85 lines
4.4 KiB
Markdown
85 lines
4.4 KiB
Markdown
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libnativeloader
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===============================================================================
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Overview
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-------------------------------------------------------------------------------
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libnativeloader is responsible for loading native shared libraries (`*.so`
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files) inside the Android Runtime (ART). The native shared libraries could be
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app-provided JNI libraries or public native libraries like `libc.so` provided
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by the platform.
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The most typical use case of this library is calling `System.loadLibrary(name)`.
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When the method is called, the ART runtime delegates the call to this library
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along with the reference to the classloader where the call was made. Then this
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library finds the linker namespace (named `classloader-namespace`) that is
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associated with the given classloader, and tries to load the requested library
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from the namespace. The actual searching, loading, and linking of the library
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is performed by the dynamic linker.
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The linker namespace is created when an APK is loaded into the process, and is
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associated with the classloader that loaded the APK. The linker namespace is
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configured so that only the JNI libraries embedded in the APK is accessible
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from the namespace, thus preventing an APK from loading JNI libraries of other
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APKs.
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The linker namespace is also configured differently depending on other
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characteristics of the APK such as whether or not the APK is bundled with the
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platform. In case of the unbundled, i.e., downloaded or updated APK, only the
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public native libraries that is listed in `/system/etc/public.libraries.txt`
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are available from the platform, whereas in case of the bundled, all libraries
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under `/system/lib` are available (i.e. shared). In case when the unbundled
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app is from `/vendor` or `/product` partition, the app is additionally provided
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with the [VNDK-SP](https://source.android.com/devices/architecture/vndk#sp-hal)
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libraries. As the platform is getting modularized with
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[APEX](https://android.googlesource.com/platform/system/apex/+/refs/heads/master/docs/README.md),
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some libraries are no longer provided from platform, but from the APEXes which
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have their own linker namespaces. For example, ICU libraries `libicuuc.so` and
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`libicui18n.so` are from the I18n APEX.
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The list of public native libraries is not static. The default set of libraries
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are defined in AOSP, but partners can extend it to include their own libraries.
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Currently, following extensions are available:
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- `/vendor/etc/public.libraries.txt`: libraries in `/vendor/lib` that are
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specific to the underlying SoC, e.g. GPU, DSP, etc.
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- `/{system|product}/etc/public.libraries-<companyname>.txt`: libraries in
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`/{system|product}/lib` that a device manufacturer has newly added. The
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libraries should be named as `lib<name>.<companyname>.so` as in
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`libFoo.acme.so`.
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Note that, due to the naming constraint requiring `.<companyname>.so` suffix, it
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is prohibited for a device manufacturer to expose an AOSP-defined private
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library, e.g. libgui.so, libart.so, etc., to APKs.
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Lastly, libnativeloader is responsible for abstracting the two types of the
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dynamic linker interface: `libdl.so` and `libnativebridge.so`. The former is
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for non-translated, e.g. ARM-on-ARM, libraries, while the latter is for
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loading libraries in a translated environment such as ARM-on-x86.
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Implementation
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-------------------------------------------------------------------------------
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Implementation wise, libnativeloader consists of four parts:
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- `native_loader.cpp`
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- `library_namespaces.cpp`
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- `native_loader_namespace.cpp`
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- `public_libraries.cpp`
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`native_loader.cpp` implements the public interface of this library. It is just
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a thin wrapper around `library_namespaces.cpp` and `native_loader_namespace.cpp`.
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`library_namespaces.cpp` implements the singleton class `LibraryNamespaces` which
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is a manager-like entity that is responsible for creating and configuring
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linker namespaces and finding an already created linker namespace for a given
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classloader.
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`native_loader_namespace.cpp` implements the class `NativeLoaderNamespace` that
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models a linker namespace. Its main job is to abstract the two types of the
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dynamic linker interface so that other parts of this library do not have to know
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the differences of the interfaces.
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`public_libraries.cpp` is responsible for reading `*.txt` files for the public
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native libraries from the various partitions. It can be considered as a part of
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`LibraryNamespaces` but is separated from it to hide the details of the parsing
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routines.
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