/usr/lib/ruby/vendor_ruby/ is in ruby-fssm 0.2.10-1.
This file is owned by root:root, with mode 0o755.
..
/usr/lib/ruby/vendor_ruby/fssm/
/usr/lib/ruby/vendor_ruby/fssm/version.rb
/usr/lib/ruby/vendor_ruby/fssm/tree.rb
/usr/lib/ruby/vendor_ruby/fssm/backends/
/usr/lib/ruby/vendor_ruby/fssm/backends/polling.rb
/usr/lib/ruby/vendor_ruby/fssm/backends/rbfsevent.rb
/usr/lib/ruby/vendor_ruby/fssm/backends/inotify.rb
/usr/lib/ruby/vendor_ruby/fssm/path.rb
/usr/lib/ruby/vendor_ruby/fssm/state/
/usr/lib/ruby/vendor_ruby/fssm/state/directory.rb
/usr/lib/ruby/vendor_ruby/fssm/state/file.rb
/usr/lib/ruby/vendor_ruby/fssm/support.rb
/usr/lib/ruby/vendor_ruby/fssm/pathname.rb
/usr/lib/ruby/vendor_ruby/fssm/monitor.rb
/usr/lib/ruby/vendor_ruby/fssm.rb