JavaScript: Servo’s only garbage collector

Keegan McAllister


by Josh Matthews and Keegan McAllister

A web browser’s purpose in life is to mediate interaction between a user and an application (which we somewhat anachronistically call a "document"). Users expect a browser to be fast and responsive, so the core layout and rendering algorithms are typically implemented in low-level native code. At the same time, JavaScript code in the document can perform complex modifications through the Document Object Model. This means the browser’s representation of a document in memory is a cross-language data structure, bridging the gap between low-level native code and the high-level, garbage-collected world of JavaScript.

We’re taking this as another opportunity in the Servo project to advance the state of the art. We have a new approach for DOM memory management, and we get to use some of the Rust language’s exciting features, like auto-generated trait implementations, lifetime checking, and custom static analysis plugins.

Memory management for the DOM

It’s essential that we never destroy a DOM object while it’s still reachable from either JavaScript or native code — such use-after-free bugs often produce exploitable security holes. To solve this problem, most existing browsers use reference counting to track the pointers between underlying low-level DOM objects. When JavaScript retrieves a DOM object (through getElementById for example), the browser builds a "reflector" object in the JavaScript VM that holds a reference to the underlying low-level object. If the JavaScript garbage collector determines that a reflector is no longer reachable, it destroys the reflector and decrements the reference count on the underlying object.

This solves the use-after-free issue. But to keep users happy, we also need to keep the browser’s memory footprint small. This means destroying objects as soon as they are no longer needed. Unfortunately, the cross-language "reflector" scheme introduces a major complication.

Consider a C++ Element object which holds a reference-counted pointer to an Event:

struct Element {
    RefPtr<Event> mEvent;

Now suppose we add an event handler to the element from JavaScript:

elem.addEventListener('load', function (event) {
    event.originalTarget = elem;

When the event fires, the handler adds a property on the Event which points back to the Element. We now have a cross-language reference cycle, with an Element pointing to an Event within C++, and an Event reflector pointing to the Element reflector in JavaScript. The C++ refcounting will never destroy a cycle, and the JavaScript garbage collector can’t trace through the C++ pointers, so these objects will never be freed.

Existing browsers resolve this problem in several ways. Some do nothing, and leak memory. Some try to manually break possible cycles, by nulling out mEvent for example. And some implement a cycle collection algorithm on top of reference counting.

None of these solutions are particularly satisfying, so we’re trying something new in Servo by choosing not to reference count DOM objects at all. Instead, we give the JavaScript garbage collector full responsibility for managing those native-code DOM objects. This requires a fairly complex interaction between Servo’s Rust code and the SpiderMonkey garbage collector, which is written in C++. Fortunately, Rust provides some cool features that let us build this in a way that’s fast, secure, and maintainable.

Auto-generating field traversals

How will the garbage collector find all the references between DOM objects? In Gecko‘s cycle collector this is done with a lot of hand-written annotations, e.g.:

NS_IMPL_CYCLE_COLLECTION(nsFrameLoader, mDocShell, mMessageManager)

This macro describes which members of a C++ class should be added to a graph of potential cycles. Forgetting an entry can produce a memory leak. In Servo the consequences would be even worse: if the garbage collector can’t see all references, it might free a node that is still in use. It’s essential for both security and programmer convenience that we get rid of this manual listing of fields.

Rust has a notion of traits, which are similar to type classes in Haskell or interfaces in many OO languages. A simple example is the Collection trait:

pub trait Collection {
    fn len(&self) -> uint;

Any type implementing the Collection trait will provide a method named len that takes a value of the type (by reference, hence &self) and returns an unsigned integer. In other words, the Collection trait describes any type which is a collection of elements, and the trait provides a way to get the collection’s length.

Now let’s look at the Encodable trait, used for serialization. Here’s a simplified version:

pub trait Encodable {
    fn encode<T: Encoder>(&self, encoder: &mut T);

Any type which can be serialized will provide an encode method. The encode method itself is generic; it takes as an argument any type T implementing the trait Encoder. The encode method visits the data type’s fields by calling Encoder methods such as emit_u32, emit_tuple, etc. The details of the particular serialization format (e.g. JSON) are handled by the Encoder implementation.

The Encodable trait is special, because the compiler can implement it for us! Although this mechanism was intended for painless serialization, it’s exactly what we need to implement garbage collector trace hooks without manually listing data fields.

Let’s look at Servo’s implementation of the DOM’s Document interface:

pub struct Document {
    pub node: Node,
    pub window: JS<Window>,
    pub is_html_document: bool,

The deriving attribute asks the compiler to write an implementation of encode that recursively calls encode on node, window, etc. The compiler will complain if we add a field to Document that doesn’t implement Encodable, so we have compile-time assurance that we’re tracing all the fields of our objects.

Note the difference between the node and window fields above. In the object hierarchy of the DOM spec, every Document is also a Node. Rust doesn’t have inheritance for data types, so we implement this by storing a Node struct within a Document struct. As in C++, the fields of Node are included in-line with the fields of Document, without any pointer indirection, and the auto-generated encode method will visit them as well.

A Document also has an associated Window, but this is not a containing or "is-a" relationship. The Document just has a pointer to a Window, one of many pointers to that object, which can live in native DOM data structures or in JavaScript reflectors. These are precisely the pointers we need to tell the garbage collector about. We do this with a custom pointer type JS<T> (for example, the JS<Window> above). The implementation of encode for JS<T> is not auto-generated; this is where we actually call the SpiderMonkey trace hooks.

Lifetime checking for safe rooting

The Rust code in Servo needs to pass DOM object pointers as function arguments, store DOM object pointers in local variables, and so forth. We need to register these additional temporary references as roots in the garbage collector’s reachability analysis. If we touch an object from Rust when it’s not rooted, that could introduce a use-after-free vulnerability.

To make this happen, we need to expand our repertoire of GC-managed pointer types. We already talked about JS<T>, which represents a reference between two GC-managed DOM objects. These are not rooted; the garbage collector only knows about them when encode reaches one as part of the tracing process.

When we want to use a DOM object from Rust code, we call the root method on JS<T>. For example:

fn load_anchor_href(&self, href: DOMString) {
    let window = self.window.root();

The root method returns a Root<T>, which is stored in a stack-allocated local variable. When the Root<T> is destroyed at the end of the function, its destructor will un-root the DOM object. This is an example of the RAII idiom, which Rust inherits from C++.

Of course, a DOM object might make its way through many function calls and local variables before we’re done with it. We want to avoid the cost of telling SpiderMonkey about each and every step. Instead, we have another type JSRef<T>, which represents a pointer to a GC-managed object which is already rooted elsewhere. Unlike Root<T>, JSRef<T> can be copied at negligible cost.

We shouldn’t un-root an object if it’s still reachable through JSRef<T>, so it’s important that a JSRef<T> can’t outlive its originating Root<T>. Situations like this are common in C++ as well. No matter how smart your smart pointer is, you can take a bare reference to the contents and then erroneously use that reference past the lifetime of the smart pointer.

Rust solves this problem with a compile-time lifetime checker. The type of a reference includes the region of code over which it is valid. In most cases, lifetimes are inferred and don’t need to be written out in the source code. Inferred or not, the presence of lifetime information allows the compiler to reject use-after-free and other dangerous bugs.

Not only do lifetimes protect Rust’s built-in reference type, we can use them in our own data structures as well. JSRef is actually defined as

pub struct JSRef<'a, T> {

T is the familiar type variable, representing the type of DOM structure we’re pointing to, e.g. Window. The somewhat odd syntax 'a is a lifetime variable, representing the region of code in which that object is rooted. Crucially, this lets us write a method on Root with the following signature:

pub fn root_ref<'a>(&'a self) -> JSRef<'a, T> {

What this syntax means is:

  • <'a>: "for any lifetime 'a",
  • (&'a self): "take a reference to a Root which is valid over lifetime 'a",
  • -> JSRef<'a, T>: "return a JSRef whose lifetime parameter is set to 'a".

The final piece of the puzzle is that we put a marker in the JSRef type saying that it’s only valid for the lifetime corresponding to that parameter 'a. This is how we extend the lifetime system to enforce our application-specific property about garbage collector rooting. If we try to compile something like this:

fn bogus_get_window<'a>(&self) -> JSRef<'a, Window> {
    let window = self.window.root();
    window.root_ref()  // return the JSRef

we get an error: 199:15 error: `window` does not live long enough     window.root_ref()
                    ^~~~~~ 200:6 note: reference must be valid for
    the lifetime 'a as defined on the block at 197:56... fn bogus_get_window<'a>(&self) -> JSRef<'a, Window> {     let window = self.window.root();     window.root_ref() } 200:6 note: ...but borrowed value is only
    valid for the block at 197:56 fn bogus_get_window<'a>(&self) -> JSRef<'a, Window> {     let window = self.window.root();     window.root_ref() }

We also implement the Deref trait for both Root<T> and JSRef<T>. This allows us to access fields of the underlying type T through a Root<T> or JSRef<T>. Because JS<T> does not implement Deref, we have to root an object before using it.

The DOM methods of Window (for example) are defined in a trait which is implemented for JSRef<Window>. This ensures that the self pointer is rooted for the duration of the method call, which would not be guaranteed if we implemented the methods on Window directly.

You can check out the Servo project wiki for more of the details that didn’t make it into this article.

Custom static analysis

To recap, the safety of our system depends on two major parts:

  • The auto-generated encode methods ensure that SpiderMonkey’s garbage collector can see all of the references between DOM objects.
  • The implementation of Root<T> and JSRef<T> guarantees that we can’t use a DOM object from Rust without telling SpiderMonkey about our temporary reference.

But there’s a hole in this scheme. We could copy an unrooted pointer — a JS<T> — to a local variable on the stack, and then at some later point, root it and use the DOM object. In the meantime, SpiderMonkey’s garbage collector won’t know about that JS<T> on the stack, so it might free the DOM object. To really be safe, we need to make sure that JS<T> only appears in traceable DOM structs, and never in local variables, function arguments, and so forth.

This rule doesn’t correspond to anything that already exists in Rust’s type system. Fortunately, the Rust compiler can load "lint plugins" providing custom static analysis. These basically take the form of new compiler warnings, although in this case we set the default severity to "error".

We have already implemented a plugin which simply forbids JS<T> from appearing at all. Because lint plugins are part of the usual warnings infrastructure, we can use the allow attribute in places where it’s okay to use JS<T>, like DOM struct definitions and the implementation of JS<T> itself.

Our plugin looks at every place where the code mentions a type. Remarkably, this adds only a fraction of a second to the compile time for Servo’s largest subcomponent, as Rust compile times are dominated by LLVM‘s back-end optimizations and code generation. The current version of the plugin is very simple and will miss some mistakes, like storing a struct containing JS<T> on the stack. However, lint plugins run at a late stage of compilation and have access to full compiler internals, including the results of type inference. So we can make the plugin incrementally more sophisticated in the future.

In the end, the plugin won’t necessarily catch every mistake. It’s hard to achieve full soundness with ad-hoc extensions to a type system. As the name "lint plugin" suggests, the idea is to catch common mistakes at a low cost to programmer productivity. By combining this with the lifetime checking built in to Rust’s type system, we hope to achieve a degree of security and reliability far beyond what’s feasible in C++. Additionally, since the checking is all done at compile time, there’s no penalty in the generated machine code.

It’s an open question how our garbage-collected DOM will perform compared to a traditional reference-counted DOM. The Blink team has performed similar experiments, but they don’t have Servo’s luxury of starting from a clean slate and using a cutting-edge language. We expect the biggest gains will come when we move to allocating DOM objects within the JavaScript reflectors themselves. Since the reflectors need to be traced no matter what, this will reduce the cost of managing native DOM structures to almost nothing.

If you find this stuff interesting, we’d love to have your help on Rust and Servo! Both are open-source projects with a large number of community contributors. Here are some resources for getting started:

1 response

Post a comment

  1. Alan Andrade wrote on :

    I learned more about Rust thanks to your nifty writing style.

    Thank you


Post Your Comment