Bach states the following:
mkdir is a typical
setuidprogram. Only a process with Effective UID root can create a folder. (Section 7.6, Page 229)
I checked the permissions on it on Ubuntu 12.04, the setuid bit isn't set.
-rwxr-xr-x 1 root root 42624 Oct 2 08:55 mkdir ^??
Then I checked other programs which are supposed to have this set, like
su and they have it set.
-rwsr-xr-x 1 root root 34740 Nov 8 2011 ping ^---there it is
So, how does
mkdir work? Have things changed since Bach wrote that?