Attacking FreeBSD with Kernel Modules – The System Call Approach. System calls can be backdoored on FreeBSD much like they can on linux, and most linux kernel modules can easily be ported to FreeBSD. Includes information on intercepting system calls, filesystem related hacks, hiding files and contests, process related hacks, file execution redirection, tty hijacking, and module hiding.
You can download it from the following link: https://packetstormsecurity.com/files/download/11095/bsdkern.htm
Source: https://packetstormsecurity.com/files/11095/bsdkern.htm.html