The Extension features files specify the different requirements for extension feature availability.
An extension feature can be any component of extension capabilities. Most notably, this includes extension APIs, but there are also more structural or behavioral features, such as web accessible resources or event pages.
There are four different feature files used:
Note that these files may be present under chrome/common/extensions/api, as well as under extensions/common/api and extensions/shell/common/api.
The feature files are written in JSON. Each file contains a single JSON object with properties for each feature.
{ "feature1": <definition>, "feature2": <definition>, ... }
Most features are known as “simple” features. These are features whose definition is a single object that contains the properties describing the criteria for availability. A simple feature might look like this:
"feature1": { "dependencies": ["permission:feature1"], "contexts": ["blessed_extension"] }
feature1
has a single definition, which says for it to be available, a permission must be present and it must be executed from a blessed context. (These concepts are covered more later in this document.)
Features can also be “complex”. A complex feature has a list of objects to specify multiple groups of possible properties. A complex feature could look like this:
"feature1": [{ "dependencies": ["permission:feature1"], "contexts": ["blessed_extension"] }, { "dependencies": ["permission:otherPermission"], "contexts": ["blessed_extension", "unblessed_extension"] }]
With complex features, if either of the definitions are matched, the feature is available (in other words, the feature definitions are logically OR'd together). Complex features should frequently be avoided, as it makes the logic more involved and slower.
By default, features inherit from parents. A feature‘s ancestry is specified by its name, where a child feature is the parent’s name followed by a ‘.’ and the child's name. That is, feature1.child
is the child of feature1
. Inheritance can carry for multiple levels (e.g. feature1.child.child
), but this is rarely (if ever) useful.
A child feature inherits all the properties of its parent, but can choose to override them or add additional properties. Take the example:
"feature1": { "dependencies": ["permission:feature1"], "contexts": ["blessed_extension"] }, "feature1.child": { "contexts": ["unblessed_extension"], "extension_types": ["extension"] }
In this case, feature1.child
will effectively have the properties
"dependencies": ["permission:feature1"], # inherited from feature1 "contexts": ["unblessed_extension"], # inherited value overridden by child "extension_types": ["extension] # specified by child
If you don‘t want a child to inherit any features from the parent, add the property "noparent": true
. This is useful if, for instance, you have a prefixed API name that isn’t dependent on the prefix, such as app.window (which is fully separate from the app API).
If the parent of a feature is a complex feature, the feature system needs to know which parent to inherit from. To do this, add the property "default_parent": true
to one of the feature definitions in the parent feature.
The following properties are supported in the feature system.
The alias
property specifies that the feature has an associated alias feature. An alias feature is a feature that provides the same functionality as it's source feature (i.e. the feature referenced by the alias). For example, an API alias provides bindings for the source API under a different name. If one wanted to declare an API alias, they would have to introduce an API alias feature - defined as a feature that has source
property, and set alias
property on the original feature. For example, the following would introduce an API alias feature named featureAlias
for API feature
:
{ "feature": { "contexts": ["blessed_extension"], "channel": "dev", "alias": "featureAlias" }, "featureAlias": { "contexts": ["blessed_extension"], "channel": "dev", "source": "feature" } }
featureAlias[source]
value specifies that featureAlias
is an alias for API feature feature
feature[alias]
value specifies that feature
API has an API alias featureAlias
When feature featureAlias
is available, feature
bindings would be accessible using feauteAlias
. In other words chrome.featureAlias
would point to an API with the bindings equivalent to the bindings of feature
API.
The alias API will inherit the schema from the source API, but it will not respect the source API child features. To accomplish parity with the source API feature children, identical child features should be added for the alias API.
Note that to properly create an alias, both source
property on the alias feature and alias
property on the aliased feature have to be set.
Alias features are only available for API features, and each API can have at most one alias. For complex features, alias
property will be set to the alias
value of the first component simple feature that has it set.
The blacklist
property specifies a list of ID hashes for extensions that cannot access a feature. See ID Hashes in this document for how to generate these hashes.
Accepted values are lists of id hashes.
The channel
property specifies a maximum channel for the feature availability. That is, specifying dev
means that the feature is available on dev
, canary
, and trunk
.
Accepted values are a single string from trunk
, canary
, dev
, beta
, and stable
.
The command_line_switch
property specifies a command line switch that must be present for the feature to be available.
Accepted values are a single string for the command line switch (without the preceeding ‘--’).
The component_extensions_auto_granted
specifies whether or not component extensions should be automatically granted access to the feature. By default, this is true
.
The only accepted value is the bool false
(since true is the default).
The contexts
property specifies which JavaScript contexts can access the feature. All API features must specify at least one context, and only API features can specify contexts.
Accepted values are a list of strings from blessed_extension
, blessed_web_page
, content_script
, extension_service_worker
, lock_screen_extension
, web_page
, webui
, and unblessed_extension
.
The lock_screen_extension
context is used instead of blessed_extension
context for extensions on the Chrome OS lock screen. Other extensions related contexts (blessed_web_page
, content_script
, extension_service_worker
, unblessed_extension
) are not expected to be present on the lock screen.
The default_parent
property specifies a feature definition from a complex feature to be used as the parent for any children. See also Inheritance.
The only accepted value is the bool true
.
The dependencies
property specifies which other features must be present in order to access this feature. This is useful so that you don't have to re-specify all the same properties on an API feature and a permission feature.
A common practice is to put as many restrictions as possible in the permission or manifest feature so that we warn at extension load, and put relatively limited properties in an API feature with a dependency on the manifest or permission feature.
To specify a dependent feature, use the prefix the feature name with the type of feature it is, followed by a colon. For example, in order to specify a dependency on a permission feature foo
, we would add the dependency entry permission:foo
.
Accepted values are lists of strings specifying the dependent features.
The extension_types
properties specifies the different classes of extensions that can use the feature. It is very common for certain features to only be allowed in certain extension classes, rather than available to all types.
Accepted values are lists of strings from extension
, hosted_app
, legacy_packaged_app
, platform_app
, shared_module
, and theme
.
The location
property specifies the required install location of the extension.
Accepted values are a single string from component
, external_component
, and policy
.
The internal
property specifies whether or not a feature is considered internal to Chromium. Internal features are not exposed to extensions, and can only be used from Chromium code.
The only accepted value is the bool true
.
The matches
property specifies url patterns which should be allowed to access the feature. Only API features may specify matches
, and matches
only make sense with a context of either webui
or web_page
.
Accepted values are a list of strings specifying the match patterns.
The max_manifest_version
property specifies the maximum manifest version to be allowed to access a feature. Extensions with a greater manifest version cannot access the feature.
The only accepted value is 1
, as currently the highest possible manifest version is 2
.
The min_manifest_version
property specifies the minimum manifest version to be allowed to access a feature. Extensions with a lesser manifest version cannot access the feature.
The only accepted value is 2
, as this is currently the highest possible manifest version.
The noparent
property specifies that a feature should not inherit any properties from a derived parent. See also Inheritance.
The only accepted value is the bool true
.
The platforms
property specifies the properties the feature should be available on.
The accepted values are lists of strings from chromeos
, mac
, linux
, and win
.
The session_types
property specifies in which types of sessions a feature should be available. The session type describes the type of user that is logged in the current session. Session types to which feature can be restricted are only supported on Chrome OS - features restricted to set of session types will be disabled on other platforms. Also, note that all currently supported session types imply that a user is logged into the session (i.e. features that use session_types
property will be disabled when a user is not logged in).
The accepted values are lists of strings from regular
, kiosk
and kiosk.autolaunched
.
regular
session is a session launched for a regular, authenticated user.
kiosk
session is a session launched for a kiosk app - an app that runs on its own, in full control of the current session.
kiosk.autolaunched
represents auto-launched kiosk session - a kiosk session that is launched automatically from Chrome OS login screen, without any user interaction. Note that allowing kiosk
session implies allowing kiosk.autolaunched
session.
The source
property specifies that the feature is an alias for the feature specified by the property value, and is only allowed for API features. For more information about alias features, see alias property documentation.
For complex features, source
property will be set to the source
value of the first component simple feature that has it set.
The whitelist
property specifies a list of ID hashes for extensions that are the only extensions allowed to access a feature.
Accepted values are lists of id hashes.
Instead of listing the ID directly in the whitelist or blacklist section, we use an uppercased SHA1 hash of the id.
To generate a new whitelisted ID for an extension ID, do the following in bash:
$ echo -n "aaaabbbbccccddddeeeeffffgggghhhh" | \ sha1sum | tr '[:lower:]' '[:upper:]'
(Replacing aaaabbbbccccddddeeeeffffgggghhhh
with your extension ID.)
The output should be something like:
9A0417016F345C934A1A88F55CA17C05014EEEBA -
Add the ID to the whitelist or blacklist for the desired feature. It is also often useful to link the crbug next to the id hash, e.g.:
"whitelist": [ "9A0417016F345C934A1A88F55CA17C05014EEEBA" // crbug.com/<num> ]
Google employees: please update http://go/chrome-api-whitelist to map hashes back to ids.
A Feature Context is the type of JavaScript context that a feature can be made available in. This allows us to restrict certain features to only being accessible in more secure contexts, or to expose features to contexts outside of extensions.
For each of these contexts, an “extension” context can refer to a context of either an app or an extension.
The blessed_extension
context refers to a JavaScript context running from an extension process. These are typically the most secure JavaScript contexts, as it reduces the likelihood that a compromised web page renderer will have access to secure APIs.
Traditionally, only pages with a top-level extension frame (with a chrome-extension://
scheme), extension popups, and app windows were blessed extension contexts. With site isolation, extension frames running in web pages are also considered blessed extension contexts, since they are running in the extension process (rather than in the same process as the web page).
The blessed_web_page
context refers to a JavaScript context running from a hosted app. These are similar to blessed extension contexts in that they are (partially) isolated from other processes, but are typically more restricted than blessed extension processes, since hosted apps generally have fewer permissions. Note that these contexts are unaffected by the matches
property.
The content_script
context refers to a JavaScript context for an extension content script. Since content scripts share a process with (and run on the same content as) web pages, these are considered very insecure contexts. Very few features should be exposed to these contexts.
The extension_service_worker
context refers to a JavaScript context for an extension‘s service worker. An extension can only register a service worker for it’s own domain, and these should only be run within an extension process. Thus, these have similar privilege levels to blessed extension processes.
The web_page
context refers to a JavaScript context for a simple web page, completely separate from extensions. This is the least secure of all contexts, and very few features should be exposed to these contexts. When specifying this context, an accompanying URL pattern should be provided with the matches
property.
The webui
context refers to a JavaScript context for a page with WebUI bindings, such as internal chrome pages like chrome://settings or chrome://extensions. These are considered secure contexts, since they are an internal part of chrome. When specifying this context, an accompanying URL pattern should be provided with the matches
property.
The unblessed_extension
context refers to a JavaScript context for an extension frame that is embedded in an external page, like a web page, and runs in the same process as the embedder. Given the limited separation between the (untrusted) embedder and the extension frame, relatively few features are exposed in these contexts. Note that with site isolation, extension frames (even those embedded in web pages) run in the trusted extension process, and become blessed extension contexts.
The feature files are compiled as part of the suite of tools in //tools/json_schema_compiler/. The output is a set of FeatureProviders that contain a mapping of all features.
In addition to being significantly more performant than parsing the JSON files at runtime, this has the added benefit of allowing us to validate at compile time rather than needing a unittest (or allowing incorrect features).
In theory, invalid features should result in a compilation failure; in practice, the compiler is probably missing some cases.
TODO(devlin): Add documentation for extension types. Probably also more on requirements for individual features.