namespace.c

Go to the documentation of this file.

/*
 *  linux/fs/namespace.c
 *
 * (C) Copyright Al Viro 2000, 2001
 *  Released under GPL v2.
 *
 * Based on code from fs/super.c, copyright Linus Torvalds and others.
 * Heavily rewritten.
 */

#include <linux/syscalls.h>
#include <linux/export.h>
#include <linux/capability.h>
#include <linux/mnt_namespace.h>
#include <linux/namei.h>
#include <linux/security.h>
#include <linux/idr.h>
#include <linux/acct.h>     /* acct_auto_close_mnt */
#include <linux/ramfs.h>    /* init_rootfs */
#include <linux/fs_struct.h>    /* get_fs_root et.al. */
#include <linux/fsnotify.h> /* fsnotify_vfsmount_delete */
#include <linux/uaccess.h>
#include "pnode.h"
#include "internal.h"

#define HASH_SHIFT ilog2(PAGE_SIZE / sizeof(struct list_head))
#define HASH_SIZE (1UL << HASH_SHIFT)

static int event;
static DEFINE_IDA(mnt_id_ida);
static DEFINE_IDA(mnt_group_ida);
static DEFINE_SPINLOCK(mnt_id_lock);
static int mnt_id_start = 0;
static int mnt_group_start = 1;

static struct list_head *mount_hashtable __read_mostly;
static struct kmem_cache *mnt_cache __read_mostly;
static struct rw_semaphore namespace_sem;

/* /sys/fs */
struct kobject *fs_kobj;
EXPORT_SYMBOL_GPL(fs_kobj);

/*
 * vfsmount lock may be taken for read to prevent changes to the
 * vfsmount hash, ie. during mountpoint lookups or walking back
 * up the tree.
 *
 * It should be taken for write in all cases where the vfsmount
 * tree or hash is modified or when a vfsmount structure is modified.
 */
DEFINE_BRLOCK(vfsmount_lock);

static inline unsigned long hash(struct vfsmount *mnt, struct dentry *dentry)
{
    unsigned long tmp = ((unsigned long)mnt / L1_CACHE_BYTES);
    tmp += ((unsigned long)dentry / L1_CACHE_BYTES);
    tmp = tmp + (tmp >> HASH_SHIFT);
    return tmp & (HASH_SIZE - 1);
}

#define MNT_WRITER_UNDERFLOW_LIMIT -(1<<16)

/*
 * allocation is serialized by namespace_sem, but we need the spinlock to
 * serialize with freeing.
 */
static int mnt_alloc_id(struct mount *mnt)
{
    int res;

retry:
    ida_pre_get(&mnt_id_ida, GFP_KERNEL);
    spin_lock(&mnt_id_lock);
    res = ida_get_new_above(&mnt_id_ida, mnt_id_start, &mnt->mnt_id);
    if (!res)
        mnt_id_start = mnt->mnt_id + 1;
    spin_unlock(&mnt_id_lock);
    if (res == -EAGAIN)
        goto retry;

    return res;
}

static void mnt_free_id(struct mount *mnt)
{
    int id = mnt->mnt_id;
    spin_lock(&mnt_id_lock);
    ida_remove(&mnt_id_ida, id);
    if (mnt_id_start > id)
        mnt_id_start = id;
    spin_unlock(&mnt_id_lock);
}

/*
 * Allocate a new peer group ID
 *
 * mnt_group_ida is protected by namespace_sem
 */
static int mnt_alloc_group_id(struct mount *mnt)
{
    int res;

    if (!ida_pre_get(&mnt_group_ida, GFP_KERNEL))
        return -ENOMEM;

    res = ida_get_new_above(&mnt_group_ida,
                mnt_group_start,
                &mnt->mnt_group_id);
    if (!res)
        mnt_group_start = mnt->mnt_group_id + 1;

    return res;
}

/*
 * Release a peer group ID
 */
void mnt_release_group_id(struct mount *mnt)
{
    int id = mnt->mnt_group_id;
    ida_remove(&mnt_group_ida, id);
    if (mnt_group_start > id)
        mnt_group_start = id;
    mnt->mnt_group_id = 0;
}

/*
 * vfsmount lock must be held for read
 */
static inline void mnt_add_count(struct mount *mnt, int n)
{
#ifdef CONFIG_SMP
    this_cpu_add(mnt->mnt_pcp->mnt_count, n);
#else
    preempt_disable();
    mnt->mnt_count += n;
    preempt_enable();
#endif
}

/*
 * vfsmount lock must be held for write
 */
unsigned int mnt_get_count(struct mount *mnt)
{
#ifdef CONFIG_SMP
    unsigned int count = 0;
    int cpu;

    for_each_possible_cpu(cpu) {
        count += per_cpu_ptr(mnt->mnt_pcp, cpu)->mnt_count;
    }

    return count;
#else
    return mnt->mnt_count;
#endif
}

static struct mount *alloc_vfsmnt(const char *name)
{
    struct mount *mnt = kmem_cache_zalloc(mnt_cache, GFP_KERNEL);
    if (mnt) {
        int err;

        err = mnt_alloc_id(mnt);
        if (err)
            goto out_free_cache;

        if (name) {
            mnt->mnt_devname = kstrdup(name, GFP_KERNEL);
            if (!mnt->mnt_devname)
                goto out_free_id;
        }

#ifdef CONFIG_SMP
        mnt->mnt_pcp = alloc_percpu(struct mnt_pcp);
        if (!mnt->mnt_pcp)
            goto out_free_devname;

        this_cpu_add(mnt->mnt_pcp->mnt_count, 1);
#else
        mnt->mnt_count = 1;
        mnt->mnt_writers = 0;
#endif

        INIT_LIST_HEAD(&mnt->mnt_hash);
        INIT_LIST_HEAD(&mnt->mnt_child);
        INIT_LIST_HEAD(&mnt->mnt_mounts);
        INIT_LIST_HEAD(&mnt->mnt_list);
        INIT_LIST_HEAD(&mnt->mnt_expire);
        INIT_LIST_HEAD(&mnt->mnt_share);
        INIT_LIST_HEAD(&mnt->mnt_slave_list);
        INIT_LIST_HEAD(&mnt->mnt_slave);
#ifdef CONFIG_FSNOTIFY
        INIT_HLIST_HEAD(&mnt->mnt_fsnotify_marks);
#endif
    }
    return mnt;

#ifdef CONFIG_SMP
out_free_devname:
    kfree(mnt->mnt_devname);
#endif
out_free_id:
    mnt_free_id(mnt);
out_free_cache:
    kmem_cache_free(mnt_cache, mnt);
    return NULL;
}

/*
 * Most r/o checks on a fs are for operations that take
 * discrete amounts of time, like a write() or unlink().
 * We must keep track of when those operations start
 * (for permission checks) and when they end, so that
 * we can determine when writes are able to occur to
 * a filesystem.
 */
/*
 * __mnt_is_readonly: check whether a mount is read-only
 * @mnt: the mount to check for its write status
 *
 * This shouldn't be used directly ouside of the VFS.
 * It does not guarantee that the filesystem will stay
 * r/w, just that it is right *now*.  This can not and
 * should not be used in place of IS_RDONLY(inode).
 * mnt_want/drop_write() will _keep_ the filesystem
 * r/w.
 */
int __mnt_is_readonly(struct vfsmount *mnt)
{
    if (mnt->mnt_flags & MNT_READONLY)
        return 1;
    if (mnt->mnt_sb->s_flags & MS_RDONLY)
        return 1;
    return 0;
}
EXPORT_SYMBOL_GPL(__mnt_is_readonly);

static inline void mnt_inc_writers(struct mount *mnt)
{
#ifdef CONFIG_SMP
    this_cpu_inc(mnt->mnt_pcp->mnt_writers);
#else
    mnt->mnt_writers++;
#endif
}

static inline void mnt_dec_writers(struct mount *mnt)
{
#ifdef CONFIG_SMP
    this_cpu_dec(mnt->mnt_pcp->mnt_writers);
#else
    mnt->mnt_writers--;
#endif
}

static unsigned int mnt_get_writers(struct mount *mnt)
{
#ifdef CONFIG_SMP
    unsigned int count = 0;
    int cpu;

    for_each_possible_cpu(cpu) {
        count += per_cpu_ptr(mnt->mnt_pcp, cpu)->mnt_writers;
    }

    return count;
#else
    return mnt->mnt_writers;
#endif
}

static int mnt_is_readonly(struct vfsmount *mnt)
{
    if (mnt->mnt_sb->s_readonly_remount)
        return 1;
    /* Order wrt setting s_flags/s_readonly_remount in do_remount() */
    smp_rmb();
    return __mnt_is_readonly(mnt);
}

/*
 * Most r/o & frozen checks on a fs are for operations that take discrete
 * amounts of time, like a write() or unlink().  We must keep track of when
 * those operations start (for permission checks) and when they end, so that we
 * can determine when writes are able to occur to a filesystem.
 */
int __mnt_want_write(struct vfsmount *m)
{
    struct mount *mnt = real_mount(m);
    int ret = 0;

    preempt_disable();
    mnt_inc_writers(mnt);
    /*
     * The store to mnt_inc_writers must be visible before we pass
     * MNT_WRITE_HOLD loop below, so that the slowpath can see our
     * incremented count after it has set MNT_WRITE_HOLD.
     */
    smp_mb();
    while (mnt->mnt.mnt_flags & MNT_WRITE_HOLD)
        cpu_relax();
    /*
     * After the slowpath clears MNT_WRITE_HOLD, mnt_is_readonly will
     * be set to match its requirements. So we must not load that until
     * MNT_WRITE_HOLD is cleared.
     */
    smp_rmb();
    if (mnt_is_readonly(m)) {
        mnt_dec_writers(mnt);
        ret = -EROFS;
    }
    preempt_enable();

    return ret;
}

int mnt_want_write(struct vfsmount *m)
{
    int ret;

    sb_start_write(m->mnt_sb);
    ret = __mnt_want_write(m);
    if (ret)
        sb_end_write(m->mnt_sb);
    return ret;
}
EXPORT_SYMBOL_GPL(mnt_want_write);

int mnt_clone_write(struct vfsmount *mnt)
{
    /* superblock may be r/o */
    if (__mnt_is_readonly(mnt))
        return -EROFS;
    preempt_disable();
    mnt_inc_writers(real_mount(mnt));
    preempt_enable();
    return 0;
}
EXPORT_SYMBOL_GPL(mnt_clone_write);

int __mnt_want_write_file(struct file *file)
{
    struct inode *inode = file->f_dentry->d_inode;

    if (!(file->f_mode & FMODE_WRITE) || special_file(inode->i_mode))
        return __mnt_want_write(file->f_path.mnt);
    else
        return mnt_clone_write(file->f_path.mnt);
}

int mnt_want_write_file(struct file *file)
{
    int ret;

    sb_start_write(file->f_path.mnt->mnt_sb);
    ret = __mnt_want_write_file(file);
    if (ret)
        sb_end_write(file->f_path.mnt->mnt_sb);
    return ret;
}
EXPORT_SYMBOL_GPL(mnt_want_write_file);

void __mnt_drop_write(struct vfsmount *mnt)
{
    preempt_disable();
    mnt_dec_writers(real_mount(mnt));
    preempt_enable();
}

void mnt_drop_write(struct vfsmount *mnt)
{
    __mnt_drop_write(mnt);
    sb_end_write(mnt->mnt_sb);
}
EXPORT_SYMBOL_GPL(mnt_drop_write);

void __mnt_drop_write_file(struct file *file)
{
    __mnt_drop_write(file->f_path.mnt);
}

void mnt_drop_write_file(struct file *file)
{
    mnt_drop_write(file->f_path.mnt);
}
EXPORT_SYMBOL(mnt_drop_write_file);

static int mnt_make_readonly(struct mount *mnt)
{
    int ret = 0;

    br_write_lock(&vfsmount_lock);
    mnt->mnt.mnt_flags |= MNT_WRITE_HOLD;
    /*
     * After storing MNT_WRITE_HOLD, we'll read the counters. This store
     * should be visible before we do.
     */
    smp_mb();

    /*
     * With writers on hold, if this value is zero, then there are
     * definitely no active writers (although held writers may subsequently
     * increment the count, they'll have to wait, and decrement it after
     * seeing MNT_READONLY).
     *
     * It is OK to have counter incremented on one CPU and decremented on
     * another: the sum will add up correctly. The danger would be when we
     * sum up each counter, if we read a counter before it is incremented,
     * but then read another CPU's count which it has been subsequently
     * decremented from -- we would see more decrements than we should.
     * MNT_WRITE_HOLD protects against this scenario, because
     * mnt_want_write first increments count, then smp_mb, then spins on
     * MNT_WRITE_HOLD, so it can't be decremented by another CPU while
     * we're counting up here.
     */
    if (mnt_get_writers(mnt) > 0)
        ret = -EBUSY;
    else
        mnt->mnt.mnt_flags |= MNT_READONLY;
    /*
     * MNT_READONLY must become visible before ~MNT_WRITE_HOLD, so writers
     * that become unheld will see MNT_READONLY.
     */
    smp_wmb();
    mnt->mnt.mnt_flags &= ~MNT_WRITE_HOLD;
    br_write_unlock(&vfsmount_lock);
    return ret;
}

static void __mnt_unmake_readonly(struct mount *mnt)
{
    br_write_lock(&vfsmount_lock);
    mnt->mnt.mnt_flags &= ~MNT_READONLY;
    br_write_unlock(&vfsmount_lock);
}

int sb_prepare_remount_readonly(struct super_block *sb)
{
    struct mount *mnt;
    int err = 0;

    /* Racy optimization.  Recheck the counter under MNT_WRITE_HOLD */
    if (atomic_long_read(&sb->s_remove_count))
        return -EBUSY;

    br_write_lock(&vfsmount_lock);
    list_for_each_entry(mnt, &sb->s_mounts, mnt_instance) {
        if (!(mnt->mnt.mnt_flags & MNT_READONLY)) {
            mnt->mnt.mnt_flags |= MNT_WRITE_HOLD;
            smp_mb();
            if (mnt_get_writers(mnt) > 0) {
                err = -EBUSY;
                break;
            }
        }
    }
    if (!err && atomic_long_read(&sb->s_remove_count))
        err = -EBUSY;

    if (!err) {
        sb->s_readonly_remount = 1;
        smp_wmb();
    }
    list_for_each_entry(mnt, &sb->s_mounts, mnt_instance) {
        if (mnt->mnt.mnt_flags & MNT_WRITE_HOLD)
            mnt->mnt.mnt_flags &= ~MNT_WRITE_HOLD;
    }
    br_write_unlock(&vfsmount_lock);

    return err;
}

static void free_vfsmnt(struct mount *mnt)
{
    kfree(mnt->mnt_devname);
    mnt_free_id(mnt);
#ifdef CONFIG_SMP
    free_percpu(mnt->mnt_pcp);
#endif
    kmem_cache_free(mnt_cache, mnt);
}

/*
 * find the first or last mount at @dentry on vfsmount @mnt depending on
 * @dir. If @dir is set return the first mount else return the last mount.
 * vfsmount_lock must be held for read or write.
 */
struct mount *__lookup_mnt(struct vfsmount *mnt, struct dentry *dentry,
                  int dir)
{
    struct list_head *head = mount_hashtable + hash(mnt, dentry);
    struct list_head *tmp = head;
    struct mount *p, *found = NULL;

    for (;;) {
        tmp = dir ? tmp->next : tmp->prev;
        p = NULL;
        if (tmp == head)
            break;
        p = list_entry(tmp, struct mount, mnt_hash);
        if (&p->mnt_parent->mnt == mnt && p->mnt_mountpoint == dentry) {
            found = p;
            break;
        }
    }
    return found;
}

/*
 * lookup_mnt - Return the first child mount mounted at path
 *
 * "First" means first mounted chronologically.  If you create the
 * following mounts:
 *
 * mount /dev/sda1 /mnt
 * mount /dev/sda2 /mnt
 * mount /dev/sda3 /mnt
 *
 * Then lookup_mnt() on the base /mnt dentry in the root mount will
 * return successively the root dentry and vfsmount of /dev/sda1, then
 * /dev/sda2, then /dev/sda3, then NULL.
 *
 * lookup_mnt takes a reference to the found vfsmount.
 */
struct vfsmount *lookup_mnt(struct path *path)
{
    struct mount *child_mnt;

    br_read_lock(&vfsmount_lock);
    child_mnt = __lookup_mnt(path->mnt, path->dentry, 1);
    if (child_mnt) {
        mnt_add_count(child_mnt, 1);
        br_read_unlock(&vfsmount_lock);
        return &child_mnt->mnt;
    } else {
        br_read_unlock(&vfsmount_lock);
        return NULL;
    }
}

static inline int check_mnt(struct mount *mnt)
{
    return mnt->mnt_ns == current->nsproxy->mnt_ns;
}

/*
 * vfsmount lock must be held for write
 */
static void touch_mnt_namespace(struct mnt_namespace *ns)
{
    if (ns) {
        ns->event = ++event;
        wake_up_interruptible(&ns->poll);
    }
}

/*
 * vfsmount lock must be held for write
 */
static void __touch_mnt_namespace(struct mnt_namespace *ns)
{
    if (ns && ns->event != event) {
        ns->event = event;
        wake_up_interruptible(&ns->poll);
    }
}

/*
 * Clear dentry's mounted state if it has no remaining mounts.
 * vfsmount_lock must be held for write.
 */
static void dentry_reset_mounted(struct dentry *dentry)
{
    unsigned u;

    for (u = 0; u < HASH_SIZE; u++) {
        struct mount *p;

        list_for_each_entry(p, &mount_hashtable[u], mnt_hash) {
            if (p->mnt_mountpoint == dentry)
                return;
        }
    }
    spin_lock(&dentry->d_lock);
    dentry->d_flags &= ~DCACHE_MOUNTED;
    spin_unlock(&dentry->d_lock);
}

/*
 * vfsmount lock must be held for write
 */
static void detach_mnt(struct mount *mnt, struct path *old_path)
{
    old_path->dentry = mnt->mnt_mountpoint;
    old_path->mnt = &mnt->mnt_parent->mnt;
    mnt->mnt_parent = mnt;
    mnt->mnt_mountpoint = mnt->mnt.mnt_root;
    list_del_init(&mnt->mnt_child);
    list_del_init(&mnt->mnt_hash);
    dentry_reset_mounted(old_path->dentry);
}

/*
 * vfsmount lock must be held for write
 */
void mnt_set_mountpoint(struct mount *mnt, struct dentry *dentry,
            struct mount *child_mnt)
{
    mnt_add_count(mnt, 1);  /* essentially, that's mntget */
    child_mnt->mnt_mountpoint = dget(dentry);
    child_mnt->mnt_parent = mnt;
    spin_lock(&dentry->d_lock);
    dentry->d_flags |= DCACHE_MOUNTED;
    spin_unlock(&dentry->d_lock);
}

/*
 * vfsmount lock must be held for write
 */
static void attach_mnt(struct mount *mnt, struct path *path)
{
    mnt_set_mountpoint(real_mount(path->mnt), path->dentry, mnt);
    list_add_tail(&mnt->mnt_hash, mount_hashtable +
            hash(path->mnt, path->dentry));
    list_add_tail(&mnt->mnt_child, &real_mount(path->mnt)->mnt_mounts);
}

/*
 * vfsmount lock must be held for write
 */
static void commit_tree(struct mount *mnt)
{
    struct mount *parent = mnt->mnt_parent;
    struct mount *m;
    LIST_HEAD(head);
    struct mnt_namespace *n = parent->mnt_ns;

    BUG_ON(parent == mnt);

    list_add_tail(&head, &mnt->mnt_list);
    list_for_each_entry(m, &head, mnt_list)
        m->mnt_ns = n;

    list_splice(&head, n->list.prev);

    list_add_tail(&mnt->mnt_hash, mount_hashtable +
                hash(&parent->mnt, mnt->mnt_mountpoint));
    list_add_tail(&mnt->mnt_child, &parent->mnt_mounts);
    touch_mnt_namespace(n);
}

static struct mount *next_mnt(struct mount *p, struct mount *root)
{
    struct list_head *next = p->mnt_mounts.next;
    if (next == &p->mnt_mounts) {
        while (1) {
            if (p == root)
                return NULL;
            next = p->mnt_child.next;
            if (next != &p->mnt_parent->mnt_mounts)
                break;
            p = p->mnt_parent;
        }
    }
    return list_entry(next, struct mount, mnt_child);
}

static struct mount *skip_mnt_tree(struct mount *p)
{
    struct list_head *prev = p->mnt_mounts.prev;
    while (prev != &p->mnt_mounts) {
        p = list_entry(prev, struct mount, mnt_child);
        prev = p->mnt_mounts.prev;
    }
    return p;
}

struct vfsmount *
vfs_kern_mount(struct file_system_type *type, int flags, const char *name, void *data)
{
    struct mount *mnt;
    struct dentry *root;

    if (!type)
        return ERR_PTR(-ENODEV);

    mnt = alloc_vfsmnt(name);
    if (!mnt)
        return ERR_PTR(-ENOMEM);

    if (flags & MS_KERNMOUNT)
        mnt->mnt.mnt_flags = MNT_INTERNAL;

    root = mount_fs(type, flags, name, data);
    if (IS_ERR(root)) {
        free_vfsmnt(mnt);
        return ERR_CAST(root);
    }

    mnt->mnt.mnt_root = root;
    mnt->mnt.mnt_sb = root->d_sb;
    mnt->mnt_mountpoint = mnt->mnt.mnt_root;
    mnt->mnt_parent = mnt;
    br_write_lock(&vfsmount_lock);
    list_add_tail(&mnt->mnt_instance, &root->d_sb->s_mounts);
    br_write_unlock(&vfsmount_lock);
    return &mnt->mnt;
}
EXPORT_SYMBOL_GPL(vfs_kern_mount);

static struct mount *clone_mnt(struct mount *old, struct dentry *root,
                    int flag)
{
    struct super_block *sb = old->mnt.mnt_sb;
    struct mount *mnt;
    int err;

    mnt = alloc_vfsmnt(old->mnt_devname);
    if (!mnt)
        return ERR_PTR(-ENOMEM);

    if (flag & (CL_SLAVE | CL_PRIVATE))
        mnt->mnt_group_id = 0; /* not a peer of original */
    else
        mnt->mnt_group_id = old->mnt_group_id;

    if ((flag & CL_MAKE_SHARED) && !mnt->mnt_group_id) {
        err = mnt_alloc_group_id(mnt);
        if (err)
            goto out_free;
    }

    mnt->mnt.mnt_flags = old->mnt.mnt_flags & ~MNT_WRITE_HOLD;
    atomic_inc(&sb->s_active);
    mnt->mnt.mnt_sb = sb;
    mnt->mnt.mnt_root = dget(root);
    mnt->mnt_mountpoint = mnt->mnt.mnt_root;
    mnt->mnt_parent = mnt;
    br_write_lock(&vfsmount_lock);
    list_add_tail(&mnt->mnt_instance, &sb->s_mounts);
    br_write_unlock(&vfsmount_lock);

    if (flag & CL_SLAVE) {
        list_add(&mnt->mnt_slave, &old->mnt_slave_list);
        mnt->mnt_master = old;
        CLEAR_MNT_SHARED(mnt);
    } else if (!(flag & CL_PRIVATE)) {
        if ((flag & CL_MAKE_SHARED) || IS_MNT_SHARED(old))
            list_add(&mnt->mnt_share, &old->mnt_share);
        if (IS_MNT_SLAVE(old))
            list_add(&mnt->mnt_slave, &old->mnt_slave);
        mnt->mnt_master = old->mnt_master;
    }
    if (flag & CL_MAKE_SHARED)
        set_mnt_shared(mnt);

    /* stick the duplicate mount on the same expiry list
     * as the original if that was on one */
    if (flag & CL_EXPIRE) {
        if (!list_empty(&old->mnt_expire))
            list_add(&mnt->mnt_expire, &old->mnt_expire);
    }

    return mnt;

 out_free:
    free_vfsmnt(mnt);
    return ERR_PTR(err);
}

static inline void mntfree(struct mount *mnt)
{
    struct vfsmount *m = &mnt->mnt;
    struct super_block *sb = m->mnt_sb;

    /*
     * This probably indicates that somebody messed
     * up a mnt_want/drop_write() pair.  If this
     * happens, the filesystem was probably unable
     * to make r/w->r/o transitions.
     */
    /*
     * The locking used to deal with mnt_count decrement provides barriers,
     * so mnt_get_writers() below is safe.
     */
    WARN_ON(mnt_get_writers(mnt));
    fsnotify_vfsmount_delete(m);
    dput(m->mnt_root);
    free_vfsmnt(mnt);
    deactivate_super(sb);
}

static void mntput_no_expire(struct mount *mnt)
{
put_again:
#ifdef CONFIG_SMP
    br_read_lock(&vfsmount_lock);
    if (likely(mnt->mnt_ns)) {
        /* shouldn't be the last one */
        mnt_add_count(mnt, -1);
        br_read_unlock(&vfsmount_lock);
        return;
    }
    br_read_unlock(&vfsmount_lock);

    br_write_lock(&vfsmount_lock);
    mnt_add_count(mnt, -1);
    if (mnt_get_count(mnt)) {
        br_write_unlock(&vfsmount_lock);
        return;
    }
#else
    mnt_add_count(mnt, -1);
    if (likely(mnt_get_count(mnt)))
        return;
    br_write_lock(&vfsmount_lock);
#endif
    if (unlikely(mnt->mnt_pinned)) {
        mnt_add_count(mnt, mnt->mnt_pinned + 1);
        mnt->mnt_pinned = 0;
        br_write_unlock(&vfsmount_lock);
        acct_auto_close_mnt(&mnt->mnt);
        goto put_again;
    }

    list_del(&mnt->mnt_instance);
    br_write_unlock(&vfsmount_lock);
    mntfree(mnt);
}

void mntput(struct vfsmount *mnt)
{
    if (mnt) {
        struct mount *m = real_mount(mnt);
        /* avoid cacheline pingpong, hope gcc doesn't get "smart" */
        if (unlikely(m->mnt_expiry_mark))
            m->mnt_expiry_mark = 0;
        mntput_no_expire(m);
    }
}
EXPORT_SYMBOL(mntput);

struct vfsmount *mntget(struct vfsmount *mnt)
{
    if (mnt)
        mnt_add_count(real_mount(mnt), 1);
    return mnt;
}
EXPORT_SYMBOL(mntget);

void mnt_pin(struct vfsmount *mnt)
{
    br_write_lock(&vfsmount_lock);
    real_mount(mnt)->mnt_pinned++;
    br_write_unlock(&vfsmount_lock);
}
EXPORT_SYMBOL(mnt_pin);

void mnt_unpin(struct vfsmount *m)
{
    struct mount *mnt = real_mount(m);
    br_write_lock(&vfsmount_lock);
    if (mnt->mnt_pinned) {
        mnt_add_count(mnt, 1);
        mnt->mnt_pinned--;
    }
    br_write_unlock(&vfsmount_lock);
}
EXPORT_SYMBOL(mnt_unpin);

static inline void mangle(struct seq_file *m, const char *s)
{
    seq_escape(m, s, " \t\n\\");
}

/*
 * Simple .show_options callback for filesystems which don't want to
 * implement more complex mount option showing.
 *
 * See also save_mount_options().
 */
int generic_show_options(struct seq_file *m, struct dentry *root)
{
    const char *options;

    rcu_read_lock();
    options = rcu_dereference(root->d_sb->s_options);

    if (options != NULL && options[0]) {
        seq_putc(m, ',');
        mangle(m, options);
    }
    rcu_read_unlock();

    return 0;
}
EXPORT_SYMBOL(generic_show_options);

/*
 * If filesystem uses generic_show_options(), this function should be
 * called from the fill_super() callback.
 *
 * The .remount_fs callback usually needs to be handled in a special
 * way, to make sure, that previous options are not overwritten if the
 * remount fails.
 *
 * Also note, that if the filesystem's .remount_fs function doesn't
 * reset all options to their default value, but changes only newly
 * given options, then the displayed options will not reflect reality
 * any more.
 */
void save_mount_options(struct super_block *sb, char *options)
{
    BUG_ON(sb->s_options);
    rcu_assign_pointer(sb->s_options, kstrdup(options, GFP_KERNEL));
}
EXPORT_SYMBOL(save_mount_options);

void replace_mount_options(struct super_block *sb, char *options)
{
    char *old = sb->s_options;
    rcu_assign_pointer(sb->s_options, options);
    if (old) {
        synchronize_rcu();
        kfree(old);
    }
}
EXPORT_SYMBOL(replace_mount_options);

#ifdef CONFIG_PROC_FS
/* iterator; we want it to have access to namespace_sem, thus here... */
static void *m_start(struct seq_file *m, loff_t *pos)
{
    struct proc_mounts *p = proc_mounts(m);

    down_read(&namespace_sem);
    return seq_list_start(&p->ns->list, *pos);
}

static void *m_next(struct seq_file *m, void *v, loff_t *pos)
{
    struct proc_mounts *p = proc_mounts(m);

    return seq_list_next(v, &p->ns->list, pos);
}

static void m_stop(struct seq_file *m, void *v)
{
    up_read(&namespace_sem);
}

static int m_show(struct seq_file *m, void *v)
{
    struct proc_mounts *p = proc_mounts(m);
    struct mount *r = list_entry(v, struct mount, mnt_list);
    return p->show(m, &r->mnt);
}

const struct seq_operations mounts_op = {
    .start  = m_start,
    .next   = m_next,
    .stop   = m_stop,
    .show   = m_show,
};
#endif  /* CONFIG_PROC_FS */

int may_umount_tree(struct vfsmount *m)
{
    struct mount *mnt = real_mount(m);
    int actual_refs = 0;
    int minimum_refs = 0;
    struct mount *p;
    BUG_ON(!m);

    /* write lock needed for mnt_get_count */
    br_write_lock(&vfsmount_lock);
    for (p = mnt; p; p = next_mnt(p, mnt)) {
        actual_refs += mnt_get_count(p);
        minimum_refs += 2;
    }
    br_write_unlock(&vfsmount_lock);

    if (actual_refs > minimum_refs)
        return 0;

    return 1;
}

EXPORT_SYMBOL(may_umount_tree);

int may_umount(struct vfsmount *mnt)
{
    int ret = 1;
    down_read(&namespace_sem);
    br_write_lock(&vfsmount_lock);
    if (propagate_mount_busy(real_mount(mnt), 2))
        ret = 0;
    br_write_unlock(&vfsmount_lock);
    up_read(&namespace_sem);
    return ret;
}

EXPORT_SYMBOL(may_umount);

void release_mounts(struct list_head *head)
{
    struct mount *mnt;
    while (!list_empty(head)) {
        mnt = list_first_entry(head, struct mount, mnt_hash);
        list_del_init(&mnt->mnt_hash);
        if (mnt_has_parent(mnt)) {
            struct dentry *dentry;
            struct mount *m;

            br_write_lock(&vfsmount_lock);
            dentry = mnt->mnt_mountpoint;
            m = mnt->mnt_parent;
            mnt->mnt_mountpoint = mnt->mnt.mnt_root;
            mnt->mnt_parent = mnt;
            m->mnt_ghosts--;
            br_write_unlock(&vfsmount_lock);
            dput(dentry);
            mntput(&m->mnt);
        }
        mntput(&mnt->mnt);
    }
}

/*
 * vfsmount lock must be held for write
 * namespace_sem must be held for write
 */
void umount_tree(struct mount *mnt, int propagate, struct list_head *kill)
{
    LIST_HEAD(tmp_list);
    struct mount *p;

    for (p = mnt; p; p = next_mnt(p, mnt))
        list_move(&p->mnt_hash, &tmp_list);

    if (propagate)
        propagate_umount(&tmp_list);

    list_for_each_entry(p, &tmp_list, mnt_hash) {
        list_del_init(&p->mnt_expire);
        list_del_init(&p->mnt_list);
        __touch_mnt_namespace(p->mnt_ns);
        p->mnt_ns = NULL;
        list_del_init(&p->mnt_child);
        if (mnt_has_parent(p)) {
            p->mnt_parent->mnt_ghosts++;
            dentry_reset_mounted(p->mnt_mountpoint);
        }
        change_mnt_propagation(p, MS_PRIVATE);
    }
    list_splice(&tmp_list, kill);
}

static void shrink_submounts(struct mount *mnt, struct list_head *umounts);

static int do_umount(struct mount *mnt, int flags)
{
    struct super_block *sb = mnt->mnt.mnt_sb;
    int retval;
    LIST_HEAD(umount_list);

    retval = security_sb_umount(&mnt->mnt, flags);
    if (retval)
        return retval;

    /*
     * Allow userspace to request a mountpoint be expired rather than
     * unmounting unconditionally. Unmount only happens if:
     *  (1) the mark is already set (the mark is cleared by mntput())
     *  (2) the usage count == 1 [parent vfsmount] + 1 [sys_umount]
     */
    if (flags & MNT_EXPIRE) {
        if (&mnt->mnt == current->fs->root.mnt ||
            flags & (MNT_FORCE | MNT_DETACH))
            return -EINVAL;

        /*
         * probably don't strictly need the lock here if we examined
         * all race cases, but it's a slowpath.
         */
        br_write_lock(&vfsmount_lock);
        if (mnt_get_count(mnt) != 2) {
            br_write_unlock(&vfsmount_lock);
            return -EBUSY;
        }
        br_write_unlock(&vfsmount_lock);

        if (!xchg(&mnt->mnt_expiry_mark, 1))
            return -EAGAIN;
    }

    /*
     * If we may have to abort operations to get out of this
     * mount, and they will themselves hold resources we must
     * allow the fs to do things. In the Unix tradition of
     * 'Gee thats tricky lets do it in userspace' the umount_begin
     * might fail to complete on the first run through as other tasks
     * must return, and the like. Thats for the mount program to worry
     * about for the moment.
     */

    if (flags & MNT_FORCE && sb->s_op->umount_begin) {
        sb->s_op->umount_begin(sb);
    }

    /*
     * No sense to grab the lock for this test, but test itself looks
     * somewhat bogus. Suggestions for better replacement?
     * Ho-hum... In principle, we might treat that as umount + switch
     * to rootfs. GC would eventually take care of the old vfsmount.
     * Actually it makes sense, especially if rootfs would contain a
     * /reboot - static binary that would close all descriptors and
     * call reboot(9). Then init(8) could umount root and exec /reboot.
     */
    if (&mnt->mnt == current->fs->root.mnt && !(flags & MNT_DETACH)) {
        /*
         * Special case for "unmounting" root ...
         * we just try to remount it readonly.
         */
        down_write(&sb->s_umount);
        if (!(sb->s_flags & MS_RDONLY))
            retval = do_remount_sb(sb, MS_RDONLY, NULL, 0);
        up_write(&sb->s_umount);
        return retval;
    }

    down_write(&namespace_sem);
    br_write_lock(&vfsmount_lock);
    event++;

    if (!(flags & MNT_DETACH))
        shrink_submounts(mnt, &umount_list);

    retval = -EBUSY;
    if (flags & MNT_DETACH || !propagate_mount_busy(mnt, 2)) {
        if (!list_empty(&mnt->mnt_list))
            umount_tree(mnt, 1, &umount_list);
        retval = 0;
    }
    br_write_unlock(&vfsmount_lock);
    up_write(&namespace_sem);
    release_mounts(&umount_list);
    return retval;
}

/*
 * Now umount can handle mount points as well as block devices.
 * This is important for filesystems which use unnamed block devices.
 *
 * We now support a flag for forced unmount like the other 'big iron'
 * unixes. Our API is identical to OSF/1 to avoid making a mess of AMD
 */

SYSCALL_DEFINE2(umount, char __user *, name, int, flags)
{
    struct path path;
    struct mount *mnt;
    int retval;
    int lookup_flags = 0;

    if (flags & ~(MNT_FORCE | MNT_DETACH | MNT_EXPIRE | UMOUNT_NOFOLLOW))
        return -EINVAL;

    if (!(flags & UMOUNT_NOFOLLOW))
        lookup_flags |= LOOKUP_FOLLOW;

    retval = user_path_at(AT_FDCWD, name, lookup_flags, &path);
    if (retval)
        goto out;
    mnt = real_mount(path.mnt);
    retval = -EINVAL;
    if (path.dentry != path.mnt->mnt_root)
        goto dput_and_out;
    if (!check_mnt(mnt))
        goto dput_and_out;

    retval = -EPERM;
    if (!capable(CAP_SYS_ADMIN))
        goto dput_and_out;

    retval = do_umount(mnt, flags);
dput_and_out:
    /* we mustn't call path_put() as that would clear mnt_expiry_mark */
    dput(path.dentry);
    mntput_no_expire(mnt);
out:
    return retval;
}

#ifdef __ARCH_WANT_SYS_OLDUMOUNT

/*
 *  The 2.0 compatible umount. No flags.
 */
SYSCALL_DEFINE1(oldumount, char __user *, name)
{
    return sys_umount(name, 0);
}

#endif

static int mount_is_safe(struct path *path)
{
    if (capable(CAP_SYS_ADMIN))
        return 0;
    return -EPERM;
#ifdef notyet
    if (S_ISLNK(path->dentry->d_inode->i_mode))
        return -EPERM;
    if (path->dentry->d_inode->i_mode & S_ISVTX) {
        if (current_uid() != path->dentry->d_inode->i_uid)
            return -EPERM;
    }
    if (inode_permission(path->dentry->d_inode, MAY_WRITE))
        return -EPERM;
    return 0;
#endif
}

struct mount *copy_tree(struct mount *mnt, struct dentry *dentry,
                    int flag)
{
    struct mount *res, *p, *q, *r;
    struct path path;

    if (!(flag & CL_COPY_ALL) && IS_MNT_UNBINDABLE(mnt))
        return ERR_PTR(-EINVAL);

    res = q = clone_mnt(mnt, dentry, flag);
    if (IS_ERR(q))
        return q;

    q->mnt_mountpoint = mnt->mnt_mountpoint;

    p = mnt;
    list_for_each_entry(r, &mnt->mnt_mounts, mnt_child) {
        struct mount *s;
        if (!is_subdir(r->mnt_mountpoint, dentry))
            continue;

        for (s = r; s; s = next_mnt(s, r)) {
            if (!(flag & CL_COPY_ALL) && IS_MNT_UNBINDABLE(s)) {
                s = skip_mnt_tree(s);
                continue;
            }
            while (p != s->mnt_parent) {
                p = p->mnt_parent;
                q = q->mnt_parent;
            }
            p = s;
            path.mnt = &q->mnt;
            path.dentry = p->mnt_mountpoint;
            q = clone_mnt(p, p->mnt.mnt_root, flag);
            if (IS_ERR(q))
                goto out;
            br_write_lock(&vfsmount_lock);
            list_add_tail(&q->mnt_list, &res->mnt_list);
            attach_mnt(q, &path);
            br_write_unlock(&vfsmount_lock);
        }
    }
    return res;
out:
    if (res) {
        LIST_HEAD(umount_list);
        br_write_lock(&vfsmount_lock);
        umount_tree(res, 0, &umount_list);
        br_write_unlock(&vfsmount_lock);
        release_mounts(&umount_list);
    }
    return q;
}

/* Caller should check returned pointer for errors */

struct vfsmount *collect_mounts(struct path *path)
{
    struct mount *tree;
    down_write(&namespace_sem);
    tree = copy_tree(real_mount(path->mnt), path->dentry,
             CL_COPY_ALL | CL_PRIVATE);
    up_write(&namespace_sem);
    if (IS_ERR(tree))
        return NULL;
    return &tree->mnt;
}

void drop_collected_mounts(struct vfsmount *mnt)
{
    LIST_HEAD(umount_list);
    down_write(&namespace_sem);
    br_write_lock(&vfsmount_lock);
    umount_tree(real_mount(mnt), 0, &umount_list);
    br_write_unlock(&vfsmount_lock);
    up_write(&namespace_sem);
    release_mounts(&umount_list);
}

int iterate_mounts(int (*f)(struct vfsmount *, void *), void *arg,
           struct vfsmount *root)
{
    struct mount *mnt;
    int res = f(root, arg);
    if (res)
        return res;
    list_for_each_entry(mnt, &real_mount(root)->mnt_list, mnt_list) {
        res = f(&mnt->mnt, arg);
        if (res)
            return res;
    }
    return 0;
}

static void cleanup_group_ids(struct mount *mnt, struct mount *end)
{
    struct mount *p;

    for (p = mnt; p != end; p = next_mnt(p, mnt)) {
        if (p->mnt_group_id && !IS_MNT_SHARED(p))
            mnt_release_group_id(p);
    }
}

static int invent_group_ids(struct mount *mnt, bool recurse)
{
    struct mount *p;

    for (p = mnt; p; p = recurse ? next_mnt(p, mnt) : NULL) {
        if (!p->mnt_group_id && !IS_MNT_SHARED(p)) {
            int err = mnt_alloc_group_id(p);
            if (err) {
                cleanup_group_ids(mnt, p);
                return err;
            }
        }
    }

    return 0;
}

/*
 *  @source_mnt : mount tree to be attached
 *  @nd         : place the mount tree @source_mnt is attached
 *  @parent_nd  : if non-null, detach the source_mnt from its parent and
 *             store the parent mount and mountpoint dentry.
 *             (done when source_mnt is moved)
 *
 *  NOTE: in the table below explains the semantics when a source mount
 *  of a given type is attached to a destination mount of a given type.
 * ---------------------------------------------------------------------------
 * |         BIND MOUNT OPERATION                                            |
 * |**************************************************************************
 * | source-->| shared        |       private  |       slave    | unbindable |
 * | dest     |               |                |                |            |
 * |   |      |               |                |                |            |
 * |   v      |               |                |                |            |
 * |**************************************************************************
 * |  shared  | shared (++)   |     shared (+) |     shared(+++)|  invalid   |
 * |          |               |                |                |            |
 * |non-shared| shared (+)    |      private   |      slave (*) |  invalid   |
 * ***************************************************************************
 * A bind operation clones the source mount and mounts the clone on the
 * destination mount.
 *
 * (++)  the cloned mount is propagated to all the mounts in the propagation
 *   tree of the destination mount and the cloned mount is added to
 *   the peer group of the source mount.
 * (+)   the cloned mount is created under the destination mount and is marked
 *       as shared. The cloned mount is added to the peer group of the source
 *       mount.
 * (+++) the mount is propagated to all the mounts in the propagation tree
 *       of the destination mount and the cloned mount is made slave
 *       of the same master as that of the source mount. The cloned mount
 *       is marked as 'shared and slave'.
 * (*)   the cloned mount is made a slave of the same master as that of the
 *   source mount.
 *
 * ---------------------------------------------------------------------------
 * |                MOVE MOUNT OPERATION                                 |
 * |**************************************************************************
 * | source-->| shared        |       private  |       slave    | unbindable |
 * | dest     |               |                |                |            |
 * |   |      |               |                |                |            |
 * |   v      |               |                |                |            |
 * |**************************************************************************
 * |  shared  | shared (+)    |     shared (+) |    shared(+++) |  invalid   |
 * |          |               |                |                |            |
 * |non-shared| shared (+*)   |      private   |    slave (*)   | unbindable |
 * ***************************************************************************
 *
 * (+)  the mount is moved to the destination. And is then propagated to
 *  all the mounts in the propagation tree of the destination mount.
 * (+*)  the mount is moved to the destination.
 * (+++)  the mount is moved to the destination and is then propagated to
 *  all the mounts belonging to the destination mount's propagation tree.
 *  the mount is marked as 'shared and slave'.
 * (*)  the mount continues to be a slave at the new location.
 *
 * if the source mount is a tree, the operations explained above is
 * applied to each mount in the tree.
 * Must be called without spinlocks held, since this function can sleep
 * in allocations.
 */
static int attach_recursive_mnt(struct mount *source_mnt,
            struct path *path, struct path *parent_path)
{
    LIST_HEAD(tree_list);
    struct mount *dest_mnt = real_mount(path->mnt);
    struct dentry *dest_dentry = path->dentry;
    struct mount *child, *p;
    int err;

    if (IS_MNT_SHARED(dest_mnt)) {
        err = invent_group_ids(source_mnt, true);
        if (err)
            goto out;
    }
    err = propagate_mnt(dest_mnt, dest_dentry, source_mnt, &tree_list);
    if (err)
        goto out_cleanup_ids;

    br_write_lock(&vfsmount_lock);

    if (IS_MNT_SHARED(dest_mnt)) {
        for (p = source_mnt; p; p = next_mnt(p, source_mnt))
            set_mnt_shared(p);
    }
    if (parent_path) {
        detach_mnt(source_mnt, parent_path);
        attach_mnt(source_mnt, path);
        touch_mnt_namespace(source_mnt->mnt_ns);
    } else {
        mnt_set_mountpoint(dest_mnt, dest_dentry, source_mnt);
        commit_tree(source_mnt);
    }

    list_for_each_entry_safe(child, p, &tree_list, mnt_hash) {
        list_del_init(&child->mnt_hash);
        commit_tree(child);
    }
    br_write_unlock(&vfsmount_lock);

    return 0;

 out_cleanup_ids:
    if (IS_MNT_SHARED(dest_mnt))
        cleanup_group_ids(source_mnt, NULL);
 out:
    return err;
}

static int lock_mount(struct path *path)
{
    struct vfsmount *mnt;
retry:
    mutex_lock(&path->dentry->d_inode->i_mutex);
    if (unlikely(cant_mount(path->dentry))) {
        mutex_unlock(&path->dentry->d_inode->i_mutex);
        return -ENOENT;
    }
    down_write(&namespace_sem);
    mnt = lookup_mnt(path);
    if (likely(!mnt))
        return 0;
    up_write(&namespace_sem);
    mutex_unlock(&path->dentry->d_inode->i_mutex);
    path_put(path);
    path->mnt = mnt;
    path->dentry = dget(mnt->mnt_root);
    goto retry;
}

static void unlock_mount(struct path *path)
{
    up_write(&namespace_sem);
    mutex_unlock(&path->dentry->d_inode->i_mutex);
}

static int graft_tree(struct mount *mnt, struct path *path)
{
    if (mnt->mnt.mnt_sb->s_flags & MS_NOUSER)
        return -EINVAL;

    if (S_ISDIR(path->dentry->d_inode->i_mode) !=
          S_ISDIR(mnt->mnt.mnt_root->d_inode->i_mode))
        return -ENOTDIR;

    if (d_unlinked(path->dentry))
        return -ENOENT;

    return attach_recursive_mnt(mnt, path, NULL);
}

/*
 * Sanity check the flags to change_mnt_propagation.
 */

static int flags_to_propagation_type(int flags)
{
    int type = flags & ~(MS_REC | MS_SILENT);

    /* Fail if any non-propagation flags are set */
    if (type & ~(MS_SHARED | MS_PRIVATE | MS_SLAVE | MS_UNBINDABLE))
        return 0;
    /* Only one propagation flag should be set */
    if (!is_power_of_2(type))
        return 0;
    return type;
}

/*
 * recursively change the type of the mountpoint.
 */
static int do_change_type(struct path *path, int flag)
{
    struct mount *m;
    struct mount *mnt = real_mount(path->mnt);
    int recurse = flag & MS_REC;
    int type;
    int err = 0;

    if (!capable(CAP_SYS_ADMIN))
        return -EPERM;

    if (path->dentry != path->mnt->mnt_root)
        return -EINVAL;

    type = flags_to_propagation_type(flag);
    if (!type)
        return -EINVAL;

    down_write(&namespace_sem);
    if (type == MS_SHARED) {
        err = invent_group_ids(mnt, recurse);
        if (err)
            goto out_unlock;
    }

    br_write_lock(&vfsmount_lock);
    for (m = mnt; m; m = (recurse ? next_mnt(m, mnt) : NULL))
        change_mnt_propagation(m, type);
    br_write_unlock(&vfsmount_lock);

 out_unlock:
    up_write(&namespace_sem);
    return err;
}

/*
 * do loopback mount.
 */
static int do_loopback(struct path *path, const char *old_name,
                int recurse)
{
    LIST_HEAD(umount_list);
    struct path old_path;
    struct mount *mnt = NULL, *old;
    int err = mount_is_safe(path);
    if (err)
        return err;
    if (!old_name || !*old_name)
        return -EINVAL;
    err = kern_path(old_name, LOOKUP_FOLLOW|LOOKUP_AUTOMOUNT, &old_path);
    if (err)
        return err;

    err = lock_mount(path);
    if (err)
        goto out;

    old = real_mount(old_path.mnt);

    err = -EINVAL;
    if (IS_MNT_UNBINDABLE(old))
        goto out2;

    if (!check_mnt(real_mount(path->mnt)) || !check_mnt(old))
        goto out2;

    if (recurse)
        mnt = copy_tree(old, old_path.dentry, 0);
    else
        mnt = clone_mnt(old, old_path.dentry, 0);

    if (IS_ERR(mnt)) {
        err = PTR_ERR(mnt);
        goto out;
    }

    err = graft_tree(mnt, path);
    if (err) {
        br_write_lock(&vfsmount_lock);
        umount_tree(mnt, 0, &umount_list);
        br_write_unlock(&vfsmount_lock);
    }
out2:
    unlock_mount(path);
    release_mounts(&umount_list);
out:
    path_put(&old_path);
    return err;
}

static int change_mount_flags(struct vfsmount *mnt, int ms_flags)
{
    int error = 0;
    int readonly_request = 0;

    if (ms_flags & MS_RDONLY)
        readonly_request = 1;
    if (readonly_request == __mnt_is_readonly(mnt))
        return 0;

    if (readonly_request)
        error = mnt_make_readonly(real_mount(mnt));
    else
        __mnt_unmake_readonly(real_mount(mnt));
    return error;
}

/*
 * change filesystem flags. dir should be a physical root of filesystem.
 * If you've mounted a non-root directory somewhere and want to do remount
 * on it - tough luck.
 */
static int do_remount(struct path *path, int flags, int mnt_flags,
              void *data)
{
    int err;
    struct super_block *sb = path->mnt->mnt_sb;
    struct mount *mnt = real_mount(path->mnt);

    if (!capable(CAP_SYS_ADMIN))
        return -EPERM;

    if (!check_mnt(mnt))
        return -EINVAL;

    if (path->dentry != path->mnt->mnt_root)
        return -EINVAL;

    err = security_sb_remount(sb, data);
    if (err)
        return err;

    down_write(&sb->s_umount);
    if (flags & MS_BIND)
        err = change_mount_flags(path->mnt, flags);
    else
        err = do_remount_sb(sb, flags, data, 0);
    if (!err) {
        br_write_lock(&vfsmount_lock);
        mnt_flags |= mnt->mnt.mnt_flags & MNT_PROPAGATION_MASK;
        mnt->mnt.mnt_flags = mnt_flags;
        br_write_unlock(&vfsmount_lock);
    }
    up_write(&sb->s_umount);
    if (!err) {
        br_write_lock(&vfsmount_lock);
        touch_mnt_namespace(mnt->mnt_ns);
        br_write_unlock(&vfsmount_lock);
    }
    return err;
}

static inline int tree_contains_unbindable(struct mount *mnt)
{
    struct mount *p;
    for (p = mnt; p; p = next_mnt(p, mnt)) {
        if (IS_MNT_UNBINDABLE(p))
            return 1;
    }
    return 0;
}

static int do_move_mount(struct path *path, const char *old_name)
{
    struct path old_path, parent_path;
    struct mount *p;
    struct mount *old;
    int err = 0;
    if (!capable(CAP_SYS_ADMIN))
        return -EPERM;
    if (!old_name || !*old_name)
        return -EINVAL;
    err = kern_path(old_name, LOOKUP_FOLLOW, &old_path);
    if (err)
        return err;

    err = lock_mount(path);
    if (err < 0)
        goto out;

    old = real_mount(old_path.mnt);
    p = real_mount(path->mnt);

    err = -EINVAL;
    if (!check_mnt(p) || !check_mnt(old))
        goto out1;

    if (d_unlinked(path->dentry))
        goto out1;

    err = -EINVAL;
    if (old_path.dentry != old_path.mnt->mnt_root)
        goto out1;

    if (!mnt_has_parent(old))
        goto out1;

    if (S_ISDIR(path->dentry->d_inode->i_mode) !=
          S_ISDIR(old_path.dentry->d_inode->i_mode))
        goto out1;
    /*
     * Don't move a mount residing in a shared parent.
     */
    if (IS_MNT_SHARED(old->mnt_parent))
        goto out1;
    /*
     * Don't move a mount tree containing unbindable mounts to a destination
     * mount which is shared.
     */
    if (IS_MNT_SHARED(p) && tree_contains_unbindable(old))
        goto out1;
    err = -ELOOP;
    for (; mnt_has_parent(p); p = p->mnt_parent)
        if (p == old)
            goto out1;

    err = attach_recursive_mnt(old, path, &parent_path);
    if (err)
        goto out1;

    /* if the mount is moved, it should no longer be expire
     * automatically */
    list_del_init(&old->mnt_expire);
out1:
    unlock_mount(path);
out:
    if (!err)
        path_put(&parent_path);
    path_put(&old_path);
    return err;
}

static struct vfsmount *fs_set_subtype(struct vfsmount *mnt, const char *fstype)
{
    int err;
    const char *subtype = strchr(fstype, '.');
    if (subtype) {
        subtype++;
        err = -EINVAL;
        if (!subtype[0])
            goto err;
    } else
        subtype = "";

    mnt->mnt_sb->s_subtype = kstrdup(subtype, GFP_KERNEL);
    err = -ENOMEM;
    if (!mnt->mnt_sb->s_subtype)
        goto err;
    return mnt;

 err:
    mntput(mnt);
    return ERR_PTR(err);
}

static struct vfsmount *
do_kern_mount(const char *fstype, int flags, const char *name, void *data)
{
    struct file_system_type *type = get_fs_type(fstype);
    struct vfsmount *mnt;
    if (!type)
        return ERR_PTR(-ENODEV);
    mnt = vfs_kern_mount(type, flags, name, data);
    if (!IS_ERR(mnt) && (type->fs_flags & FS_HAS_SUBTYPE) &&
        !mnt->mnt_sb->s_subtype)
        mnt = fs_set_subtype(mnt, fstype);
    put_filesystem(type);
    return mnt;
}

/*
 * add a mount into a namespace's mount tree
 */
static int do_add_mount(struct mount *newmnt, struct path *path, int mnt_flags)
{
    int err;

    mnt_flags &= ~(MNT_SHARED | MNT_WRITE_HOLD | MNT_INTERNAL);

    err = lock_mount(path);
    if (err)
        return err;

    err = -EINVAL;
    if (unlikely(!check_mnt(real_mount(path->mnt)))) {
        /* that's acceptable only for automounts done in private ns */
        if (!(mnt_flags & MNT_SHRINKABLE))
            goto unlock;
        /* ... and for those we'd better have mountpoint still alive */
        if (!real_mount(path->mnt)->mnt_ns)
            goto unlock;
    }

    /* Refuse the same filesystem on the same mount point */
    err = -EBUSY;
    if (path->mnt->mnt_sb == newmnt->mnt.mnt_sb &&
        path->mnt->mnt_root == path->dentry)
        goto unlock;

    err = -EINVAL;
    if (S_ISLNK(newmnt->mnt.mnt_root->d_inode->i_mode))
        goto unlock;

    newmnt->mnt.mnt_flags = mnt_flags;
    err = graft_tree(newmnt, path);

unlock:
    unlock_mount(path);
    return err;
}

/*
 * create a new mount for userspace and request it to be added into the
 * namespace's tree
 */
static int do_new_mount(struct path *path, const char *type, int flags,
            int mnt_flags, const char *name, void *data)
{
    struct vfsmount *mnt;
    int err;

    if (!type)
        return -EINVAL;

    /* we need capabilities... */
    if (!capable(CAP_SYS_ADMIN))
        return -EPERM;

    mnt = do_kern_mount(type, flags, name, data);
    if (IS_ERR(mnt))
        return PTR_ERR(mnt);

    err = do_add_mount(real_mount(mnt), path, mnt_flags);
    if (err)
        mntput(mnt);
    return err;
}

int finish_automount(struct vfsmount *m, struct path *path)
{
    struct mount *mnt = real_mount(m);
    int err;
    /* The new mount record should have at least 2 refs to prevent it being
     * expired before we get a chance to add it
     */
    BUG_ON(mnt_get_count(mnt) < 2);

    if (m->mnt_sb == path->mnt->mnt_sb &&
        m->mnt_root == path->dentry) {
        err = -ELOOP;
        goto fail;
    }

    err = do_add_mount(mnt, path, path->mnt->mnt_flags | MNT_SHRINKABLE);
    if (!err)
        return 0;
fail:
    /* remove m from any expiration list it may be on */
    if (!list_empty(&mnt->mnt_expire)) {
        down_write(&namespace_sem);
        br_write_lock(&vfsmount_lock);
        list_del_init(&mnt->mnt_expire);
        br_write_unlock(&vfsmount_lock);
        up_write(&namespace_sem);
    }
    mntput(m);
    mntput(m);
    return err;
}

void mnt_set_expiry(struct vfsmount *mnt, struct list_head *expiry_list)
{
    down_write(&namespace_sem);
    br_write_lock(&vfsmount_lock);

    list_add_tail(&real_mount(mnt)->mnt_expire, expiry_list);

    br_write_unlock(&vfsmount_lock);
    up_write(&namespace_sem);
}
EXPORT_SYMBOL(mnt_set_expiry);

/*
 * process a list of expirable mountpoints with the intent of discarding any
 * mountpoints that aren't in use and haven't been touched since last we came
 * here
 */
void mark_mounts_for_expiry(struct list_head *mounts)
{
    struct mount *mnt, *next;
    LIST_HEAD(graveyard);
    LIST_HEAD(umounts);

    if (list_empty(mounts))
        return;

    down_write(&namespace_sem);
    br_write_lock(&vfsmount_lock);

    /* extract from the expiration list every vfsmount that matches the
     * following criteria:
     * - only referenced by its parent vfsmount
     * - still marked for expiry (marked on the last call here; marks are
     *   cleared by mntput())
     */
    list_for_each_entry_safe(mnt, next, mounts, mnt_expire) {
        if (!xchg(&mnt->mnt_expiry_mark, 1) ||
            propagate_mount_busy(mnt, 1))
            continue;
        list_move(&mnt->mnt_expire, &graveyard);
    }
    while (!list_empty(&graveyard)) {
        mnt = list_first_entry(&graveyard, struct mount, mnt_expire);
        touch_mnt_namespace(mnt->mnt_ns);
        umount_tree(mnt, 1, &umounts);
    }
    br_write_unlock(&vfsmount_lock);
    up_write(&namespace_sem);

    release_mounts(&umounts);
}

EXPORT_SYMBOL_GPL(mark_mounts_for_expiry);

/*
 * Ripoff of 'select_parent()'
 *
 * search the list of submounts for a given mountpoint, and move any
 * shrinkable submounts to the 'graveyard' list.
 */
static int select_submounts(struct mount *parent, struct list_head *graveyard)
{
    struct mount *this_parent = parent;
    struct list_head *next;
    int found = 0;

repeat:
    next = this_parent->mnt_mounts.next;
resume:
    while (next != &this_parent->mnt_mounts) {
        struct list_head *tmp = next;
        struct mount *mnt = list_entry(tmp, struct mount, mnt_child);

        next = tmp->next;
        if (!(mnt->mnt.mnt_flags & MNT_SHRINKABLE))
            continue;
        /*
         * Descend a level if the d_mounts list is non-empty.
         */
        if (!list_empty(&mnt->mnt_mounts)) {
            this_parent = mnt;
            goto repeat;
        }

        if (!propagate_mount_busy(mnt, 1)) {
            list_move_tail(&mnt->mnt_expire, graveyard);
            found++;
        }
    }
    /*
     * All done at this level ... ascend and resume the search
     */
    if (this_parent != parent) {
        next = this_parent->mnt_child.next;
        this_parent = this_parent->mnt_parent;
        goto resume;
    }
    return found;
}

/*
 * process a list of expirable mountpoints with the intent of discarding any
 * submounts of a specific parent mountpoint
 *
 * vfsmount_lock must be held for write
 */
static void shrink_submounts(struct mount *mnt, struct list_head *umounts)
{
    LIST_HEAD(graveyard);
    struct mount *m;

    /* extract submounts of 'mountpoint' from the expiration list */
    while (select_submounts(mnt, &graveyard)) {
        while (!list_empty(&graveyard)) {
            m = list_first_entry(&graveyard, struct mount,
                        mnt_expire);
            touch_mnt_namespace(m->mnt_ns);
            umount_tree(m, 1, umounts);
        }
    }
}

/*
 * Some copy_from_user() implementations do not return the exact number of
 * bytes remaining to copy on a fault.  But copy_mount_options() requires that.
 * Note that this function differs from copy_from_user() in that it will oops
 * on bad values of `to', rather than returning a short copy.
 */
static long exact_copy_from_user(void *to, const void __user * from,
                 unsigned long n)
{
    char *t = to;
    const char __user *f = from;
    char c;

    if (!access_ok(VERIFY_READ, from, n))
        return n;

    while (n) {
        if (__get_user(c, f)) {
            memset(t, 0, n);
            break;
        }
        *t++ = c;
        f++;
        n--;
    }
    return n;
}

int copy_mount_options(const void __user * data, unsigned long *where)
{
    int i;
    unsigned long page;
    unsigned long size;

    *where = 0;
    if (!data)
        return 0;

    if (!(page = __get_free_page(GFP_KERNEL)))
        return -ENOMEM;

    /* We only care that *some* data at the address the user
     * gave us is valid.  Just in case, we'll zero
     * the remainder of the page.
     */
    /* copy_from_user cannot cross TASK_SIZE ! */
    size = TASK_SIZE - (unsigned long)data;
    if (size > PAGE_SIZE)
        size = PAGE_SIZE;

    i = size - exact_copy_from_user((void *)page, data, size);
    if (!i) {
        free_page(page);
        return -EFAULT;
    }
    if (i != PAGE_SIZE)
        memset((char *)page + i, 0, PAGE_SIZE - i);
    *where = page;
    return 0;
}

int copy_mount_string(const void __user *data, char **where)
{
    char *tmp;

    if (!data) {
        *where = NULL;
        return 0;
    }

    tmp = strndup_user(data, PAGE_SIZE);
    if (IS_ERR(tmp))
        return PTR_ERR(tmp);

    *where = tmp;
    return 0;
}

/*
 * Flags is a 32-bit value that allows up to 31 non-fs dependent flags to
 * be given to the mount() call (ie: read-only, no-dev, no-suid etc).
 *
 * data is a (void *) that can point to any structure up to
 * PAGE_SIZE-1 bytes, which can contain arbitrary fs-dependent
 * information (or be NULL).
 *
 * Pre-0.97 versions of mount() didn't have a flags word.
 * When the flags word was introduced its top half was required
 * to have the magic value 0xC0ED, and this remained so until 2.4.0-test9.
 * Therefore, if this magic number is present, it carries no information
 * and must be discarded.
 */
long do_mount(const char *dev_name, const char *dir_name,
        const char *type_page, unsigned long flags, void *data_page)
{
    struct path path;
    int retval = 0;
    int mnt_flags = 0;

    /* Discard magic */
    if ((flags & MS_MGC_MSK) == MS_MGC_VAL)
        flags &= ~MS_MGC_MSK;

    /* Basic sanity checks */

    if (!dir_name || !*dir_name || !memchr(dir_name, 0, PAGE_SIZE))
        return -EINVAL;

    if (data_page)
        ((char *)data_page)[PAGE_SIZE - 1] = 0;

    /* ... and get the mountpoint */
    retval = kern_path(dir_name, LOOKUP_FOLLOW, &path);
    if (retval)
        return retval;

    retval = security_sb_mount(dev_name, &path,
                   type_page, flags, data_page);
    if (retval)
        goto dput_out;

    /* Default to relatime unless overriden */
    if (!(flags & MS_NOATIME))
        mnt_flags |= MNT_RELATIME;

    /* Separate the per-mountpoint flags */
    if (flags & MS_NOSUID)
        mnt_flags |= MNT_NOSUID;
    if (flags & MS_NODEV)
        mnt_flags |= MNT_NODEV;
    if (flags & MS_NOEXEC)
        mnt_flags |= MNT_NOEXEC;
    if (flags & MS_NOATIME)
        mnt_flags |= MNT_NOATIME;
    if (flags & MS_NODIRATIME)
        mnt_flags |= MNT_NODIRATIME;
    if (flags & MS_STRICTATIME)
        mnt_flags &= ~(MNT_RELATIME | MNT_NOATIME);
    if (flags & MS_RDONLY)
        mnt_flags |= MNT_READONLY;

    flags &= ~(MS_NOSUID | MS_NOEXEC | MS_NODEV | MS_ACTIVE | MS_BORN |
           MS_NOATIME | MS_NODIRATIME | MS_RELATIME| MS_KERNMOUNT |
           MS_STRICTATIME);

    if (flags & MS_REMOUNT)
        retval = do_remount(&path, flags & ~MS_REMOUNT, mnt_flags,
                    data_page);
    else if (flags & MS_BIND)
        retval = do_loopback(&path, dev_name, flags & MS_REC);
    else if (flags & (MS_SHARED | MS_PRIVATE | MS_SLAVE | MS_UNBINDABLE))
        retval = do_change_type(&path, flags);
    else if (flags & MS_MOVE)
        retval = do_move_mount(&path, dev_name);
    else
        retval = do_new_mount(&path, type_page, flags, mnt_flags,
                      dev_name, data_page);
dput_out:
    path_put(&path);
    return retval;
}

static struct mnt_namespace *alloc_mnt_ns(void)
{
    struct mnt_namespace *new_ns;

    new_ns = kmalloc(sizeof(struct mnt_namespace), GFP_KERNEL);
    if (!new_ns)
        return ERR_PTR(-ENOMEM);
    atomic_set(&new_ns->count, 1);
    new_ns->root = NULL;
    INIT_LIST_HEAD(&new_ns->list);
    init_waitqueue_head(&new_ns->poll);
    new_ns->event = 0;
    return new_ns;
}

/*
 * Allocate a new namespace structure and populate it with contents
 * copied from the namespace of the passed in task structure.
 */
static struct mnt_namespace *dup_mnt_ns(struct mnt_namespace *mnt_ns,
        struct fs_struct *fs)
{
    struct mnt_namespace *new_ns;
    struct vfsmount *rootmnt = NULL, *pwdmnt = NULL;
    struct mount *p, *q;
    struct mount *old = mnt_ns->root;
    struct mount *new;

    new_ns = alloc_mnt_ns();
    if (IS_ERR(new_ns))
        return new_ns;

    down_write(&namespace_sem);
    /* First pass: copy the tree topology */
    new = copy_tree(old, old->mnt.mnt_root, CL_COPY_ALL | CL_EXPIRE);
    if (IS_ERR(new)) {
        up_write(&namespace_sem);
        kfree(new_ns);
        return ERR_CAST(new);
    }
    new_ns->root = new;
    br_write_lock(&vfsmount_lock);
    list_add_tail(&new_ns->list, &new->mnt_list);
    br_write_unlock(&vfsmount_lock);

    /*
     * Second pass: switch the tsk->fs->* elements and mark new vfsmounts
     * as belonging to new namespace.  We have already acquired a private
     * fs_struct, so tsk->fs->lock is not needed.
     */
    p = old;
    q = new;
    while (p) {
        q->mnt_ns = new_ns;
        if (fs) {
            if (&p->mnt == fs->root.mnt) {
                fs->root.mnt = mntget(&q->mnt);
                rootmnt = &p->mnt;
            }
            if (&p->mnt == fs->pwd.mnt) {
                fs->pwd.mnt = mntget(&q->mnt);
                pwdmnt = &p->mnt;
            }
        }
        p = next_mnt(p, old);
        q = next_mnt(q, new);
    }
    up_write(&namespace_sem);

    if (rootmnt)
        mntput(rootmnt);
    if (pwdmnt)
        mntput(pwdmnt);

    return new_ns;
}

struct mnt_namespace *copy_mnt_ns(unsigned long flags, struct mnt_namespace *ns,
        struct fs_struct *new_fs)
{
    struct mnt_namespace *new_ns;

    BUG_ON(!ns);
    get_mnt_ns(ns);

    if (!(flags & CLONE_NEWNS))
        return ns;

    new_ns = dup_mnt_ns(ns, new_fs);

    put_mnt_ns(ns);
    return new_ns;
}

static struct mnt_namespace *create_mnt_ns(struct vfsmount *m)
{
    struct mnt_namespace *new_ns = alloc_mnt_ns();
    if (!IS_ERR(new_ns)) {
        struct mount *mnt = real_mount(m);
        mnt->mnt_ns = new_ns;
        new_ns->root = mnt;
        list_add(&new_ns->list, &mnt->mnt_list);
    } else {
        mntput(m);
    }
    return new_ns;
}

struct dentry *mount_subtree(struct vfsmount *mnt, const char *name)
{
    struct mnt_namespace *ns;
    struct super_block *s;
    struct path path;
    int err;

    ns = create_mnt_ns(mnt);
    if (IS_ERR(ns))
        return ERR_CAST(ns);

    err = vfs_path_lookup(mnt->mnt_root, mnt,
            name, LOOKUP_FOLLOW|LOOKUP_AUTOMOUNT, &path);

    put_mnt_ns(ns);

    if (err)
        return ERR_PTR(err);

    /* trade a vfsmount reference for active sb one */
    s = path.mnt->mnt_sb;
    atomic_inc(&s->s_active);
    mntput(path.mnt);
    /* lock the sucker */
    down_write(&s->s_umount);
    /* ... and return the root of (sub)tree on it */
    return path.dentry;
}
EXPORT_SYMBOL(mount_subtree);

SYSCALL_DEFINE5(mount, char __user *, dev_name, char __user *, dir_name,
        char __user *, type, unsigned long, flags, void __user *, data)
{
    int ret;
    char *kernel_type;
    struct filename *kernel_dir;
    char *kernel_dev;
    unsigned long data_page;

    ret = copy_mount_string(type, &kernel_type);
    if (ret < 0)
        goto out_type;

    kernel_dir = getname(dir_name);
    if (IS_ERR(kernel_dir)) {
        ret = PTR_ERR(kernel_dir);
        goto out_dir;
    }

    ret = copy_mount_string(dev_name, &kernel_dev);
    if (ret < 0)
        goto out_dev;

    ret = copy_mount_options(data, &data_page);
    if (ret < 0)
        goto out_data;

    ret = do_mount(kernel_dev, kernel_dir->name, kernel_type, flags,
        (void *) data_page);

    free_page(data_page);
out_data:
    kfree(kernel_dev);
out_dev:
    putname(kernel_dir);
out_dir:
    kfree(kernel_type);
out_type:
    return ret;
}

/*
 * Return true if path is reachable from root
 *
 * namespace_sem or vfsmount_lock is held
 */
bool is_path_reachable(struct mount *mnt, struct dentry *dentry,
             const struct path *root)
{
    while (&mnt->mnt != root->mnt && mnt_has_parent(mnt)) {
        dentry = mnt->mnt_mountpoint;
        mnt = mnt->mnt_parent;
    }
    return &mnt->mnt == root->mnt && is_subdir(dentry, root->dentry);
}

int path_is_under(struct path *path1, struct path *path2)
{
    int res;
    br_read_lock(&vfsmount_lock);
    res = is_path_reachable(real_mount(path1->mnt), path1->dentry, path2);
    br_read_unlock(&vfsmount_lock);
    return res;
}
EXPORT_SYMBOL(path_is_under);

/*
 * pivot_root Semantics:
 * Moves the root file system of the current process to the directory put_old,
 * makes new_root as the new root file system of the current process, and sets
 * root/cwd of all processes which had them on the current root to new_root.
 *
 * Restrictions:
 * The new_root and put_old must be directories, and  must not be on the
 * same file  system as the current process root. The put_old  must  be
 * underneath new_root,  i.e. adding a non-zero number of /.. to the string
 * pointed to by put_old must yield the same directory as new_root. No other
 * file system may be mounted on put_old. After all, new_root is a mountpoint.
 *
 * Also, the current root cannot be on the 'rootfs' (initial ramfs) filesystem.
 * See Documentation/filesystems/ramfs-rootfs-initramfs.txt for alternatives
 * in this situation.
 *
 * Notes:
 *  - we don't move root/cwd if they are not at the root (reason: if something
 *    cared enough to change them, it's probably wrong to force them elsewhere)
 *  - it's okay to pick a root that isn't the root of a file system, e.g.
 *    /nfs/my_root where /nfs is the mount point. It must be a mountpoint,
 *    though, so you may need to say mount --bind /nfs/my_root /nfs/my_root
 *    first.
 */
SYSCALL_DEFINE2(pivot_root, const char __user *, new_root,
        const char __user *, put_old)
{
    struct path new, old, parent_path, root_parent, root;
    struct mount *new_mnt, *root_mnt;
    int error;

    if (!capable(CAP_SYS_ADMIN))
        return -EPERM;

    error = user_path_dir(new_root, &new);
    if (error)
        goto out0;

    error = user_path_dir(put_old, &old);
    if (error)
        goto out1;

    error = security_sb_pivotroot(&old, &new);
    if (error)
        goto out2;

    get_fs_root(current->fs, &root);
    error = lock_mount(&old);
    if (error)
        goto out3;

    error = -EINVAL;
    new_mnt = real_mount(new.mnt);
    root_mnt = real_mount(root.mnt);
    if (IS_MNT_SHARED(real_mount(old.mnt)) ||
        IS_MNT_SHARED(new_mnt->mnt_parent) ||
        IS_MNT_SHARED(root_mnt->mnt_parent))
        goto out4;
    if (!check_mnt(root_mnt) || !check_mnt(new_mnt))
        goto out4;
    error = -ENOENT;
    if (d_unlinked(new.dentry))
        goto out4;
    if (d_unlinked(old.dentry))
        goto out4;
    error = -EBUSY;
    if (new.mnt == root.mnt ||
        old.mnt == root.mnt)
        goto out4; /* loop, on the same file system  */
    error = -EINVAL;
    if (root.mnt->mnt_root != root.dentry)
        goto out4; /* not a mountpoint */
    if (!mnt_has_parent(root_mnt))
        goto out4; /* not attached */
    if (new.mnt->mnt_root != new.dentry)
        goto out4; /* not a mountpoint */
    if (!mnt_has_parent(new_mnt))
        goto out4; /* not attached */
    /* make sure we can reach put_old from new_root */
    if (!is_path_reachable(real_mount(old.mnt), old.dentry, &new))
        goto out4;
    br_write_lock(&vfsmount_lock);
    detach_mnt(new_mnt, &parent_path);
    detach_mnt(root_mnt, &root_parent);
    /* mount old root on put_old */
    attach_mnt(root_mnt, &old);
    /* mount new_root on / */
    attach_mnt(new_mnt, &root_parent);
    touch_mnt_namespace(current->nsproxy->mnt_ns);
    br_write_unlock(&vfsmount_lock);
    chroot_fs_refs(&root, &new);
    error = 0;
out4:
    unlock_mount(&old);
    if (!error) {
        path_put(&root_parent);
        path_put(&parent_path);
    }
out3:
    path_put(&root);
out2:
    path_put(&old);
out1:
    path_put(&new);
out0:
    return error;
}

static void __init init_mount_tree(void)
{
    struct vfsmount *mnt;
    struct mnt_namespace *ns;
    struct path root;

    mnt = do_kern_mount("rootfs", 0, "rootfs", NULL);
    if (IS_ERR(mnt))
        panic("Can't create rootfs");

    ns = create_mnt_ns(mnt);
    if (IS_ERR(ns))
        panic("Can't allocate initial namespace");

    init_task.nsproxy->mnt_ns = ns;
    get_mnt_ns(ns);

    root.mnt = mnt;
    root.dentry = mnt->mnt_root;

    set_fs_pwd(current->fs, &root);
    set_fs_root(current->fs, &root);
}

void __init mnt_init(void)
{
    unsigned u;
    int err;

    init_rwsem(&namespace_sem);

    mnt_cache = kmem_cache_create("mnt_cache", sizeof(struct mount),
            0, SLAB_HWCACHE_ALIGN | SLAB_PANIC, NULL);

    mount_hashtable = (struct list_head *)__get_free_page(GFP_ATOMIC);

    if (!mount_hashtable)
        panic("Failed to allocate mount hash table\n");

    printk(KERN_INFO "Mount-cache hash table entries: %lu\n", HASH_SIZE);

    for (u = 0; u < HASH_SIZE; u++)
        INIT_LIST_HEAD(&mount_hashtable[u]);

    br_lock_init(&vfsmount_lock);

    err = sysfs_init();
    if (err)
        printk(KERN_WARNING "%s: sysfs_init error: %d\n",
            __func__, err);
    fs_kobj = kobject_create_and_add("fs", NULL);
    if (!fs_kobj)
        printk(KERN_WARNING "%s: kobj create error\n", __func__);
    init_rootfs();
    init_mount_tree();
}

void put_mnt_ns(struct mnt_namespace *ns)
{
    LIST_HEAD(umount_list);

    if (!atomic_dec_and_test(&ns->count))
        return;
    down_write(&namespace_sem);
    br_write_lock(&vfsmount_lock);
    umount_tree(ns->root, 0, &umount_list);
    br_write_unlock(&vfsmount_lock);
    up_write(&namespace_sem);
    release_mounts(&umount_list);
    kfree(ns);
}

struct vfsmount *kern_mount_data(struct file_system_type *type, void *data)
{
    struct vfsmount *mnt;
    mnt = vfs_kern_mount(type, MS_KERNMOUNT, type->name, data);
    if (!IS_ERR(mnt)) {
        /*
         * it is a longterm mount, don't release mnt until
         * we unmount before file sys is unregistered
        */
        real_mount(mnt)->mnt_ns = MNT_NS_INTERNAL;
    }
    return mnt;
}
EXPORT_SYMBOL_GPL(kern_mount_data);

void kern_unmount(struct vfsmount *mnt)
{
    /* release long term mount so mount point can be released */
    if (!IS_ERR_OR_NULL(mnt)) {
        br_write_lock(&vfsmount_lock);
        real_mount(mnt)->mnt_ns = NULL;
        br_write_unlock(&vfsmount_lock);
        mntput(mnt);
    }
}
EXPORT_SYMBOL(kern_unmount);

bool our_mnt(struct vfsmount *mnt)
{
    return check_mnt(real_mount(mnt));
}