Permissions on /usr/src

[Evan Champion <evanc@concer.to>]

I would like to take advantage of the sudo make build feature so I can do
builds as a non-root user.  To accomplish this, /usr/src is 775 root:wsrc,
and the files within are 664 user:wsrc (user depends on who did the last
file update).  umask is 002.

The problem I am seeing is that when a file is cvs update'd, the
permissions are set to 644 instead of 664.  Is this a "feature" or am I
doing something wrong?
Evan