Mojo “Style” Guide

Mojo is Chrome's new IPC system and provides lots of useful abstractions. These abstractions can make it easier to write code that makes interprocess calls, but can also add significant complexity. Below are some recommendation from Mojo and IPC reviewers for best practices.

For questions, concerns, or suggestions, reach out to mojo@chromium.org.

For legacy IPC, please see security tips for IPC.

Simplicity

Strive to write simple interfaces. Minimize the amount of cross-process state that needs to be maintained in sync.

Good

interface TeleporterFactory {
  Create(Location start, Location end) => (Teleporter);
};

interface Teleporter {
  TeleportGoat(Goat) = ();
};

Bad

interface Teleporter {
  // Bad: comments will need to explicitly call out that both locations need to
  // be bound before calling TeleportGoat()!
  //
  // In addition, if untrustworthy processes can talk to trustworthy processes,
  // the Teleporter implementation will need to also handle the case where the
  // Location objects are not yet bound.
  SetStart(Location);
  SetEnd(Location);
  TeleportGoat(Goat) = ();
};

Similarly, strive to make methods focused. Do not overuse optional types.

Good

struct TeleporterStats {
  AnimalStats animal_stats;
  FungiStats fungi_stats;
  GoatStats goat_stats;
  PlantStats plant_stats;
};

interface Teleporter {
  TeleportAnimal(Animal) => ();
  TeleportFungi(Fungi) => ();
  TeleportGoat(Goat) = ();
  TeleportPlant(Plant) => ();

  // TeleportStats is only non-null if success is true.
  GetStats() => (bool success, TeleporterStats?);
};

Bad

interface Teleporter {
  // The intent of four optional arguments is unclear: can this call teleport
  // multiple objects of different types at once, or is the caller only
  // supposed to only pass one non-null argument per call?
  Teleport(Animal?, Fungi?, Goat?, Plant?) => ();

  // Does this return all stats if sucess is true? Or just the categories that
  // the teleporter already has stats for? The intent is uncertain, so wrapping
  // the disparate values into a result struct would be cleaner.
  GetStats() =>
      (bool success, AnimalStats?, FungiStats?, PlantStats?, FungiStats?);
};

Documentation

Mojo structs, interfaces, and methods should all have comments. Make sure the comments cover the “how” and the “why” of using an interface and its methods, and not just the “what”. Document preconditions, postconditions, and trust: if an interface is implemented in the browser process and handles requests from the renderer process, this should be mentioned in the comments. Complex features should also have an external README.md that covers the high-level flow of information through interfaces and how they interact to implement the feature.

Good

// Interface for controlling a teleporter. Lives in the browser process, and
// used to implement the Teleportation over Mojo IPC RFC.
interface Teleporter {
  // Teleportation helpers for different taxonomic kingdoms. Teleportation is
  // not complete until the reply callback is invoked. The caller must NOT
  // release the sender side resources until the reply callback runs; releasing
  // the resources early will cause splinching.
  TeleportAnimal(Animal) => ();
  TeleportFungi(Fungi) => ();
  // Goats require a specialized teleportation channel distinct from
  // TeleportAnimal to ensure goatiness isolation.
  TeleportGoat(Goat) => ();
  TeleportPlant(Plant) => ();

  // Returns current teleportation stats. On failure (e.g. a teleportation
  // operation is currently in progress) success will be false and a null stats
  // object will be returned.
  GetStats() =>
      (bool success, TeleportationStats?);
};

Security

Policy should be controlled solely by the browser process. “Policy” can mean any number of things, such as sizes, addresses, permissions, URLs, origins, etc. In an ideal world:

  1. Unprivileged process asks for a capability from the privileged process that owns the resource.
  2. Privileged process applies policy to find an implementation for the capability.
  3. Unprivileged process performs operations on the capability, constrained in scope.

The privileged process must own the capability lifecycle.

Do not trust less privileged processes

This is the overriding principle for all guidelines in this section. When receiving data from a less trusted process, treat the data as if it were generated by a malicious adversary. Message handlers cannot assume that offsets are valid, calculations won't overflow, et cetera.

In general:

  • the browser process is the most privileged process type and therefore, must be maximally suspicious of its IPC inputs
  • the renderer and the ARC++ processes are the least privileged and least trustworthy process types
  • other process types, such as GPU and plugin, fall in between

When passing objects up a privilege gradient (from less ā†’ more privileged), the callee must validate the inputs before acting on them. When passing objects down a privilege gradient, such as from browser ā†’ renderer, it is OK for the callee to trust the caller.

See also: Do not Handle Impossible Situations

Do not send unnecessary or privilege-presuming data

Note: there is currently work in progress to associate origins with the InterfaceProviders for frames and workers: https://crbug.com/734210 and https://crbug.com/775792/.

For example, the browser process must not (fully) trust the renderer's claims about origins. The browser process should already know what origin the renderer is evaluating, and thus should already have this data (for example, see RenderFrameHost::GetLastCommittedOrigin()). Thus, a method that requires passing an origin from the renderer to the browser process has a conceptual error, and quite possibly, a vulnerability.

Note: there are currently subtle races when using GetLastCommittedOrigin() that will be resolved by fixing https://crbug.com/729021.

Similarly, the browser process must not trust the renderer's claims about file pathnames. It would be unsafe for the browser process to save a downloaded file to ~/.bashrc just because the renderer asked. Instead, it would be better for the browser process to:

  1. Kill the renderer if basename(pathname) != pathname, since the renderer is obviously compromised if it makes this mistake.
  2. Defang the basename, by removing leading dots, et cetera. Note that the definition of proper defanging varies per platform.
  3. Prepend its own parent directory to the basename, e.g. ~/Downloads.

TODO(https://crbug.com/779196): Even better would be to implement a C++ type performs the appropriate sanitizations and recommend its usage directly here.

Do not define unused or unimplemented things

Mojo interfaces often cross privilege boundaries. Having well-defined interfaces that don't contain stubbed out methods or unused parameters makes it easier to understand and evaluate the implications of crossing these boundaries. Several common areas to watch out for:

Do use EnableIf to guard platform-specific constructs

Platform-specific functionality should only be defined on the platforms where it is implemented. Use the Mojo EnableIf annotation to guard definitions that should only be visible in certain build configurations.

Good

// GN file:
mojom("view_bindings") {
  // ...

  enabled_features = []
  if (is_android) {
    enabled_features += [ "is_android" ];
  }
}

// mojom definition:
interface View {
  // ...

  [EnableIf=is_android]
  UpdateBrowserControlsState(bool enable_hiding, bool enable_showing,
                             bool animate);
};

// C++ implementation:
class View : public mojom::View {
 public:
  // ...

#if defined(OS_ANDROID)
  void UpdateBrowserControlsState(bool enable_hiding, bool enable_showing,
                                  bool animate);
#endif
};

Bad

// mojom definition:
interface View {
  // ...

  UpdateBrowserControlsState(bool enable_hiding, bool enable_showing,
                             bool animate);
};

// C++ implementation:
class View : public mojom::View {
 public:
  // ...

#if defined(OS_ANDROID)
  void UpdateBrowserControlsState(bool enable_hiding, bool enable_showing,
                                  bool animate) override;
#else
  void UpdateBrowserControlsState(bool enable_hiding, bool enable_showing,
                                  bool animate) override {
    NOTREACHED();
  }
#endif
};

The EnableIf annotation can be applied to almost anything: imports, interfaces, methods, arguments, constants, structs, struct members, enums, enumerator values, et cetera.

Do not define unimplemented methods

Reviewing IPC requires reviewing a concrete implementation of the Mojo interface, to evaluate how the (possibly untrustworthy) inputs are used, what outputs are produced, et cetera. If a method is not yet implemented, do not define it in the interface.

Bad

// mojom definition:
interface Spaceship {
  EnterHyperspace();
  ExitHyperspace();
};

// C++ implementation:
class SpaceshipPrototype : public mojom::Spaceship {
  void EnterHyperspace() { /* TODO(dcheng): Implement. */ }
  void ExitHyperspace() { /* TODO(dcheng): Implement. */ }
};

Do not define placeholder enumerator values

Do not define placeholder enumerator values like kLast, kMax, kCount, et cetera. Instead, rely on the autogenerated kMaxValue enumerator emitted for Mojo C++ bindings.

For UMA histograms, logging a Mojo enum is simple: simply use the two argument version of UMA_HISTOGRAM_ENUMERATION:

Good

// mojom definition:
enum GoatStatus {
  kHappy,
  kSad,
  kHungry,
  kGoaty,
};

// C++:
UMA_HISTOGRAM_ENUMERATION("Goat.Status", status);

Using a kCount sentinel complicates switch statements and makes it harder to enforce invariants: code needs to actively enforce that the otherwise invalid kCount sentinel value is not incorrectly passed around.

Bad

// mojom definition:
enum CatStatus {
  kAloof,
  kCount,
};

// C++
switch (cat_status) {
  case CatStatus::kAloof:
    IgnoreHuman();
    break;
  case CatStatus::kCount:
    // this should never happen
}

Defining kLast manually results in ugly casts to perform arithmetic:

Bad

// mojom definition:
enum WhaleStatus {
  kFail,
  kNotFail,
  kLast = kNotFail,
};

// C++:
UMA_HISTOGRAM_ENUMERATION("Whale.Status", status,
                          static_cast<int>(WhaleStatus::kLast) + 1);

For interoperation with legacy IPC, also use kMaxValue rather than defining a custom kLast:

Good

IPC_ENUM_TRAITS_MAX_VALUE(GoatStatus, GoatStatus::kMaxValue);

Use structured types

Where possible, use structured types: this allows the type system to help enforce that the input data is valid. Common ones to watch out for:

  • Files: use mojo_base.mojom.File, not raw descriptor types like HANDLE and int.
  • File paths: use mojo_base.mojom.FilePath, not string.
  • JSON: use mojo_base.mojom.Value, not string.
  • Mojo interfaces: use Interface or Interface&, not handle or handle<message_pipe>.
  • Nonces: use mojo_base.mojom.UnguessableToken, not string.
  • Origins: use url.mojom.Origin, not url.mojom.Url and certainly not string.
  • Time types: use mojo_base.mojom.TimeDelta / mojo_base.mojom.TimeTicks / mojo_base.mojom.Time, not int64 / uint64 / double / et cetera.
  • URLs: use url.mojom.Url, not string.
  • array<uint8> or string and memcpy(): use a Mojo struct and statically define the serialized fields. While memcpy() may be tempting for its simplicity, it can leak info in padding. Even worse, memcpy() can easily copy undocumented fields or newly introduced fields that were never evaluated for safety by the developer or reviewer.

Good

interface ReportingService {
  ReportDeprecation(mojo_base.mojom.TimeTicks time,
                    url.mojom.Url resource,
                    uint32 line_number);
};

Bad

interface ReportingService {
  // Bad: unclear what units |time| is or what |data| contains.
  ReportDeprecation(double time, mojo_base.mojom.Value data);
};

Avoid parallel arrays of data that require the receiver to validate that the arrays have matching lengths. Instead, bundle the data together in a struct so it is impossible to have a mismatch:

Good

struct Pixel {
  int8 reds;
  int8 greens;
  int8 blues;
  int8 alphas;
};

struct Bitmap {
  // Good: it is impossible for there to be mismatched data.
  array<Pixel> pixels;
};

Bad

// Bad: code using this struct will need to validate that all the arrays have
// matching sizes.
struct Bitmap {
  array<int8> reds;
  array<int8> greens;
  array<int8> blues;
  array<int8> alphas;
};

Beware of arithmetic overflow

TODO(dcheng): Import the guidance from the legacy IPC doc.

Signed overflow is undefined in C++. If unsure about whether or not something will overflow, use the safe numeric helpers from //base/numerics!

Good

base::CheckedNumeric<int32_t> size = mojo_rect->width();
size *= mojo_rect.height();
if (!size.IsValid()) {
  mojo::ReportBadMessage("Bad size from renderer!");
}

Bad

// Bad: Signed overflow is undefined in C++!
int32_t size = mojo_rect->width() * mojo_rect.height();

Note that even if the types have defined overflow semantics, it is almost always a good idea to check for overflow.

Good

uint32_t alloc_size;
if (!CheckMul(request->elements(), request->element_size())
         .AssignIfValid(&alloc_size)) {
  // Safe: avoids allocating with a bogus size that overflowed to a smaller than
  // expected value.
  mojo::ReportBadMessge("Invalid allocation size");
}

Element* array = CreateArray(alloc_size);
for (size_t i = 0; i < request->element_size(); ++i) {
  array[i] = PopulateArray(i);
}

Bad

uint32_t alloc_size = request->elements() * request->element_size();

// Dangerous: alloc_size can overflow so that CreateArray allocates too little
// memory. Subsequent assignments will turn into an out-of-bound write!
Element* array = CreateArray(alloc_size);
for (size_t i = 0; i < request->element_size(); ++i) {
  array[i] = PopulateArray(i);
}

C++ Best Practices

Use mojo::WrapCallbackWithDefaultInvokeIfNotRun And mojo::WrapCallbackWithDropHandler sparingly

Mojo provides several convenience helpers to automatically invoke a callback if the callback has not already been invoked in some other way when the callback is destroyed, e.g.:

  {
    base::Callback<int> cb = mojo::WrapCallbackWithDefaultInvokeIfNotRun(
        base::Bind([](int) { ... }), -1);
  }  // |cb| is automatically invoked with an argument of -1.

This can be useful for detecting interface errors:

  process->GetMemoryStatistics(
      mojo::WrapCallbackWithDefaultInvokeIfNotRun(
          base::Bind(&MemoryProfiler::OnReplyFromRenderer), <failure args>));
  // If the remote process dies, &MemoryProfiler::OnReplyFromRenderer will be
  // invoked with <failure args> when Mojo drops outstanding callbacks due to
  // a connection error on |process|.

However, due to limitations of the current implementation, it's difficult to tell if a callback object has invoke-on-destroy behavior. In general:

  1. Prefer error connection handlers where possible.
  2. Only use the callback helpers for detecting interface errors. These callbacks may be invoked during destruction and must carefully consider receiver object lifetime. For more information, please see the Mojo documentation.

Note that using the callback wrappers in the renderer is often unnecessary. Message pipes are typically closed as part of a Document shutting down; since many Blink objects already inherit blink::ContextLifecycleObserver, it is usually more idiomatic to use this signal to perform any needed cleanup work.

Use StructTraits

Creating a typemap and defining a StructTraits specialization moves the complexity of serialization, deserialization, and validation into a central location. We universally recommend this over defining TypeConverter specializations: when a value fails deserialization, the receiver method will never even be invoked. As a bonus, it also reduces the number of copies during serialization and deserialization. šŸ˜„

Good

// In url_gurl_mojom_traits.h:
template <>
struct StructTraits<url::mojom::UrlDataView, GURL> {
  static base::StringPiece url(const GURL& r);

  // If Read() returns false, Mojo will discard the message.
  static bool Read(url::mojom::UrlDataView data, GURL* out);
};

// In url_gurl_mojom_traits.cc:
// Note that methods that aren't simple getters should be defined
// out-of-line to avoid code bloat.
base::StringPiece StructTraits<url::mojom::UrlDataView, GURL>::url(
    const GURL& r) {
  if (r.possibly_invalid_spec().length() > url::kMaxURLChars ||
      !r.is_valid()) {
    return base::StringPiece();
  }
  return base::StringPiece(r.possibly_invalid_spec().c_str(),
                           r.possibly_invalid_spec().length());
}

bool StructTraits<url::mojom::UrlDataView, GURL>::Read(
    url::mojom::UrlDataView data, GURL* out) {
  base::StringPiece url_string;
  if (!data.ReadUrl(&url_string))
    return false;
  if (url_string.length() > url::kMaxURLChars)
    return false;
  *out = GURL(url_string);
  return !url_string.empty() && out->is_valid();
}

Bad

template <>
struct TypeConverter<url::mojom::UrlPtr, GURL> {
  // Inefficient: this copies data once off the wire to create a
  // url.mojom.Url object, then copies it again to create a GURL.
  static GURL Convert(const url::mojom::UrlPtr url) {
    if (url.url.is_empty()) return GURL();
    // Not good: no way to signal errors, so any code that converts the
    // Mojo struct to a GURL will somehow need to check for errorsā€¦
    // but it can't even be distinguished from the empty URL case!
    if (url.url.size() > url::kMaxURLChars) return GURL();
    return GURL(url.url);
  }
};

There are also corresponding EnumTraits and UnionTraits specializations for mojo enums and unions respectively.

StructTraits getters should be simple

Where possible, StructTraits should be returning const references or simple read-only views of the data. Having to create temporary data structures during serialization should be rare, and it should be even rarer to mutate the input argument.

Out-of-line complex serialization/deserialization logic

A StructTraits specialization is almost always fully specialized. Only define StructTraits methods inline in the header if the method is a simple getter that returns a reference, pointer, or other simple POD. Define all other methods out-of-line to avoid code bloat.

Do not write one-off functions to convert to/from Mojo types

There are some instances where it is simply not possible to define a StructTraits for type mapping: this commonly occurs with Blink IDL and Oilpan types. In these instances, add a TypeConverter specialization rather than defining a one-off conversion function. This makes it easier to search for and audit code that does potentially risky type conversions.

The use of TypeConverter should be limited as much as possible: ideally, only use it in renderers.

Good

template <>
struct TypeConverter<IDLDictionary, mojom::blink::DictionaryPtr> {
  static IDLDictionary* Convert(const mojom::blink::DictionaryPtr& in) {
    // Note that unlike StructTraits, there is no out-of-band way to signal
    // failure.
    IDLDictionary* out = new IDLDictionary;
    out->int_value = in->int_value;
    out->str_value = in->str_value;
    return out;
  }
};

Bad

// Using a custom one-off function makes this hard to discover in security
// audits.
IDLDictionary* FromMojo(const mojom::blink::DictionaryPtr& in) {
  IDLDictionary* out = new IDLDictionary;
  out->int_value = in->int_value;
  out->str_value = in->str_value;
  return out;
}

Use the proper abstractions

mojo::BindingSet implies multiple clients may connect. If this actually isn't the case, please do not use it. For example, if an interface can be rebound, then use the singular mojo::Binding and simply Close() the existing binding before reusing it.

Explicitly reject bad input

While validation should be done inside StructTraits specializations when possible, there are situations where additional checks, e.g. overflow checks, are needed outside of StructTraits specializations. Use mojo::ReportBadMessage() or mojo::GetBadMessageCallback() to reject bad input in these situations. Under the hood, this may record UMAs, kill the process sending bad input, et cetera.

  • mojo::ReportBadMessage(): use to report bad IPC input while a message is being dispatched on the stack.
  • mojo::GetBadMessageCallback(): use to generate a callback to report bad IPC input. The callback must be generated while a message is being dispatched on the stack; however, the returned callback may be invoked be freely invoked in asynchronously posted callbacks.

Java Best Practices

Unfortunately, there are no strongly established conventions here. Most code tends to write manual conversion helpers and throw an exception on conversion failure. See NfcTypeConverter.java as one example of how to write conversion code.

General Code Health

Naming Conventions

Place mojo types in <top-level namespace>.mojom. Directories for Mojo traits should be named mojom (preferable) or ipc. Legacy names that are also encountered are public/interfaces, interfaces, or just mojo.

mojom is preferred for consistency between the directory name and the nested namespace name.

Do not handle impossible situations

Do not clutter the code by handling impossible situations. Omitting it makes the invariants clear. This takes two different forms:

  • A less trustworthy process can be compromised by an adversary and send arbitrary data. When processing data from a less trustworthy process, do not attempt to handle this invalid data: just call mojo::ReportBadMessage(). When invoked in the context of processing an IPC from the renderer, this will kill the renderer process.
  • A more trustworthy process must be trusted, by definition. Do not write code to handle impossible situations “just in case” there's a bug. For example, the renderer class content::RenderFrameImpl must always be connected to certain control interfaces in the browser. It does not makes sense to handle a Mojo connection error and try to reconnect: a connection error signals that the browser process is in the process of deleting the frame, and any attempt at reconnecting will never succeed.

Using mojo enums directly when possible

EnumTraits generally do not add much value: incoming Mojo enum values are already validated before typemapping, so it is guaranteed that the input value to EnumTraits<T>::FromMojom() is already a valid enum value, so the method itself is just a bunch of boilerplate to map between two very similarly named, yet slightly different, enums.

To avoid this, prefer to use the Mojo enum directly when possible.

Consider the cost of setting up message pipes

Message pipes are fairly inexpensive, but they are not free either: it takes 6 control messages to establish a message pipe. Keep this in mind: if the interface is used relatively frequently, connecting once and reusing the interface pointer is probably a good idea.

Ensure An Explicit Grant For WebUI Bindings

WebUI renderers sometimes need to call special, powerful IPC endpoints in a privileged process. It is important to enforce the constraint that the privileged callee previously created and blessed the calling process as a WebUI process, and not as a (potentially compromised) web renderer or other low-privilege process.

  • Use the standard pattern for instantiating MojoWebUIController. WebUI Mojo interfaces must only be exposed through a MojoWebUIController subclass.
  • If there is external functionality that the WebUI needs, make sure to route it through the Mojo interfaces implemented by the MojoWebUIController, to avoid circumventing access checks.

Not-Yet-Shipped Features Should Be Feature-Checked On The Privileged Side

Sometimes, there will be powerful new features that are not yet turned on by default, such as behind a flag, Finch trial, or origin trial. It is not safe to check for the feature's availability on the renderer side (or in another low-privilege process type). Instead, ensure that the check is done in the process that has power to actually enact the feature. Otherwise, a compromised renderer could opt itself in to the feature! If the feature might not yet be fully developed and safe, vulnerabilities could arise.