4 files changed, 350 insertions, 50 deletions

diff --git a/fs/namespace.c b/fs/namespace.c
index f91a60e6039d..382edc76aee7 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -926,6 +926,33 @@ void mnt_set_mountpoint(struct mount *mnt,
 	hlist_add_head(&child_mnt->mnt_mp_list, &mp->m_list);
 }
 
+/**
+ * mnt_set_mountpoint_beneath - mount a mount beneath another one
+ *
+ * @new_parent: the source mount
+ * @top_mnt:    the mount beneath which @new_parent is mounted
+ * @new_mp:     the new mountpoint of @top_mnt on @new_parent
+ *
+ * Remove @top_mnt from its current mountpoint @top_mnt->mnt_mp and
+ * parent @top_mnt->mnt_parent and mount it on top of @new_parent at
+ * @new_mp. And mount @new_parent on the old parent and old
+ * mountpoint of @top_mnt.
+ *
+ * Context: This function expects namespace_lock() and lock_mount_hash()
+ *          to have been acquired in that order.
+ */
+static void mnt_set_mountpoint_beneath(struct mount *new_parent,
+				       struct mount *top_mnt,
+				       struct mountpoint *new_mp)
+{
+	struct mount *old_top_parent = top_mnt->mnt_parent;
+	struct mountpoint *old_top_mp = top_mnt->mnt_mp;
+
+	mnt_set_mountpoint(old_top_parent, old_top_mp, new_parent);
+	mnt_change_mountpoint(new_parent, new_mp, top_mnt);
+}
+
+
 static void __attach_mnt(struct mount *mnt, struct mount *parent)
 {
 	hlist_add_head_rcu(&mnt->mnt_hash,
@@ -933,15 +960,42 @@ static void __attach_mnt(struct mount *mnt, struct mount *parent)
 	list_add_tail(&mnt->mnt_child, &parent->mnt_mounts);
 }
 
-/*
- * vfsmount lock must be held for write
+/**
+ * attach_mnt - mount a mount, attach to @mount_hashtable and parent's
+ *              list of child mounts
+ * @parent:  the parent
+ * @mnt:     the new mount
+ * @mp:      the new mountpoint
+ * @beneath: whether to mount @mnt beneath or on top of @parent
+ *
+ * If @beneath is false, mount @mnt at @mp on @parent. Then attach @mnt
+ * to @parent's child mount list and to @mount_hashtable.
+ *
+ * If @beneath is true, remove @mnt from its current parent and
+ * mountpoint and mount it on @mp on @parent, and mount @parent on the
+ * old parent and old mountpoint of @mnt. Finally, attach @parent to
+ * @mnt_hashtable and @parent->mnt_parent->mnt_mounts.
+ *
+ * Note, when __attach_mnt() is called @mnt->mnt_parent already points
+ * to the correct parent.
+ *
+ * Context: This function expects namespace_lock() and lock_mount_hash()
+ *          to have been acquired in that order.
  */
-static void attach_mnt(struct mount *mnt,
-			struct mount *parent,
-			struct mountpoint *mp)
+static void attach_mnt(struct mount *mnt, struct mount *parent,
+		       struct mountpoint *mp, bool beneath)
 {
-	mnt_set_mountpoint(parent, mp, mnt);
-	__attach_mnt(mnt, parent);
+	if (beneath)
+		mnt_set_mountpoint_beneath(mnt, parent, mp);
+	else
+		mnt_set_mountpoint(parent, mp, mnt);
+	/*
+	 * Note, @mnt->mnt_parent has to be used. If @mnt was mounted
+	 * beneath @parent then @mnt will need to be attached to
+	 * @parent's old parent, not @parent. IOW, @mnt->mnt_parent
+	 * isn't the same mount as @parent.
+	 */
+	__attach_mnt(mnt, mnt->mnt_parent);
 }
 
 void mnt_change_mountpoint(struct mount *parent, struct mountpoint *mp, struct mount *mnt)
@@ -953,7 +1007,7 @@ void mnt_change_mountpoint(struct mount *parent, struct mountpoint *mp, struct m
 	hlist_del_init(&mnt->mnt_mp_list);
 	hlist_del_init_rcu(&mnt->mnt_hash);
 
-	attach_mnt(mnt, parent, mp);
+	attach_mnt(mnt, parent, mp, false);
 
 	put_mountpoint(old_mp);
 	mnt_add_count(old_parent, -1);
@@ -1954,7 +2008,7 @@ struct mount *copy_tree(struct mount *mnt, struct dentry *dentry,
 				goto out;
 			lock_mount_hash();
 			list_add_tail(&q->mnt_list, &res->mnt_list);
-			attach_mnt(q, parent, p->mnt_mp);
+			attach_mnt(q, parent, p->mnt_mp, false);
 			unlock_mount_hash();
 		}
 	}
@@ -2163,12 +2217,17 @@ int count_mounts(struct mnt_namespace *ns, struct mount *mnt)
 	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)
+enum mnt_tree_flags_t {
+	MNT_TREE_MOVE = BIT(0),
+	MNT_TREE_BENEATH = BIT(1),
+};
+
+/**
+ * attach_recursive_mnt - attach a source mount tree
+ * @source_mnt: mount tree to be attached
+ * @top_mnt:    mount that @source_mnt will be mounted on or mounted beneath
+ * @dest_mp:    the mountpoint @source_mnt will be mounted at
+ * @flags:      modify how @source_mnt is supposed to be attached
  *
  *  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.
@@ -2225,22 +2284,28 @@ int count_mounts(struct mnt_namespace *ns, struct mount *mnt)
  * applied to each mount in the tree.
  * Must be called without spinlocks held, since this function can sleep
  * in allocations.
+ *
+ * Context: The function expects namespace_lock() to be held.
+ * Return: If @source_mnt was successfully attached 0 is returned.
+ *         Otherwise a negative error code is returned.
  */
 static int attach_recursive_mnt(struct mount *source_mnt,
-			struct mount *dest_mnt,
-			struct mountpoint *dest_mp,
-			bool moving)
+				struct mount *top_mnt,
+				struct mountpoint *dest_mp,
+				enum mnt_tree_flags_t flags)
 {
 	struct user_namespace *user_ns = current->nsproxy->mnt_ns->user_ns;
 	HLIST_HEAD(tree_list);
-	struct mnt_namespace *ns = dest_mnt->mnt_ns;
+	struct mnt_namespace *ns = top_mnt->mnt_ns;
 	struct mountpoint *smp;
-	struct mount *child, *p;
+	struct mount *child, *dest_mnt, *p;
 	struct hlist_node *n;
-	int err;
+	int err = 0;
+	bool moving = flags & MNT_TREE_MOVE, beneath = flags & MNT_TREE_BENEATH;
 
-	/* Preallocate a mountpoint in case the new mounts need
-	 * to be tucked under other mounts.
+	/*
+	 * Preallocate a mountpoint in case the new mounts need to be
+	 * mounted beneath mounts on the same mountpoint.
 	 */
 	smp = get_mountpoint(source_mnt->mnt.mnt_root);
 	if (IS_ERR(smp))
@@ -2253,29 +2318,41 @@ static int attach_recursive_mnt(struct mount *source_mnt,
 			goto out;
 	}
 
+	if (beneath)
+		dest_mnt = top_mnt->mnt_parent;
+	else
+		dest_mnt = top_mnt;
+
 	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;
+	}
+	lock_mount_hash();
+	if (err)
+		goto out_cleanup_ids;
+
+	if (IS_MNT_SHARED(dest_mnt)) {
 		for (p = source_mnt; p; p = next_mnt(p, source_mnt))
 			set_mnt_shared(p);
-	} else {
-		lock_mount_hash();
 	}
+
 	if (moving) {
+		if (beneath)
+			dest_mp = smp;
 		unhash_mnt(source_mnt);
-		attach_mnt(source_mnt, dest_mnt, dest_mp);
+		attach_mnt(source_mnt, top_mnt, dest_mp, beneath);
 		touch_mnt_namespace(source_mnt->mnt_ns);
 	} else {
 		if (source_mnt->mnt_ns) {
 			/* move from anon - the caller will destroy */
 			list_del_init(&source_mnt->mnt_ns->list);
 		}
-		mnt_set_mountpoint(dest_mnt, dest_mp, source_mnt);
+		if (beneath)
+			mnt_set_mountpoint_beneath(source_mnt, top_mnt, smp);
+		else
+			mnt_set_mountpoint(dest_mnt, dest_mp, source_mnt);
 		commit_tree(source_mnt);
 	}
 
@@ -2315,28 +2392,80 @@ static int attach_recursive_mnt(struct mount *source_mnt,
 	return err;
 }
 
-static struct mountpoint *lock_mount(struct path *path)
+/**
+ * do_lock_mount - lock mount and mountpoint
+ * @path:    target path
+ * @beneath: whether the intention is to mount beneath @path
+ *
+ * Follow the mount stack on @path until the top mount @mnt is found. If
+ * the initial @path->{mnt,dentry} is a mountpoint lookup the first
+ * mount stacked on top of it. Then simply follow @{mnt,mnt->mnt_root}
+ * until nothing is stacked on top of it anymore.
+ *
+ * Acquire the inode_lock() on the top mount's ->mnt_root to protect
+ * against concurrent removal of the new mountpoint from another mount
+ * namespace.
+ *
+ * If @beneath is requested, acquire inode_lock() on @mnt's mountpoint
+ * @mp on @mnt->mnt_parent must be acquired. This protects against a
+ * concurrent unlink of @mp->mnt_dentry from another mount namespace
+ * where @mnt doesn't have a child mount mounted @mp. A concurrent
+ * removal of @mnt->mnt_root doesn't matter as nothing will be mounted
+ * on top of it for @beneath.
+ *
+ * In addition, @beneath needs to make sure that @mnt hasn't been
+ * unmounted or moved from its current mountpoint in between dropping
+ * @mount_lock and acquiring @namespace_sem. For the !@beneath case @mnt
+ * being unmounted would be detected later by e.g., calling
+ * check_mnt(mnt) in the function it's called from. For the @beneath
+ * case however, it's useful to detect it directly in do_lock_mount().
+ * If @mnt hasn't been unmounted then @mnt->mnt_mountpoint still points
+ * to @mnt->mnt_mp->m_dentry. But if @mnt has been unmounted it will
+ * point to @mnt->mnt_root and @mnt->mnt_mp will be NULL.
+ *
+ * Return: Either the target mountpoint on the top mount or the top
+ *         mount's mountpoint.
+ */
+static struct mountpoint *do_lock_mount(struct path *path, bool beneath)
 {
-	struct vfsmount *mnt;
+	struct vfsmount *mnt = path->mnt;
 	struct dentry *dentry;
-	struct mountpoint *mp;
+	struct mountpoint *mp = ERR_PTR(-ENOENT);
 
 	for (;;) {
-		dentry = path->dentry;
+		struct mount *m;
+
+		if (beneath) {
+			m = real_mount(mnt);
+			read_seqlock_excl(&mount_lock);
+			dentry = dget(m->mnt_mountpoint);
+			read_sequnlock_excl(&mount_lock);
+		} else {
+			dentry = path->dentry;
+		}
+
 		inode_lock(dentry->d_inode);
 		if (unlikely(cant_mount(dentry))) {
 			inode_unlock(dentry->d_inode);
-			return ERR_PTR(-ENOENT);
+			goto out;
 		}
 
 		namespace_lock();
 
+		if (beneath && (!is_mounted(mnt) || m->mnt_mountpoint != dentry)) {
+			namespace_unlock();
+			inode_unlock(dentry->d_inode);
+			goto out;
+		}
+
 		mnt = lookup_mnt(path);
 		if (likely(!mnt))
 			break;
 
 		namespace_unlock();
 		inode_unlock(dentry->d_inode);
+		if (beneath)
+			dput(dentry);
 		path_put(path);
 		path->mnt = mnt;
 		path->dentry = dget(mnt->mnt_root);
@@ -2348,9 +2477,18 @@ static struct mountpoint *lock_mount(struct path *path)
 		inode_unlock(dentry->d_inode);
 	}
 
+out:
+	if (beneath)
+		dput(dentry);
+
 	return mp;
 }
 
+static inline struct mountpoint *lock_mount(struct path *path)
+{
+	return do_lock_mount(path, false);
+}
+
 static void unlock_mount(struct mountpoint *where)
 {
 	struct dentry *dentry = where->m_dentry;
@@ -2372,7 +2510,7 @@ static int graft_tree(struct mount *mnt, struct mount *p, struct mountpoint *mp)
 	      d_is_dir(mnt->mnt.mnt_root))
 		return -ENOTDIR;
 
-	return attach_recursive_mnt(mnt, p, mp, false);
+	return attach_recursive_mnt(mnt, p, mp, 0);
 }
 
 /*
@@ -2854,7 +2992,110 @@ out:
 	return err;
 }
 
-static int do_move_mount(struct path *old_path, struct path *new_path)
+/**
+ * path_overmounted - check if path is overmounted
+ * @path: path to check
+ *
+ * Check if path is overmounted, i.e., if there's a mount on top of
+ * @path->mnt with @path->dentry as mountpoint.
+ *
+ * Context: This function expects namespace_lock() to be held.
+ * Return: If path is overmounted true is returned, false if not.
+ */
+static inline bool path_overmounted(const struct path *path)
+{
+	rcu_read_lock();
+	if (unlikely(__lookup_mnt(path->mnt, path->dentry))) {
+		rcu_read_unlock();
+		return true;
+	}
+	rcu_read_unlock();
+	return false;
+}
+
+/**
+ * can_move_mount_beneath - check that we can mount beneath the top mount
+ * @from: mount to mount beneath
+ * @to:   mount under which to mount
+ *
+ * - Make sure that @to->dentry is actually the root of a mount under
+ *   which we can mount another mount.
+ * - Make sure that nothing can be mounted beneath the caller's current
+ *   root or the rootfs of the namespace.
+ * - Make sure that the caller can unmount the topmost mount ensuring
+ *   that the caller could reveal the underlying mountpoint.
+ * - Ensure that nothing has been mounted on top of @from before we
+ *   grabbed @namespace_sem to avoid creating pointless shadow mounts.
+ * - Prevent mounting beneath a mount if the propagation relationship
+ *   between the source mount, parent mount, and top mount would lead to
+ *   nonsensical mount trees.
+ *
+ * Context: This function expects namespace_lock() to be held.
+ * Return: On success 0, and on error a negative error code is returned.
+ */
+static int can_move_mount_beneath(const struct path *from,
+				  const struct path *to,
+				  const struct mountpoint *mp)
+{
+	struct mount *mnt_from = real_mount(from->mnt),
+		     *mnt_to = real_mount(to->mnt),
+		     *parent_mnt_to = mnt_to->mnt_parent;
+
+	if (!mnt_has_parent(mnt_to))
+		return -EINVAL;
+
+	if (!path_mounted(to))
+		return -EINVAL;
+
+	if (IS_MNT_LOCKED(mnt_to))
+		return -EINVAL;
+
+	/* Avoid creating shadow mounts during mount propagation. */
+	if (path_overmounted(from))
+		return -EINVAL;
+
+	/*
+	 * Mounting beneath the rootfs only makes sense when the
+	 * semantics of pivot_root(".", ".") are used.
+	 */
+	if (&mnt_to->mnt == current->fs->root.mnt)
+		return -EINVAL;
+	if (parent_mnt_to == current->nsproxy->mnt_ns->root)
+		return -EINVAL;
+
+	for (struct mount *p = mnt_from; mnt_has_parent(p); p = p->mnt_parent)
+		if (p == mnt_to)
+			return -EINVAL;
+
+	/*
+	 * If the parent mount propagates to the child mount this would
+	 * mean mounting @mnt_from on @mnt_to->mnt_parent and then
+	 * propagating a copy @c of @mnt_from on top of @mnt_to. This
+	 * defeats the whole purpose of mounting beneath another mount.
+	 */
+	if (propagation_would_overmount(parent_mnt_to, mnt_to, mp))
+		return -EINVAL;
+
+	/*
+	 * If @mnt_to->mnt_parent propagates to @mnt_from this would
+	 * mean propagating a copy @c of @mnt_from on top of @mnt_from.
+	 * Afterwards @mnt_from would be mounted on top of
+	 * @mnt_to->mnt_parent and @mnt_to would be unmounted from
+	 * @mnt->mnt_parent and remounted on @mnt_from. But since @c is
+	 * already mounted on @mnt_from, @mnt_to would ultimately be
+	 * remounted on top of @c. Afterwards, @mnt_from would be
+	 * covered by a copy @c of @mnt_from and @c would be covered by
+	 * @mnt_from itself. This defeats the whole purpose of mounting
+	 * @mnt_from beneath @mnt_to.
+	 */
+	if (propagation_would_overmount(parent_mnt_to, mnt_from, mp))
+		return -EINVAL;
+
+	return 0;
+}
+
+static int do_move_mount(struct path *old_path, struct path *new_path,
+			 bool beneath)
 {
 	struct mnt_namespace *ns;
 	struct mount *p;
@@ -2863,8 +3104,9 @@ static int do_move_mount(struct path *old_path, struct path *new_path)
 	struct mountpoint *mp, *old_mp;
 	int err;
 	bool attached;
+	enum mnt_tree_flags_t flags = 0;
 
-	mp = lock_mount(new_path);
+	mp = do_lock_mount(new_path, beneath);
 	if (IS_ERR(mp))
 		return PTR_ERR(mp);
 
@@ -2872,6 +3114,8 @@ static int do_move_mount(struct path *old_path, struct path *new_path)
 	p = real_mount(new_path->mnt);
 	parent = old->mnt_parent;
 	attached = mnt_has_parent(old);
+	if (attached)
+		flags |= MNT_TREE_MOVE;
 	old_mp = old->mnt_mp;
 	ns = old->mnt_ns;
 
@@ -2902,6 +3146,17 @@ static int do_move_mount(struct path *old_path, struct path *new_path)
 	 */
 	if (attached && IS_MNT_SHARED(parent))
 		goto out;
+
+	if (beneath) {
+		err = can_move_mount_beneath(old_path, new_path, mp);
+		if (err)
+			goto out;
+
+		err = -EINVAL;
+		p = p->mnt_parent;
+		flags |= MNT_TREE_BENEATH;
+	}
+
 	/*
 	 * Don't move a mount tree containing unbindable mounts to a destination
 	 * mount which is shared.
@@ -2915,8 +3170,7 @@ static int do_move_mount(struct path *old_path, struct path *new_path)
 		if (p == old)
 			goto out;
 
-	err = attach_recursive_mnt(old, real_mount(new_path->mnt), mp,
-				   attached);
+	err = attach_recursive_mnt(old, real_mount(new_path->mnt), mp, flags);
 	if (err)
 		goto out;
 
@@ -2948,7 +3202,7 @@ static int do_move_mount_old(struct path *path, const char *old_name)
 	if (err)
 		return err;
 
-	err = do_move_mount(&old_path, path);
+	err = do_move_mount(&old_path, path, false);
 	path_put(&old_path);
 	return err;
 }
@@ -3114,13 +3368,10 @@ int finish_automount(struct vfsmount *m, const struct path *path)
 		err = -ENOENT;
 		goto discard_locked;
 	}
-	rcu_read_lock();
-	if (unlikely(__lookup_mnt(path->mnt, dentry))) {
-		rcu_read_unlock();
+	if (path_overmounted(path)) {
 		err = 0;
 		goto discard_locked;
 	}
-	rcu_read_unlock();
 	mp = get_mountpoint(dentry);
 	if (IS_ERR(mp)) {
 		err = PTR_ERR(mp);
@@ -3812,6 +4063,10 @@ SYSCALL_DEFINE5(move_mount,
 	if (flags & ~MOVE_MOUNT__MASK)
 		return -EINVAL;
 
+	if ((flags & (MOVE_MOUNT_BENEATH | MOVE_MOUNT_SET_GROUP)) ==
+	    (MOVE_MOUNT_BENEATH | MOVE_MOUNT_SET_GROUP))
+		return -EINVAL;
+
 	/* If someone gives a pathname, they aren't permitted to move
 	 * from an fd that requires unmount as we can't get at the flag
 	 * to clear it afterwards.
@@ -3841,7 +4096,8 @@ SYSCALL_DEFINE5(move_mount,
 	if (flags & MOVE_MOUNT_SET_GROUP)
 		ret = do_set_group(&from_path, &to_path);
 	else
-		ret = do_move_mount(&from_path, &to_path);
+		ret = do_move_mount(&from_path, &to_path,
+				    (flags & MOVE_MOUNT_BENEATH));
 
 out_to:
 	path_put(&to_path);
@@ -3974,9 +4230,9 @@ SYSCALL_DEFINE2(pivot_root, const char __user *, new_root,
 		root_mnt->mnt.mnt_flags &= ~MNT_LOCKED;
 	}
 	/* mount old root on put_old */
-	attach_mnt(root_mnt, old_mnt, old_mp);
+	attach_mnt(root_mnt, old_mnt, old_mp, false);
 	/* mount new_root on / */
-	attach_mnt(new_mnt, root_parent, root_mp);
+	attach_mnt(new_mnt, root_parent, root_mp, false);
 	mnt_add_count(root_parent, -1);
 	touch_mnt_namespace(current->nsproxy->mnt_ns);
 	/* A moved mount should not expire automatically */
diff --git a/fs/pnode.c b/fs/pnode.c
index 3cede8b18c8b..e4d0340393d5 100644
--- a/fs/pnode.c
+++ b/fs/pnode.c
@@ -216,7 +216,7 @@ static struct mount *next_group(struct mount *m, struct mount *origin)
 static struct mount *last_dest, *first_source, *last_source, *dest_master;
 static struct hlist_head *list;
 
-static inline bool peers(struct mount *m1, struct mount *m2)
+static inline bool peers(const struct mount *m1, const struct mount *m2)
 {
 	return m1->mnt_group_id == m2->mnt_group_id && m1->mnt_group_id;
 }
@@ -354,6 +354,46 @@ static inline int do_refcount_check(struct mount *mnt, int count)
 	return mnt_get_count(mnt) > count;
 }
 
+/**
+ * propagation_would_overmount - check whether propagation from @from
+ *                               would overmount @to
+ * @from: shared mount
+ * @to:   mount to check
+ * @mp:   future mountpoint of @to on @from
+ *
+ * If @from propagates mounts to @to, @from and @to must either be peers
+ * or one of the masters in the hierarchy of masters of @to must be a
+ * peer of @from.
+ *
+ * If the root of the @to mount is equal to the future mountpoint @mp of
+ * the @to mount on @from then @to will be overmounted by whatever is
+ * propagated to it.
+ *
+ * Context: This function expects namespace_lock() to be held and that
+ *          @mp is stable.
+ * Return: If @from overmounts @to, true is returned, false if not.
+ */
+bool propagation_would_overmount(const struct mount *from,
+				 const struct mount *to,
+				 const struct mountpoint *mp)
+{
+	if (!IS_MNT_SHARED(from))
+		return false;
+
+	if (IS_MNT_NEW(to))
+		return false;
+
+	if (to->mnt.mnt_root != mp->m_dentry)
+		return false;
+
+	for (const struct mount *m = to; m; m = m->mnt_master) {
+		if (peers(from, m))
+			return true;
+	}
+
+	return false;
+}
+
 /*
  * check if the mount 'mnt' can be unmounted successfully.
  * @mnt: the mount to be checked for unmount
diff --git a/fs/pnode.h b/fs/pnode.h
index 988f1aa9b02a..0b02a6393891 100644
--- a/fs/pnode.h
+++ b/fs/pnode.h
@@ -53,4 +53,7 @@ struct mount *copy_tree(struct mount *, struct dentry *, int);
 bool is_path_reachable(struct mount *, struct dentry *,
 			 const struct path *root);
 int count_mounts(struct mnt_namespace *ns, struct mount *mnt);
+bool propagation_would_overmount(const struct mount *from,
+				 const struct mount *to,
+				 const struct mountpoint *mp);
 #endif /* _LINUX_PNODE_H */
diff --git a/include/uapi/linux/mount.h b/include/uapi/linux/mount.h
index 4d93967f8aea..8eb0d7b758d2 100644
--- a/include/uapi/linux/mount.h
+++ b/include/uapi/linux/mount.h
@@ -74,7 +74,8 @@
 #define MOVE_MOUNT_T_AUTOMOUNTS		0x00000020 /* Follow automounts on to path */
 #define MOVE_MOUNT_T_EMPTY_PATH		0x00000040 /* Empty to path permitted */
 #define MOVE_MOUNT_SET_GROUP		0x00000100 /* Set sharing group instead */
-#define MOVE_MOUNT__MASK		0x00000177
+#define MOVE_MOUNT_BENEATH		0x00000200 /* Mount beneath top mount */
+#define MOVE_MOUNT__MASK		0x00000377
 
 /*
  * fsopen() flags.