FFM not reliable on 5.1 and 5.1B?

We're creating a chroot environment on 4.0D and 5.1 to perform
software builds from. We want to perform FFM mounts of /dev/tty,
/dev/mem, and /dev/kmem. However, the 4.0D/5.1 systems keep rebooting.
The 5.1 system reboots with:
  trap: invalid memory write access from kernel mode

      faulting virtual address:     0x0000000000000038
      pc of faulting instruction:   0xfffffc000030fbd0
      ra contents at time of fault: 0xfffffc000030fbb4
      sp contents at time of fault: 0xfffffe04288475c0

  panic (cpu 0): kernel memory fault
  syncing disks... done

We can consistently force this with the following shell command:
  # a=1; while [[ $a -lt 10000 ]]; do a=$(( $a + 1 )); echo $a; \
  mount -t ffm /dev/tty /mnt/foo; umount /mnt/foo; done

Are FFM mounts unreliable? We have a 5.1B system and get the exact
same error with the same shell command.

The 5.1 system (Rev. 732) has the latest patches from
T64V51B20AS0006-20030210.tar.

The 5.1B system (Rev. 2650) has the latest patches from
T64V51BB27AS0006-20061208.tar.

-- 
albert chin ([email protected])