setuid cleared on write

[utzoo!decvax!pur-ee!bruner]

I tried to send this out before, but it was smashed somehow:

I would propose that the setuid bit be cleared if the file is written
by someone other than its owner, and similarly that the setgid bit
be cleared if the group-id of the writer doesn't match the group id
of the file.  This way, a user could write upon his own files and
not have to remember to "chmod" them back after each write.  Also,
members of a group (who, in general, cannot "chmod" the file) can
change its contents without clearing the setgid bit.  Users other
than the owner (for setuid) or users outside of the group (for setgid)
could not take advantage of a file accidentally left writable.

--John Bruner
(ucbvax!pur-ee!bruner)