pre ubuntu
http://manpages.ubuntu.com/manpages/xenial/man1/chmod.1.html
http://manpages.ubuntu.com/manpages/xenial/man2/chmod.2.html
Pisu tam aj o tych SETUID a SETGID

(p.s. alebo mozno ma blbo nieco uplne ine, napr. nastaveny iny datovy adresar alebo neviem tie vrchne bity nepoznam)

chmod preserves a directory's set-user-ID and set-group-ID bits unless
you explicitly specify otherwise. You can set or clear the bits with
symbolic modes like u+s and g-s, and you can set (but not clear) the
bits with a numeric mode.