Fuchsia Namespaces

Namespaces are the backbone of file access and service discovery in Fuchsia.

Definition

A namespace is a composite hierarchy of files, directories, sockets, services, devices, and other named objects provided to a component.

Let's unpack that a little bit.

Objects are named: The namespace contains objects which can be enumerated and accessed by name, much like listing a directory or opening a file.

Composite hierarchy: The namespace is a tree of objects that has been assembled by combining together subtrees of objects from other namespaces into a composite structure where each part has been assigned a path prefix by convention.

Namespace per component: Every component receives its own namespace tailored to meet its own needs. It can also publish objects of its own to be included in other namespaces.

Namespaces can also be created and used independently from components although this document focuses on typical component-bound usage.

Namespaces in Action

You have probably already spent some time exploring a Fuchsia namespace; they are everywhere. If you type ls / at a command-line shell prompt you will see a list of some of the objects that are accessible from the shell's namespace.

Unlike other operating systems, Fuchsia does not have a "root filesystem". As described earlier, namespaces are defined per-component rather than globally or per-process.

This has some interesting implications:

  • There is no global "root" namespace.
  • There is no concept of "running in a chroot-ed environment" because every component effectively has its own private "root".
  • Components receive namespaces tailored to their specific needs.
  • Object paths may not be meaningful across namespace boundaries.
  • A process may have access to several distinct namespaces at once.
  • The mechanisms used to control access to files can also be used to control access to services and other named objects on a per-component basis.

Objects

The items within a namespace are called objects. They come in various flavors, including:

  • Files: objects that contain binary data
  • Directories: objects that contain other objects
  • Sockets: objects that establish connections when opened, like named pipes
  • Services: objects that provide FIDL services when opened
  • Devices: objects that provide access to hardware resources

Accessing Objects

To access an object within a namespace, you must already have another object in your possession. A component typically receives channel handles for objects in the scope of its namespace during Namespace Transfer.

You can also create new objects out of thin air by implementing the appropriate FIDL protocols.

Given an object's channel, you can open a channel for one of its sub-objects by sending it a FIDL message that includes an object relative path expression which identifies the desired sub-object. This is much like opening files in a directory.

Notice that you can only access objects that are reachable from the ones you already have access to. There is no ambient authority.

We will now define how object names and paths are constructed.

Object Names

An object name is a locally unique label by which an object can be located within a container (such as a directory). Note that the name is a property of the container's table of sub-objects rather than a property of the object itself.

For example, cat designates a furry object located within some unspecified recipient of an Open() request.

Objects are fundamentally nameless but they may be called many names by others.

Object names are represented as binary octet strings (arbitrary sequences of bytes) subject to the following constraints:

  • Minimum length of 1 byte.
  • Maximum length of 255 bytes.
  • Does not contain NULs (zero-valued bytes).
  • Does not contain /.
  • Does not equal . or ...
  • Always compared using byte-for-byte equality (implies case-sensitive).

Object names are valid arguments to a container's Open() method. See FIDL Protocols.

It is intended that object names be encoded and interpreted as human-readable sequences of UTF-8 graphic characters, however this property is not enforced by the namespace itself.

Consequently clients are responsible for deciding how to present names which contain invalid, undisplayable, or ambiguous character sequences to the user.

Object Relative Path Expressions

An object relative path expression is an object name or a /-delimited sequence of object names designating a sequence of nested objects to be traversed in order to locate an object within a container (such as a directory).

For example, house/box/cat designates a furry object located within its containing object called box located within its containing object called house located within some unspecified recipient of an Open() request.

An object relative path expression always traverses deeper into the namespace. Notably, the namespace does not directly support upwards traversal out of containers (e.g. via ..) but this feature may be partially emulated by clients (see below).

Object relative path expressions have the following additional constraints:

  • Minimum length of 1 byte.
  • Maximum length of 4095 bytes.
  • Does not begin or end with /.
  • All segments are valid object names.
  • Always compared using byte-for-byte equality (implies case-sensitive).

Object relative path expressions are valid arguments to a container's Open() method. See FIDL Protocols.

Client Interpreted Path Expressions

A client interpreted path expression is a generalization of object relative path expressions, but includes optional features that may be emulated by client code to enhance compatibility with programs that expect a rooted file-like interface.

Technically these features are beyond the scope of the Fuchsia namespace protocol itself but they are often used so we describe them here.

  • A client may designate one of its namespaces to function as its "root". This namespace is denoted /.
  • A client may construct paths relative to its designated root namespace by prepending a single /.
  • A client may construct paths that traverse upwards from containers using .. path segments by folding segments together (assuming the container's path is known) through a process known as client-side "canonicalization".
  • These features may be combined together.

For example, /places/house/box/../sofa/cat designates a furry object located at places/house/sofa/cat within some client designated "root" container.

Client interpreted path expressions that contain these optional features are not valid arguments to a container's Open() method; they must be translated by the client prior to communicating with the namespace. See FIDL Protocols.

For example, fdio implements client-side interpretation of .. paths in file manipulation APIs such as open(), stat(), unlink(), etc.

Namespace Transfer

When a component is started (e.g. its process is started), it receives a table that maps one or more namespace path prefixes to object handles.

The path prefixes in the table encode the intended significance of their associated objects by convention. For example, the pkg prefix should be associated with a directory object, which contains the component's own binaries and assets.

More on this in the next section.

Namespace Conventions

This section describes the conventional layout of namespaces for typical components running on Fuchsia.

The precise contents and organization of a component's namespace varies greatly depending on the component's role, type, identity, scope, relation to other components, and rights. See Sandboxing for information about how namespaces are used to create sandboxes for components.

For more information about the namespace your component can expect to receive, please consult the documentation related to the component type you are implementing.

Typical Objects

There are some typical objects that a component namespace might contain:

  • Read-only executables and assets from the component's package.
  • Private local persistent storage.
  • Private temporary storage.
  • Services offered to the component by the system, the component framework, or by the client that started it.
  • Device nodes (for drivers and privileged components).
  • Configuration information.

Typical Directory Structure

  • pkg/: the contents of the current program's package as they were when the package was signed
    • bin/: executable binaries within the package
    • lib/: shared libraries within the package
    • data/: data, such as assets, within the package
  • data/: local persistent storage (read-write, private to the component)
  • tmp/: temporary storage (read-write, private to the component)
  • svc/: protocols and services offered to the component
    • fuchsia.process.Launcher: launch processes
    • fuchsia.logger.Log: log messages
    • vendor.topic.Interface: service defined by a vendor
  • dev/: device tree (relevant portions visible to privileged components as needed)
    • class/, ...
  • config/: configuration data for the component
    • build-info/: the canonical path for the build information files.
    • ssl/: the canonical path for the SSL certificates
    • tzdata/: time zone data files. The first sub-path should be the data format name, such as tzif2/..., or icudata/....

Namespace Participants

Here is some more information about a few abstractions that interact with and support the Fuchsia namespace protocol.

Filesystems

Filesystems make files available in namespaces.

A filesystem is simply a component that publishes file-like objects from someone else's namespace.

Protocols

Protocols are accessed through namespaces.

A protocol handle is a channel object backed by an implementation of a FIDL protocol, which can be discovered using the namespace. A protocol name corresponds to a path within the /svc branch of the namespace from which a component can access the implementation.

For example, the default Fuchsia logging protocol is fuchsia.logger.Log and its path in the namespace is /svc/fuchsia.logger.Log.

For more details on protocols and components, see protocol capabilities.

Services

Services are accessed through namespaces.

A service handle is a directory that contains related FIDL protocols. Implementations of those protocols can be discovered using the namespace. A service name corresponds to a path within the /svc branch of the namespace from which a component can access the implementation.

For more details on services and components, see service capabilities.

Components

Components consume and extend namespaces.

A component is an executable program object that has been instantiated within some topology and given a namespace.

A component participates in the Fuchsia namespace in two ways:

  1. It can use objects from the namespace provided to it, such as incoming protocols and services or its own package contents.
  2. It can publish objects through its outgoing directory in the form of a namespace to other components.