__do_loopback.md

__do_loopback

__do_loopback() create a new struct mount and init its members according to the info of the source directory and its parent mnt.

static struct mount *__do_loopback(struct path *old_path, int recurse)
{
	struct mount *mnt = ERR_PTR(-EINVAL), *old = real_mount(old_path->mnt);

	if (IS_MNT_UNBINDABLE(old))
		return mnt;

	if (!check_mnt(old) && old_path->dentry->d_op != &ns_dentry_operations)
		return mnt;

	if (!recurse && has_locked_children(old, old_path->dentry))
		return mnt;

	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))
		mnt->mnt.mnt_flags &= ~MNT_LOCKED;

	return mnt;
}

Assume recurse is false which means we are not doing "rbind". the mean work is done by clone_mnt(), here old is the parent mnt of the source directory and old_path->dentry is the dentry of the source directory.

clone_mnt()

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->mnt.mnt_flags &= ~(MNT_WRITE_HOLD|MNT_MARKED|MNT_INTERNAL);

	atomic_inc(&sb->s_active);
	mnt->mnt.mnt_idmap = mnt_idmap_get(mnt_idmap(&old->mnt));

	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;
	} else {
		CLEAR_MNT_SHARED(mnt);
	}
	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);
}

Let's ignore the code for mount propagation and focus on the new allocated struct mount

	mnt->mnt.mnt_flags = old->mnt.mnt_flags;
	mnt->mnt.mnt_flags &= ~(MNT_WRITE_HOLD|MNT_MARKED|MNT_INTERNAL);

	atomic_inc(&sb->s_active);
	mnt->mnt.mnt_idmap = mnt_idmap_get(mnt_idmap(&old->mnt));

	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();

We can see the content of the new mount is from the parent mount of the source dir.