Initial import

1 files changed, 3296 insertions, 0 deletions

diff --git a/fs/namespace.c b/fs/namespace.c
new file mode 100644
index 000000000..18fe976ea
--- /dev/null
+++ b/fs/namespace.c
@@ -0,0 +1,3296 @@
+/*
+ *  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/user_namespace.h>
+#include <linux/namei.h>
+#include <linux/security.h>
+#include <linux/idr.h>
+#include <linux/init.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 <linux/proc_ns.h>
+#include <linux/magic.h>
+#include <linux/bootmem.h>
+#include <linux/task_work.h>
+#include "pnode.h"
+#include "internal.h"
+
+static unsigned int m_hash_mask __read_mostly;
+static unsigned int m_hash_shift __read_mostly;
+static unsigned int mp_hash_mask __read_mostly;
+static unsigned int mp_hash_shift __read_mostly;
+
+static __initdata unsigned long mhash_entries;
+static int __init set_mhash_entries(char *str)
+{
+	if (!str)
+		return 0;
+	mhash_entries = simple_strtoul(str, &str, 0);
+	return 1;
+}
+__setup("mhash_entries=", set_mhash_entries);
+
+static __initdata unsigned long mphash_entries;
+static int __init set_mphash_entries(char *str)
+{
+	if (!str)
+		return 0;
+	mphash_entries = simple_strtoul(str, &str, 0);
+	return 1;
+}
+__setup("mphash_entries=", set_mphash_entries);
+
+static u64 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 hlist_head *mount_hashtable __read_mostly;
+static struct hlist_head *mountpoint_hashtable __read_mostly;
+static struct kmem_cache *mnt_cache __read_mostly;
+static DECLARE_RWSEM(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.
+ */
+__cacheline_aligned_in_smp DEFINE_SEQLOCK(mount_lock);
+
+static inline struct hlist_head *m_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 >> m_hash_shift);
+	return &mount_hashtable[tmp & m_hash_mask];
+}
+
+static inline struct hlist_head *mp_hash(struct dentry *dentry)
+{
+	unsigned long tmp = ((unsigned long)dentry / L1_CACHE_BYTES);
+	tmp = tmp + (tmp >> mp_hash_shift);
+	return &mountpoint_hashtable[tmp & mp_hash_mask];
+}
+
+/*
+ * 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 void drop_mountpoint(struct fs_pin *p)
+{
+	struct mount *m = container_of(p, struct mount, mnt_umount);
+	dput(m->mnt_ex_mountpoint);
+	pin_remove(p);
+	mntput(&m->mnt);
+}
+
+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_const(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_HLIST_NODE(&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);
+		INIT_HLIST_NODE(&mnt->mnt_mp_list);
+#ifdef CONFIG_FSNOTIFY
+		INIT_HLIST_HEAD(&mnt->mnt_fsnotify_marks);
+#endif
+		init_fs_pin(&mnt->mnt_umount, drop_mountpoint);
+	}
+	return mnt;
+
+#ifdef CONFIG_SMP
+out_free_devname:
+	kfree_const(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.
+ */
+/**
+ * __mnt_want_write - get write access to a mount without freeze protection
+ * @m: the mount on which to take a write
+ *
+ * This tells the low-level filesystem that a write is about to be performed to
+ * it, and makes sure that writes are allowed (mnt it read-write) before
+ * returning success. This operation does not protect against filesystem being
+ * frozen. When the write operation is finished, __mnt_drop_write() must be
+ * called. This is effectively a refcount.
+ */
+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 (ACCESS_ONCE(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;
+}
+
+/**
+ * mnt_want_write - get write access to a mount
+ * @m: the mount on which to take a write
+ *
+ * This tells the low-level filesystem that a write is about to be performed to
+ * it, and makes sure that writes are allowed (mount is read-write, filesystem
+ * is not frozen) before returning success.  When the write operation is
+ * finished, mnt_drop_write() must be called.  This is effectively a refcount.
+ */
+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);
+
+/**
+ * mnt_clone_write - get write access to a mount
+ * @mnt: the mount on which to take a write
+ *
+ * This is effectively like mnt_want_write, except
+ * it must only be used to take an extra write reference
+ * on a mountpoint that we already know has a write reference
+ * on it. This allows some optimisation.
+ *
+ * After finished, mnt_drop_write must be called as usual to
+ * drop the reference.
+ */
+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);
+
+/**
+ * __mnt_want_write_file - get write access to a file's mount
+ * @file: the file who's mount on which to take a write
+ *
+ * This is like __mnt_want_write, but it takes a file and can
+ * do some optimisations if the file is open for write already
+ */
+int __mnt_want_write_file(struct file *file)
+{
+	if (!(file->f_mode & FMODE_WRITER))
+		return __mnt_want_write(file->f_path.mnt);
+	else
+		return mnt_clone_write(file->f_path.mnt);
+}
+
+/**
+ * mnt_want_write_file - get write access to a file's mount
+ * @file: the file who's mount on which to take a write
+ *
+ * This is like mnt_want_write, but it takes a file and can
+ * do some optimisations if the file is open for write already
+ */
+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);
+
+/**
+ * __mnt_drop_write - give up write access to a mount
+ * @mnt: the mount on which to give up write access
+ *
+ * Tells the low-level filesystem that we are done
+ * performing writes to it.  Must be matched with
+ * __mnt_want_write() call above.
+ */
+void __mnt_drop_write(struct vfsmount *mnt)
+{
+	preempt_disable();
+	mnt_dec_writers(real_mount(mnt));
+	preempt_enable();
+}
+EXPORT_SYMBOL_GPL(__mnt_drop_write);
+
+/**
+ * mnt_drop_write - give up write access to a mount
+ * @mnt: the mount on which to give up write access
+ *
+ * Tells the low-level filesystem that we are done performing writes to it and
+ * also allows filesystem to be frozen again.  Must be matched with
+ * mnt_want_write() call above.
+ */
+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;
+
+	lock_mount_hash();
+	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;
+	unlock_mount_hash();
+	return ret;
+}
+
+static void __mnt_unmake_readonly(struct mount *mnt)
+{
+	lock_mount_hash();
+	mnt->mnt.mnt_flags &= ~MNT_READONLY;
+	unlock_mount_hash();
+}
+
+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;
+
+	lock_mount_hash();
+	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;
+	}
+	unlock_mount_hash();
+
+	return err;
+}
+
+static void free_vfsmnt(struct mount *mnt)
+{
+	kfree_const(mnt->mnt_devname);
+#ifdef CONFIG_SMP
+	free_percpu(mnt->mnt_pcp);
+#endif
+	kmem_cache_free(mnt_cache, mnt);
+}
+
+static void delayed_free_vfsmnt(struct rcu_head *head)
+{
+	free_vfsmnt(container_of(head, struct mount, mnt_rcu));
+}
+
+/* call under rcu_read_lock */
+bool legitimize_mnt(struct vfsmount *bastard, unsigned seq)
+{
+	struct mount *mnt;
+	if (read_seqretry(&mount_lock, seq))
+		return false;
+	if (bastard == NULL)
+		return true;
+	mnt = real_mount(bastard);
+	mnt_add_count(mnt, 1);
+	if (likely(!read_seqretry(&mount_lock, seq)))
+		return true;
+	if (bastard->mnt_flags & MNT_SYNC_UMOUNT) {
+		mnt_add_count(mnt, -1);
+		return false;
+	}
+	rcu_read_unlock();
+	mntput(bastard);
+	rcu_read_lock();
+	return false;
+}
+
+/*
+ * find the first mount at @dentry on vfsmount @mnt.
+ * call under rcu_read_lock()
+ */
+struct mount *__lookup_mnt(struct vfsmount *mnt, struct dentry *dentry)
+{
+	struct hlist_head *head = m_hash(mnt, dentry);
+	struct mount *p;
+
+	hlist_for_each_entry_rcu(p, head, mnt_hash)
+		if (&p->mnt_parent->mnt == mnt && p->mnt_mountpoint == dentry)
+			return p;
+	return NULL;
+}
+
+/*
+ * find the last mount at @dentry on vfsmount @mnt.
+ * mount_lock must be held.
+ */
+struct mount *__lookup_mnt_last(struct vfsmount *mnt, struct dentry *dentry)
+{
+	struct mount *p, *res = NULL;
+	p = __lookup_mnt(mnt, dentry);
+	if (!p)
+		goto out;
+	if (!(p->mnt.mnt_flags & MNT_UMOUNT))
+		res = p;
+	hlist_for_each_entry_continue(p, mnt_hash) {
+		if (&p->mnt_parent->mnt != mnt || p->mnt_mountpoint != dentry)
+			break;
+		if (!(p->mnt.mnt_flags & MNT_UMOUNT))
+			res = p;
+	}
+out:
+	return res;
+}
+
+/*
+ * 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;
+	struct vfsmount *m;
+	unsigned seq;
+
+	rcu_read_lock();
+	do {
+		seq = read_seqbegin(&mount_lock);
+		child_mnt = __lookup_mnt(path->mnt, path->dentry);
+		m = child_mnt ? &child_mnt->mnt : NULL;
+	} while (!legitimize_mnt(m, seq));
+	rcu_read_unlock();
+	return m;
+}
+
+/*
+ * __is_local_mountpoint - Test to see if dentry is a mountpoint in the
+ *                         current mount namespace.
+ *
+ * The common case is dentries are not mountpoints at all and that
+ * test is handled inline.  For the slow case when we are actually
+ * dealing with a mountpoint of some kind, walk through all of the
+ * mounts in the current mount namespace and test to see if the dentry
+ * is a mountpoint.
+ *
+ * The mount_hashtable is not usable in the context because we
+ * need to identify all mounts that may be in the current mount
+ * namespace not just a mount that happens to have some specified
+ * parent mount.
+ */
+bool __is_local_mountpoint(struct dentry *dentry)
+{
+	struct mnt_namespace *ns = current->nsproxy->mnt_ns;
+	struct mount *mnt;
+	bool is_covered = false;
+
+	if (!d_mountpoint(dentry))
+		goto out;
+
+	down_read(&namespace_sem);
+	list_for_each_entry(mnt, &ns->list, mnt_list) {
+		is_covered = (mnt->mnt_mountpoint == dentry);
+		if (is_covered)
+			break;
+	}
+	up_read(&namespace_sem);
+out:
+	return is_covered;
+}
+
+static struct mountpoint *lookup_mountpoint(struct dentry *dentry)
+{
+	struct hlist_head *chain = mp_hash(dentry);
+	struct mountpoint *mp;
+
+	hlist_for_each_entry(mp, chain, m_hash) {
+		if (mp->m_dentry == dentry) {
+			/* might be worth a WARN_ON() */
+			if (d_unlinked(dentry))
+				return ERR_PTR(-ENOENT);
+			mp->m_count++;
+			return mp;
+		}
+	}
+	return NULL;
+}
+
+static struct mountpoint *new_mountpoint(struct dentry *dentry)
+{
+	struct hlist_head *chain = mp_hash(dentry);
+	struct mountpoint *mp;
+	int ret;
+
+	mp = kmalloc(sizeof(struct mountpoint), GFP_KERNEL);
+	if (!mp)
+		return ERR_PTR(-ENOMEM);
+
+	ret = d_set_mounted(dentry);
+	if (ret) {
+		kfree(mp);
+		return ERR_PTR(ret);
+	}
+
+	mp->m_dentry = dentry;
+	mp->m_count = 1;
+	hlist_add_head(&mp->m_hash, chain);
+	INIT_HLIST_HEAD(&mp->m_list);
+	return mp;
+}
+
+static void put_mountpoint(struct mountpoint *mp)
+{
+	if (!--mp->m_count) {
+		struct dentry *dentry = mp->m_dentry;
+		BUG_ON(!hlist_empty(&mp->m_list));
+		spin_lock(&dentry->d_lock);
+		dentry->d_flags &= ~DCACHE_MOUNTED;
+		spin_unlock(&dentry->d_lock);
+		hlist_del(&mp->m_hash);
+		kfree(mp);
+	}
+}
+
+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);
+	}
+}
+
+/*
+ * vfsmount lock must be held for write
+ */
+static void unhash_mnt(struct mount *mnt)
+{
+	mnt->mnt_parent = mnt;
+	mnt->mnt_mountpoint = mnt->mnt.mnt_root;
+	list_del_init(&mnt->mnt_child);
+	hlist_del_init_rcu(&mnt->mnt_hash);
+	hlist_del_init(&mnt->mnt_mp_list);
+	put_mountpoint(mnt->mnt_mp);
+	mnt->mnt_mp = NULL;
+}
+
+/*
+ * 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;
+	unhash_mnt(mnt);
+}
+
+/*
+ * vfsmount lock must be held for write
+ */
+static void umount_mnt(struct mount *mnt)
+{
+	/* old mountpoint will be dropped when we can do that */
+	mnt->mnt_ex_mountpoint = mnt->mnt_mountpoint;
+	unhash_mnt(mnt);
+}
+
+/*
+ * vfsmount lock must be held for write
+ */
+void mnt_set_mountpoint(struct mount *mnt,
+			struct mountpoint *mp,
+			struct mount *child_mnt)
+{
+	mp->m_count++;
+	mnt_add_count(mnt, 1);	/* essentially, that's mntget */
+	child_mnt->mnt_mountpoint = dget(mp->m_dentry);
+	child_mnt->mnt_parent = mnt;
+	child_mnt->mnt_mp = mp;
+	hlist_add_head(&child_mnt->mnt_mp_list, &mp->m_list);
+}
+
+/*
+ * vfsmount lock must be held for write
+ */
+static void attach_mnt(struct mount *mnt,
+			struct mount *parent,
+			struct mountpoint *mp)
+{
+	mnt_set_mountpoint(parent, mp, mnt);
+	hlist_add_head_rcu(&mnt->mnt_hash, m_hash(&parent->mnt, mp->m_dentry));
+	list_add_tail(&mnt->mnt_child, &parent->mnt_mounts);
+}
+
+static void attach_shadowed(struct mount *mnt,
+			struct mount *parent,
+			struct mount *shadows)
+{
+	if (shadows) {
+		hlist_add_behind_rcu(&mnt->mnt_hash, &shadows->mnt_hash);
+		list_add(&mnt->mnt_child, &shadows->mnt_child);
+	} else {
+		hlist_add_head_rcu(&mnt->mnt_hash,
+				m_hash(&parent->mnt, mnt->mnt_mountpoint));
+		list_add_tail(&mnt->mnt_child, &parent->mnt_mounts);
+	}
+}
+
+/*
+ * vfsmount lock must be held for write
+ */
+static void commit_tree(struct mount *mnt, struct mount *shadows)
+{
+	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);
+
+	attach_shadowed(mnt, parent, shadows);
+	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)) {
+		mnt_free_id(mnt);
+		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;
+	lock_mount_hash();
+	list_add_tail(&mnt->mnt_instance, &root->d_sb->s_mounts);
+	unlock_mount_hash();
+	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 | CL_SHARED_TO_SLAVE))
+		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|MNT_MARKED);
+	/* Don't allow unprivileged users to change mount flags */
+	if (flag & CL_UNPRIVILEGED) {
+		mnt->mnt.mnt_flags |= MNT_LOCK_ATIME;
+
+		if (mnt->mnt.mnt_flags & MNT_READONLY)
+			mnt->mnt.mnt_flags |= MNT_LOCK_READONLY;
+
+		if (mnt->mnt.mnt_flags & MNT_NODEV)
+			mnt->mnt.mnt_flags |= MNT_LOCK_NODEV;
+
+		if (mnt->mnt.mnt_flags & MNT_NOSUID)
+			mnt->mnt.mnt_flags |= MNT_LOCK_NOSUID;
+
+		if (mnt->mnt.mnt_flags & MNT_NOEXEC)
+			mnt->mnt.mnt_flags |= MNT_LOCK_NOEXEC;
+	}
+
+	/* Don't allow unprivileged users to reveal what is under a mount */
+	if ((flag & CL_UNPRIVILEGED) &&
+	    (!(flag & CL_EXPIRE) || list_empty(&old->mnt_expire)))
+		mnt->mnt.mnt_flags |= MNT_LOCKED;
+
+	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;
+	lock_mount_hash();
+	list_add_tail(&mnt->mnt_instance, &sb->s_mounts);
+	unlock_mount_hash();
+
+	if ((flag & CL_SLAVE) ||
+	    ((flag & CL_SHARED_TO_SLAVE) && IS_MNT_SHARED(old))) {
+		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:
+	mnt_free_id(mnt);
+	free_vfsmnt(mnt);
+	return ERR_PTR(err);
+}
+
+static void cleanup_mnt(struct mount *mnt)
+{
+	/*
+	 * 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));
+	if (unlikely(mnt->mnt_pins.first))
+		mnt_pin_kill(mnt);
+	fsnotify_vfsmount_delete(&mnt->mnt);
+	dput(mnt->mnt.mnt_root);
+	deactivate_super(mnt->mnt.mnt_sb);
+	mnt_free_id(mnt);
+	call_rcu(&mnt->mnt_rcu, delayed_free_vfsmnt);
+}
+
+static void __cleanup_mnt(struct rcu_head *head)
+{
+	cleanup_mnt(container_of(head, struct mount, mnt_rcu));
+}
+
+static LLIST_HEAD(delayed_mntput_list);
+static void delayed_mntput(struct work_struct *unused)
+{
+	struct llist_node *node = llist_del_all(&delayed_mntput_list);
+	struct llist_node *next;
+
+	for (; node; node = next) {
+		next = llist_next(node);
+		cleanup_mnt(llist_entry(node, struct mount, mnt_llist));
+	}
+}
+static DECLARE_DELAYED_WORK(delayed_mntput_work, delayed_mntput);
+
+static void mntput_no_expire(struct mount *mnt)
+{
+	rcu_read_lock();
+	mnt_add_count(mnt, -1);
+	if (likely(mnt->mnt_ns)) { /* shouldn't be the last one */
+		rcu_read_unlock();
+		return;
+	}
+	lock_mount_hash();
+	if (mnt_get_count(mnt)) {
+		rcu_read_unlock();
+		unlock_mount_hash();
+		return;
+	}
+	if (unlikely(mnt->mnt.mnt_flags & MNT_DOOMED)) {
+		rcu_read_unlock();
+		unlock_mount_hash();
+		return;
+	}
+	mnt->mnt.mnt_flags |= MNT_DOOMED;
+	rcu_read_unlock();
+
+	list_del(&mnt->mnt_instance);
+
+	if (unlikely(!list_empty(&mnt->mnt_mounts))) {
+		struct mount *p, *tmp;
+		list_for_each_entry_safe(p, tmp, &mnt->mnt_mounts,  mnt_child) {
+			umount_mnt(p);
+		}
+	}
+	unlock_mount_hash();
+
+	if (likely(!(mnt->mnt.mnt_flags & MNT_INTERNAL))) {
+		struct task_struct *task = current;
+		if (likely(!(task->flags & PF_KTHREAD))) {
+			init_task_work(&mnt->mnt_rcu, __cleanup_mnt);
+			if (!task_work_add(task, &mnt->mnt_rcu, true))
+				return;
+		}
+		if (llist_add(&mnt->mnt_llist, &delayed_mntput_list))
+			schedule_delayed_work(&delayed_mntput_work, 1);
+		return;
+	}
+	cleanup_mnt(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);
+
+struct vfsmount *mnt_clone_internal(struct path *path)
+{
+	struct mount *p;
+	p = clone_mnt(real_mount(path->mnt), path->dentry, CL_PRIVATE);
+	if (IS_ERR(p))
+		return ERR_CAST(p);
+	p->mnt.mnt_flags |= MNT_INTERNAL;
+	return &p->mnt;
+}
+
+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);
+	if (p->cached_event == p->ns->event) {
+		void *v = p->cached_mount;
+		if (*pos == p->cached_index)
+			return v;
+		if (*pos == p->cached_index + 1) {
+			v = seq_list_next(v, &p->ns->list, &p->cached_index);
+			return p->cached_mount = v;
+		}
+	}
+
+	p->cached_event = p->ns->event;
+	p->cached_mount = seq_list_start(&p->ns->list, *pos);
+	p->cached_index = *pos;
+	return p->cached_mount;
+}
+
+static void *m_next(struct seq_file *m, void *v, loff_t *pos)
+{
+	struct proc_mounts *p = proc_mounts(m);
+
+	p->cached_mount = seq_list_next(v, &p->ns->list, pos);
+	p->cached_index = *pos;
+	return p->cached_mount;
+}
+
+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 */
+
+/**
+ * may_umount_tree - check if a mount tree is busy
+ * @mnt: root of mount tree
+ *
+ * This is called to check if a tree of mounts has any
+ * open files, pwds, chroots or sub mounts that are
+ * busy.
+ */
+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 */
+	lock_mount_hash();
+	for (p = mnt; p; p = next_mnt(p, mnt)) {
+		actual_refs += mnt_get_count(p);
+		minimum_refs += 2;
+	}
+	unlock_mount_hash();
+
+	if (actual_refs > minimum_refs)
+		return 0;
+
+	return 1;
+}
+
+EXPORT_SYMBOL(may_umount_tree);
+
+/**
+ * may_umount - check if a mount point is busy
+ * @mnt: root of mount
+ *
+ * This is called to check if a mount point has any
+ * open files, pwds, chroots or sub mounts. If the
+ * mount has sub mounts this will return busy
+ * regardless of whether the sub mounts are busy.
+ *
+ * Doesn't take quota and stuff into account. IOW, in some cases it will
+ * give false negatives. The main reason why it's here is that we need
+ * a non-destructive way to look for easily umountable filesystems.
+ */
+int may_umount(struct vfsmount *mnt)
+{
+	int ret = 1;
+	down_read(&namespace_sem);
+	lock_mount_hash();
+	if (propagate_mount_busy(real_mount(mnt), 2))
+		ret = 0;
+	unlock_mount_hash();
+	up_read(&namespace_sem);
+	return ret;
+}
+
+EXPORT_SYMBOL(may_umount);
+
+static HLIST_HEAD(unmounted);	/* protected by namespace_sem */
+
+static void namespace_unlock(void)
+{
+	struct hlist_head head;
+
+	hlist_move_list(&unmounted, &head);
+
+	up_write(&namespace_sem);
+
+	if (likely(hlist_empty(&head)))
+		return;
+
+	synchronize_rcu();
+
+	group_pin_kill(&head);
+}
+
+static inline void namespace_lock(void)
+{
+	down_write(&namespace_sem);
+}
+
+enum umount_tree_flags {
+	UMOUNT_SYNC = 1,
+	UMOUNT_PROPAGATE = 2,
+	UMOUNT_CONNECTED = 4,
+};
+/*
+ * mount_lock must be held
+ * namespace_sem must be held for write
+ */
+static void umount_tree(struct mount *mnt, enum umount_tree_flags how)
+{
+	LIST_HEAD(tmp_list);
+	struct mount *p;
+
+	if (how & UMOUNT_PROPAGATE)
+		propagate_mount_unlock(mnt);
+
+	/* Gather the mounts to umount */
+	for (p = mnt; p; p = next_mnt(p, mnt)) {
+		p->mnt.mnt_flags |= MNT_UMOUNT;
+		list_move(&p->mnt_list, &tmp_list);
+	}
+
+	/* Hide the mounts from mnt_mounts */
+	list_for_each_entry(p, &tmp_list, mnt_list) {
+		list_del_init(&p->mnt_child);
+	}
+
+	/* Add propogated mounts to the tmp_list */
+	if (how & UMOUNT_PROPAGATE)
+		propagate_umount(&tmp_list);
+
+	while (!list_empty(&tmp_list)) {
+		bool disconnect;
+		p = list_first_entry(&tmp_list, struct mount, mnt_list);
+		list_del_init(&p->mnt_expire);
+		list_del_init(&p->mnt_list);
+		__touch_mnt_namespace(p->mnt_ns);
+		p->mnt_ns = NULL;
+		if (how & UMOUNT_SYNC)
+			p->mnt.mnt_flags |= MNT_SYNC_UMOUNT;
+
+		disconnect = !(((how & UMOUNT_CONNECTED) &&
+				mnt_has_parent(p) &&
+				(p->mnt_parent->mnt.mnt_flags & MNT_UMOUNT)) ||
+			       IS_MNT_LOCKED_AND_LAZY(p));
+
+		pin_insert_group(&p->mnt_umount, &p->mnt_parent->mnt,
+				 disconnect ? &unmounted : NULL);
+		if (mnt_has_parent(p)) {
+			mnt_add_count(p->mnt_parent, -1);
+			if (!disconnect) {
+				/* Don't forget about p */
+				list_add_tail(&p->mnt_child, &p->mnt_parent->mnt_mounts);
+			} else {
+				umount_mnt(p);
+			}
+		}
+		change_mnt_propagation(p, MS_PRIVATE);
+	}
+}
+
+static void shrink_submounts(struct mount *mnt);
+
+static int do_umount(struct mount *mnt, int flags)
+{
+	struct super_block *sb = mnt->mnt.mnt_sb;
+	int retval;
+
+	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.
+		 */
+		lock_mount_hash();
+		if (mnt_get_count(mnt) != 2) {
+			unlock_mount_hash();
+			return -EBUSY;
+		}
+		unlock_mount_hash();
+
+		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.
+		 */
+		if (!capable(CAP_SYS_ADMIN))
+			return -EPERM;
+		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;
+	}
+
+	namespace_lock();
+	lock_mount_hash();
+	event++;
+
+	if (flags & MNT_DETACH) {
+		if (!list_empty(&mnt->mnt_list))
+			umount_tree(mnt, UMOUNT_PROPAGATE);
+		retval = 0;
+	} else {
+		shrink_submounts(mnt);
+		retval = -EBUSY;
+		if (!propagate_mount_busy(mnt, 2)) {
+			if (!list_empty(&mnt->mnt_list))
+				umount_tree(mnt, UMOUNT_PROPAGATE|UMOUNT_SYNC);
+			retval = 0;
+		}
+	}
+	unlock_mount_hash();
+	namespace_unlock();
+	return retval;
+}
+
+/*
+ * __detach_mounts - lazily unmount all mounts on the specified dentry
+ *
+ * During unlink, rmdir, and d_drop it is possible to loose the path
+ * to an existing mountpoint, and wind up leaking the mount.
+ * detach_mounts allows lazily unmounting those mounts instead of
+ * leaking them.
+ *
+ * The caller may hold dentry->d_inode->i_mutex.
+ */
+void __detach_mounts(struct dentry *dentry)
+{
+	struct mountpoint *mp;
+	struct mount *mnt;
+
+	namespace_lock();
+	mp = lookup_mountpoint(dentry);
+	if (IS_ERR_OR_NULL(mp))
+		goto out_unlock;
+
+	lock_mount_hash();
+	while (!hlist_empty(&mp->m_list)) {
+		mnt = hlist_entry(mp->m_list.first, struct mount, mnt_mp_list);
+		if (mnt->mnt.mnt_flags & MNT_UMOUNT) {
+			struct mount *p, *tmp;
+			list_for_each_entry_safe(p, tmp, &mnt->mnt_mounts,  mnt_child) {
+				hlist_add_head(&p->mnt_umount.s_list, &unmounted);
+				umount_mnt(p);
+			}
+		}
+		else umount_tree(mnt, UMOUNT_CONNECTED);
+	}
+	unlock_mount_hash();
+	put_mountpoint(mp);
+out_unlock:
+	namespace_unlock();
+}
+
+/* 
+ * Is the caller allowed to modify his namespace?
+ */
+static inline bool may_mount(void)
+{
+	return ns_capable(current->nsproxy->mnt_ns->user_ns, CAP_SYS_ADMIN);
+}
+
+/*
+ * 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 (!may_mount())
+		return -EPERM;
+
+	if (!(flags & UMOUNT_NOFOLLOW))
+		lookup_flags |= LOOKUP_FOLLOW;
+
+	retval = user_path_mountpoint_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;
+	if (mnt->mnt.mnt_flags & MNT_LOCKED)
+		goto dput_and_out;
+	retval = -EPERM;
+	if (flags & MNT_FORCE && !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 bool is_mnt_ns_file(struct dentry *dentry)
+{
+	/* Is this a proxy for a mount namespace? */
+	return dentry->d_op == &ns_dentry_operations &&
+	       dentry->d_fsdata == &mntns_operations;
+}
+
+struct mnt_namespace *to_mnt_ns(struct ns_common *ns)
+{
+	return container_of(ns, struct mnt_namespace, ns);
+}
+
+static bool mnt_ns_loop(struct dentry *dentry)
+{
+	/* Could bind mounting the mount namespace inode cause a
+	 * mount namespace loop?
+	 */
+	struct mnt_namespace *mnt_ns;
+	if (!is_mnt_ns_file(dentry))
+		return false;
+
+	mnt_ns = to_mnt_ns(get_proc_ns(dentry->d_inode));
+	return current->nsproxy->mnt_ns->seq >= mnt_ns->seq;
+}
+
+struct mount *copy_tree(struct mount *mnt, struct dentry *dentry,
+					int flag)
+{
+	struct mount *res, *p, *q, *r, *parent;
+
+	if (!(flag & CL_COPY_UNBINDABLE) && IS_MNT_UNBINDABLE(mnt))
+		return ERR_PTR(-EINVAL);
+
+	if (!(flag & CL_COPY_MNT_NS_FILE) && is_mnt_ns_file(dentry))
+		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)) {
+			struct mount *t = NULL;
+			if (!(flag & CL_COPY_UNBINDABLE) &&
+			    IS_MNT_UNBINDABLE(s)) {
+				s = skip_mnt_tree(s);
+				continue;
+			}
+			if (!(flag & CL_COPY_MNT_NS_FILE) &&
+			    is_mnt_ns_file(s->mnt.mnt_root)) {
+				s = skip_mnt_tree(s);
+				continue;
+			}
+			while (p != s->mnt_parent) {
+				p = p->mnt_parent;
+				q = q->mnt_parent;
+			}
+			p = s;
+			parent = q;
+			q = clone_mnt(p, p->mnt.mnt_root, flag);
+			if (IS_ERR(q))
+				goto out;
+			lock_mount_hash();
+			list_add_tail(&q->mnt_list, &res->mnt_list);
+			mnt_set_mountpoint(parent, p->mnt_mp, q);
+			if (!list_empty(&parent->mnt_mounts)) {
+				t = list_last_entry(&parent->mnt_mounts,
+					struct mount, mnt_child);
+				if (t->mnt_mp != p->mnt_mp)
+					t = NULL;
+			}
+			attach_shadowed(q, parent, t);
+			unlock_mount_hash();
+		}
+	}
+	return res;
+out:
+	if (res) {
+		lock_mount_hash();
+		umount_tree(res, UMOUNT_SYNC);
+		unlock_mount_hash();
+	}
+	return q;
+}
+
+/* Caller should check returned pointer for errors */
+
+struct vfsmount *collect_mounts(struct path *path)
+{
+	struct mount *tree;
+	namespace_lock();
+	if (!check_mnt(real_mount(path->mnt)))
+		tree = ERR_PTR(-EINVAL);
+	else
+		tree = copy_tree(real_mount(path->mnt), path->dentry,
+				 CL_COPY_ALL | CL_PRIVATE);
+	namespace_unlock();
+	if (IS_ERR(tree))
+		return ERR_CAST(tree);
+	return &tree->mnt;
+}
+
+void drop_collected_mounts(struct vfsmount *mnt)
+{
+	namespace_lock();
+	lock_mount_hash();
+	umount_tree(real_mount(mnt), UMOUNT_SYNC);
+	unlock_mount_hash();
+	namespace_unlock();
+}
+
+/**
+ * clone_private_mount - create a private clone of a path
+ *
+ * This creates a new vfsmount, which will be the clone of @path.  The new will
+ * not be attached anywhere in the namespace and will be private (i.e. changes
+ * to the originating mount won't be propagated into this).
+ *
+ * Release with mntput().
+ */
+struct vfsmount *clone_private_mount(struct path *path)
+{
+	struct mount *old_mnt = real_mount(path->mnt);
+	struct mount *new_mnt;
+
+	if (IS_MNT_UNBINDABLE(old_mnt))
+		return ERR_PTR(-EINVAL);
+
+	down_read(&namespace_sem);
+	new_mnt = clone_mnt(old_mnt, path->dentry, CL_PRIVATE);
+	up_read(&namespace_sem);
+	if (IS_ERR(new_mnt))
+		return ERR_CAST(new_mnt);
+
+	return &new_mnt->mnt;
+}
+EXPORT_SYMBOL_GPL(clone_private_mount);
+
+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;
+}
+EXPORT_SYMBOL(iterate_mounts);
+
+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 mount *dest_mnt,
+			struct mountpoint *dest_mp,
+			struct path *parent_path)
+{
+	HLIST_HEAD(tree_list);
+	struct mount *child, *p;
+	struct hlist_node *n;
+	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_mp, source_mnt, &tree_list);
+		lock_mount_hash();
+		if (err)
+			goto out_cleanup_ids;
+		for (p = source_mnt; p; p = next_mnt(p, source_mnt))
+			set_mnt_shared(p);
+	} else {
+		lock_mount_hash();
+	}
+	if (parent_path) {
+		detach_mnt(source_mnt, parent_path);
+		attach_mnt(source_mnt, dest_mnt, dest_mp);
+		touch_mnt_namespace(source_mnt->mnt_ns);
+	} else {
+		mnt_set_mountpoint(dest_mnt, dest_mp, source_mnt);
+		commit_tree(source_mnt, NULL);
+	}
+
+	hlist_for_each_entry_safe(child, n, &tree_list, mnt_hash) {
+		struct mount *q;
+		hlist_del_init(&child->mnt_hash);
+		q = __lookup_mnt_last(&child->mnt_parent->mnt,
+				      child->mnt_mountpoint);
+		commit_tree(child, q);
+	}
+	unlock_mount_hash();
+
+	return 0;
+
+ out_cleanup_ids:
+	while (!hlist_empty(&tree_list)) {
+		child = hlist_entry(tree_list.first, struct mount, mnt_hash);
+		umount_tree(child, UMOUNT_SYNC);
+	}
+	unlock_mount_hash();
+	cleanup_group_ids(source_mnt, NULL);
+ out:
+	return err;
+}
+
+static struct mountpoint *lock_mount(struct path *path)
+{
+	struct vfsmount *mnt;
+	struct dentry *dentry = path->dentry;
+retry:
+	mutex_lock(&dentry->d_inode->i_mutex);
+	if (unlikely(cant_mount(dentry))) {
+		mutex_unlock(&dentry->d_inode->i_mutex);
+		return ERR_PTR(-ENOENT);
+	}
+	namespace_lock();
+	mnt = lookup_mnt(path);
+	if (likely(!mnt)) {
+		struct mountpoint *mp = lookup_mountpoint(dentry);
+		if (!mp)
+			mp = new_mountpoint(dentry);
+		if (IS_ERR(mp)) {
+			namespace_unlock();
+			mutex_unlock(&dentry->d_inode->i_mutex);
+			return mp;
+		}
+		return mp;
+	}
+	namespace_unlock();
+	mutex_unlock(&path->dentry->d_inode->i_mutex);
+	path_put(path);
+	path->mnt = mnt;
+	dentry = path->dentry = dget(mnt->mnt_root);
+	goto retry;
+}
+
+static void unlock_mount(struct mountpoint *where)
+{
+	struct dentry *dentry = where->m_dentry;
+	put_mountpoint(where);
+	namespace_unlock();
+	mutex_unlock(&dentry->d_inode->i_mutex);
+}
+
+static int graft_tree(struct mount *mnt, struct mount *p, struct mountpoint *mp)
+{
+	if (mnt->mnt.mnt_sb->s_flags & MS_NOUSER)
+		return -EINVAL;
+
+	if (d_is_dir(mp->m_dentry) !=
+	      d_is_dir(mnt->mnt.mnt_root))
+		return -ENOTDIR;
+
+	return attach_recursive_mnt(mnt, p, mp, 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 (path->dentry != path->mnt->mnt_root)
+		return -EINVAL;
+
+	type = flags_to_propagation_type(flag);
+	if (!type)
+		return -EINVAL;
+
+	namespace_lock();
+	if (type == MS_SHARED) {
+		err = invent_group_ids(mnt, recurse);
+		if (err)
+			goto out_unlock;
+	}
+
+	lock_mount_hash();
+	for (m = mnt; m; m = (recurse ? next_mnt(m, mnt) : NULL))
+		change_mnt_propagation(m, type);
+	unlock_mount_hash();
+
+ out_unlock:
+	namespace_unlock();
+	return err;
+}
+
+static bool has_locked_children(struct mount *mnt, struct dentry *dentry)
+{
+	struct mount *child;
+	list_for_each_entry(child, &mnt->mnt_mounts, mnt_child) {
+		if (!is_subdir(child->mnt_mountpoint, dentry))
+			continue;
+
+		if (child->mnt.mnt_flags & MNT_LOCKED)
+			return true;
+	}
+	return false;
+}
+
+/*
+ * do loopback mount.
+ */
+static int do_loopback(struct path *path, const char *old_name,
+				int recurse)
+{
+	struct path old_path;
+	struct mount *mnt = NULL, *old, *parent;
+	struct mountpoint *mp;
+	int err;
+	if (!old_name || !*old_name)
+		return -EINVAL;
+	err = kern_path(old_name, LOOKUP_FOLLOW|LOOKUP_AUTOMOUNT, &old_path);
+	if (err)
+		return err;
+
+	err = -EINVAL;
+	if (mnt_ns_loop(old_path.dentry))
+		goto out; 
+
+	mp = lock_mount(path);
+	err = PTR_ERR(mp);
+	if (IS_ERR(mp))
+		goto out;
+
+	old = real_mount(old_path.mnt);
+	parent = real_mount(path->mnt);
+
+	err = -EINVAL;
+	if (IS_MNT_UNBINDABLE(old))
+		goto out2;
+
+	if (!check_mnt(parent))
+		goto out2;
+
+	if (!check_mnt(old) && old_path.dentry->d_op != &ns_dentry_operations)
+		goto out2;
+
+	if (!recurse && has_locked_children(old, old_path.dentry))
+		goto out2;
+
+	if (recurse)
+		mnt = copy_tree(old, old_path.dentry, CL_COPY_MNT_NS_FILE);
+	else
+		mnt = clone_mnt(old, old_path.dentry, 0);
+
+	if (IS_ERR(mnt)) {
+		err = PTR_ERR(mnt);
+		goto out2;
+	}
+
+	mnt->mnt.mnt_flags &= ~MNT_LOCKED;
+
+	err = graft_tree(mnt, parent, mp);
+	if (err) {
+		lock_mount_hash();
+		umount_tree(mnt, UMOUNT_SYNC);
+		unlock_mount_hash();
+	}
+out2:
+	unlock_mount(mp);
+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 (!check_mnt(mnt))
+		return -EINVAL;
+
+	if (path->dentry != path->mnt->mnt_root)
+		return -EINVAL;
+
+	/* Don't allow changing of locked mnt flags.
+	 *
+	 * No locks need to be held here while testing the various
+	 * MNT_LOCK flags because those flags can never be cleared
+	 * once they are set.
+	 */
+	if ((mnt->mnt.mnt_flags & MNT_LOCK_READONLY) &&
+	    !(mnt_flags & MNT_READONLY)) {
+		return -EPERM;
+	}
+	if ((mnt->mnt.mnt_flags & MNT_LOCK_NODEV) &&
+	    !(mnt_flags & MNT_NODEV)) {
+		/* Was the nodev implicitly added in mount? */
+		if ((mnt->mnt_ns->user_ns != &init_user_ns) &&
+		    !(sb->s_type->fs_flags & FS_USERNS_DEV_MOUNT)) {
+			mnt_flags |= MNT_NODEV;
+		} else {
+			return -EPERM;
+		}
+	}
+	if ((mnt->mnt.mnt_flags & MNT_LOCK_NOSUID) &&
+	    !(mnt_flags & MNT_NOSUID)) {
+		return -EPERM;
+	}
+	if ((mnt->mnt.mnt_flags & MNT_LOCK_NOEXEC) &&
+	    !(mnt_flags & MNT_NOEXEC)) {
+		return -EPERM;
+	}
+	if ((mnt->mnt.mnt_flags & MNT_LOCK_ATIME) &&
+	    ((mnt->mnt.mnt_flags & MNT_ATIME_MASK) != (mnt_flags & MNT_ATIME_MASK))) {
+		return -EPERM;
+	}
+
+	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 if (!capable(CAP_SYS_ADMIN))
+		err = -EPERM;
+	else
+		err = do_remount_sb(sb, flags, data, 0);
+	if (!err) {
+		lock_mount_hash();
+		mnt_flags |= mnt->mnt.mnt_flags & ~MNT_USER_SETTABLE_MASK;
+		mnt->mnt.mnt_flags = mnt_flags;
+		touch_mnt_namespace(mnt->mnt_ns);
+		unlock_mount_hash();
+	}
+	up_write(&sb->s_umount);
+	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;
+	struct mountpoint *mp;
+	int err;
+	if (!old_name || !*old_name)
+		return -EINVAL;
+	err = kern_path(old_name, LOOKUP_FOLLOW, &old_path);
+	if (err)
+		return err;
+
+	mp = lock_mount(path);
+	err = PTR_ERR(mp);
+	if (IS_ERR(mp))
+		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 (old->mnt.mnt_flags & MNT_LOCKED)
+		goto out1;
+
+	err = -EINVAL;
+	if (old_path.dentry != old_path.mnt->mnt_root)
+		goto out1;
+
+	if (!mnt_has_parent(old))
+		goto out1;
+
+	if (d_is_dir(path->dentry) !=
+	      d_is_dir(old_path.dentry))
+		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, real_mount(path->mnt), mp, &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(mp);
+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);
+}
+
+/*
+ * add a mount into a namespace's mount tree
+ */
+static int do_add_mount(struct mount *newmnt, struct path *path, int mnt_flags)
+{
+	struct mountpoint *mp;
+	struct mount *parent;
+	int err;
+
+	mnt_flags &= ~MNT_INTERNAL_FLAGS;
+
+	mp = lock_mount(path);
+	if (IS_ERR(mp))
+		return PTR_ERR(mp);
+
+	parent = real_mount(path->mnt);
+	err = -EINVAL;
+	if (unlikely(!check_mnt(parent))) {
+		/* 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 (!parent->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 (d_is_symlink(newmnt->mnt.mnt_root))
+		goto unlock;
+
+	newmnt->mnt.mnt_flags = mnt_flags;
+	err = graft_tree(newmnt, parent, mp);
+
+unlock:
+	unlock_mount(mp);
+	return err;
+}
+
+static bool fs_fully_visible(struct file_system_type *fs_type, int *new_mnt_flags);
+
+/*
+ * 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 *fstype, int flags,
+			int mnt_flags, const char *name, void *data)
+{
+	struct file_system_type *type;
+	struct user_namespace *user_ns = current->nsproxy->mnt_ns->user_ns;
+	struct vfsmount *mnt;
+	int err;
+
+	if (!fstype)
+		return -EINVAL;
+
+	type = get_fs_type(fstype);
+	if (!type)
+		return -ENODEV;
+
+	if (user_ns != &init_user_ns) {
+		if (!(type->fs_flags & FS_USERNS_MOUNT)) {
+			put_filesystem(type);
+			return -EPERM;
+		}
+		/* Only in special cases allow devices from mounts
+		 * created outside the initial user namespace.
+		 */
+		if (!(type->fs_flags & FS_USERNS_DEV_MOUNT)) {
+			flags |= MS_NODEV;
+			mnt_flags |= MNT_NODEV | MNT_LOCK_NODEV;
+		}
+		if (type->fs_flags & FS_USERNS_VISIBLE) {
+			if (!fs_fully_visible(type, &mnt_flags))
+				return -EPERM;
+		}
+	}
+
+	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);
+	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)) {
+		namespace_lock();
+		list_del_init(&mnt->mnt_expire);
+		namespace_unlock();
+	}
+	mntput(m);
+	mntput(m);
+	return err;
+}
+
+/**
+ * mnt_set_expiry - Put a mount on an expiration list
+ * @mnt: The mount to list.
+ * @expiry_list: The list to add the mount to.
+ */
+void mnt_set_expiry(struct vfsmount *mnt, struct list_head *expiry_list)
+{
+	namespace_lock();
+
+	list_add_tail(&real_mount(mnt)->mnt_expire, expiry_list);
+
+	namespace_unlock();
+}
+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);
+
+	if (list_empty(mounts))
+		return;
+
+	namespace_lock();
+	lock_mount_hash();
+
+	/* 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, UMOUNT_PROPAGATE|UMOUNT_SYNC);
+	}
+	unlock_mount_hash();
+	namespace_unlock();
+}
+
+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
+ *
+ * mount_lock must be held for write
+ */
+static void shrink_submounts(struct mount *mnt)
+{
+	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, UMOUNT_PROPAGATE|UMOUNT_SYNC);
+		}
+	}
+}
+
+/*
+ * 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;
+}
+
+char *copy_mount_string(const void __user *data)
+{
+	return data ? strndup_user(data, PAGE_SIZE) : NULL;
+}
+
+/*
+ * 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 __user *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 (data_page)
+		((char *)data_page)[PAGE_SIZE - 1] = 0;
+
+	/* ... and get the mountpoint */
+	retval = user_path(dir_name, &path);
+	if (retval)
+		return retval;
+
+	retval = security_sb_mount(dev_name, &path,
+				   type_page, flags, data_page);
+	if (!retval && !may_mount())
+		retval = -EPERM;
+	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;
+
+	/* The default atime for remount is preservation */
+	if ((flags & MS_REMOUNT) &&
+	    ((flags & (MS_NOATIME | MS_NODIRATIME | MS_RELATIME |
+		       MS_STRICTATIME)) == 0)) {
+		mnt_flags &= ~MNT_ATIME_MASK;
+		mnt_flags |= path.mnt->mnt_flags & MNT_ATIME_MASK;
+	}
+
+	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 void free_mnt_ns(struct mnt_namespace *ns)
+{
+	ns_free_inum(&ns->ns);
+	put_user_ns(ns->user_ns);
+	kfree(ns);
+}
+
+/*
+ * Assign a sequence number so we can detect when we attempt to bind
+ * mount a reference to an older mount namespace into the current
+ * mount namespace, preventing reference counting loops.  A 64bit
+ * number incrementing at 10Ghz will take 12,427 years to wrap which
+ * is effectively never, so we can ignore the possibility.
+ */
+static atomic64_t mnt_ns_seq = ATOMIC64_INIT(1);
+
+static struct mnt_namespace *alloc_mnt_ns(struct user_namespace *user_ns)
+{
+	struct mnt_namespace *new_ns;
+	int ret;
+
+	new_ns = kmalloc(sizeof(struct mnt_namespace), GFP_KERNEL);
+	if (!new_ns)
+		return ERR_PTR(-ENOMEM);
+	ret = ns_alloc_inum(&new_ns->ns);
+	if (ret) {
+		kfree(new_ns);
+		return ERR_PTR(ret);
+	}
+	new_ns->ns.ops = &mntns_operations;
+	new_ns->seq = atomic64_add_return(1, &mnt_ns_seq);
+	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;
+	new_ns->user_ns = get_user_ns(user_ns);
+	return new_ns;
+}
+
+struct mnt_namespace *copy_mnt_ns(unsigned long flags, struct mnt_namespace *ns,
+		struct user_namespace *user_ns, struct fs_struct *new_fs)
+{
+	struct mnt_namespace *new_ns;
+	struct vfsmount *rootmnt = NULL, *pwdmnt = NULL;
+	struct mount *p, *q;
+	struct mount *old;
+	struct mount *new;
+	int copy_flags;
+
+	BUG_ON(!ns);
+
+	if (likely(!(flags & CLONE_NEWNS))) {
+		get_mnt_ns(ns);
+		return ns;
+	}
+
+	old = ns->root;
+
+	new_ns = alloc_mnt_ns(user_ns);
+	if (IS_ERR(new_ns))
+		return new_ns;
+
+	namespace_lock();
+	/* First pass: copy the tree topology */
+	copy_flags = CL_COPY_UNBINDABLE | CL_EXPIRE;
+	if (user_ns != ns->user_ns)
+		copy_flags |= CL_SHARED_TO_SLAVE | CL_UNPRIVILEGED;
+	new = copy_tree(old, old->mnt.mnt_root, copy_flags);
+	if (IS_ERR(new)) {
+		namespace_unlock();
+		free_mnt_ns(new_ns);
+		return ERR_CAST(new);
+	}
+	new_ns->root = new;
+	list_add_tail(&new_ns->list, &new->mnt_list);
+
+	/*
+	 * 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 (new_fs) {
+			if (&p->mnt == new_fs->root.mnt) {
+				new_fs->root.mnt = mntget(&q->mnt);
+				rootmnt = &p->mnt;
+			}
+			if (&p->mnt == new_fs->pwd.mnt) {
+				new_fs->pwd.mnt = mntget(&q->mnt);
+				pwdmnt = &p->mnt;
+			}
+		}
+		p = next_mnt(p, old);
+		q = next_mnt(q, new);
+		if (!q)
+			break;
+		while (p->mnt.mnt_root != q->mnt.mnt_root)
+			p = next_mnt(p, old);
+	}
+	namespace_unlock();
+
+	if (rootmnt)
+		mntput(rootmnt);
+	if (pwdmnt)
+		mntput(pwdmnt);
+
+	return new_ns;
+}
+
+/**
+ * create_mnt_ns - creates a private namespace and adds a root filesystem
+ * @mnt: pointer to the new root filesystem mountpoint
+ */
+static struct mnt_namespace *create_mnt_ns(struct vfsmount *m)
+{
+	struct mnt_namespace *new_ns = alloc_mnt_ns(&init_user_ns);
+	if (!IS_ERR(new_ns)) {
+		struct mount *mnt = real_mount(m);
+		mnt->mnt_ns = new_ns;
+		new_ns->root = mnt;
+		list_add(&mnt->mnt_list, &new_ns->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;
+	char *kernel_dev;
+	unsigned long data_page;
+
+	kernel_type = copy_mount_string(type);
+	ret = PTR_ERR(kernel_type);
+	if (IS_ERR(kernel_type))
+		goto out_type;
+
+	kernel_dev = copy_mount_string(dev_name);
+	ret = PTR_ERR(kernel_dev);
+	if (IS_ERR(kernel_dev))
+		goto out_dev;
+
+	ret = copy_mount_options(data, &data_page);
+	if (ret < 0)
+		goto out_data;
+
+	ret = do_mount(kernel_dev, dir_name, kernel_type, flags,
+		(void *) data_page);
+
+	free_page(data_page);
+out_data:
+	kfree(kernel_dev);
+out_dev:
+	kfree(kernel_type);
+out_type:
+	return ret;
+}
+
+/*
+ * Return true if path is reachable from root
+ *
+ * namespace_sem or mount_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;
+	read_seqlock_excl(&mount_lock);
+	res = is_path_reachable(real_mount(path1->mnt), path1->dentry, path2);
+	read_sequnlock_excl(&mount_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, *old_mnt;
+	struct mountpoint *old_mp, *root_mp;
+	int error;
+
+	if (!may_mount())
+		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);
+	old_mp = lock_mount(&old);
+	error = PTR_ERR(old_mp);
+	if (IS_ERR(old_mp))
+		goto out3;
+
+	error = -EINVAL;
+	new_mnt = real_mount(new.mnt);
+	root_mnt = real_mount(root.mnt);
+	old_mnt = real_mount(old.mnt);
+	if (IS_MNT_SHARED(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;
+	if (new_mnt->mnt.mnt_flags & MNT_LOCKED)
+		goto out4;
+	error = -ENOENT;
+	if (d_unlinked(new.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 */
+	root_mp = root_mnt->mnt_mp;
+	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(old_mnt, old.dentry, &new))
+		goto out4;
+	/* make certain new is below the root */
+	if (!is_path_reachable(new_mnt, new.dentry, &root))
+		goto out4;
+	root_mp->m_count++; /* pin it so it won't go away */
+	lock_mount_hash();
+	detach_mnt(new_mnt, &parent_path);
+	detach_mnt(root_mnt, &root_parent);
+	if (root_mnt->mnt.mnt_flags & MNT_LOCKED) {
+		new_mnt->mnt.mnt_flags |= MNT_LOCKED;
+		root_mnt->mnt.mnt_flags &= ~MNT_LOCKED;
+	}
+	/* mount old root on put_old */
+	attach_mnt(root_mnt, old_mnt, old_mp);
+	/* mount new_root on / */
+	attach_mnt(new_mnt, real_mount(root_parent.mnt), root_mp);
+	touch_mnt_namespace(current->nsproxy->mnt_ns);
+	/* A moved mount should not expire automatically */
+	list_del_init(&new_mnt->mnt_expire);
+	unlock_mount_hash();
+	chroot_fs_refs(&root, &new);
+	put_mountpoint(root_mp);
+	error = 0;
+out4:
+	unlock_mount(old_mp);
+	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;
+	struct file_system_type *type;
+
+	type = get_fs_type("rootfs");
+	if (!type)
+		panic("Can't find rootfs type");
+	mnt = vfs_kern_mount(type, 0, "rootfs", NULL);
+	put_filesystem(type);
+	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;
+	mnt->mnt_flags |= MNT_LOCKED;
+
+	set_fs_pwd(current->fs, &root);
+	set_fs_root(current->fs, &root);
+}
+
+void __init mnt_init(void)
+{
+	unsigned u;
+	int err;
+
+	mnt_cache = kmem_cache_create("mnt_cache", sizeof(struct mount),
+			0, SLAB_HWCACHE_ALIGN | SLAB_PANIC, NULL);
+
+	mount_hashtable = alloc_large_system_hash("Mount-cache",
+				sizeof(struct hlist_head),
+				mhash_entries, 19,
+				0,
+				&m_hash_shift, &m_hash_mask, 0, 0);
+	mountpoint_hashtable = alloc_large_system_hash("Mountpoint-cache",
+				sizeof(struct hlist_head),
+				mphash_entries, 19,
+				0,
+				&mp_hash_shift, &mp_hash_mask, 0, 0);
+
+	if (!mount_hashtable || !mountpoint_hashtable)
+		panic("Failed to allocate mount hash table\n");
+
+	for (u = 0; u <= m_hash_mask; u++)
+		INIT_HLIST_HEAD(&mount_hashtable[u]);
+	for (u = 0; u <= mp_hash_mask; u++)
+		INIT_HLIST_HEAD(&mountpoint_hashtable[u]);
+
+	kernfs_init();
+
+	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)
+{
+	if (!atomic_dec_and_test(&ns->count))
+		return;
+	drop_collected_mounts(&ns->root->mnt);
+	free_mnt_ns(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)) {
+		real_mount(mnt)->mnt_ns = NULL;
+		synchronize_rcu();	/* yecchhh... */
+		mntput(mnt);
+	}
+}
+EXPORT_SYMBOL(kern_unmount);
+
+bool our_mnt(struct vfsmount *mnt)
+{
+	return check_mnt(real_mount(mnt));
+}
+
+bool current_chrooted(void)
+{
+	/* Does the current process have a non-standard root */
+	struct path ns_root;
+	struct path fs_root;
+	bool chrooted;
+
+	/* Find the namespace root */
+	ns_root.mnt = &current->nsproxy->mnt_ns->root->mnt;
+	ns_root.dentry = ns_root.mnt->mnt_root;
+	path_get(&ns_root);
+	while (d_mountpoint(ns_root.dentry) && follow_down_one(&ns_root))
+		;
+
+	get_fs_root(current->fs, &fs_root);
+
+	chrooted = !path_equal(&fs_root, &ns_root);
+
+	path_put(&fs_root);
+	path_put(&ns_root);
+
+	return chrooted;
+}
+
+static bool fs_fully_visible(struct file_system_type *type, int *new_mnt_flags)
+{
+	struct mnt_namespace *ns = current->nsproxy->mnt_ns;
+	int new_flags = *new_mnt_flags;
+	struct mount *mnt;
+	bool visible = false;
+
+	if (unlikely(!ns))
+		return false;
+
+	down_read(&namespace_sem);
+	list_for_each_entry(mnt, &ns->list, mnt_list) {
+		struct mount *child;
+		if (mnt->mnt.mnt_sb->s_type != type)
+			continue;
+
+		/* This mount is not fully visible if it's root directory
+		 * is not the root directory of the filesystem.
+		 */
+		if (mnt->mnt.mnt_root != mnt->mnt.mnt_sb->s_root)
+			continue;
+
+		/* Verify the mount flags are equal to or more permissive
+		 * than the proposed new mount.
+		 */
+		if ((mnt->mnt.mnt_flags & MNT_LOCK_READONLY) &&
+		    !(new_flags & MNT_READONLY))
+			continue;
+		if ((mnt->mnt.mnt_flags & MNT_LOCK_NODEV) &&
+		    !(new_flags & MNT_NODEV))
+			continue;
+		if ((mnt->mnt.mnt_flags & MNT_LOCK_ATIME) &&
+		    ((mnt->mnt.mnt_flags & MNT_ATIME_MASK) != (new_flags & MNT_ATIME_MASK)))
+			continue;
+
+		/* This mount is not fully visible if there are any
+		 * locked child mounts that cover anything except for
+		 * empty directories.
+		 */
+		list_for_each_entry(child, &mnt->mnt_mounts, mnt_child) {
+			struct inode *inode = child->mnt_mountpoint->d_inode;
+			/* Only worry about locked mounts */
+			if (!(mnt->mnt.mnt_flags & MNT_LOCKED))
+				continue;
+			/* Is the directory permanetly empty? */
+			if (!is_empty_dir_inode(inode))
+				goto next;
+		}
+		/* Preserve the locked attributes */
+		*new_mnt_flags |= mnt->mnt.mnt_flags & (MNT_LOCK_READONLY | \
+							MNT_LOCK_NODEV    | \
+							MNT_LOCK_ATIME);
+		visible = true;
+		goto found;
+	next:	;
+	}
+found:
+	up_read(&namespace_sem);
+	return visible;
+}
+
+static struct ns_common *mntns_get(struct task_struct *task)
+{
+	struct ns_common *ns = NULL;
+	struct nsproxy *nsproxy;
+
+	task_lock(task);
+	nsproxy = task->nsproxy;
+	if (nsproxy) {
+		ns = &nsproxy->mnt_ns->ns;
+		get_mnt_ns(to_mnt_ns(ns));
+	}
+	task_unlock(task);
+
+	return ns;
+}
+
+static void mntns_put(struct ns_common *ns)
+{
+	put_mnt_ns(to_mnt_ns(ns));
+}
+
+static int mntns_install(struct nsproxy *nsproxy, struct ns_common *ns)
+{
+	struct fs_struct *fs = current->fs;
+	struct mnt_namespace *mnt_ns = to_mnt_ns(ns);
+	struct path root;
+
+	if (!ns_capable(mnt_ns->user_ns, CAP_SYS_ADMIN) ||
+	    !ns_capable(current_user_ns(), CAP_SYS_CHROOT) ||
+	    !ns_capable(current_user_ns(), CAP_SYS_ADMIN))
+		return -EPERM;
+
+	if (fs->users != 1)
+		return -EINVAL;
+
+	get_mnt_ns(mnt_ns);
+	put_mnt_ns(nsproxy->mnt_ns);
+	nsproxy->mnt_ns = mnt_ns;
+
+	/* Find the root */
+	root.mnt    = &mnt_ns->root->mnt;
+	root.dentry = mnt_ns->root->mnt.mnt_root;
+	path_get(&root);
+	while(d_mountpoint(root.dentry) && follow_down_one(&root))
+		;
+
+	/* Update the pwd and root */
+	set_fs_pwd(fs, &root);
+	set_fs_root(fs, &root);
+
+	path_put(&root);
+	return 0;
+}
+
+const struct proc_ns_operations mntns_operations = {
+	.name		= "mnt",
+	.type		= CLONE_NEWNS,
+	.get		= mntns_get,
+	.put		= mntns_put,
+	.install	= mntns_install,
+};