/etc/rc.conf

linux_enable=YES

rebootして、linux64 fdescfs linprocfs linsysfs tmpfs がロードされていることを確認

# kldstat
Id Refs Address                Size Name
 1   63 0xffffffff80200000  1f3e2d0 kernel
 2    1 0xffffffff82140000   59dfa8 zfs.ko
 3    1 0xffffffff826de000     a4a0 cryptodev.ko
 4    1 0xffffffff82b18000     3218 intpm.ko
 5    1 0xffffffff82b1c000     2180 smbus.ko
 6    1 0xffffffff82b1f000     3340 uhid.ko
 7    1 0xffffffff82b23000     4350 ums.ko
 8    1 0xffffffff82b28000     3380 usbhid.ko
 9    1 0xffffffff82b2c000     31f8 hidbus.ko
10    1 0xffffffff82b30000    31a80 linux.ko
11    4 0xffffffff82b62000     be88 linux_common.ko
12    1 0xffffffff82b6e000    14b98 netlink.ko
13    1 0xffffffff82b83000    2dca0 linux64.ko
14    1 0xffffffff82bb1000     2260 pty.ko
15    1 0xffffffff82bb4000     3530 fdescfs.ko
16    1 0xffffffff82bb8000     73b0 linprocfs.ko
17    1 0xffffffff82bc0000     3284 linsysfs.ko

# kldload tmpfs
kldload: can't load tmpfs: module already loaded or in kernel

Jail+zfsあたりを参照ください

# pkg install debootstrap

• https://forums.freebsd.org/threads/setting-up-a-debian-linux-jail-on-freebsd.68434/
• https://wiki.freebsd.org/LinuxJails
• https://forums.freebsd.org/threads/jails-rc-conf-bug-combo-of-settings-prevents-jail-creation.76721/
• https://people.allbsd.org/~hrs/FreeBSD/sato-FBSDW20200925-jail.pdf