iops;
static const struct inode_operations hostfs_dir_iops;
static const struct inode_operations hostfs_link_iops;

#ifndef MODULE
static int __init hostfs_args(char *options, int *add)
{
    char *ptr;

    ptr = strchr(options, ',');
    if (ptr != NULL)
        *ptr++ = '\0';
    if (*options != '\0')
        root_ino = options;

    options = ptr;
    while (options) {
        ptr = strchr(options, ',');
        if (ptr != NULL)
            *ptr++ = '\0';
        if (*options != '\0') {
            if (!strcmp(options, "append"))
                append = 1;
            else printf("hostfs_args - unsupported option - %s\n",
                    options);
        }
        options = ptr;
    }
    return 0;
}

__uml_setup("hostfs=", hostfs_args,
"hostfs=<root dir>,<flags>,...\n"
"    This is used to set hostfs parameters.  The root directory argument\n"
"    is used to confine all hostfs mounts to within the specified directory\n"
"    tree on the host.  If this isn't specified, then a user inside UML can\n"
"    mount anything on the host that's accessible to the user that's running\n"
"    it.\n"
"    The only flag currently supported is 'append', which specifies that all\n"
"    files opened by hostfs will be opened in append mode.\n\n"
);
#endif

static char *__dentry_name(struct dentry *dentry, char *name)
{
    char *p = dentry_path_raw(dentry, name, PATH_MAX);
    char *root;
    size_t len;

    root = dentry->d_sb->s_fs_info;
    len = strlen(root);
    if (IS_ERR(p)) {
        __putname(name);
        return NULL;
    }
    strlcpy(name, root, PATH_MAX);
    if (len > p - name) {
        __putname(name);
        return NULL;
    }
    if (p > name + len) {
        char *s = name + len;
        while ((*s++ = *p++) != '\0')
            ;
    }
    return name;
}

static char *dentry_name(struct dentry *dentry)
{
    char *name = __getname();
    if (!name)
        return NULL;

    return __dentry_name(dentry, name); /* will unlock */
}

static char *inode_name(struct inode *ino)
{
    struct dentry *dentry;
    char *name;

    dentry = d_find_alias(ino);
    if (!dentry)
        return NULL;

    name = dentry_name(dentry);

    dput(dentry);

    return name;
}

static char *follow_link(char *link)
{
    int len, n;
    char *name, *resolved, *end;

    len = 64;
    while (1) {
        n = -ENOMEM;
        name = kmalloc(len, GFP_KERNEL);
        if (name == NULL)
            goto out;

        n = hostfs_do_readlink(link, name, len);
        if (n < len)
            break;
        len *= 2;
        kfree(name);
    }
    if (n < 0)
        goto out_free;

    if (*name == '/')
        return name;

    end = strrchr(link, '/');
    if (end == NULL)
        return name;

    *(end + 1) = '\0';
    len = strlen(link) + strlen(name) + 1;

    resolved = kmalloc(len, GFP_KERNEL);
    if (resolved == NULL) {
        n = -ENOMEM;
        goto out_free;
    }

    sprintf(resolved, "%s%s", link, name);
    kfree(name);
    kfree(link);
    return resolved;

 out_free:
    kfree(name);
 out:
    return ERR_PTR(n);
}

static struct inode *hostfs_iget(struct super_block *sb)
{
    struct inode *inode = new_inode(sb);
    if (!inode)
        return ERR_PTR(-ENOMEM);
    return inode;
}

int hostfs_statfs(struct dentry *dentry, struct kstatfs *sf)
{
    /*
     * do_statfs uses struct statfs64 internally, but the linux kernel
     * struct statfs still has 32-bit versions for most of these fields,
     * so we convert them here
     */
    int err;
    long long f_blocks;
    long long f_bfree;
    long long f_bavail;
    long long f_files;
    long long f_ffree;

    err = do_statfs(dentry->d_sb->s_fs_info,
            &sf->f_bsize, &f_blocks, &f_bfree, &f_bavail, &f_files,
            &f_ffree, &sf->f_fsid, sizeof(sf->f_fsid),
            &sf->f_namelen);
    if (err)
        return err;
    sf->f_blocks = f_blocks;
    sf->f_bfree = f_bfree;
    sf->f_bavail = f_bavail;
    sf->f_files = f_files;
    sf->f_ffree = f_ffree;
    sf->f_type = HOSTFS_SUPER_MAGIC;
    return 0;
}

static struct inode *hostfs_alloc_inode(struct super_block *sb)
{
    struct hostfs_inode_info *hi;

    hi = kzalloc(sizeof(*hi), GFP_KERNEL);
    if (hi == NULL)
        return NULL;
    hi->fd = -1;
    inode_init_once(&hi->vfs_inode);
    return &hi->vfs_inode;
}

static void hostfs_evict_inode(struct inode *inode)
{
    truncate_inode_pages(&inode->i_data, 0);
    clear_inode(inode);
    if (HOSTFS_I(inode)->fd != -1) {
        close_file(&HOSTFS_I(inode)->fd);
        HOSTFS_I(inode)->fd = -1;
    }
}

static void hostfs_i_callback(struct rcu_head *head)
{
    struct inode *inode = container_of(head, struct inode, i_rcu);
    kfree(HOSTFS_I(inode));
}

static void hostfs_destroy_inode(struct inode *inode)
{
    call_rcu(&inode->i_rcu, hostfs_i_callback);
}

static int hostfs_show_options(struct seq_file *seq, struct dentry *root)
{
    const char *root_path = root->d_sb->s_fs_info;
    size_t offset = strlen(root_ino) + 1;

    if (strlen(root_path) > offset)
        seq_printf(seq, ",%s", root_path + offset);

    return 0;
}

static const struct super_operations hostfs_sbops = {
    .alloc_inode    = hostfs_alloc_inode,
    .destroy_inode  = hostfs_destroy_inode,
    .evict_inode    = hostfs_evict_inode,
    .statfs     = hostfs_statfs,
    .show_options   = hostfs_show_options,
};

int hostfs_readdir(struct file *file, void *ent, filldir_t filldir)
{
    void *dir;
    char *name;
    unsigned long long next, ino;
    int error, len;
    unsigned int type;

    name = dentry_name(file->f_path.dentry);
    if (name == NULL)
        return -ENOMEM;
    dir = open_dir(name, &error);
    __putname(name);
    if (dir == NULL)
        return -error;
    next = file->f_pos;
    while ((name = read_dir(dir, &next, &ino, &len, &type)) != NULL) {
        error = (*filldir)(ent, name, len, file->f_pos,
                   ino, type);
        if (error) break;
        file->f_pos = next;
    }
    close_dir(dir);
    return 0;
}

int hostfs_file_open(struct inode *ino, struct file *file)
{
    static DEFINE_MUTEX(open_mutex);
    char *name;
    fmode_t mode = 0;
    int err;
    int r = 0, w = 0, fd;

    mode = file->f_mode & (FMODE_READ | FMODE_WRITE);
    if ((mode & HOSTFS_I(ino)->mode) == mode)
        return 0;

    mode |= HOSTFS_I(ino)->mode;

retry:
    if (mode & FMODE_READ)
        r = 1;
    if (mode & FMODE_WRITE)
        w = 1;
    if (w)
        r = 1;

    name = dentry_name(file->f_path.dentry);
    if (name == NULL)
        return -ENOMEM;

    fd = open_file(name, r, w, append);
    __putname(name);
    if (fd < 0)
        return fd;

    mutex_lock(&open_mutex);
    /* somebody else had handled it first? */
    if ((mode & HOSTFS_I(ino)->mode) == mode) {
        mutex_unlock(&open_mutex);
        return 0;
    }
    if ((mode | HOSTFS_I(ino)->mode) != mode) {
        mode |= HOSTFS_I(ino)->mode;
        mutex_unlock(&open_mutex);
        close_file(&fd);
        goto retry;
    }
    if (HOSTFS_I(ino)->fd == -1) {
        HOSTFS_I(ino)->fd = fd;
    } else {
        err = replace_file(fd, HOSTFS_I(ino)->fd);
        close_file(&fd);
        if (err < 0) {
            mutex_unlock(&open_mutex);
            return err;
        }
    }
    HOSTFS_I(ino)->mode = mode;
    mutex_unlock(&open_mutex);

    return 0;
}

int hostfs_fsync(struct file *file, loff_t start, loff_t end, int datasync)
{
    struct inode *inode = file->f_mapping->host;
    int ret;

    ret = filemap_write_and_wait_range(inode->i_mapping, start, end);
    if (ret)
        return ret;

    mutex_lock(&inode->i_mutex);
    ret = fsync_file(HOSTFS_I(inode)->fd, datasync);
    mutex_unlock(&inode->i_mutex);

    return ret;
}

static const struct file_operations hostfs_file_fops = {
    .llseek     = generic_file_llseek,
    .read       = do_sync_read,
    .splice_read    = generic_file_splice_read,
    .aio_read   = generic_file_aio_read,
    .aio_write  = generic_file_aio_write,
    .write      = do_sync_write,
    .mmap       = generic_file_mmap,
    .open       = hostfs_file_open,
    .release    = NULL,
    .fsync      = hostfs_fsync,
};

static const struct file_operations hostfs_dir_fops = {
    .llseek     = generic_file_llseek,
    .readdir    = hostfs_readdir,
    .read       = generic_read_dir,
};

int hostfs_writepage(struct page *page, struct writeback_control *wbc)
{
    struct address_space *mapping = page->mapping;
    struct inode *inode = mapping->host;
    char *buffer;
    unsigned long long base;
    int count = PAGE_CACHE_SIZE;
    int end_index = inode->i_size >> PAGE_CACHE_SHIFT;
    int err;

    if (page->index >= end_index)
        count = inode->i_size & (PAGE_CACHE_SIZE-1);

    buffer = kmap(page);
    base = ((unsigned long long) page->index) << PAGE_CACHE_SHIFT;

    err = write_file(HOSTFS_I(inode)->fd, &base, buffer, count);
    if (err != count) {
        ClearPageUptodate(page);
        goto out;
    }

    if (base > inode->i_size)
        inode->i_size = base;

    if (PageError(page))
        ClearPageError(page);
    err = 0;

 out:
    kunmap(page);

    unlock_page(page);
    return err;
}

int hostfs_readpage(struct file *file, struct page *page)
{
    char *buffer;
    long long start;
    int err = 0;

    start = (long long) page->index << PAGE_CACHE_SHIFT;
    buffer = kmap(page);
    err = read_file(FILE_HOSTFS_I(file)->fd, &start, buffer,
            PAGE_CACHE_SIZE);
    if (err < 0)
        goto out;

    memset(&buffer[err], 0, PAGE_CACHE_SIZE - err);

    flush_dcache_page(page);
    SetPageUptodate(page);
    if (PageError(page)) ClearPageError(page);
    err = 0;
 out:
    kunmap(page);
    unlock_page(page);
    return err;
}

int hostfs_write_begin(struct file *file, struct address_space *mapping,
            loff_t pos, unsigned len, unsigned flags,
            struct page **pagep, void **fsdata)
{
    pgoff_t index = pos >> PAGE_CACHE_SHIFT;

    *pagep = grab_cache_page_write_begin(mapping, index, flags);
    if (!*pagep)
        return -ENOMEM;
    return 0;
}

int hostfs_write_end(struct file *file, struct address_space *mapping,
            loff_t pos, unsigned len, unsigned copied,
            struct page *page, void *fsdata)
{
    struct inode *inode = mapping->host;
    void *buffer;
    unsigned from = pos & (PAGE_CACHE_SIZE - 1);
    int err;

    buffer = kmap(page);
    err = write_file(FILE_HOSTFS_I(file)->fd, &pos, buffer + from, copied);
    kunmap(page);

    if (!PageUptodate(page) && err == PAGE_CACHE_SIZE)
        SetPageUptodate(page);

    /*
     * If err > 0, write_file has added err to pos, so we are comparing
     * i_size against the last byte written.
     */
    if (err > 0 && (pos > inode->i_size))
        inode->i_size = pos;
    unlock_page(page);
    page_cache_release(page);

    return err;
}

static const struct address_space_operations hostfs_aops = {
    .writepage  = hostfs_writepage,
    .readpage   = hostfs_readpage,
    .set_page_dirty = __set_page_dirty_nobuffers,
    .write_begin    = hostfs_write_begin,
    .write_end  = hostfs_write_end,
};

static int read_name(struct inode *ino, char *name)
{
    dev_t rdev;
    struct hostfs_stat st;
    int err = stat_file(name, &st, -1);
    if (err)
        return err;

    /* Reencode maj and min with the kernel encoding.*/
    rdev = MKDEV(st.maj, st.min);

    switch (st.mode & S_IFMT) {
    case S_IFLNK:
        ino->i_op = &hostfs_link_iops;
        break;
    case S_IFDIR:
        ino->i_op = &hostfs_dir_iops;
        ino->i_fop = &hostfs_dir_fops;
        break;
    case S_IFCHR:
    case S_IFBLK:
    case S_IFIFO:
    case S_IFSOCK:
        init_special_inode(ino, st.mode & S_IFMT, rdev);
        ino->i_op = &hostfs_iops;
        break;

    default:
        ino->i_op = &hostfs_iops;
        ino->i_fop = &hostfs_file_fops;
        ino->i_mapping->a_ops = &hostfs_aops;
    }

    ino->i_ino = st.ino;
    ino->i_mode = st.mode;
    set_nlink(ino, st.nlink);
    i_uid_write(ino, st.uid);
    i_gid_write(ino, st.gid);
    ino->i_atime = st.atime;
    ino->i_mtime = st.mtime;
    ino->i_ctime = st.ctime;
    ino->i_size = st.size;
    ino->i_blocks = st.blocks;
    return 0;
}

int hostfs_create(struct inode *dir, struct dentry *dentry, umode_t mode,
          bool excl)
{
    struct inode *inode;
    char *name;
    int error, fd;

    inode = hostfs_iget(dir->i_sb);
    if (IS_ERR(inode)) {
        error = PTR_ERR(inode);
        goto out;
    }

    error = -ENOMEM;
    name = dentry_name(dentry);
    if (name == NULL)
        goto out_put;

    fd = file_create(name,
             mode & S_IRUSR, mode & S_IWUSR, mode & S_IXUSR,
             mode & S_IRGRP, mode & S_IWGRP, mode & S_IXGRP,
             mode & S_IROTH, mode & S_IWOTH, mode & S_IXOTH);
    if (fd < 0)
        error = fd;
    else
        error = read_name(inode, name);

    __putname(name);
    if (error)
        goto out_put;

    HOSTFS_I(inode)->fd = fd;
    HOSTFS_I(inode)->mode = FMODE_READ | FMODE_WRITE;
    d_instantiate(dentry, inode);
    return 0;

 out_put:
    iput(inode);
 out:
    return error;
}

struct dentry *hostfs_lookup(struct inode *ino, struct dentry *dentry,
                 unsigned int flags)
{
    struct inode *inode;
    char *name;
    int err;

    inode = hostfs_iget(ino->i_sb);
    if (IS_ERR(inode)) {
        err = PTR_ERR(inode);
        goto out;
    }

    err = -ENOMEM;
    name = dentry_name(dentry);
    if (name == NULL)
        goto out_put;

    err = read_name(inode, name);

    __putname(name);
    if (err == -ENOENT) {
        iput(inode);
        inode = NULL;
    }
    else if (err)
        goto out_put;

    d_add(dentry, inode);
    return NULL;

 out_put:
    iput(inode);
 out:
    return ERR_PTR(err);
}

int hostfs_link(struct dentry *to, struct inode *ino, struct dentry *from)
{
    char *from_name, *to_name;
    int err;

    if ((from_name = dentry_name(from)) == NULL)
        return -ENOMEM;
    to_name = dentry_name(to);
    if (to_name == NULL) {
        __putname(from_name);
        return -ENOMEM;
    }
    err = link_file(to_name, from_name);
    __putname(from_name);
    __putname(to_name);
    return err;
}

int hostfs_unlink(struct inode *ino, struct dentry *dentry)
{
    char *file;
    int err;

    if (append)
        return -EPERM;

    if ((file = dentry_name(dentry)) == NULL)
        return -ENOMEM;

    err = unlink_file(file);
    __putname(file);
    return err;
}

int hostfs_symlink(struct inode *ino, struct dentry *dentry, const char *to)
{
    char *file;
    int err;

    if ((file = dentry_name(dentry)) == NULL)
        return -ENOMEM;
    err = make_symlink(file, to);
    __putname(file);
    return err;
}

int hostfs_mkdir(struct inode *ino, struct dentry *dentry, umode_t mode)
{
    char *file;
    int err;

    if ((file = dentry_name(dentry)) == NULL)
        return -ENOMEM;
    err = do_mkdir(file, mode);
    __putname(file);
    return err;
}

int hostfs_rmdir(struct inode *ino, struct dentry *dentry)
{
    char *file;
    int err;

    if ((file = dentry_name(dentry)) == NULL)
        return -ENOMEM;
    err = do_rmdir(file);
    __putname(file);
    return err;
}

static int hostfs_mknod(struct inode *dir, struct dentry *dentry, umode_t mode, dev_t dev)
{
    struct inode *inode;
    char *name;
    int err;

    inode = hostfs_iget(dir->i_sb);
    if (IS_ERR(inode)) {
        err = PTR_ERR(inode);
        goto out;
    }

    err = -ENOMEM;
    name = dentry_name(dentry);
    if (name == NULL)
        goto out_put;

    init_special_inode(inode, mode, dev);
    err = do_mknod(name, mode, MAJOR(dev), MINOR(dev));
    if (!err)
        goto out_free;

    err = read_name(inode, name);
    __putname(name);
    if (err)
        goto out_put;
    if (err)
        goto out_put;

    d_instantiate(dentry, inode);
    return 0;

 out_free:
    __putname(name);
 out_put:
    iput(inode);
 out:
    return err;
}

int hostfs_rename(struct inode *from_ino, struct dentry *from,
          struct inode *to_ino, struct dentry *to)
{
    char *from_name, *to_name;
    int err;

    if ((from_name = dentry_name(from)) == NULL)
        return -ENOMEM;
    if ((to_name = dentry_name(to)) == NULL) {
        __putname(from_name);
        return -ENOMEM;
    }
    err = rename_file(from_name, to_name);
    __putname(from_name);
    __putname(to_name);
    return err;
}

int hostfs_permission(struct inode *ino, int desired)
{
    char *name;
    int r = 0, w = 0, x = 0, err;

    if (desired & MAY_NOT_BLOCK)
        return -ECHILD;

    if (desired & MAY_READ) r = 1;
    if (desired & MAY_WRITE) w = 1;
    if (desired & MAY_EXEC) x = 1;
    name = inode_name(ino);
    if (name == NULL)
        return -ENOMEM;

    if (S_ISCHR(ino->i_mode) || S_ISBLK(ino->i_mode) ||
        S_ISFIFO(ino->i_mode) || S_ISSOCK(ino->i_mode))
        err = 0;
    else
        err = access_file(name, r, w, x);
    __putname(name);
    if (!err)
        err = generic_permission(ino, desired);
    return err;
}

int hostfs_setattr(struct dentry *dentry, struct iattr *attr)
{
    struct inode *inode = dentry->d_inode;
    struct hostfs_iattr attrs;
    char *name;
    int err;

    int fd = HOSTFS_I(inode)->fd;

    err = inode_change_ok(inode, attr);
    if (err)
        return err;

    if (append)
        attr->ia_valid &= ~ATTR_SIZE;

    attrs.ia_valid = 0;
    if (attr->ia_valid & ATTR_MODE) {
        attrs.ia_valid |= HOSTFS_ATTR_MODE;
        attrs.ia_mode = attr->ia_mode;
    }
    if (attr->ia_valid & ATTR_UID) {
        attrs.ia_valid |= HOSTFS_ATTR_UID;
        attrs.ia_uid = from_kuid(&init_user_ns, attr->ia_uid);
    }
    if (attr->ia_valid & ATTR_GID) {
        attrs.ia_valid |= HOSTFS_ATTR_GID;
        attrs.ia_gid = from_kgid(&init_user_ns, attr->ia_gid);
    }
    if (attr->ia_valid & ATTR_SIZE) {
        attrs.ia_valid |= HOSTFS_ATTR_SIZE;
        attrs.ia_size = attr->ia_size;
    }
    if (attr->ia_valid & ATTR_ATIME) {
        attrs.ia_valid |= HOSTFS_ATTR_ATIME;
        attrs.ia_atime = attr->ia_atime;
    }
    if (attr->ia_valid & ATTR_MTIME) {
        attrs.ia_valid |= HOSTFS_ATTR_MTIME;
        attrs.ia_mtime = attr->ia_mtime;
    }
    if (attr->ia_valid & ATTR_CTIME) {
        attrs.ia_valid |= HOSTFS_ATTR_CTIME;
        attrs.ia_ctime = attr->ia_ctime;
    }
    if (attr->ia_valid & ATTR_ATIME_SET) {
        attrs.ia_valid |= HOSTFS_ATTR_ATIME_SET;
    }
    if (attr->ia_valid & ATTR_MTIME_SET) {
        attrs.ia_valid |= HOSTFS_ATTR_MTIME_SET;
    }
    name = dentry_name(dentry);
    if (name == NULL)
        return -ENOMEM;
    err = set_attr(name, &attrs, fd);
    __putname(name);
    if (err)
        return err;

    if ((attr->ia_valid & ATTR_SIZE) &&
        attr->ia_size != i_size_read(inode)) {
        int error;

        error = inode_newsize_ok(inode, attr->ia_size);
        if (error)
            return error;

        truncate_setsize(inode, attr->ia_size);
    }

    setattr_copy(inode, attr);
    mark_inode_dirty(inode);
    return 0;
}

static const struct inode_operations hostfs_iops = {
    .create     = hostfs_create,
    .link       = hostfs_link,
    .unlink     = hostfs_unlink,
    .symlink    = hostfs_symlink,
    .mkdir      = hostfs_mkdir,
    .rmdir      = hostfs_rmdir,
    .mknod      = hostfs_mknod,
    .rename     = hostfs_rename,
    .permission = hostfs_permission,
    .setattr    = hostfs_setattr,
};

static const struct inode_operations hostfs_dir_iops = {
    .create     = hostfs_create,
    .lookup     = hostfs_lookup,
    .link       = hostfs_link,
    .unlink     = hostfs_unlink,
    .symlink    = hostfs_symlink,
    .mkdir      = hostfs_mkdir,
    .rmdir      = hostfs_rmdir,
    .mknod      = hostfs_mknod,
    .rename     = hostfs_rename,
    .permission = hostfs_permission,
    .setattr    = hostfs_setattr,
};

static void *hostfs_follow_link(struct dentry *dentry, struct nameidata *nd)
{
    char *link = __getname();
    if (link) {
        char *path = dentry_name(dentry);
        int err = -ENOMEM;
        if (path) {
            err = hostfs_do_readlink(path, link, PATH_MAX);
            if (err == PATH_MAX)
                err = -E2BIG;
            __putname(path);
        }
        if (err < 0) {
            __putname(link);
            link = ERR_PTR(err);
        }
    } else {
        link = ERR_PTR(-ENOMEM);
    }

    nd_set_link(nd, link);
    return NULL;
}

static void hostfs_put_link(struct dentry *dentry, struct nameidata *nd, void *cookie)
{
    char *s = nd_get_link(nd);
    if (!IS_ERR(s))
        __putname(s);
}

static const struct inode_operations hostfs_link_iops = {
    .readlink   = generic_readlink,
    .follow_link    = hostfs_follow_link,
    .put_link   = hostfs_put_link,
};

static int hostfs_fill_sb_common(struct super_block *sb, void *d, int silent)
{
    struct inode *root_inode;
    char *host_root_path, *req_root = d;
    int err;

    sb->s_blocksize = 1024;
    sb->s_blocksize_bits = 10;
    sb->s_magic = HOSTFS_SUPER_MAGIC;
    sb->s_op = &hostfs_sbops;
    sb->s_d_op = &hostfs_dentry_ops;
    sb->s_maxbytes = MAX_LFS_FILESIZE;

    /* NULL is printed as <NULL> by sprintf: avoid that. */
    if (req_root == NULL)
        req_root = "";

    err = -ENOMEM;
    sb->s_fs_info = host_root_path =
        kmalloc(strlen(root_ino) + strlen(req_root) + 2, GFP_KERNEL);
    if (host_root_path == NULL)
        goto out;

    sprintf(host_root_path, "%s/%s", root_ino, req_root);

    root_inode = new_inode(sb);
    if (!root_inode)
        goto out;

    err = read_name(root_inode, host_root_path);
    if (err)
        goto out_put;

    if (S_ISLNK(root_inode->i_mode)) {
        char *name = follow_link(host_root_path);
        if (IS_ERR(name))
            err = PTR_ERR(name);
        else
            err = read_name(root_inode, name);
        kfree(name);
        if (err)
            goto out_put;
    }

    err = -ENOMEM;
    sb->s_root = d_make_root(root_inode);
    if (sb->s_root == NULL)
        goto out;

    return 0;

out_put:
    iput(root_inode);
out:
    return err;
}

static struct dentry *hostfs_read_sb(struct file_system_type *type,
              int flags, const char *dev_name,
              void *data)
{
    return mount_nodev(type, flags, data, hostfs_fill_sb_common);
}

static void hostfs_kill_sb(struct super_block *s)
{
    kill_anon_super(s);
    kfree(s->s_fs_info);
}

static struct file_system_type hostfs_type = {
    .owner      = THIS_MODULE,
    .name       = "hostfs",
    .mount      = hostfs_read_sb,
    .kill_sb    = hostfs_kill_sb,
    .fs_flags   = 0,
};

static int __init init_hostfs(void)
{
    return register_filesystem(&hostfs_type);
}

static void __exit exit_hostfs(void)
{
    unregister_filesystem(&hostfs_type);
}

module_init(init_hostfs)
module_exit(exit_hostfs)
MODULE_LICENSE("GPL");