Ruby and syscalls: `Fiddle` vs `Kernel.syscall`

memfd.rb

# docker run --rm -it -v "${PWD}":"${PWD}" -w "${PWD}" ruby:3.3 ruby -w -I. memfd.rb kernel
# docker run --rm -it -v "${PWD}":"${PWD}" -w "${PWD}" ruby:3.3 ruby -w -I. memfd.rb fiddle

raise unless RUBY_PLATFORM =~ /linux/

SYS_MEMFD_CREATE = case RUBY_PLATFORM
               when /x86_64/
                 319
               when /i386/
                 356
               when /armv7l/
                 385
               when /(arm|aarch)64/
                 279
               end

MFD_CLOEXEC = 0x0001

require 'json'

flags = MFD_CLOEXEC
filename = 'datadog.tracing_library.json'
data = { 'foo' => 'bar' }
payload = JSON.dump(data)

if ARGV[0] == 'fiddle'
  # https://www.man7.org/linux/man-pages/man2/syscall.2.html

  # needs libffi

  require 'fiddle'

  # https://docs.ruby-lang.org/en/master/Fiddle.html
  libc = Fiddle.dlopen(nil)
  c_syscall = Fiddle::Function.new(libc['syscall'], [Fiddle::TYPE_LONG, Fiddle::TYPE_VOIDP, Fiddle::TYPE_UINT], Fiddle::TYPE_LONG)

  # https://docs.ruby-lang.org/en/master/Fiddle/Pointer.html
  filename_ptr = Fiddle::Pointer.malloc(filename.bytesize + 1)
  Fiddle::Pointer.write(filename_ptr, filename)
  fd = c_syscall.call(SYS_MEMFD_CREATE, filename_ptr, flags)
  Fiddle.free filename_ptr
  if fd == -1
    raise "error: #{Fiddle.last_error}"
  end
elsif ARGV[0] == 'kernel'
  # https://docs.ruby-lang.org/en/master/Kernel.html#method-i-syscall
  # emits warning: deprecated

  begin
    fd = Kernel.syscall(SYS_MEMFD_CREATE, filename, flags)
  rescue SystemCallError => e
    raise "error: #{e.errno}"
  end
else
  raise 'error: missing arg'
end

if fd < 2
  raise "unexpected fd value for : #{fd}"
else
  puts "success #{fd}"
  # https://docs.ruby-lang.org/en/master/IO.html#method-c-new
  io = IO.new(fd, 'w')

  # underlying has no encoding, ensure write UTF-8 bytes
  count = io.write(payload.encode('UTF-8'))
  p count
  io.flush
end