src/docs/updater/functional_spec.md

Chromium Updater Functional Specification

This is the functional specification for Chromium Updater. It describes the externally observable behavior of the updater, including APIs and UI.

[TOC]

Installation

Metainstaller

The metainstaller (UpdaterSetup) is a small executable that contains a compressed copy of the updater as a resource, extracts it, and triggers installation of the updater / an app. The metainstaller is downloaded by the user and can be run from any directory.

The metainstaller may have a tag attached to it. The tag is a piece of unsigned data from which the metainstaller extracts the ID of the application to be installed, along with the application's brand code, usage-stats opt-in status, and any additional parameters to be associated with the application.

After the metainstaller installs the updater, the updater installs an application by connecting to update servers and downloading and executing an application installer.

On Windows, the tag is embedded in one of the certificates in the metainstaller PE. The tag is supported for both EXE and MSI formats.

Tag Format

Tags have a format of a UTF-8 string Gact2.0Omaha{length}{tag}, where {length} is a big-endian uint16, and {tag} is {length} bytes long.

The format of the {tag} piece is further documented in tag.h.

The project also contains code used by non-Google embedders to support UTF-16 tags of the format Gact2.0Omaha{tag}ahamO0.2tcaG, but Chromium-branded and Google-branded builds assume the first case.

Brand code

The brand code is a string of up to 4 characters long. The brand code is persisted during the install, over-installs, and updates.

Elevation (Windows)

The metainstaller parses its tag and re-launches itself at high integrity if installing an application with needsadmin=true or needsadmin=prefers.

Localization

Metainstaller localization presents the metainstaller UI with the user's preferred language on the current system. Every string shown in the UI is translated.

Bundle Installer

TODO(crbug.com/1035895): Implement bundle installers.

The bundle installer allows installation of more than one application. The bundle installer is typically used in software distribution scenarios.

Standalone Installer

TODO(crbug.com/1281688): Implement standalone installers.

TODO(crbug.com/1035895): Document the standalone installer, including building a standalone installer for a given application.

Standalone installers embed all data required to install the application, including the payload and various configuration data needed by the application setup. Such an install completes even if a network connection is not available.

Standalone installers are used:

1. when an interactive user experience is not needed, such as automated deployments in an enterprise.
2. when downloading the application payload is not desirable for any reason.
3. during OEM installation.

Applications on macOS frequently install via "drag-install", and then install the updater using a standalone installer on the application's first-run. The updater app can be embedded in a macOS application bundle as a helper and then invoked with appropriate command line arguments to install itself.

MSI Wrapper

TODO(crbug.com/1327497) - Implement and document.

Scope

The updater is installed in one of the following modes (or scopes):

1. per-system (or per-machine). This mode requires administrator privileges.
2. per-user

Per-machine and per-user instances of the updater can run side by side.

Depending on the scope, the updater is installed at:

• (Windows, User): %LOCAL_APP_DATA%\{COMPANY}\{UPDATERNAME}\{VERSION}\updater.exe
• (Windows, System): %PROGRAM_FILES%\{COMPANY}\{UPDATERNAME}\{VERSION}\updater.exe
• (macOS, User): ~/Library/{COMPANY}/{UPDATERNAME}/{VERSION}/{UPDATERNAME}.app
• (macOS, System): /Library/{COMPANY}/{UPDATERNAME}/{VERSION}/{UPDATERNAME}.app

Command Line

The updater's functionality is split between several processes. The mode of a process is determined by command-line arguments:

• --install [--app-id=...]
  • Install and activate this version of the updater if there is no active updater.
  • --app-id=...
    • Also install the given application.
  • --tag=...
    • Supplies the install metadata needed when installing an application. Typically, a tagged metainstaller invokes the updater with this command line argument.
    • If --tag is specified, --install is assumed.
  • --handoff=...
    • As --tag.
  • --appargs="appguid=...&installerdata=..."
    • Allows extra data (installerdata) to be communicated to the application installer. One per application.
    • appguid must be the first arg followed by the installerdata. The same appguid must appear in tag value or handoff value.
    • This is an alternative to using installdataindex in the tag. Where installdataindex selects from a pre-defined set of installerdata options, this value specifies the exact installerdata to use.
    • The value of installerdata needs to be URL encoded.
    • The data will be decoded and written to a file same as in installdataindex.
  • --offlinedir={absolute dir} or {GUID} relative to the "Offline" dir
    • Performs offline install, which means no update check or file download is performed against the server during installation. All data is read from the files in the offline directory instead.
    • Files in offline directory:
      • Manifest file, named OfflineManifest.gup or <app-id>.gup. The file contains the update check response in XML format.
      • App installer.
      • See the "Offline installs" section below for more information.
    • The switch can be combined with --handoff above.
    • --enterprise
      • Suppresses transmission of pings from the offline install.
• --uninstall
  • Uninstall all versions of the updater.
• --uninstall-self
  • Uninstall this version of the updater.
• --uninstall-if-unused
  • Uninstall all versions of the updater, only if there are no apps being kept up to date by the updater.
• --wake
  • Trigger the updater's periodic background tasks. If this version of the updater is inactive, it may qualify and activate, or uninstall itself. If this version of the updater is active, it may check for updates for applications, unregister uninstalled applications, and more.
• --wake-all
  • Runs --wake for every updater version installed in this scope.
• --crash-me
  • Record a backtrace in the log, crash the program, save a crash dump, and report the crash.
• --crash-handler
  • Starts a crash handler for the parent process.
• --server
  • Launch the updater RPC server. The server answers RPC messages on the UpdateService interface only.
  • --service=update|update-internal
    • If update, the server answers RPC messages on the UpdateService interface only.
    • If update-internal, the server answers RPC messages on the UpdateServiceInternal interface only.
• --windows-service
  • This switch starts the Windows service. This switch is invoked by the SCM either as a part of system startup (SERVICE_AUTO_START) or when CoCreate is called on one of several CLSIDs that the server supports.
  • --console
    • Run in interactive mode.
  • -–com-service
    • If present, run in a mode analogous to --server --service=update. This switch is passed to ServiceMain by the SCM when CoCreate is called on one of several CLSIDs that the server supports. This is used for:
      • The Server for the UI when installing Machine applications.
      • The On-Demand COM Server for Machine applications.
      • COM Server for launching processes at System Integrity, i.e., an Elevator.
• --update
  • Install this version of the updater as an inactive instance.
• --recover
  • Repair the installation of the updater.
  • --appguid=...
    • After recovery, register an application with this id.
  • --browser-version=...
    • Register an application with this version.
    • If --browser-version is specified, --recover can be omitted.
  • --sessionid=...
    • Specifies the sesionid associated with this recovery attempt.
• --test
  • Exit immediately with no error.
• --healthcheck
  • Exit immediately with no error.

If none of the above arguments are set, the updater exits with an error.

Additionally, the mode may be modified by combining it with:

• --system
  • The updater operates in system scope if and only if this switch is present.

Backward-Compatible Updater Shims

To maintain backwards compatibility with Omaha and Keystone, the updater installs small versions of those programs that implement a subset of their APIs.

Keystone Shims

The updater installs a Keystone-like application that contains these shims:

1. The Keystone app executable.
2. The ksadmin helper executable.
3. The ksinstall helper executable.
4. The Keystone Agent helper app executable.

Both the Keystone and Keystone Agent executables simply exit immediately when run.

The ksinstall executable expects to be called with --uninstall and possibly --system. If so, it deletes the Keystone shim (but not the overall updater) from the file system. Otherwise, it exits with a non-zero exit code.

The ksadmin shim is frequently called by applications and handles a variety of command lines:

• --delete, -d
  • Delete a ticket.
  • Accepts -P.
• --install, -i
  • Check for and apply updates.
• --ksadmin-version, -k
  • Print the version of ksadmin.
• --print
  • An alias for --print-tickets.
• --print-tag, -G
  • Print a ticket's tag.
  • Accepts -P.
• --print-tickets, -p
  • Print all tickets.
  • Accepts -P.
• --register, -r
  • Register a new ticket or update an existing one.
  • Accepts -P, -v, -x, -e, -a, -K, -H, -g.

Some of these actions accept parameters:

• --brand-key, -b plistKeyName
  • Set the brand code key. Use with -P and -B. Value must be empty or KSBrandID.
• --brand-path, -B pathToPlist
  • Set the brand code path. Use with -P and -b.
• --productid, -P id
  • Specifies the application ID.
• --system-store, -S
  • Use the system-scope updater, even if not running as root.
  • Not all operations can be performed with -S if not running as root.
• --tag, -g ap
  • Set the application's additional parameters. Use with -P.
• --tag-key, -K plistKeyName
  • Set the additional parameters path key. Use with -P and -H.
• --tag-path, -H pathToPlist
  • Set the tag path. Use with -P and -K.
• --user-initiated, -F
  • Set foreground priority for this operation.
• --user-store, -U
  • Use a per-user ticket store, even if running as root.
• --version, -v version
  • Set the application's version. Use with -P.
• --version-key, -e plistKeyName
  • Set the version path key. Use with -P and -a.
• --version-path, -a pathToPlist
  • Set the version path. Use with -P and -e.
• --xcpath, -x PATH
  • Set a path to use as an existence checker.

Omaha Shims

On Windows, the updater replaces Omaha's files with a copy of the updater, and keeps the Omaha registry entry (CLIENTS/{430FD4D0-B729-4F61-AA34-91526481799D}) up-to-date with the latest pv value. Additionally, the updater replaces the Omaha uninstall command line with its own.

Installer User Interface

During installation, the user is presented with a UI that displays the progress of the download and installation. The user may close the dialog, which cancels the installation. A cancelled installation still results in an event ping to the server indicating an installation failure.

The user interface is localized in the same languages as the Chromium project.

TODO(crbug.com/1286580): Implement and document silent mode.

Help Button

If the installation fails, the updater shows an error message with a "Help" button. Clicking the help button opens a web page in the user's default browser. The page is opened with a query string: ?product={AppId}&errorcode={ErrorCode}.

Updates

There is no limit for the number of retries to update an application if the update fails repeatedly.

Protocol

The updater communicates with update servers using the Omaha Protocol.

The updater uses platform-native network stacks (WinHTTP on Windows and NSURLSession on macOS).

Security

It is not possible to MITM the updater even if the network (including TLS) is compromised. The integrity of the client-server communication is guaranteed by the Client Update Protocol (CUP).

COM Security

The legacy COM classes in updater_legacy_idl.template allow non-admin callers because the interfaces only expose functionality that non-admin callers need.

The new COM classes in updater_internal_idl.template and updater_idl.template require the callers to be admin. This is because the corresponding interfaces allow for unrestricted functionality, such as installing any app that the updater supports. For non-admins, COM creation will fail with E_ACCESSDENIED.

Retries

The updater does not retry an update check that transacted with the backend, even if the response was erroneous (misformatted or unparsable), until the next normally scheduled update check.

DOS Mitigation

The updater sends DoS mitigation headers in requests to the server.

When the server responds with an X-Retry-After header, the client does not issue another update check until the specified period has passed (maximum 24 hours).

• The updater distinguishes between foreground and background priority: if an X-Retry-After was received in the background case, a foreground update is still permitted (but not if it was received in response to a foreground update).

Usage Counts

The updater implements date-last counting, allowing servers to anonymously count the number of active updaters and applications.

Cohort Tracking

The client records the cohort, cohortname, and cohorthint values from the server in each update response (even if there is no-update) and reports them on subsequent update checks.

Installer APIs

As part of installing or updating an application, the updater executes the application's installer. The API for the application installer is platform- specific.

Application installers are run with a 15-minute timeout. If the installer runs for longer than this, the updater assumes failure and continues operation. However, the updater does not kill the installer process.

The application installer API varies by platform. macOS, Windows.

TODO(crbug.com/1339454): Implement running installers at BELOW_NORMAL_PRIORITY_CLASS if the update flow is a background flow.

Installer Setup

To maintain backwards compatibility with Omaha, the updater setup signals a shutdown event that Omaha listens to, so that Omaha processes can shut down gracefully. The updater then proceeds to overinstall the Omaha binaries with the updater binaries.

Offline installs

The updater supports offline installations, for which no update check or file download is performed against the server during installation. All data is read from the files in the directory instead.

Offline installs include:

• an offline manifest file, which contains the update check response in XML format.
• app installer.

For online app installs, the update server checks the compatibility between the application and the host OS that the install is attempted on.

The updater client has equivalent support for offline installs, where no update server is involved.

The platform, arch, and min_os_version attributes in the offline update response are used to determine compatibility of the app being installed with the host OS.

Omaha 3 offline manifests have arch as "x64", but the Chromium functions return "x86_64" as the architecture for amd64. The updater accounts for this by treating "x64" the same as "x86_64".

The offline dir can be specified as a relative path when it is in the format "/offlinedir {GUID}". In this case:

• the actual offline directory is {CURRENT_PROCESS_DIR}\Offline\{GUID},
• the offline manifest is {CURRENT_PROCESS_DIR}\Offline\{GUID}\OfflineManifest.gup, and
• the installer is at {CURRENT_PROCESS_DIR}\Offline\{GUID}\{app_id}\installer.exe.
  • installer.exe may not correspond exactly to the value of the manifest's run attribute, so the code picks the first file it finds in the directory if that is the case.

For more information, see the protocol document.

Enterprise Enrollment

The updater may be enrolled with a particular enterprise. Enrollment is coordinated with a device management server by means of an enrollment token and a device management token. The enrollment token is placed on the device by other programs or the enterprise administrator and serves as an indicator of which enterprise the device should attempt to enroll with. The updater sends the enrollment token, along with the device's machine name, os information, and (on Windows) BIOS serial number. If the server accepts the enrollment, it responds with a device-specific device management token, which is used in future requests to fetch device-specific policies from the device management server.

By default, if enrollment fails, for example if the enrollment token is invalid or revoked, the updater will start in an unmanaged state. Instead, if you want to prevent the updater from starting if enrollment fails, set EnrollmentMandatory to 1.

After the updater sets itself up, the FetchPolicies RPC is invoked on the updater server to register with device management and fetch policies.

The updater also checks for policy updates when the RunPeriodicTasks RPC is invoked at periodic intervals.

Windows

The EnrollmentToken REG_SZ value is read from HKLM\Software\Policies\{COMPANY_SHORTNAME}\CloudManagement.

The EnrollmentMandatory REG_DWORD value is also read from HKLM\Software\Policies\{COMPANY_SHORTNAME}\CloudManagement.

macOS

The enrollment token is searched in the order:

• Managed Preference value with key EnrollmentToken in domain {MAC_BROWSER_BUNDLE_IDENTIFIER}.
• File /Library/{COMPANY_SHORTNAME}/{BROWSER_NAME}/CloudManagementEnrollmentToken.

CBCM enterprise enrollment and policy fetches are done every time an install or or update happens, as well as when the updater periodic background task --wake runs.

Enterprise Policies

Enterprise policies can prevent the installation of applications:

• A per-application setting may specify whether an application is installable.
• If no per-application setting specifies otherwise, the default install policy is used.
• If the default install policy is unset, the application may be installed.

Refer to chrome/updater/protos/omaha_settings.proto for more details.

Policies may be set by platform-specific means (group policy on Windows, managed preferences on macOS), or by communication with the device management server.

For device management, the enterprise policies for Google applications are downloaded from the device management server periodically and stored at a fixed secure location. The path on Windows is %ProgramFiles(x86)%\Google\Policies and on macOS is /Library/Google/GoogleSoftwareUpdate/DeviceManagement.

The policy service searches all active policy providers in pre-determined order for any policy value. When a policy value is configured in multiple providers, the service always returns the first active valid value.

The policy searching order:

Windows

• Policy dictionary defined in External constants(testing overrides)
• Group Policy
• Device Management policy
• Policy from default value provider

macOS

• Policy dictionary defined in External constants(testing overrides)
• Device management policy
• Policy from Managed Preferences
• Policy from default value provider

Deploying enterprise applications via updater policy

For each application that needs to be deployed via the updater, the policy for that application can be set to either Force installs (system wide) or Force installs (per user).

The updater then downloads and installs the application on all machines where the policy is deployed, and where the application is not already installed.

Dynamic Install Parameters

needsadmin

needsadmin is one of the install parameters that can be specified for first installs via the metainstaller tag. needsadmin is used to indicate whether the application needs admin rights to install.

For example, here is a command line for the Updater on Windows that includes:

UpdaterSetup.exe --install --tag="appguid=YourAppID&needsadmin=False"

In this case, the updater client understands that the application installer needs to install the application on a per-user basis for the current user.

needsadmin has the following supported values:

• true: the application supports being installed systemwide and once installed, is available to all users on the system.
• false: the application supports only user installs.
• prefers: the application installation is first attempted systemwide. If the user refuses the UAC prompt however, the application is then only installed for the current user. The application installer needs to be able to support the installation as system, or per-user, or both modes.

installdataindex

installdataindex is one of the install parameters that can be specified for first installs on the command line or via the metainstaller tag.

For example, here is a typical command line for the Updater on Windows:

UpdaterSetup.exe /install "appguid=YourAppID&appname=YourAppName&needsadmin=False&lang=en&installdataindex =verboselog"

In this case, the updater client sends the installdataindex of verboselog to the update server.

This is how a JSON request from the updater client may look like:

{
   "request":{
      "@os":"win",
      "@updater":"updater",
      "acceptformat":"crx3",
      "app":[
         {
            "appid":"YourAppID",
            "data":[
               {
                  "index":"verboselog",
                  "name":"install"
               }
            ],
            "enabled":true,
            "installsource":"ondemand",
            "ping":{
               "r":-2
            },
            "updatecheck":{
               "sameversionupdate":true
            },
            "version":"0.1"
         }
      ],
      "arch":"x86",
      "dedup":"cr",
      "domainjoined":true,
      "hw":{
         "avx":true,
         "physmemory":32,
         "sse":true,
         "sse2":true,
         "sse3":true,
         "sse41":true,
         "sse42":true,
         "ssse3":true
      },
      "ismachine":false,
      "lang":"en-US",
      "nacl_arch":"x86-64",
      "os":{
         "arch":"x86_64",
         "platform":"Windows",
         "version":"10.0.19042.1586"
      },
      "prodversion":"101.0.4949.0",
      "protocol":"3.1",
      "requestid":"{6b417770-1f68-4d52-8843-356760c84d33}",
      "sessionid":"{37775211-4487-48d5-845d-35a1d71b03bc}",
      "updaterversion":"101.0.4949.0",
      "wow64":true
   }
}

The server retrieves the data corresponding to installdataindex=verboselog and returns it back to the updater client.

This is how a JSON response from the update server may look like:

  "response":{
   "protocol":"3.1",
   "app":[
    {"appid":"12345",
     "data":[{
      "status":"ok",
      "name":"install",
      "index":"verboselog",
      "#text":"{\"logging\":{\"verbose\":true}}"
     }],
     "updatecheck":{
     "status":"ok",
     "urls":{"url":[{"codebase":"http://example.com/"},
                    {"codebasediff":"http://diff.example.com/"}]},
     "manifest":{
      "version":"1.2.3.4",
      "prodversionmin":"2.0.143.0",
      "run":"UpdaterSetup.exe",
      "arguments":"--arg1 --arg2",
      "packages":{"package":[{"name":"extension_1_2_3_4.crx"}]}}
     }
    }
   ]
  }

The updater client writes this data to a temporary file in the same directory as the application installer. This is for security reasons, since writing the data to the temp directory could potentially allow a man-in-the-middle attack.

The updater client provides the temporary file as a parameter to the application installer.

Let's say, as shown above, that the update server responds with these example file contents:

{"logging":{"verbose":true}}

The updater client creates a temporary file, say c:\my path\temporaryfile.dat (assuming the application installer is running from c:\my path\YesExe.exe), with the following file contents:

\xEF\xBB\xBF{"logging":{"verbose":true}}

and then provide the file as a parameter to the application installer:

"c:\my path\YesExe.exe" --installerdata="c:\my path\temporaryfile.dat"

• Notice above that the temp file contents are prefixed with an UTF-8 Byte Order Mark of EF BB BF.
• For MSI installers, a property is passed to the installer: INSTALLERDATA="pathtofile".
• For exe-based installers, as shown above, a command line parameter is passed to the installer: --installerdata="pathtofile".
• For Mac installers, an environment variable is set: INSTALLERDATA="pathtofile".
• Ownership of the temp file is the responsibility of the application installer. The updater does not delete this file.
• This installerdata is not persisted anywhere else, and it is not sent as a part of pings to the update server.

Application logo shown in the UI

The app logo is expected to be hosted at {APP_LOGO_URL}{url escaped app_id_}.bmp.

If {url escaped app_id_}.bmp exists, a logo is shown in the updater UI for that app install.

For example, if app_id_ is {8A69D345-D564-463C-AFF1-A69D9E530F96}, the {url escaped app_id_}.bmp is %7b8A69D345-D564-463C-AFF1-A69D9E530F96%7d.bmp.

APP_LOGO_URL is specified in chrome/updater/branding.gni. branding.gni

Update Formats

The updater accepts updates packaged as CRX₃ files. All files are signed with a publisher key. The corresponding public key is hardcoded into the updater.

Differential Updates

TODO(crbug.com/1331030): Implement and document differential update support.

Update Timing

The updater runs periodic tasks every hour, checking its own status, detecting application uninstalls, and potentially checking for updates.

The updater has a base update check period of 4.5 hours (though this can be overridden by policy). Each time the updater runs routine tasks, the update check is only run if the period has elapsed since the last check.

Since the updater's periodic tasks are run every hour, in practice the update check period is always rounded up to the nearest hour.

To prevent multiple updaters from synchronizing their update checks (for example, if a large cohort of machines is powered on at the the same time), the updater will randomly use a longer update check period (120% of the normal period) with 10% probability.

The updater will always check for updates if the time since the last check is negative (e.g. due to clock wander).

Once the updater commits to checking for updates, it will delay the actual check by a random number of milliseconds up to one minute. This avoids sychronizing traffic to the first second of each minute (or the first millisecond of each second).

Background updates can be disabled entirely through policy.

Windows Scheduling of Updates

The update tasks are scheduled using the OS task scheduler.

The time resolution for tasks is 1 minute. Tasks are set to run 5 minutes after they've been created. If a task execution is missed, it will run as soon as the system is able to.

The updater also runs at user login.

On-Demand Updates

The updater exposes an RPC interface for any user to trigger an update check. The update can be triggered by any user on the system, even in the system scope.

The caller provides the ID of the item to update, the install data index to request, the priority, whether a same-version update (repair) is permitted, and callbacks to monitor the progress and completion of the operation.

The interface provides the version of the update, if an update is available.

Regardless of the normal update check timing, the update check is attempted immediately.

App Registration

The updater exposes an RPC interface for users to register an application with the updater. Unlike on-demand updates, cross-user application registration is not permitted.

If the application is already installed, it is registered with the version present on disk. If it has not yet been installed, a version of 0 is used.

App Activity Reporting

Applications can report whether they are actively used or not through the updater. Update servers can then aggregate this information to produce user counts.

Windows:

• When active, the application sets HKCU\SOFTWARE{Company}\Update\ClientState{AppID} → dr (REG_SZ): 1.
  • Note: both user-scoped and system-scoped updaters use HKCU.
• When reporting active use, the updater resets the value to 0.

macOS:

• The application touches ~/Library/{Company}/{Company}SoftwareUpdate/Actives/{APPID}.
• The updater deletes the file when reporting active use.

EULA/ToS Acceptance

Software can be installed or updated only if the user has agreed to the Terms of Service. The updater only runs if the user has accepted the ToS for at least one application.

TODO(crbug.com/1035895): Document EULA signals.

Usage Stats Acceptance

The updater may upload its crash reports and send usage stats if and only if any piece of software it manages is permitted to send usage stats.

Windows:

• Applications enable usage stats by writing: HKCU\SOFTWARE\{Company}\Update\ClientState\{APPID} → usagestats (DWORD): 1 or HKLM\SOFTWARE\{Company}\Update\ClientStateMedium\{APPID} → usagestats (DWORD): 1
• Applications rescind this permission by writing a value of 0.

macOS:

• Application enable usage stats by setting IsUploadEnabled to true for a crashpad database maintained in a "Crashpad" subdirectory of their application data directory.
• The updater searches the file system for Crashpad directories belonging to {Company}.

Telemetry

When the updater installs an application (an installer is run) it sends an event with "eventtype": 2 indicating the outcome of installation. The updater does not send such a ping for its own installation.

When the updater updates an application (including itself) it sends an event with "eventtype": 3 indicating the outcome of update operation.

When the updater detects the uninstallation of an application, it sends an event with "eventtype": 4 to notify the server of the uninstallation.

When the updater attempts to download a file, it sends an event with "eventtype": 14 describing the parameters and outcome of the download.

Multiple events associated with an update session are bundled together into a single request.

Downloading

There could be multiple URLs for a given application payload. The URLs are tried in the order they are returned in the update response.

The integrity of the payload is verified.

There is no download cache. Payloads are re-downloaded for applications which fail to install.

Logging

All updater logs are written to {UPDATER_DATA_DIR}\updater.log.

After the log reaches 5 MiB in size, the updater will attempt to move it to {UPDATER_DATA_DIR}\updater.log.old when starting, replacing any existing file there. The log rotation may be delayed if another updater process is running.

On macOS for system-scope updaters, {UPDATER_DATA_DIR} is /Library/Application Support/{COMPANY_SHORTNAME}/{PRODUCT_FULLNAME}.

On macOS for user-scope updaters, {UPDATER_DATA_DIR} is ~/Library/Application Support/{COMPANY_SHORTNAME}/{PRODUCT_FULLNAME}.

On Windows for system-scope updaters, {UPDATER_DATA_DIR} is %PROGRAMFILES%\{COMPANY_SHORTNAME}\{PRODUCT_FULLNAME}. (A 32-bit updater uses use %PROGRAMFILESX86% if appropriate instead.)

On Windows for user-scope updaters, {UPDATER_DATA_DIR} is %LOCALAPPDATA%\{COMPANY_SHORTNAME}\{PRODUCT_FULLNAME}.

Services

Crash Reporting

The updater uses Crashpad for crash reporting. Each updater process spawns a crash handler child process. Each crash handler process is capable of uploading crashes.

Process Launcher

The feature allows installed products to pre-register and later run elevated command lines in the format c:\program files\foo\exe.exe params. Multiple command lines can be registered per app_id.

This feature is only for system applications.

The program path is always an absolute path. Additionally, the program path is also a child of %ProgramFiles% or %ProgramFiles(x86)%. For instance:

• c:\path-to-exe\exe.exe is an invalid path.
• "c:\Program Files\subdir\exe.exe" is a valid path.
• "c:\Program Files (x86)\subdir\exe.exe" is also a valid path.

Registration

Registration is the same as for App commands, except there are no replaceable parameters. See App command registration.

Usage

Once registered, commands may be invoked using the LaunchCmdElevated method in the IProcessLauncher interface.

Application Commands (applicable to the Windows version of the Updater)

The feature allows installed products to pre-register and later run command lines in the format c:\path-to-exe\exe.exe {params} (elevated for system applications). {params} is optional and can also include replaceable parameters substituted at runtime. Multiple app commands can be registered per app_id.

The program path is always an absolute path. Additionally, for system applications, the program path is also a child of %ProgramFiles% or %ProgramFiles(x86)%. For instance:

• c:\path-to-exe\exe.exe is an invalid path.
• "c:\Program Files\subdir\exe.exe" is a valid path.
• "c:\Program Files (x86)\subdir\exe.exe" is also a valid path.

Registration

App commands are registered in the registry with the following format:

    Update\Clients\{`app_id`}\Commands\`command_id`
        REG_SZ "CommandLine" == {command format}
        {optional} REG_DWORD "AutoRunOnOSUpgrade" == {1}

• There is a deprecated command layout format that is only supported for versions of Google Chrome 110.0.5435.0 and below with the cmd command id.

    Update\Clients\{`app_id`}
        REG_SZ `command_id` == {command format}

Example {command format}: c:\path-to\echo.exe %1 %2 %3 StaticParam4

As shown above, {command format} needs to be the complete path to an executable followed by optional parameters.

If "AutoRunOnOSUpgrade" is non-zero, the command is invoked when the updater detects an OS upgrade. In this case, command format can optionally contain a single substitutible parameter, which is filled in with the OS versions in the format {Previous OS Version}-{Current OS Version}. It is ok to have a static command line as well if the OS versions information is not required.

Usage

Once registered, commands may be invoked using the execute method in the IAppCommandWeb interface.

interface IAppCommandWeb : IDispatch {
  // Use values from the AppCommandStatus enum.
  [propget] HRESULT status([out, retval] UINT*);
  [propget] HRESULT exitCode([out, retval] DWORD*);
  [propget] HRESULT output([out, retval] BSTR*);
  HRESULT execute([in, optional] VARIANT substitution1,
                  [in, optional] VARIANT substitution2,
                  [in, optional] VARIANT substitution3,
                  [in, optional] VARIANT substitution4,
                  [in, optional] VARIANT substitution5,
                  [in, optional] VARIANT substitution6,
                  [in, optional] VARIANT substitution7,
                  [in, optional] VARIANT substitution8,
                  [in, optional] VARIANT substitution9);
};

Here is a code snippet a client may use, with error checking omitted:

var bundle = update3WebServer.createAppBundleWeb();
bundle.initialize();
bundle.createInstalledApp(appGuid);
var app = bundle.appWeb(0);
cmd = app.command(command);
cmd.execute();

Parameters placeholders (%1-%9) are filled by the numbered substitutions in IAppCommandWeb::execute. Placeholders without corresponding substitutions cause the execution to fail.

Clients may poll for the execution status of commands that they have invoked by using the status method of IAppCommandWeb. When the status is COMMAND_STATUS_COMPLETE, the exitCode method can be used to get the process exit code.

Command-Line Format

• for system applications, the executable path is in a secure location such as %ProgramFiles% for security, since it runs elevated.
• placeholders are not permitted in the executable path.
• placeholders take the form of a percent character % followed by a digit. Literal % characters are escaped by doubling them.

For example, if substitutions to IAppCommandWeb::execute are AA and BB respectively, a command format of: echo.exe %1 %%2 %%%2 becomes the command line echo.exe AA %2 %BB

Signature verification

Signature verification is not done on the app command executables, since the AppCommands will always be running from a secure location for system, and the key that defines the application command path is in HKLM, both of which mitigate the threat of a non-admin attacker. An Admin attacker would already be able to bypass any signature checking by binplanting a DLL, or just by performing whatever changes they like on the system, so is outside the threat model.

Policy Status API

The feature allows Chrome and other applications to query the policies that are currently in effect.

Chrome Browser Enterprise (CBE) admins sometimes want to understand if the update policies they have set have propagated to the clients.

Without this API, the only way they can do this is to open up regedit to see if the GPO has propagated correctly.

In addition there is a delay between when the GPO is set on the server and when the value is propagated on the client so being able to verify that the updater picks up the policy can help debug propagation issues as well.

The IPolicyStatus/IPolicyStatus2/IPolicyStatus3 interfaces therefore expose this functionality that can be queried and shown in chrome://policy.

IPolicyStatus/IPolicyStatus2/IPolicyStatus3 interface definition

Uninstallation

On Mac and Linux, if the application was registered with an existence path checker and no file at that path exists (or if the file at that path is owned by another user), the updater considers the application uninstalled, sends the ping, and stops trying to keep it up to date.

On Windows, if the ClientState entry for for the application is deleted, the app is considered uninstalled.

On Windows, the updater registers a "UninstallCmdLine" under the Software\ {Company}\Updater key. This command line can be invoked by application uninstallers to cause the updater to update its registrations. The updater also checks for uninstallations in every periodic task execution.

When the last registered application is uninstalled, the updater uninstalls itself immediately. The updater also uninstalls itself if it has started 24 times but never had a product (besides itself) registered for updates.

The updater uninstaller removes all updater files, registry keys, RPC hooks, scheduled tasks, and so forth from the file system, except that it leaves a small log file in its data directory.

Associated Tools

External Constants Overrides

Building the updater produces both a production-ready updater executable and a version of the executable used for the purposes of testing. The test executable is identical to the production one except that it allows cetain constants to be overridden by the execution environment:

• url: Update check & ping-back URL.
• use_cup: Whether CUP is used at all.
• cup_public_key: An unarmored PEM-encoded ASN.1 SubjectPublicKeyInfo with the ecPublicKey algorithm and containing a named elliptic curve.
• group_policies: Allows setting group policies, such as install and update policies.

Overrides are specified in an overrides.json file placed in the updater data directory.

Tagging Tools

The project contains a helper tool for tagging called certificate_tag.exe. This tool can be used to inject a superfluous certificate into a signed binary to support the creation of tagged binaries.