Preserve permissions after backup?

[Kai Großjohann]

A Tramp user is reporting that the permissions of a root-owned file
change after he edits it with Tramp (the su method).  I'm guessing
that's because Tramp does backup by renaming and creates a new file
with default permissions.

Is there some machinery in Emacs which is designed to prevent things
like this from happening?  I guess there must be a bug in Tramp which
prevents this machinery from working.

I investigated the backup-by-copying variables but these don't talk
about the permissions, only the owner and group.

Or is backup-by-copying the right approach and I (and the user) just
need to make sure that it is used for the file(s) in question?
-- 
Ambibibentists unite!