Distribution/Version: 3.99.5

GDM is completely unable to start a session for a user since the update to
2.8.0.0.  After entering a user name and a password, the login dialog appears
again and no error is reported on the screen (although it's sent to syslog).  It
just says "Cannot set effective user/group id".

I guess this is a NetBSD specific problem.

The issue arises due to the fact that daemon/slave.c code makes two subsequent
calls to setegid/seteuid, although the second pair of them is not strictly
required.  The problem is that these two extra calls are only needed in a branch
of the conditional used to check for the home directory, as it raises privileges
to the superuser and must be dropped again later.  However, if everything is
correct, no extra calls should be made to change privileges.

Consider the following example, which illustrates what GDM is trying to do
(modulo the egid stuff):

#include <stdio.h>
#include <unistd.h>

int main() {
    if (seteuid(100) == -1) printf("setting 1 failed\n");
    if (seteuid(100) == -1) printf("setting 2 failed\n");
    return 0;
}

When run as root under NetBSD, it says:

dawn# ./a.out                                                                  
setting 2 failed

Moving the offending code block inside the home directory conditional makes GDM
work nicely again.