csup permissions issue + truss core dump

[Galen Sampson]

All,

I am running 7.1-RELEASE-p2.  I use csup to update my base source tree 
and my ports tree.  I want to be able to update my sources while running 
as a user that doesn't own the files being updated.  I have /usr/ports 
set as a symlink to /home/port_builder/ports.  The files there are owned 
by the user "port_builder" and group "port_builder".  I then add my user 
to the "port_builder" group.  When I run csup I see the following output 
when a file is changed:

Updater failed: Cannot install 
"/home/port_builder/ports/audio/last.fm/#cvs.csup-14913.1" to 
"/home/port_builder/ports/audio/last.fm/Makefile": Operation not permitted

However if I mv the #cvs.csup-14913.1 to Makefile as the same user this 
works just fine.

To debug the problem I tried to run csup under truss.  This did not work 
as expected as truss dumps core when the permission denied occurs and 
generates no output to help.

To debug the truss core dump I tried the following:

 cd /usr/src/usr.bin/truss;
 make CFLAGS="-g -pipe" depend
 make CFLAGS="-g -pipe"
 gdb -core ~/truss.core truss

Unfortunately the backtrace is garbled:
 > gdb -core ~/truss.core truss
GNU gdb 6.1.1 [FreeBSD]
Copyright 2004 Free Software Foundation, Inc.
GDB is free software, covered by the GNU General Public License, and you are
welcome to change it and/or distribute copies of it under certain 
conditions.
Type "show copying" to see the conditions.
There is absolutely no warranty for GDB.  Type "show warranty" for details.
This GDB was configured as "i386-marcel-freebsd"...

warning: exec file is newer than core file.
Core was generated by `truss'.
Program terminated with signal 11, Segmentation fault.
#0  0x28113199 in ?? ()
(gdb) bt
#0  0x28113199 in ?? ()
#1  0x0000000c in ?? ()
#2  0x00003989 in ?? ()
#3  0xbfbfe7b8 in ?? ()
#4  0x00000000 in ?? ()
#5  0x28202000 in ?? ()
#6  0x00000000 in ?? ()
#7  0x00000001 in ?? ()
#8  0x00000000 in ?? ()
#9  0xbf4fa000 in ?? ()
#10 0x37382332 in ?? ()
#11 0xbfbfeac8 in ?? ()
#12 0x08049ea5 in get_string (pid=0, offset=0x37382332, max=-1085300736)
    at syscalls.c:475
Previous frame identical to this frame (corrupt stack?)
(gdb)

Can anyone else reproduce the problem with truss?  Is what I am trying 
to do supported by csup?

Regards,
Galen