When a process transfers data to or from the kernel, and those data are
organized into structures whose size depends on whether or not the size
of a pointer is 4 or 8 bytes, an LP64 kernel always uses 64-bit structures,
even with 32-bit processes. I know this is true of structures returned from
sysctl(3), but I also think it is true of structures used with ioctl(2)
and fcntl(2). Because 32-bit processes expect 32-bit structures, attempts
to use the 64-bit structures fail at best and cause massive data corruption
at worst.

This is already affecting real-world code. A patch of mine that was
recently committed to Wine (http://www.winehq.org) exposed this issue
with the xtcpcb and xinpcb structs returned by the sysctl(3)s
"net.inet.tcp.pcblist" and "net.inet.udp.pcblist" (cf. Wine bug 28857:
http://bugs.winehq.org/show_bug.cgi?id=28857).

Fix: 

FreeBSD should detect if a process is a 32-bit one, and if so, it should
use 32-bit structures instead of 64-bit ones. I don't know how the former
can be done; a cursory look through the <sys/proc.h> header reveals
nothing. The latter is easy, but tedious: definitions for the 32-bit
structs must be added to the kernel headers.

Another way to solve this problem (at the cost of binary compatibility
for 32-bit programs) is to simply make the 32-bit and 64-bit structures
the same.
How-To-Repeat: Run any 32-bit program under a 64-bit kernel that calls sysctl(3),
ioctl(2), or fcntl(2) and uses a data structure with those calls whose
size depends on the pointer size.