Version:           unspecified (using KDE 4.8.0) 
OS:                Linux

Yes, logging in as root is not recommended and not really supported, but a crash is a crash...

# kdmrc
AllowRootLogin=true 

I also cleaned out root's home directory completely to make sure no old config files were causing a problem. The crash was the same before and after cleaning home.

Backtrace:

markuspc / # gdb /usr/bin/kdm --core core
gdb /usr/bin/kdm --core core
GNU gdb (Gentoo 7.4 p1) 7.4
Copyright (C) 2012 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.  Type "show copying"
and "show warranty" for details.
This GDB was configured as "x86_64-pc-linux-gnu".
For bug reporting instructions, please see:
<http://bugs.gentoo.org/>...
Reading symbols from /usr/bin/kdm...Reading symbols from /usr/lib64/debug/usr/bin/kdm.debug...done.
done.
[New LWP 26527]
Core was generated by `-:0'        '.
Program terminated with signal 11, Segmentation fault.
#0  0x000000000040d80c in startClient (pid=0x0) at /var/tmp/portage/kde-base/kdm-4.8.0-r1/work/kdm-4.8.0/kdm/backend/client.c:1496
1496            mergeSessionArgs(True);
(gdb) bt
#0  0x000000000040d80c in startClient (pid=0x0) at /var/tmp/portage/kde-base/kdm-4.8.0-r1/work/kdm-4.8.0/kdm/backend/client.c:1496
#1  0x0000000000413131 in anyReserveDisplays () at /var/tmp/portage/kde-base/kdm-4.8.0-r1/work/kdm-4.8.0/kdm/backend/dpylist.c:72
#2  0x00000000014674b8 in ?? ()
#3  0x0000000000000010 in ?? ()
#4  0x0000000001467460 in ?? ()
#5  0x0000000001467460 in ?? ()
#6  0x0000000001468c20 in ?? ()
#7  0x0000000001468bb0 in ?? ()
#8  0x00007fff311ead70 in ?? ()
#9  0x00000000004127af in storePid () at /var/tmp/portage/kde-base/kdm-4.8.0-r1/work/kdm-4.8.0/kdm/backend/dm.c:1640
#10 main (argc=<optimized out>, argv=<optimized out>) at /var/tmp/portage/kde-base/kdm-4.8.0-r1/work/kdm-4.8.0/kdm/backend/dm.c:270
(gdb) 

Reproducible: Always

Steps to Reproduce:
Set AllowRootLogin=true in kdmrc

Attempt to login as root

Actual Results:  
Screen goes black and returns to login screen. 

Expected Results:  
Was kinda hoping to get a desktop... :)

I don't usually login as root. This came about as a part of trying to fix an issue with KAuth authentication. I'm unable to change system settings requiring root (Date/Time, Kdm settings etc). One poster said doing a root login fixed it for him.

Original thread here:

http://forum.kde.org/viewtopic.php?f=66&t=94544

One thing that may not be typical is that the home directory for root is located at /home/root instead of the typical location.