Add README file for libnativeloader

Bug: 130388701
Test: N/A
Change-Id: Idb6ef31d72041759d42b9e8fb504af2103c149e3

libnativeloader/README.md

@@ -0,0 +1,84 @@
+libnativeloader
+===============================================================================
+
+Overview
+-------------------------------------------------------------------------------
+libnativeloader is responsible for loading native shared libraries (`*.so`
+files) inside the Android Runtime (ART). The native shared libraries could be
+app-provided JNI libraries or public native libraries like `libc.so` provided
+by the platform.
+
+The most typical use case of this library is calling `System.loadLibrary(name)`.
+When the method is called, the ART runtime delegates the call to this library
+along with the reference to the classloader where the call was made.  Then this
+library finds the linker namespace (named `classloader-namespace`) that is
+associated with the given classloader, and tries to load the requested library
+from the namespace. The actual searching, loading, and linking of the library
+is performed by the dynamic linker.
+
+The linker namespace is created when an APK is loaded into the process, and is
+associated with the classloader that loaded the APK. The linker namespace is
+configured so that only the JNI libraries embedded in the APK is accessible
+from the namespace, thus preventing an APK from loading JNI libraries of other
+APKs.
+
+The linker namespace is also configured differently depending on other
+characteristics of the APK such as whether or not the APK is bundled with the
+platform. In case of the unbundled, i.e., downloaded or updated APK, only the
+public native libraries that is listed in `/system/etc/public.libraries.txt`
+are available from the platform, whereas in case of the bundled, all libraries
+under `/system/lib` are available (i.e. shared). In case when the unbundled
+app is from `/vendor` or `/product` partition, the app is additionally provided
+with the [VNDK-SP](https://source.android.com/devices/architecture/vndk#sp-hal)
+libraries. As the platform is getting modularized with
+[APEX](https://android.googlesource.com/platform/system/apex/+/refs/heads/master/docs/README.md),
+some libraries are no longer provided from platform, but from the APEXes which
+have their own linker namespaces. For example, ICU libraries `libicuuc.so` and
+`libicui18n.so` are from the runtime APEX.
+
+The list of public native libraries is not static. The default set of libraries
+are defined in AOSP, but partners can extend it to include their own libraries.
+Currently, following extensions are available:
+
+- `/vendor/etc/public.libraries.txt`: libraries in `/vendor/lib` that are
+specific to the underlying SoC, e.g. GPU, DSP, etc.
+- `/{system|product}/etc/public.libraries-<companyname>.txt`: libraries in
+`/{system|system}/lib` that a device manufacturer has newly added. The
+libraries should be named as `lib<name>.<companyname>.so` as in
+`libFoo.acme.so`.
+
+Note that, due to the naming constraint requiring `.<companyname>.so` suffix, it
+is prohibited for a device manufacturer to expose an AOSP-defined private
+library, e.g. libgui.so, libart.so, etc., to APKs.
+
+Lastly, libnativeloader is responsible for abstracting the two types of the
+dynamic linker interface: `libdl.so` and `libnativebridge.so`. The former is
+for non-translated, e.g. ARM-on-ARM, libraries, while the latter is for
+loading libraries in a translated environment such as ARM-on-x86.
+
+Implementation
+-------------------------------------------------------------------------------
+Implementation wise, libnativeloader consists of four parts:
+
+- `native_loader.cpp`
+- `library_namespaces.cpp`
+- `native_loader_namespace.cpp`
+- `public_libraries.cpp`
+
+`native_loader.cpp` implements the public interface of this library. It is just
+a thin wrapper around `library_namespaces.cpp` and `native_loader_namespace.cpp`.
+
+`library_namespaces.cpp` implements the singleton class `LibraryNamespaces` which
+is a manager-like entity that is responsible for creating and configuring
+linker namespaces and finding an already created linker namespace for a given
+classloader.
+
+`native_loader_namesapces.cpp` implements the class `NativeLoaderNamespace` that
+models a linker namespace. It's main job is to abstract the two types of the
+dynamic linker interface so that other parts of this library do not have to know
+the differences of the interfaces.
+
+`public_libraries.cpp` is responsible for reading `*.txt` files for the public
+native libraries from the various partitions. It can be considered as a part of
+`LibraryNamespaces` but is separated from it to hide the details of the parsing
+routines.