VirtualFileSystem able to mkdir and chdir to non rooted directory

[Al Ho]

Hi SSHD team,

Not sure if this is a bug or not, but when I instantiate a new FileSystem
using the VirtualFileSystemFactory and RootedFileSystemProvider, as a user
on the box, I am able to mkdir and get/put files in parent (i.e.
non-subpath) paths of the supposed "root" if I do something like

sftp> pwd
Remote working directory: /

$ put ../thisismyfile

It seems like the resolveLocalPath which is supposed to throw an
InvalidPathException if the path is not a proper subpath of the rooted file
system needs to normalize the path in addition to doing it's nullity
checks.  I was able to prevent this behavior by doing something like this,
but not sure if this is the best approach.

Any guidance/explanation would be appreciated.  Thanks.

public class FixedRootedFileSystemProvider extends RootedFileSystemProvider {

  private static final Logger LOG =
LoggerFactory.getLogger(FixedRootedFileSystemProvider.class);

  public FixedRootedFileSystemProvider() { super(); }

  @Override
  protected Path resolveLocalPath(RootedPath path) {
    Path resolvedLocalPath = super.resolveLocalPath(path);
    return validateParent(path, resolvedLocalPath);
  }

  private Path validateParent(RootedPath path, Path localPath) throws
InvalidPathException {
    RootedFileSystem rfs = path.getFileSystem();
    Path root = rfs.getRoot();

    if 
(!localPath.toAbsolutePath().normalize().startsWith(root.toAbsolutePath().normalize()))
{ //i.e. is not a REAL subpath
      LOG.info("{} is not a subpath of the root FS path " +
root.toAbsolutePath().normalize(),
localPath.toAbsolutePath().normalize());
      throw new InvalidPathException(localPath.toString(), "Invalid path");
    }
    return localPath;
  }
}