Directory capabilities

Directory capabilities allow components to connect to directories provided by other components.

Creating directory capabilities

When a component wants to make one of its directories available to other components, it specifies the path of that directory in its outgoing directory in one of the following ways:

• Exposing the directory to the containing realm.

  {
      "expose": [{
          "directory": "/data",
          "from": "self",
          "rights": ["r*"],
      }],
  }
  

• Or offering the directory to some of the component's children with read-write rights.

  {
      "offer": [{
          "directory": "/data",
          "from": "self",
          "rights": ["rw*"],
          "to": [{
              { "dest": "#child-a" },
              { "dest": "#child-b" },
          }],
      }],
  }
  

Consuming directory capabilities

When a component wants to make use of a directory from its containing realm, it does so by using the directory. This will make the directory accessible from the component's namespace.

This example shows a directory named /data that is included in the component‘s namespace. If the component instance accesses this directory during its execution, the component framework performs capability routing to find the component that provides it. Then, the framework connects the directory from the component’s namespace to this provider.

{
    "use": [{
        "directory": "/data",
        "rights": ["r*"],
    }],
}

See //examples/components/routing for a working example of routing a directory capability from one component to another.

Directory capability rights

As directories are offered and exposed throughout the system a user may want to restrict what components who have access to this directory may do. For example, a component could expose a directory as read-write to its parent realm which could expose that directory it to its children as read-write but to its parent as read-only.

Directory rights allow any directory declaration to specify a rights field that indicates the set of rights that the directory would like to offer, expose or use.

Example

This example shows component A requesting access to /data from its namespace with read-write rights:

A.cml
{
    "use": [{
        "directory": "/data",
        "rights": ["rw*"],
    }],
}

Furthermore, parent component B offers the directory /data to component A but with only read-only rights. In this case the routing fails and /data wouldn‘t be present in A’s namespace.

B.cml
{
  "offer": [{
      "directory": "/data",
      "from": "self",
      "rights": ["r*"],
      "to": [{
        { "dest": "#A"}
      }],
  }],
}

Inference Rules

Directory rights are required in the following situations:

• use - All directories use statements must specify their directory rightsn.
• offer and expose from self - All directories that are provided by components must specify their directory rights.

If an expose or offer directory declaration does not specify optional rights, it will inherit the rights from the source of the expose or offer. Rights specified in a use, offer, or expose declaration must be a subset of the rights set on the capability's source.

Framework directory capabilities

Some directory capabilities are available to all components through the framework. When a component wants to use one of these directories, it does so by using the directory with a source of framework.

{
    "use": [{
        "directory": "/hub",
        "from": "framework",
        "rights": ["r*"],
    }],
}

Directory paths

The paths used to refer to directories can be renamed when being offered, exposed, or used.

In the following example, there are three components, A, B, and C, with the following layout:

 A  <- offers directory "/data" from "self" to B as "/intermediary"
 |
 B  <- offers directory "/intermediary" from "realm" to B as "/intermediary2"
 |
 C  <- uses directory "/intermediary2" as "/config"

Each component in this example changes the path used to reference the directory when passing it along in this chain. When component C accesses the /config directory in its namespace, it will be connected to directory /data in component A's outgoing directory.

A.cml:
{
    "offer": [{
        "directory": "/data",
        "from": "self",
        "to": [{
            { "dest": "#B", "as": "/intermediary" },
        }],
        "rights": ["rw*"],
    }],
    "children": [{
        "name": "B",
        "url": "fuchsia-pkg://fuchsia.com/B#meta/B.cm",
    }],
}

B.cml:
{
    "offer": [{
        "directory": "/intermediary",
        "from": "self",
        "to": [{
            { "dest": "#C", "as": "/intermediary2" },
        }],
        "rights": ["r*"],
    }],
    "children": [{
        "name": "C",
        "url": "fuchsia-pkg://fuchsia.com/C#meta/C.cm",
    }],
}

C.cml:
{
    "use": [{
        "directory": "/intermediary2",
        "as": "/config",
    }],
}

If any of the names didn't match in this chain, any attempts by C to list or open items in this directory would fail.