Добавляем в /boot/loader.conf:
    linux_load="YES"
    linprocfs_load="YES"

В /etc/fstab:
    linprocfs /compat/linux/proc linprocfs rw 0 0

/etc/rc.conf
    linux_enable="YES"

/etc/sysctl.conf
    kern.fallback_elf_brand=3