Testing the Lua kernel interpreter

JuvenalUrbino

I was curious to see lua(4) in action, so I decided to perform some quick test.

printing a "hello, world!" script

First of all, load the relevant kernel modules:

$ modload lua
$ modload luasystm

To have them loaded automatically at boot:

printf '%s\n%s\n' 'lua' 'luasystm' >> /etc/modules.conf

If everything's right the kernel buffer will show:

[     6.176500] lua0: Lua 5.3.5  Copyright (C) 1994-2018 Lua.org, PUC-Rio

And modstat will return:

NAME                    CLASS    SOURCE   FLAG  REFS    SIZE REQUIRES 
lua                     misc     filesys  -        1       - -
luasystm                misc     filesys  -        0       - lua

Let's enhance the verbosity of the lua(4) module:

$ sysctl -w kern.lua.verbose=1
kern.lua.verbose: 0 -> 1

Without further ado, we can now proceed to test the built-in kernel parser, using luactl(8):

create a state to load our scripts

$ luactl create s1 testing
s1 created
$ luactl
Name             Creator  Description
s1               user     testing

load luasystm bindings on state s1 (required for print())

$ luactl require s1 systm
systm required by s1

Write a sample 'hello world' script:

$ cat << eof > hello.lua
-- hello world!
systm.print("hello world!\n")
eof
$ luactl load s1 ./hello.lua
./hello.lua loaded into s1
$ dmesg | tail -1
[  2769.687618] hello world!

Notice that the absolute path is required for luactl to read the script

more testing

1) parsing simple functions

-- sum func
function add(x, y)
  return x + y
end
systm.print("the sum of 16 and 32 is:\n")
systm.print (add(16, 32))

systm.print ("\n")

-- factorial func
function fact (n)
  if n == 0 then
    return 1
  else
    return n * fact(n-1)
  end
end
systm.print("the factorial of 7 is:\n")
systm.print (fact(7))

$ luactl load s1 ./sample.lua
./sample.lua loaded into s1
 $ dmesg | grep -A 5 sample
[  4283.160428] lua0: loading ./sample.lua into state s1
[  4283.170432] the sum of 16 and 32 is:
[  4283.170432] 48
[  4283.170432] the factorial of 7 is:
[  4283.180433] 5040

2) displaying OS info

-- print system info
function uname()
  tbl = {"copyright", "cpu_model", "machine", "machine_arch", "osrelease", "os
type", "kernel_ident", "version"}
  for i = 1, #tbl do
    systm.print(systm[tbl[i]] .. "\n")
  end
end

uname()

Which produces:

[  5519.710421] lua0: loading ./sys.lua into state s1
[  5519.720425] Copyright (c) 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005,
[  5519.730424]     2006, 2007, 2008, 2009, 2010, 2011, 2012, 2013, 2014, 2015, 2016, 2017,
[  5519.730424]     2018, 2019, 2020 The NetBSD Foundation, Inc.  All rights reserved.
[  5519.740424] Copyright (c) 1982, 1986, 1989, 1991, 1993
[  5519.750425]     The Regents of the University of California.  All rights reserved.

[  5519.760427] raspberrypi,3-model-b
[  5519.760427] evbarm
[  5519.760427] aarch64
[  5519.760427] 9.2_STABLE
[  5519.770428] NetBSD
[  5519.770428] GENERIC64
[  5519.770428] NetBSD 9.2_STABLE (GENERIC64) #0: Fri Aug 20 19:33:44 UTC 2021
[  5519.780428] mkrepro@mkrepro.NetBSD.org:/usr/src/sys/arch/evbarm/compile/GENERIC64

3) display hostname

function hostname()
  local f = io.popen ("/bin/hostname")
  local host = f:read("*a") or ""
  f:close()
  host =string.gsub(host, "\n$", "")
  systm.print (host)
end
systm.print("Welcome to:\n")
hostname()

This is not going to work:

$ luactl load s1 ./host.lua
luactl: LUALOAD: Invalid argument