User Impersonation in FreeBSD, by Scott Deming

This was a research project I did a number of years ago. The idea was to
quickly switch from one uid to another and then back again during the
execution of any process, via a simple system call. Under the hood a 
simple kernel module (kld) would take care of house-keeping and security
by enforcing a specific set of rules.

The original PDF document for this project is included here as uimp.pdf
though it is missing a couple of images.

All of this code is released under the MIT license. Feel free to do as you
wish.