Thank you very much for the in-depth explanation.

I have to apologize that I left out the use English; statement at the begin of the example code. Of course, it is in my test program :-) It fell off the table during copy-and-paste.

I also would like to state that my test scenario works. For example, when I make the script setuid-root at the file system level, then log in as a normal user with user id 1016 and execute the script, $EUID is 0 and $UID is 1016 at the beginning (before executing line 4).

It also works if I make the script setuid-<user with id 1015>, log in with user id 1016 and run the script. Then $EUID is 1015 and $UID is 1016 at the beginning (before executing line 4).

With another scenario, I have found a bug in the meantime (I can't judge if Perl or if the script is the culprit). I'll describe it in a separate post below.

Hoping to gain some deeper understanding, I have studied the man pages of the library functions you mentioned. But I seem to be blind: I still can't understand the sense of lines 7 and 8. They would only make any sense if the assignments in lines 4 and 5 would change not only the left hand side, but also the right hand side.

IMHO, dropping privileges actually happens in lines 4 and 5, not in lines 7 and 8 as the comment implies. What lines 7 and 8 do will probably remain the author's secret.

Anyway, due to the bug I have found, I am unsure whether I should trust that script at all. Too sad that it's just perlsec that shows a buggy and totally incomprehensible script as reference for how to securely drop privileges.

In reply to Re^2: Not understanding the code to drop privileges in perlsec by Nocturnus
in thread Not understanding the code to drop privileges in perlsec by Nocturnus

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