19 KiB

Raw Blame History Unescape Escape

How To APEX

This doc reflects the current implementation status, and thus is expected to change regularly.

Reference

To understand the design rationale, visit this public doc and go/android-apex (internal).

Building an APEX

A cheat sheet:

apex {
    name: "com.android.my.apex",

    manifest: "apex_manifest.json",

    // optional. if unspecified, a default one is auto-generated
    androidManifest: "AndroidManifest.xml",

    // libc.so and libcutils.so are included in the apex
    native_shared_libs: ["libc", "libcutils"],
    binaries: ["vold"],
    java_libs: ["core-all"],
    apps: ["myapk"],
    prebuilts: ["my_prebuilt"],

    compile_multilib: "both",

    key: "com.android.my.apex.key",
    certificate: ":com.android.my.apex.certificate",
}

apex_manifest.json should look like:

{
  "name": "com.android.my.apex",
  "version": 1
}

The file contexts files should be created at /system/sepolicy/apex/com.android.my.apex-file_contexts:

(/.*)?           u:object_r:system_file:s0
/sub(/.*)?       u:object_r:sub_file:s0
/sub/file3       u:object_r:file3_file:s0

The file should describe the contents of your apex. Note that the file is amended by the build system so that the apexd can access the root directory of your apex and the apex_manifest.pb file. (Technically, they are labeled as system_file.) So if you're building the apex without Soong, please be sure that apexd can access the root directory and the apex_manifest.pb file. (In the example above, the first line does that.)

A script to create a skeleton of APEX

For convenience, you might want to use a script that creates a skeleton (Android.bp, keys, etc.) of an APEX for you. You only need to adjust the APEX_NAME variable to be your actual APEX name.

File types and places where they are installed in apex

file type	place in apex
shared libs	`/lib` and `/lib64` (`/lib/arm` for translated arm in x86)
executables	`/bin`
java libraries	`/javalib`
android apps	`/app` or `/priv-app`
prebuilts	`/etc`

Transitive dependencies

Transitive dependencies of a native shared lib or an executable are automatically included in the APEX. For example, if libFoo depends on libBar, then the two libs are included even when only libFoo is listed in native_shared_libs property.

However, if a transitive dependency has a stable ABI, it is not included transitively. It can be included in an APEX only by directly being referenced. Currently (2019/08/05), the only module type that can provide stable ABI is cc_library. To do so, add stubs.* property as shown below:

cc_library {
    name: "foo",
    srcs: [...],
    stubs: {
        symbol_file: "foo.map.txt",
        versions: ["29", "30"],
    },
}

Use this when a lib has to be accessed across the APEX boundary, e.g. between APEXes or between an APEX and the platform.

apex_available

Any module that is “included” (not just referenced) in an APEX either via the direct dependency or the transitive dependency has to correctly set the apex_available property in its Android.bp file. The property can have one or more of the following values:

<name_of_an_apex>: Like com.android.adbd. By specifying the APEX names explicitly, the module is guaranteed to be included in those APEXes. This is useful when a module has to be kept as an implementation detail of an APEX and therefore shouldn’t be used from outside.
//apex_available:anyapex: This means that the module can be included in any APEX. This is useful for general-purpose utility libraries like libbase, libcutils, etc.
//apex_available:platform: The module can be installed to the platform, outside of APEXes. This is the default value. However, if apex_available is set to either of <name_of_an_apex or //apex_available:anyapex, the default is removed. If a module has to be included in both APEX and the platform, //apex_available:platform and//apex_available:anyapex should be specified together.

The act of adding an APEX name to the apex_available property of a module has to be done or be reviewed by the author(s) of the module. Being included in an APEX means that the module will be portable, i.e., running on multiple versions of the current and previous platforms, whereas it usually was expected to run on the current (the up-to-date) platform. Therefore, the module might have to be prepared to not have version-specific dependencies to the platform, like the existence of a dev node, a system call, etc.

Handling multiple ABIs

compile_multilib: specifies the ABI(s) that this APEX will compile native modules for. Can be either of both, first, 32, 64, prefer32. For most of the cases, this should be both.

native_shared_libs: installed for both primary and secondary ABIs of the device. Of course, if the APEX is built for a target having single ABI (i.e. 32-bit only or 64-bit only), only libraries with the corresponding ABI are installed.

binaries: installed only for the primary ABI of the device. In other words,

If the device is 32-bit only, only the 32-bit variant of the binary is installed.
If the device supports both 32/64 ABIs, but with TARGET_PREFER_32_BIT_EXECUTABLES=true, then only the 32-bit variant of the binary is installed.
If the device is 64-bit only, then only the 64-bit variant of the binary is installed.
If the device supports both 32/64 ABIs, but without TARGET_PREFER_32_BIT_EXECUTABLES=true, then only the 64-bit variant of the binary is installed.

first: matches with the primary ABI of the device. This is the default for binaries.
lib32: matches with the 32-bit ABI of the device, if supported
lib64: matches with the 64-bit ABI of the device, it supported
prefer32: matches with the 32-bit ABI of the device, if support. If 32-bit ABI is not supported, it is matched with the 64-bit ABI.
both: matches with the both ABIs. This is the default for native_shared_libraries.
java libraries and prebuilts: ABI-agnostic

Example: (let’s assume that the device supports 32/64 and does not prefer32)

apex {
    // other properties are omitted
    compile_multilib: "both",
    native_shared_libs: ["libFoo"], // installed for 32 and 64
    binaries: ["exec1"], // installed for 64, but not for 32
    multilib: {
        first: {
            native_shared_libs: ["libBar"], // installed for 64, but not for 32
            binaries: ["exec2"], // same as binaries without multilib.first
        },
        both: {
            native_shared_libs: ["libBaz"], // same as native_shared_libs without multilib
            binaries: ["exec3"], // installed for 32 and 64
        },
        prefer32: {
            native_shared_libs: ["libX"], // installed for 32, but not for 64
        },
        lib64: {
            native_shared_libs: ["libY"], // installed for 64, but not for 32
        },
    },
}

APEX image signing

Note: the APEX skeleton creation script automates this step.

Each APEX must be signed with different keys. There is no concept of the platform key. apexd in the future might reject if multiple APEXes are signed with the same key. When a new key is needed, create a public-private key pair and make an apex_key module. Use key property to sign an APEX using the key. The public key is included in the zip container of the APEX as a file entry apex_pubkey.

How to generate the key pair:

# create an rsa key pair
$ openssl genrsa -out com.android.my.apex.pem 4096

# extract the public key from the key pair
$ avbtool extract_public_key --key com.android.my.apex.pem \
--output com.android.my.apex.avbpubkey

# in Android.bp
apex_key {
    name: "com.android.my.apex.key",
    public_key: "com.android.my.apex.avbpubkey",
    private_key: "com.android.my.apex.pem",
}

Important: In the above example, the name of the public key (that is com.android.my.apex) becomes the ID of the key. The ID of the key used to sign an APEX is recorded in the APEX. At runtime, a public key with the same ID in the device is used to verify the APEX.

APK (APEX container) signing

Note: the APEX skeleton creation script automates this step.

An APEX should also be signed just like APKs. So, an APEX is signed twice; once for the mini file system (apex_payload.img file) and once for the entire file.

Just like APK, the file-level signing is done via the certificate property. It can be set in three ways.

not set: if unset, the APEX is signed with the certificate located at PRODUCT_DEFAULT_DEV_CERTIFICATE. If the flag is also unset, it defaults to build/target/product/security/testkey
<name>: the APEX is signed with the certificate named <name> in the same directory as PRODUCT_DEFAULT_DEV_CERTIFICATE
<name>: the APEX signed with the certificate which is defined by a Soong module named <name>. The certificate module can be defined as follows.

android_app_certificate {
    name: "com.android.my.apex.certificate",
    // This will use com.android.my.apex.x509.pem (the cert) and
    // com.android.my.apex.pk8 (the private key)
    certificate: "com.android.my.apex",
}

How to generate the certificate/private key pair:

# Create certificate and private in PEM form
$ openssl req -x509 -newkey rsa:4096 -nodes -days 999999 -keyout key.pem -out com.android.my.apex.x509.pem

# Enter following info via the interactive prompts
# Country Name: US
# State: California
# Locality Name: Mountain View
# Organization Name: Android
# Organization Unit Name: Android
# Common Name: <your-apk-name>
# Email address: android@android.com

# Convert the private to pkcs8 format
$ openssl pkcs8 -topk8 -inform PEM -outform DER -in key.pem -out com.android.my.apex.pk8 -nocrypt

Signing APEXs with release keys

The procedures described in the APEX image signing and APK (APEX container) signing sections require the private keys to be present in the tree. This is not suitable for public release. Please refer to the APEX signing key replacement documentation to prepare the APEX packages for release.

For the Google-specific procedure for release keys, the documentation is available at go/android-apex-howto-internal (internal only).

Linker namespaces for native libraries and binaries

The linker needs to be set up with separate namespaces for each APEX, for isolation. It is done through ld.config.txt files, which are autogenerated by linkerconfig. Normally you only need to ensure that the APEX manifest correctly lists the native libraries it requires (from platform or other APEXes) and provides, which by default is taken from the build system.

Refer to the design doc for more information about linkerconfig and apex.

Installing an APEX

Use

adb install --staged <path_to_apex> && adb reboot

The adb install --staged command triggers a verification for the staged APEX which might fail when the APEX is signed incorrectly.

Note that on Q devices when the adb install --staged command completes you still will have to wait until the verification for the staged APEX is finished before issuing adb reboot.

On R devices we added the --wait option to adb install to wait until the verification is completed before returning. On S devices the --wait option is implicit.

Hot swapping an APEX (development only)

Use

adb sync && adb shell cmd -w apexservice remountPackages

Note that for this command to remount your APEX, you must ensure that all processes that have reference to your APEX are killed. E.g. if you are developing an APEX that contributes to system_server, you can use the following:

adb root
adb remount
adb shell stop
adb sync
adb shell cmd -w apexservice remountPackages
adb shell start

Using an APEX

After the reboot, the apex will be mounted at /apex/<apex_name>@<version> directory. Multiple versions of the same APEX can be mounted at the same time. A mount point that always points to the latest version of an APEX is provided: /apex/<apex_name>.

Clients can use the latter path to read or execute something from APEX.

So, typical usage of APEX is as follows.

an APEX is pre-loaded under /system/apexwhen the device is shipped.
Files in it are accessed via the /apex/<apex_name>/path.
When an updated version of the APEX is installed in /data/apex/active, the path will point to the new APEX after the reboot.

Updating service with APEX

Using APEX, you can update a service. To do so, you need …

Mark the service in system partition as updatable. Add the new option ‘updatable’ to the service definition.

/system/etc/init/myservice.rc:

service myservice /system/bin/myservice
    class core
    user system
    …
    updatable

Create a new .rc file for the updated service. Use ‘override’ option to redefine the existing service.

/apex/my.apex/etc/init.rc:

service myservice /apex/my.apex/bin/myservice
    class core
    user system
    …
    override

Note that you can only have service definitions in the rc file in APEX. You cannot have action triggers in APEXes.

Also note that if a service marked as updatable is started before APEXes are activated, the start is delayed until the activation of APEXes is finished.

Configuring system to support APEX updates

Set the following system property to true to support APEX file updates.

<device.mk>:

PRODUCT_PROPERTY_OVERRIDES += ro.apex.updatable=true

BoardConfig.mk:
TARGET_FLATTEN_APEX := false

or just
<device.mk>:

$(call inherit-product, $(SRC_TARGET_DIR)/product/updatable_apex.mk)

Flattened APEX

For legacy devices, it is sometimes impossible or infeasible to update the old kernel to fully support APEX. For example, the kernel might have been built without CONFIG_BLK_DEV_LOOP=Y, which is crucial for mounting the file system image inside an APEX.

Flattened APEX is a specially built APEX that can be activated on devices with a legacy kernel. Files in a flattened APEX are directly installed to a directory under the built-in partition. For example, lib/libFoo.so in a flattened APEX my.apex is installed to /system/apex/my.apex/lib/libFoo.so.

Activating a flattened APEX doesn't involve the loop device. The entire directory /system/apex/my.apex is directly bind-mounted to /apex/name@ver.

Flattened APEXs can‘t be updated by downloading updated versions of the APEXs from network because the downloaded APEXs can’t be flattened. Flattened APEXs can be updated only via a regular OTA.

Note that flattened APEX is the default configuration for now (2019/Aug). This means all APEXes are by default flattened unless you explicitly configure your device to support updatable APEX (explained above).

Also note that, mixing flattened and non-flattened APEXes in a device is NOT supported. It should be either all non-flattened or all flattened. This is especially important when shipping pre-signed APEX prebuilts for the projects like Mainline. APEXes that are not pre-signed (i.e. built from the source) should also be non-flattened and signed with proper keys in that case. The device should inherit from updatable_apex.mk as explained above.

Building APEX without Soong

An APEX can be built without relying on the build commands generated by Soong.

Prepare following files:

APEX manifest file (in JSON)
AndroidManifest file (in XML, optional)
AVB private key
APK certificate (*.x509.pem)
APK private key (*.pk8)
file_contexts file
files to be packaged into the APEX

Create canned_fs_config file

It is a file that specifies access bits and uid/gid of each file in the APEX.

/ 1000 1000 0755
/apex_manifest.json 1000 1000 0644
/apex_manifest.pb 1000 1000 0644
/file1 1000 1000 0644
/file2 1000 1000 0644
/dir 0 2000 0755
/dir/file3 1000 1000 0644
...

Note that ALL files AND directories must be specified. And don’t forget to have a line for /and /apex_manifest.pb. (/apex_manifest.json line is for Q-targeting modules)

Invoke apexer

$ apexer \
  --manifest <apex_manifest_file> \
  --file_contexts <file_contexts_file> \
  --canned_fs_config <canned_fs_config_file> \
  --key <avb_private_key_file> \
  --payload_type image \
  --android_manifest <android_manifest_file> \
  --override_apk_package_name com.google.foo \
  <input_directory> \
  <output_apex_file>

--android_manifest and --override_apk_package are optional arguments and thus can be omitted if not needed.

Note: The <apex_manifest_file> shouldn’t be under <input_directory>.

Sign it

apexer signs the apex_payload.img file only. The entire apex (which is a zip file) has to be signed with Signapk.

$ java \
  -Djava.library.path=$(dirname out/soong/host/linux-x86/lib64/libconscrypt_openjdk_jni.so)\
  -jar out/soong/host/linux-x86/framework/signapk.jar \
  -a 4096 \
  <apk_certificate_file> \
  <apk_private_key_file> \
  <unsigned_input_file> \
  <signed_output_file>

This will sign the input file with the cert/privkey pairs to produce the output file.

Re-packaging an existing APEX

If an APEX has been build by passing --include_build_info to apexer (this is the default when building via Soong), it will then include a file named apex_build_info.pb which will store as much information as possible about how the apex was built (see the ApexBuildInfo proto definition for more info) with the exception of the signing keys.

We also provide a tool named deapexer to extract the payload content of an APEX in a local directory.

By using these tools, you can then adapt the procedure described in the building the apex without Soong section and pass the --build_info apex_build_info.pb file where apex_build_info.pb contains all the build parameters that you would otherwise pass via flag to apexer.

We do this programmatically in some unit test code to generate "unusual" APEX files, see for example here and here.

19 KiB Raw Blame History Unescape Escape