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@ -1,20 +1,21 @@
#import "../lib.typ": todo, APK, JAR, AXML, ART, SDK, JNI, NDK, DEX, XML, API
#import "../lib.typ": todo, num, APK, JAR, AXML, ART, SDK, JNI, NDK, DEX, XML, API, ZIP, jfl-note
== Android <sec:bg-android>
Android is the smartphone operating system develloped by Google.
It is based on a Long Term Support Linux Kernel, to which patches are added patches develloped by the Android community.
It is based on a Long Term Support Linux Kernel, to which are added patches develloped by the Android community.
On top of the kernel, Android redeveloped many of the usual components used by linux-based operating systems, and added new ones.
Those change make Android a verry unique operating system.
#jfl-note[][Chiffres pour illustrer?]
=== Android Applications <sec:bg-android>
Application in the Android ecosystem are distributed in the #APK format.
#APK files are #JAR files with additionnal features, which are themself ZIP files with additionnal features.
#APK files are #JAR files with additionnal features, which are themself #ZIP files with additionnal features.
A minimal #APK file is a ZIP archive containing a file `AndroidManifest.xml`, the `META-INF/` folder containing the #JAR manifest and signature files, and an #APK Signing Block at the end of the ZIP file.
Other files are then added.
Dalvik bytecode is stored in the `classes.dex`, `classes2.dex`, `classes3.dex`, ... and native code is stored in `lib/<arch>/*.so`.
A minimal #APK file contains a file `AndroidManifest.xml`, the `META-INF/` folder containing the #JAR manifest and signature files, and an #APK Signing Block at the end of the #ZIP file.
The code of the application is then store in a custom format, the Dalvik bytecode, or in the binary ELF format, called native code in the Android ecosystem, or both.
Dalvik bytecode is stored in the `classes.dex`, `classes2.dex`, `classes3.dex`, ... while native code is stored in `lib/<arch>/*.so`.
The `res/` folder contains the ressources required for the user interface.
When ressources are present in `res/`, the file `resources.arsc` is also present at the root of the archive.
The `assets/` folder contains the files that are used directly by the code application.
@ -23,15 +24,15 @@ Depending on the application and compilation process, any kind of other files an
==== Signature
Android applications are cryptographically signed to prove the autorship.
Applicatations signed with same key are considered develloped by the same entity.
This allow to securelly update applications, and application can declare security permission to restrict access to some feature to only application with the same author.
Applicatations signed with same key are considered developed by the same entity.
This allow to securely update applications, and applications can declare security permission to restrict access to some feature to only application with the same author.
Android has several signature schemes coexisting:
- The v1 signature scheme is the #JAR signing scheme, where the signature data is stored in the `META-INF/` folder.
- The v2, v3 and v3.1 signature scheme are store in the '#APK Signing Block' of the #APK.
The v2 signature scheme was introduce in Android 7.0, and to keep retrocompatibility with older version, the v1 scheme is still used in addition to the #APK Signing Block.
The Signing block is an unindexed binary section added to the ZIP file, between the ZIP entries and the Central Directory.
The signature was added in an unindexed section of the ZIP to avoid interferring with the v1 signature scheme that sign the files inside the archive, and not the archive itself.
The v2 signature scheme was introduced in Android 7.0, and to keep retrocompatibility with older version, the v1 scheme is still used in addition to the #APK Signing Block.
The Signing block is an unindexed binary section added to the #ZIP file, between the #ZIP entries and the Central Directory.
The signature was added in an unindexed section of the #ZIP to avoid interferring with the v1 signature scheme that sign the files inside the archive, and not the archive itself.
- The v4 signature scheme is complementary to the v2/v3 signature scheme.
Signature data are stored in an external, `.apk.idsig` file.
@ -39,22 +40,23 @@ Android has several signature schemes coexisting:
The Android Manifest is stored in the `AndroidManifest.xml`, encoded in the binary #AXML format.
The manifest declare important informations about the application:
- generic informations like the application name, id, icon
- The Android compatibility of the applications, in the form of 3 values: the Android `min-sdk`, `target-sdk` and `max-sdk`. Those are the minimum, targeted and maximum version of the Android SDK supported by the application
- The application componants (Activity, Service, Receiver and Provider) of the application and the classes they are associated to
- Intent filters to list the itents that can start or be sent to the application componants
- Security permissions required by the application
- Generic informations like the application name, id, icon.
- The Android compatibility of the applications, in the form of 3 values: the Android `min-sdk`, `target-sdk` and `max-sdk`. Those are the minimum, targeted and maximum version of the Android SDK supported by the application.
- The application componants (Activity, Service, Receiver and Provider) of the application and their associated classes.
- Intent filters to list the intents that can start or be sent to the application componants.
- Security permissions required by the application.
==== Code
==== Code <sec:bg-android-code-format>
An application usually contains at least a `classes.dex` file containing Dalvik bytecode.
This is the format executed by the Android #ART.
It is common for an application to have more thant one #DEX file, when application need to reference more methods than the format allows in one file.
It is common for an application to have more thant one #DEX file, when application need to reference more methods than the format allows in one file
(each method referenced inside a #DEX is associated to a 16 bits number, limiting their number to #num(65536)).
Support for multiple #DEX files was added in the #SDK 21 version of Android, and applications that have multiple #DEX file are sometimes refered to as 'multi-dex'.
In addition to #DEX files, and sometimes instead of #DEX files, applications can contain `.so` ELF (Executable and Linkable Format) files in the `lib/` folder.
In the Android echosystem, binary code is called native code.
Because native code is compile for a specific architecture, `.so` files are present in different versions, stored in different subfolders, depending on the targetted architecture.
In the Android ecosystem, binary code is called native code.
Because native code is compiled for a specific architecture, `.so` files are present in different versions, stored in different subfolders, depending on the targetted architecture.
For example `lib/arm64-v8a/libexample.so` is the version of the `example` library compiled for an ARM 64 architecture.
Because smartphones mostly use ARM processors, it is not rare to see applications that only have the ARM version of their native code.
@ -77,28 +79,28 @@ The source code is then compile.
The most common programming langages used for Android application are Java and Kotlin.
Both are first compiled to java bytecode in `.class` files using the langage compiler.
To allow access to the Android #API, the `.class` are linked during the compilation to an `android.jar` file that contains classes with the same signatures as the one in the Android #API for the targeted SDK.
The `.class` files are the converted to #DEX files using `d8`.
During those steeps, both the original langage compiler and `d8` can perform optimizations on the classes.
The `.class` files are then converted into the #DEX format using `d8`.
During those steeps, both the original langage compiler and `d8` can perform optimizations on the classes, like code shrinking, inlining, etc.
If the application contains native code, the original C or C++ code is compile using tools Android #NDK to target the different architecture target.
If the application contains native code, the original C or C++ code is compile using tools Android from the #NDK to target the different possible architectures.
`aapt` is then used once again to package all the generated #AXML, #DEX, `.so` files, as well as the other ressources files, assets, `resources.arsc`, and any additionnal files deemed necessary in ZIP file.
`aapt` ensures that the generated ZIP is compatible with the requirement from Android.
For instance, the `resources.arsc` will be mapped directly in memory at runtime, so it must not be compressed inside the ZIP file.
`aapt` is then used once again to package all the generated #AXML, #DEX, `.so` files, as well as the other ressources files, assets, `resources.arsc`, and any additionnal files deemed necessary to form the final #ZIP file.
`aapt` ensures that the generated #ZIP is compatible with the requirement from Android.
For instance, the `resources.arsc` will be mapped directly in memory at runtime, so it must not be compressed inside the #ZIP file.
If necessary, the ZIP file is then aligned using `zipalign`.
Again, this is to ensure compatibility with android optimizations: files like `resources.arsc` need to be 4 bits alligned to be mapped in memory.
If necessary, the #ZIP file is then aligned using `zipalign`.
Again, this is to ensure compatibility with android optimizations: some files like `resources.arsc` need to be 4 bits alligned to be mapped in memory.
The last step is to sign the application using the `apksigner` utility.
Since 2021, Google require that new applications in the Google Play app store to be uploaded in a new format called Android App Bundles.
Since 2021, Google requires that new applications in the Google Play app store to be uploaded in a new format called Android App Bundles.
The main difference is that Google will perform the last packaging steps and generate (and sign) the application itself.
This allow Google to generate different applications for different target, and avoid including unnecessary files in the application like native code targetting the wrong architecture.
=== Android Runtime <sec:bg-art>
Android runtime environement has many specificities that sets it appart from other platforms.
An heavy heavy empasis is put on isolating the applications from one another as well from the systems critical capabilities.
An heavy emphasis is put on isolating the applications from one another as well from the systems critical capabilities.
The code execution itself can be confusing at first.
Instead of the usual linear model with a single entry point, applications have many entrypoints that are called by the Android framework in accordance to external events.
@ -106,23 +108,22 @@ Instead of the usual linear model with a single entry point, applications have m
#todo[Subsection name?]
Android application expose their componants to the Android Runtime (#ART) via classes inheriting specific classes from the Android SDK.
They are four type of application commponents, that serves as entry points for application.
Each has a class associated to it, and serves a different role.
Android application expose their componants to the Android Runtime (#ART) via classes inheriting specific classes from the Android #SDK.
Four classes represent application components that can be used as entry points:
/ Activities: An activity represent a single screen with a user interface. This is the componant used to interact with a user.
/ Activities: An activity represent a single screen with a user interface. This is the component used to interact with a user.
/ Services: A service serves as en entrypoint to run the application in the background.
/ Broadcast receivers: A broadcast receiver is an entry point used when a matching event is broadcasted by the system. They allow to application responce to event event when not started.
/ Content providers: A content provider is a componant that manage data accessible by other app through the content provider.
/ Broadcast receivers: A broadcast receiver is an entry point used when a matching event is broadcasted by the system.
/ Content providers: A content provider is a component that manage data accessible by other app through the content provider.
Componant must be listed in the `AndroidManifest.xml` of the application so that the system nows of them.
In the course of a componant live cicle, the system will call specifics methods defined by the classes associated to each componant type.
Those methods are to be overrident by the classes defined in the application if they are specific action to be perfomed.
For instance, an activitymight compute some values in `onCreate()`, called when the activity is created, save the value of those variable to the file system in `onStop()`, called when the acitivity stop being visible to the user, and recover the saved values in `onRestart()`, called when the user navigate back to the activity.
Components must be listed in the `AndroidManifest.xml` of the application so that the system knows of them.
In the live cicle of a component, the system will call specific methods defined by the classes associated to each componant type.
Those methods are to be overridden by the classes defined in the application if they are specific action to be perfomed.
For instance, an activity might compute some values in `onCreate()`, called when the activity is created, save the value of those variable to the file system in `onStop()`, called when the acitivity stop being visible to the user, and recover the saved values in `onRestart()`, called when the user navigate back to the activity.
In addition to the componants declared in the manifest that act as entry points, the Android #API heavily relies on callbacks.
The most obvious cases are for the user interface, for example a button will call a callback method defined by the application when clicked.
Other part of the #API also rely on non-linear execution, for example when an application send an itent (see @sec:bg-sandbox), the intent sent in responce is transmitted to back to the application by calling another method.
Other part of the #API also rely on non-linear execution, for example when an application sends an intent (see @sec:bg-sandbox), the intent sent in responce is transmitted back to the application by calling another method.
==== Application Isolation and Interprocess Communication <sec:bg-sandbox>