Hello!
If VDR is started with the "-u USERNAME" option to make it change from root to a less privileged user, VDR first checks with getuid(), if the current UID is 0 / root. I suggest to change VDR to use geteuid() here to check for the "effective UID".
Background: In the c't-VDR Debian package, VDR runs with it's own user "vdr". In order to allow VDR to restart the vdr daemon from within its commands menu, a small setuid wrapper is used. This wrapper only changes the effective UID to "root", not the real UID. Because VDR does not check the effective UID, it will not call SetUserid() and happily runs with root privileges, even if provided with a username in the "-u" option.
The effectice UID is used to evaluate the privileges of a process to perform a particular action. So I think it is more correct to let VDR check this UID, when deciding whether to change to a different user or not.
bye,
Tobias