This FTP is rejected

Rejection rationale

This FTP was rejected because having a hard limit was felt to be too heavy-handed. The types.h file has definitions for ZX_CHANNEL_MAX_MSG_BYTES and ZX_CHANNEL_MAX_MSG_HANDLES that can be examined by the implementor and used to limit the resource consumption.

Possible directions discussed were annotation based constraints (today we have [MaxHandles] and [MaxBytes] attributes).

There are also use cases where we need to express (in the language) possibly unbounded messages (size, or handles). Typically, such messages are dynamically assembled to meet the requirements of the underlying transport (e.g. optimize throughput, adapt to older clients).

FTP-005: Method Impossible



It should be an error to declare an interface method that may require more than the maximum number of handles or bytes allowed in a Zircon channel message.

It's easy to declare FIDL types that may be impossible to send in a single Zircon channel message. Developers should be able to avoid defining types that may cause unexpected runtime errors.

Since we foresee other transports for FIDL data like shared memory, persistent storage and the network, the limit is on interface methods rather than on types.


Edge-cases are hard to test well and hard to reason about. FIDL messages that could be impossible to transmit in exceptional circumstances are likely to expose poorly tested parts of FIDL services. They may be exposed to untrusted code.

For example, in fuchsia.sys, currently a FlatNamespace may contain any number of handles. A LaunchInfo may contain a FlatNamespace. A call to Launcher.CreateComponent() could be crafted (maliciously or accidentally) that would succeed, but then when the appmgr went to pass the supplied LaunchInfo to Runner.StartComponent() the additional handles supplied would block the successful encoding of the message.


This modifies the FIDL compiler but not the FIDL language, bindings or wire format.

The FIDL frontend compiler now keeps track of how many handles and bytes may be required to represent a message. Currently a Zircon channel message may only contain up to 64 handles. The compiler should print an error and fail if any method is defined that could require more than 64 handles to encode its request or response. Currently a Zircon channel message may only contain up to 64k bytes. The compiler should print an error and fail if any method is defined that could require more than 64k bytes to encode its request or response.

There are two main patterns that fall afoul of this restriction: recursive types with handles and unbounded vectors of handles (or types that contain them). It's fairly straight-forward to avoid these.

Documentation and examples

FIDL documentation should document this constraint and ensure that all examples are valid. The error messages produced by the compiler should be clear and useful.

Backwards compatibility

This breaks FIDL source compatibility. Many existing interfaces (such as FlatNamespace described above) fail to constrain the number of handles or bytes that may be required. These interfaces would have to be tightened up.


There should be no direct performance impact. Some interfaces and types may need to be changed if they really want to support arbitrarily many handles or bytes but they never would have worked anyway.


This change reduces the amount of unexpected, under-tested behavior that applications will experience so it will improve security.


The fidlc compiler should get some new tests to ensure that its calculations of handle counts are correct.

Drawbacks, alternatives, and unknowns

This exposes some additional details of the Zircon channel IPC mechanism to FIDL interface authors. This seems like a fair trade-off because currently anyone using those interfaces needs to be aware of those trade-offs.

We could require all strings and vectors to have explicit bounds. This would encourage interface authors to design types that might be safely usable across Zircon channels but might limit the flexibility of FIDL types across other media.

While the only transport for handles is channel messages, there are other transports for FIDL encoded bytes such as VMOs, network sockets and persistent storage. It may be useful to allow some types to opt-out (for example with an attribute) from this constraint, even though those types would never be able to be transmitted over a zircon channel.

It may be worth considering incorporating other mechanism into the FIDL language (pagination / streaming / etc.) before imposing this constraint on byte lengths.

Prior art and references