Shared Libraries on Android

This doc outlines some tricks / gotchas / features of how we ship native code in Chrome on Android.

Library Packaging

• Android L & M (ChromeModernPublic.aab):
  • libchrome.so is stored uncompressed within the apk (with the name crazy.libchrome.so to avoid extraction).
  • It is loaded directly from the apk via libchromium_android_linker.so.
  • Only JNI_OnLoad is exported, since manual JNI registration is required (see //base/android/jni_generator/README.md).
• Android N, O & P (MonochromePublic.aab):
  • libmonochrome.so is stored uncompressed within the apk (an AndroidManifest.xml attribute disables extraction).
  • It is loaded directly from the apk by the system linker.
  • It exports all JNI symbols and does not use explicit JNI registration.
  • It is not loaded by libchromium_android_linker.so and relies on the system's webview zygote for RELRO sharing.
• Android Q (TrichromeChrome.aab + TrichromeLibrary.apk):
  • Trichrome uses the exact same native library as Monochrome: libmonochrome.so.
  • libmonochrome.so is stored in the shared APK (TrichromeLibrary.apk) so that it can be shared with TrichromeWebView.
  • It is loaded by libchromium_android_linker.so using android_dlopen_ext() to enable RELRO sharing.

Build Variants (eg. monochrome_64_32_apk)

The packaging above extends to cover both 32-bit and 64-bit device configurations.

Chrome support 64-bit builds, but these do not ship to Stable. The system Webview APK that ships to those devices contains a 32-bit library, and for 64-bit devices, a 64-bit library as well (32-bit Webview client apps will use the 32-bit library, and vice-versa).

Monochrome

Monochrome's intent was to eliminate the duplication between the 32-bit Chrome and Webview libraries (most of the library is identical). In 32-bit Monochrome, a single combined library serves both Chrome and Webview needs. The 64-bit version adds an extra Webview-only library.

More recently, additional Monochrome permutations have arrived. First, Google Play will eventually require that apps offer a 64-bit version to compatible devices. In Monochrome, this implies swapping the architecture of the Chrome and Webview libraries (64-bit combined lib, and extra 32-bit Webview lib). Further down the road, silicon vendors may drop 32-bit support from their chips, after which a pure 64-bit version of Monochrome will apply. In each of these cases, the library name of the combined and Webview-only libraries must match (an Android platform requirement), so both libs are named libmonochrome.so (or libmonochrome_64.so in the 64-bit browser case).

Since 3 of these variations require a 64-bit build config, it makes sense to also support the 4th variant on 64-bit, thus allowing a single builder to build all variants (if desired). Further, a naming scheme must exist to disambiguate the various targets:

monochrome_(browser ABI)_(extra_webview ABI)

For example, the 64-bit browser version with extra 32-bit Webview is monochrome_64_32_apk. The combinations are as follows:

Trichrome

Trichrome has the same 4 permutations as Monochrome, but adds another dimension. Trichrome returns to separate apps for Chrome and Webview, but places shared resources in a third shared-library APK. The table below shows which native libraries are packaged where. Note that dummy placeholder libraries are inserted where needed, since Android determines supported ABIs from the presence of native libraries, and the ABIs of a shared library APK must match its client app.

Crashpad Packaging

• Crashpad is a native library providing out-of-process crash dumping. When a dump is requested (e.g. after a crash), a Crashpad handler process is started to produce a dump.
• Chrome (Android L through M):
  • libchrome_crashpad_handler.so is a standalone executable containing all of the crash dumping code. It is stored compressed and extracted automatically by the system, allowing it to be directly executed to produce a crash dump.
• Monochrome (N through P) and SystemWebView (L through P):
  • All of the Crashpad code is linked into the package‘s main native library (e.g. libmonochrome.so). When a dump is requested, /system/bin/app_process is executed, loading CrashpadMain.java which in turn uses JNI to call into the native crash dumping code. This approach requires building CLASSPATH and LD_LIBRARY_PATH variables to ensure app_process can locate CrashpadMain.java and any native libraries (e.g. system libraries, shared libraries, split apks, etc.) the package’s main native library depends on.
• Monochrome, Trichrome, and SystemWebView (Q+):
  • All of the Crashpad handler code is linked into the package‘s native library. libcrashpad_handler_trampoline.so is a minimal executable packaged with the main native library, stored uncompressed and left unextracted. When a dump is requested, /system/bin/linker is executed to load the trampoline from the APK, which in turn dlopen()s the main native library to load the remaining Crashpad handler code. A trampoline is used to de-duplicate shared code between Crashpad and the main native library packaged with it. This approach isn’t used for P- because the linker doesn't support loading executables on its command line until Q. This approach also requires building a suitable LD_LIBRARY_PATH to locate any shared libraries Chrome/WebView depends on.

Debug Information

What is it?

• Sections of an ELF that provide debugging and symbolization information (e.g. ability convert addresses to function & line numbers).

How we use it:

• ELF debug information is too big to push to devices, even for local development.
• All of our APKs include .so files with debug information removed via strip.
• Unstripped libraries are stored at out/Default/lib.unstripped.
  • Many of our scripts are hardcoded to look for them there.

Unwind Info & Frame Pointers

What are they:

• Unwind info is data that describes how to unwind the stack. It is:
  • It is required to support C++ exceptions (which Chrome doesn't use).
  • It can also be used to produce stack traces.
  • It is generally stored in an ELF section called .eh_frame & .eh_frame_hdr, but arm32 stores it in .ARM.exidx and .ARM.extab.
    • You can see these sections via: readelf -S libchrome.so
• “Frame Pointers” is a calling convention that ensures every function call has the return address pushed onto the stack.
  • Frame Pointers can also be used to produce stack traces (but without entries for inlined functions).

How we use them:

• We disable unwind information (search for exclude_unwind_tables).
• For all architectures except arm64, we disable frame pointers in order to reduce binary size (search for enable_frame_pointers).
• Crashes are unwound offline using minidump_stackwalk, which can create a stack trace given a snapshot of stack memory and the unstripped library (see //docs/testing/using_breakpad_with_content_shell.md)
• To facilitate heap profiling, we ship unwind information to arm32 canary & dev channels as a separate file: assets/unwind_cfi_32

JNI Native Methods Resolution

• For ChromePublic.apk:
  • JNI_OnLoad() is the only exported symbol (enforced by a linker script).
  • Native methods registered explicitly during start-up by generated code.
    • Explicit generation is required because the Android runtime uses the system‘s dlsym(), which doesn’t know about Crazy-Linker-opened libraries.
• For MonochromePublic.apk and TrichromeChrome.aab:
  • JNI_OnLoad() and Java_* symbols are exported by linker script.
  • No manual JNI registration is done. Symbols are resolved lazily by the runtime.

Packed Relocations

• All flavors of lib(mono)chrome.so enable “packed relocations”, or “APS2 relocations” in order to save binary size.
  • Refer to this source file for an explanation of the format.
• To process these relocations:
  • Pre-M Android: Our custom linker must be used.
  • M+ Android: The system linker understands the format.
• To see if relocations are packed, look for LOOS+# when running: readelf -S libchrome.so
• Android P+ supports an even better format known as RELR.
  • We'll likely switch non-Monochrome apks over to using it once it is implemented in lld.

RELRO Sharing

What is it?

• RELRO refers to the ELF segment GNU_RELRO. It contains data that the linker marks as read-only after it applies relocations.
  • To inspect the size of the segment: readelf --segments libchrome.so
  • For lib(mono)chrome.so on arm32, it's about 2mb.
• If two processes map this segment to the same virtual address space, then pages of memory within the segment which contain only relative relocations (99% of them) will be byte-for-byte identical.
  • Note: For fork()ed processes, all pages are already shared (via fork()'s copy-on-write semantics), so RELRO sharing does not apply to them.
• “RELRO sharing” is when this segment is copied into shared memory and shared by multiple processes.

How does it work?

• For Android < N (crazy linker):
  1. Browser Process: libchrome.so loaded normally.
  2. Browser Process: GNU_RELRO segment copied into ashmem (shared memory).
  3. Browser Process (low-end only): RELRO private memory pages swapped out for ashmem ones (using munmap() & mmap()).
  4. Browser Process: Load address and shared memory fd passed to renderers / gpu process.
  5. Renderer Process: Crazy linker tries to load to the given load address.
     • Loading can fail due to address space randomization causing something else to already by loaded at the address.
  6. Renderer Process: If loading to the desired address succeeds:
     • Linker puts GNU_RELRO into private memory and applies relocations as per normal.
     • Afterwards, memory pages are compared against the shared memory and all identical pages are swapped out for ashmem ones (using munmap() & mmap()).
• For a more detailed description, refer to comments in Linker.java.
• For Android N-P:
  • The OS maintains a RELRO file on disk with the contents of the GNU_RELRO segment.
  • All Android apps that contain a WebView load libmonochrome.so at the same virtual address and apply RELRO sharing against the memory-mapped RELRO file.
  • Chrome uses MonochromeLibraryPreloader to call into the same WebView library loading code.
    • When Monochrome is the WebView provider, libmonochrome.so is loaded with the system‘s cached RELRO’s applied.
  • System.loadLibrary() is called afterwards.
    • When Monochrome is the WebView provider, this only calls JNI_OnLoad, since the library is already loaded. Otherwise, this loads the library and no RELRO sharing occurs.
• For non-low-end Android O-P (where there's a WebView zygote):
  • For non-renderer processes, the above Android N+ logic applies.
  • For renderer processes, the OS starts all Monochrome renderer processes by fork()ing the WebView zygote rather than the normal application zygote.
    • In this case, RELRO sharing would be redundant since the entire process' memory is shared with the zygote with copy-on-write semantics.
• For Android Q+ (Trichrome):
  • TrichromeWebView works the same way as on Android N-P.
  • TrichromeChrome uses android_dlopen_ext() and ASharedMemory_create() to perform RELRO sharing, and then relies on a subsequent call to System.loadLibrary() to enable JNI method resolution without loading the library a second time.
  • For renderer processes, TrichromeChrome fork()s from a chrome-specific app zygote. libmonochrome.so is loaded in the zygote before fork().
    • Similar to O-P, app zygote provides copy-on-write memory semantics so RELRO sharing is redundant.

Partitioned libraries

Some Chrome code is placed in feature-specific libraries and delivered via Dynamic Feature Modules.

A linker-assisted partitioning system automates the placement of code into either the main Chrome library or feature-specific .so libraries. Feature code may continue to make use of core Chrome code (eg. base::) without modification, but Chrome must call feature code through a virtual interface.

How partitioning works

The lld linker is now capable of producing a partitioned library, which is effectively an intermediate single file containing multiple libraries. A separate tool (llvm-objcopy) then splits the file into standalone .so files, invoked through a partitioned shared library GN template.

The primary partition is Chrome's main library (eg. libchrome.so), and other partitions may contain feature code (eg. libvr.so). By specifying a list of C/C++ symbols to use as entrypoints, the linker can collect all code used only through these entrypoints, and place it in a particular partition.

To facilitate partitioning, all references from Chrome to the feature entrypoints must be indirect. That is, Chrome must obtain a symbol from the feature library through dlsym(), cast the pointer to its actual type, and call through the resulting pointer.

Feature code retains the ability to freely call back into Chrome‘s core code. When loading the library, the feature module system uses the feature name to look up a partition name (libfoo.so) in an address offset table built into the main library. The resulting offset is supplied to android_dlopen_ext(), which instructs Android to load the library in a particular reserved address region. This allows the feature library’s relative references back to the main library to work, as if the feature code had been linked into the main library originally. No dynamic symbol resolution is required here.

Implications on code placement

• Any symbol referenced by multiple partitions ends up in the main library (even if all calling libraries are feature partitions).
• Symbols that aren‘t feature code (eg. base::) will be pulled into the feature’s library if only that feature uses the code. This is a benefit, but can be unexpected.

Builds that support partitioned libraries

Partitioned libraries are usable when all of the following are true:

• Component build is disabled (component build splits code across GN component target boundaries instead).
• The compiler is Clang.
• The linker is lld.

Library Prefetching

• During start-up, we fork() a process that reads a byte from each page of the library's memory (or just the ordered range of the library).
  • See //base/android/library_loader/.

Historical Tidbits

• We used to use the system linker on M (ModernLinker.java).
  • This was removed due to poor performance.
• We used to use relocation_packer to pack relocations after linking, which complicated our build system and caused many problems for our tools because it caused logical addresses to differ from physical addresses.
  • We now link with lld, which supports packed relocations natively and doesn't have these problems.

See Also

• //docs/android_build_instructions.md#Multiple-Chrome-APK-Targets
• //third_party/android_crazy_linker/README.chromium
• //base/android/linker/BUILD.gn