Modular foreign function bindings

One of the most frequent questions about MirageOS from developers is "do I really need to write all my code in OCaml"? There are, of course, very good reasons to build the core system in pure OCaml: the module system permits reusing algorithmic abstractions at scale, and OCaml's static type checking makes it possible to enforce lightweight invariants across interfaces. However, it's ultimately necessary to support interfacing to existing code, and this blog post will describe what we're doing to make this possible this without sacrificing the security benefits afforded by unikernels.

A MirageOS application works by abstracting the logic of the application from the details of platform that it is compiled for. The mirage CLI tool parses a configuration file that represents the desired hardware target, which can be a Unix binary or a specialized Xen guest OS. Our foreign function interface design elaborates on these design principles by separating the description of the C foreign functions from how we link to that code. For instance, a Unix unikernel could use the normal ld.so to connect to a shared library, while in Xen we would need to interface to that C library through some other mechanism (for instance, a separate VM could be spawned to run the untrusted OpenSSL code). If you're curious about how this works, this blog post is for you!

Introducing ctypes

ocaml-ctypes ("ctypes" for short) is a library for gluing together OCaml code and C code without writing any C. This post introduces the ctypes library with a couple of simple examples, and outlines how OCaml's module system makes it possible to write high-level bindings to C that are independent of any particular linking mechanism.

Hello, C

Binding a C function using ctypes involves two steps.

• First, construct an OCaml value that represents the type of the function
• Second, use the type representation and the function name to resolve and bind the function

For example, here's a binding to C's puts function, which prints a string to standard output and returns the number of characters written:

let puts = foreign "puts" (string @-> returning int)

After the call to foreign the bound function is available to OCaml immediately. Here's a call to puts from the interactive top level:

# puts "Hello, world";;
Hello, world
- : int = 13

<Hello-C/>

Now that we've had a taste of ctypes, let's look at a more realistic example: a program that defines bindings to the expat XML parsing library, then uses them to display the structure of an XML document.

We'll start by describing the types used by expat. Since ctypes represents C types as OCaml values, each of the types we need becomes a value binding in our OCaml program. The parser object involves an incomplete (abstract) struct definition and a typedef for a pointer to a struct:

struct xml_ParserStruct;
typedef xml_ParserStruct *xml_Parser;

In ctypes these become calls to the structure and ptr functions:

let parser_struct : [`XML_ParserStruct] structure typ = structure "xml_ParserStruct"
let xml_Parser = ptr parser_struct

Next, we'll use the type representations to bind some functions. The XML_ParserCreate and XML_ParserFree functions construct and destroy parser objects. As with puts, each function binding involves a simple call to foreign:

let parser_create = foreign "XML_ParserCreate"
  (ptr void @-> returning xml_Parser)
let parser_free = foreign "XML_ParserFree"
  (xml_Parser @-> returning void)

Expat operates primarily through callbacks: when start and end elements are encountered the parser invokes user-registered functions, passing the tag names and attributes (along with a piece of user data):

typedef void (*start_handler)(void *, char *, char **);
typedef void (*end_handler)(void *, char *);

In ctypes function pointer types are built using the funptr function:

let start_handler =
  funptr (ptr void @-> string @-> ptr string @-> returning void)
let end_handler =
  funptr (ptr void @-> string @-> returning void)

We can use the start_handler and end_handler type representations to bind XML_SetElementHandler, the callback-registration function:

let set_element_handler = foreign "XML_SetElementHandler"
  (xml_Parser @-> start_handler @-> end_handler @-> returning void)

The type that OCaml infers for set_element_handler reveals that the function accepts regular OCaml functions as arguments, since the argument types are normal OCaml function types:

val set_element_handler :
  [ `XML_ParserStruct ] structure ptr ->
  (unit ptr -> string -> string ptr -> unit) ->
  (unit ptr -> string -> unit) -> unit

There's one remaining function to bind, then we're ready to use the library. The XML_Parse function performs the actual parsing, invoking the callbacks when tags are encountered:

let parse = foreign "XML_Parse"
  (xml_Parser @-> string @-> int @-> int @-> returning int)

As before, all the functions that we've bound are available for use immediately. We'll start by using them to define a more idiomatic OCaml entry point to the library. The parse_string function accepts the start and end callbacks as labelled arguments, along with a string to parse:

let parse_string ~start_handler ~end_handler s =
  let p = parser_create null in
  let () = set_element_handler p start_handler end_handler in
  let _ = parse p s (String.length s) 1 in
  parser_free p

Using parse_string we can write a program that prints out the names of each element in an XML document, indented according to nesting depth:

let depth = ref 0

let start_handler _ name _ =
  Printf.printf "%*s%s\\n" (!depth * 3) "" name;
  incr depth

let end_handler _ _ =
  decr depth

let () =
  parse_string ~start_handler ~end_handler (In_channel.input_all stdin)

The full source of the program is available on github.

Here's the program in action:

$ ocamlfind opt -thread -package core,ctypes.foreign expat_example.ml \\
   -linkpkg -cclib -lexpat -o expat_example
$ wget -q https://mirage.io/blog/atom.xml -O /dev/stdout \\
  | ./expat_example
feed
   id
   title
   subtitle
   rights
   updated
   link
   link
   contributor
      email
      uri
      name
[...]

Since this is just a high-level overview we've passed over a number of details. The interested reader can find a more comprehensive introduction to using ctypes in Chapter 19: Foreign Function Interface of Real World OCaml.

Dynamic vs static

Up to this point we've been using a single function, foreign, to make C functions available to OCaml. Although foreign is simple to use, there's quite a lot going on behind the scenes. The two arguments to foreign are used to dynamically construct an OCaml function value that wraps the C function: the name is used to resolve the code for the C function, and the type representation is used to construct a call frame appropriate to the C types invovled and to the underlying platform.

The dynamic nature of foreign that makes it convenient for interactive use, also makes it unsuitable for some environments. There are three main drawbacks:

• Binding functions dynamically involves a certain loss of safety: since C libraries typically don't maintain information about the types of the functions they contain, there's no way to check whether the type representation passed to foreign matches the actual type of the C function.

• Dynamically constructing calls introduces a certain interpretative overhead. In mitigation, this overhead is much less than might be supposed, since much of the work can be done when the function is bound rather than when the call is made, and foreign has been used to bind C functions in performance-sensitive applications without problems.

• The implementation of foreign uses a low-level library, libffi, to deal with calling conventions across platforms. While libffi is mature and widely supported, it's not appropriate for use in every environment. For example, introducing such a (relatively) large and complex library into Mirage would compromise many of the benefits of writing the rest of the system in OCaml.

Happily, there's a solution at hand. As the introduction hints, foreign is one of a number of binding strategies, and OCaml's module system makes it easy to defer the choice of which strategy to use when writing the actual code. Placing the expat bindings in a functor (parameterised module) makes it possible to abstract over the linking strategy:

module Bindings(F : FOREIGN) =
struct
  let parser_create = F.foreign "XML_ParserCreate"
    (ptr void @-> returning xml_Parser)
  let parser_free = F.foreign "XML_ParserFree"
    (xml_Parser @-> returning void)
  let set_element_handler = F.foreign "XML_SetElementHandler"
    (xml_Parser @-> start_handler @-> end_handler @-> returning void)
  let parse = F.foreign "XML_Parse"
    (xml_Parser @-> string @-> int @-> int @-> returning int)
end

The Bindings module accepts a single parameter of type FOREIGN, which encodes the binding strategy to use. Instantiating Bindings with a module containing the foreign function used above recovers the dynamically-constructed bindings that we've been using so far. However, there are now other possibilities available. In particular, we can instantiate Bindings with code generators that output code to expose the bound functions to OCaml. The actual instantiation is hidden behind a couple of convenient functions, write_c and write_ml, which accept Bindings as a parameter and write to a formatter:

Cstubs.write_c formatter ~prefix:"expat" ~bindings:(module Bindings)
Cstubs.write_ml formatter ~prefix:"expat" ~bindings:(module Bindings)

Generating code in this way eliminates the concerns associated with constructing calls dynamically:

• The C compiler checks the types of the generated calls against the C headers (the API), so the safety concerns associated with linking directly against the C library binaries (the ABI) don't apply.

• There's no interpretative overhead, since the generated code is (statically) compiled.

• The dependency on libffi disappears altogether.

How easy is it in practice to switch between dynamic and static binding strategies? It turns out that it's quite straightforward, even for code that was originally written without parameterisation. Bindings written using early releases of ctypes used the dynamic strategy exclusively, since dynamic binding was then the only option available. The commit logs for projects that switched over to static generation and linking (e.g. ocaml-lz4 and async-ssl) when it became available show that moving to the new approach involved only straightforward and localised changes.

Local vs remote

Generating code is safer than constructing calls dynamically, since it allows the C compiler to check the types of function calls against declarations. However, there are some safety problems that even C's type checking doesn't detect. For instance, the following call is type correct (given suitable definitions of p and q), but is likely to misbehave at run time:

memcpy(p, q, SIZE_MAX)

In contrast, code written purely in OCaml detects and prevents attempts to write beyond the bounds of allocated objects:

# StringLabels.blit ~src ~dst ~src_pos:0 ~dst_pos:0 ~len:max_int;;
Exception: Invalid_argument "String.blit".

It seems a shame to weaken OCaml's safety guarantees by linking in C code that can potentially write to any region of memory, but what is the alternative?

One possibility is to use privilege separation to separate trusted OCaml code from untrusted C functions. The modular design of ctypes means that privilege separation can be treated as one more linking strategy: we can run C code in an entirely separate process (or for Mirage/Xen, in a separate virtual machine), and instantiate Bindings with a strategy that forwards calls to the process using standard inter-process communication. The remote calling strategy is not supported in the current release of ctypes, but it's scheduled for a future version. As with the switch from dynamic to static bindings, we anticipate that updating existing bindings to use cross-process calls will be straightforward.

This introductory post should give you a sense of the power of the unikernel approach in Mirage. By turning the FFI into just another library (for the C interface description) and protocol (for the linkage model), we can use code generation to map application logic onto the privilege model most suitable for the target hardware platform. This starts with Unix processes, continues onto Xen paravirtualization, and could even extend into CHERI fine-grained compartmentalization.

Further examples

Although ctypes is a fairly new library, it's already in use in a number of projects across a variety of domains: graphics, multimedia, compression, cryptography, security, geospatial data, communication, and many others. Further resources (documentation, forums, etc.) are available via the home page.