[Pkg-shadow-devel] su privilege dropping missing

[U.Mutlu]

User ps switches to user mo:

ps at mach:~/tmp$ su mo
Password:
mo at mach:/tmp$ cd
mo at mach:~$ ps aux | grep mo
root      8635  0.1  0.0  47812  3280 pts/4    S    03:08   0:00 su mo
mo        8636  0.7  0.1  26932  7420 pts/4    S    03:08   0:00 bash

Why is su running as root?
It should have dropped its root privilege since neither of the
two users is root.
IMO a bug in su, isn't it?

$ ls -l /bin/su
-rwsr-xr-x 1 root root 40168 Nov 20  2014 /bin/su

OS: Debian 8