Simple tool to get a romFS image out of a .NRO file

dump_romfs.rb

#!/usr/bin/env ruby
#
# dump_romfs.rb by carstene1ns, 2018 - under ISC License
#
# See http://switchbrew.org/index.php?title=NRO for reference

MAGIC = "NRO0"
HEADER = "ASET"

# helper
def error_out(msg)
  puts "ERROR: #{msg}"
  exit 1
end

# no args?
if ARGV.size != 1
  puts "Usage: #{__FILE__} file.nro"
  exit 0
end

# no file?
input = ARGV[0]
if not File.file?(input)
  error_out "#{input} is not a file!"
end

# wrong type?
archive = open(input, 'rb')
archive.seek(16)
if archive.read(4) != MAGIC
  error_out "#{input} is not a NRO file!"
end

# get size and jump to asset section
archive.seek(4, :CUR)
size = archive.read(4).unpack('L<')[0]
puts "Skipping to asset section (offset 0x%x)..." % size
archive.seek(size)

# size >= file size (no asset section)
if archive.eof? || archive.read(4) != HEADER
  error_out "Did not find asset section!"
end

# format check
if archive.read(4).to_i != 0
  error_out "Unknown asset format version!"
end

# get romfs
archive.seek(32, :CUR)
romfs_offset = archive.read(8).unpack('Q<')[0]
romfs_size = archive.read(8).unpack('Q<')[0]
if romfs_size == 0
  puts "Romfs is empty :("
else
  puts "Found romfs (%d bytes) at offset 0x%x," % [romfs_size, romfs_offset + size]

  print "dumping to \"romfs.bin\"..."
  archive.seek(romfs_offset + size)
  output = open("romfs.bin", 'wb')
  output.write(archive.read(romfs_size))
  output.close
  puts "done."
end

# cleanup
archive.close
exit 0

# EOF