platform_bionic/android-changes-for-ndk-developers.md

Android changes for NDK developers

This document details important changes related to native code loading in various Android releases.

Required tools: the NDK has an arch-linux-android-readelf binary (e.g. arm-linux-androideabi-readelf or i686-linux-android-readelf) for each architecture (under toolchains/), but you can use readelf for any architecture, as we will be doing basic inspection only. On Linux you need to have the “binutils” package installed for readelf, and “pax-utils” for scanelf.

Changes to library search order

We have made various fixes to library search order when resolving symbols.

With API 22, load order switched from depth-first to breadth-first to fix dlsym(3).

Before API 23, the default search order was to try the main executable, LD_PRELOAD libraries, the library itself, and its DT_NEEDED libraries in that order. For API 23 and later, for any given library, the dynamic linker divides other libraries into the global group and the local group. The global group is shared by all libraries and contains the main executable, LD_PRELOAD libraries, and any library with the DF_1_GLOBAL flag set (by passing “-z global” to ld(1)). The local group is the breadth-first transitive closure of the library and its DT_NEEDED libraries. The M dynamic linker searches the global group followed by the local group. This allows ASAN, for example, to ensure that it can intercept any symbol.

RTLD_LOCAL (Available in API level >= 23)

The dlopen(3) RTLD_LOCAL flag used to be ignored but is implemented correctly in API 23 and later. Note that RTLD_LOCAL is the default, so even calls to dlopen(3) that didn’t explicitly use RTLD_LOCAL will be affected (unless they explicitly used RTLD_GLOBAL). With RTLD_LOCAL, symbols will not be made available to libraries loaded by later calls to dlopen(3) (as opposed to being referenced by DT_NEEDED entries).

GNU hashes (Availible in API level >= 23)

The GNU hash style available with --hash-style=gnu allows faster symbol lookup and is now supported by the dynamic linker in API 23 and above. (Use --hash-style=both if you want to build code that uses this feature >= Android M but still works on older releases.)

Correct soname/path handling (Available in API level >= 23)

The dynamic linker now understands the difference between a library’s soname and its path (public bug https://code.google.com/p/android/issues/detail?id=6670). API level 23 is the first release where search by soname is implemented. Earlier releases would assume that the basename of the library was the soname, and used that to search for already-loaded libraries. For example, dlopen("/this/directory/does/not/exist/libc.so", RTLD_NOW) would find /system/lib/libc.so because it’s already loaded. This also meant that it was impossible to have two libraries "dir1/libx.so" and "dir2/libx.so" --- the dynamic linker couldn’t tell the difference and would always use whichever was loaded first, even if you explicitly tried to load both. This also applied to DT_NEEDED entries.

Some apps have bad DT_NEEDED entries (usually absolute paths on the build machine’s file system) that used to work because we ignored everything but the basename. These apps will fail to load on API level 23 and above.

Symbol versioning (Available in API level >= 23)

Symbol versioning allows libraries to provide better backwards compatibility. For example, if a library author knowingly changes the behavior of a function, they can provide two versions in the same library so that old code gets the old version and new code gets the new version. This is supported in API level 23 and above.

Opening shared libraries directly from an APK

In API level 23 and above, it’s possible to open a .so file directly from your APK. Just use System.loadLibrary("foo") exactly as normal but set android:extractNativeLibs="false" in your AndroidManifest.xml. In older releases, the .so files were extracted from the APK file at install time. This meant that they took up space in your APK and again in your installation directory (and this was counted against you and reported to the user as space taken up by your app). Any .so file that you want to load directly from your APK must be page aligned (on a 4096-byte boundary) in the zip file and stored uncompressed. Current versions of the zipalign tool take care of alignment.

Note that in API level 23 and above dlopen(3) will open a library from any zip file, not just your APK. Just give dlopen(3) a path of the form "my_zip_file.zip!/libs/libstuff.so". As with APKs, the library must be page-aligned and stored uncompressed for this to work.

Private API (Enforced for API level >= 24)

Native libraries must use only public API, and must not link against non-NDK platform libraries. Starting with API 24 this rule is enforced and applications are no longer able to load non-NDK platform libraries. The rule is enforced by the dynamic linker, so non-public libraries are not accessible regardless of the way code tries to load them: System.loadLibrary, DT_NEEDED entries, and direct calls to dlopen(3) will all work exactly the same.

Users should have a consistent app experience across updates, and developers shouldn't have to make emergency app updates to handle platform changes. For that reason, we recommend against using private C/C++ symbols. Private symbols aren't tested as part of the Compatibility Test Suite (CTS) that all Android devices must pass. They may not exist, or they may behave differently. This makes apps that use them more likely to fail on specific devices, or on future releases --- as many developers found when Android 6.0 Marshmallow switched from OpenSSL to BoringSSL.

In order to reduce the user impact of this transition, we've identified a set of libraries that see significant use from Google Play's most-installed apps, and that are feasible for us to support in the short term (including libandroid_runtime.so, libcutils.so, libcrypto.so, and libssl.so). In order to give you more time to transition, we will temporarily support these libraries; so if you see a warning that means your code will not work in a future release -- please fix it now!

$ readelf --dynamic libBroken.so | grep NEEDED
 0x00000001 (NEEDED)                     Shared library: [libnativehelper.so]
 0x00000001 (NEEDED)                     Shared library: [libutils.so]
 0x00000001 (NEEDED)                     Shared library: [libstagefright_foundation.so]
 0x00000001 (NEEDED)                     Shared library: [libmedia_jni.so]
 0x00000001 (NEEDED)                     Shared library: [liblog.so]
 0x00000001 (NEEDED)                     Shared library: [libdl.so]
 0x00000001 (NEEDED)                     Shared library: [libz.so]
 0x00000001 (NEEDED)                     Shared library: [libstdc++.so]
 0x00000001 (NEEDED)                     Shared library: [libm.so]
 0x00000001 (NEEDED)                     Shared library: [libc.so]

Potential problems: starting from API 24 the dynamic linker will not load private libraries, preventing the application from loading.

Resolution: rewrite your native code to rely only on public API. As a short term workaround, platform libraries without complex dependencies (libcutils.so) can be copied to the project. As a long term solution the relevant code must be copied to the project tree. SSL/Media/JNI internal/binder APIs should not be accessed from the native code. When necessary, native code should call appropriate public Java API methods.

A complete list of public libraries is available within the NDK, under platforms/android-API/usr/lib.

Note: SSL/crypto is a special case, applications must NOT use platform libcrypto and libssl libraries directly, even on older platforms. All applications should use GMS Security Provider to ensure they are protected from known vulnerabilities.

Missing Section Headers (Enforced for API level >= 24)

Each ELF file has additional information contained in the section headers. These headers must be present now, because the dynamic linker uses them for sanity checking. Some developers strip them in an attempt to obfuscate the binary and prevent reverse engineering. (This doesn't really help because it is possible to reconstruct the stripped information using widely-available tools.)

$ readelf --header libBroken.so | grep 'section headers'
  Start of section headers:          0 (bytes into file)
  Size of section headers:           0 (bytes)
  Number of section headers:         0

Resolution: remove the extra steps from your build that strip section headers.

Text Relocations (Enforced for API level >= 23)

Starting with API 23, shared objects must not contain text relocations. That is, the code must be loaded as is and must not be modified. Such an approach reduces load time and improves security.

The usual reason for text relocations is non-position independent hand-written assembler. This is not common. Use the scanelf tool as described in our documentation for further diagnostics:

$ scanelf -qT libTextRel.so
  libTextRel.so: (memory/data?) [0x15E0E2] in (optimized out: previous simd_broken_op1) [0x15E0E0]
  libTextRel.so: (memory/data?) [0x15E3B2] in (optimized out: previous simd_broken_op2) [0x15E3B0]
  ...

If you have no scanelf tool available, it is possible to do a basic check with readelf instead, look for either a TEXTREL entry or the TEXTREL flag. Either alone is sufficient. (The value corresponding to the TEXTREL entry is irrelevant and typically 0 --- simply the presence of the TEXTREL entry declares that the .so contains text relocations). This example has both indicators present:

$ readelf --dynamic libTextRel.so | grep TEXTREL
 0x00000016 (TEXTREL)                    0x0
 0x0000001e (FLAGS)                      SYMBOLIC TEXTREL BIND_NOW

Note: it is technically possible to have a shared object with the TEXTREL entry/flag but without any actual text relocations. This doesn't happen with the NDK, but if you're generating ELF files yourself make sure you're not generating ELF files that claim to have text relocations, because the Android dynamic linker trusts the entry/flag.

Potential problems: Relocations enforce code pages being writable, and wastefully increase the number of dirty pages in memory. The dynamic linker has issued warnings about text relocations since Android K (API 19), but on API 23 and above it refuses to load code with text relocations.

Resolution: rewrite assembler to be position independent to ensure no text relocations are necessary. The Gentoo Textrels guide has instructions for fixing text relocations, and more detailed scanelf documentation.

Invalid DT_NEEDED Entries (Enforced for API level >= 23)

While library dependencies (DT_NEEDED entries in the ELF headers) can be absolute paths, that doesn't make sense on Android because you have no control over where your library will be installed by the system. A DT_NEEDED entry should be the same as the needed library's SONAME, leaving the business of finding the library at runtime to the dynamic linker.

Before API 23, Android's dynamic linker ignored the full path, and used only the basename (the part after the last ‘/') when looking up the required libraries. Since API 23 the runtime linker will honor the DT_NEEDED exactly and so it won't be able to load the library if it is not present in that exact location on the device.

Even worse, some build systems have bugs that cause them to insert DT_NEEDED entries that point to a file on the build host, something that cannot be found on the device.

$ readelf --dynamic libSample.so | grep NEEDED
 0x00000001 (NEEDED)                     Shared library: [libm.so]
 0x00000001 (NEEDED)                     Shared library: [libc.so]
 0x00000001 (NEEDED)                     Shared library: [libdl.so]
 0x00000001 (NEEDED)                     Shared library:
[C:\Users\build\Android\ci\jni\libBroken.so]

Potential problems: before API 23 the DT_NEEDED entry's basename was used, but starting from API 23 the Android runtime will try to load the library using the path specified, and that path won't exist on the device. There are broken third-party toolchains/build systems that use a path on a build host instead of the SONAME.

Resolution: make sure all required libraries are referenced by SONAME only. It is better to let the runtime linker to find and load those libraries as the location may change from device to device.

Missing SONAME (Enforced for API level >= 23)

Each ELF shared object (“native library”) must have a SONAME (Shared Object Name) attribute. The NDK toolchain adds this attribute by default, so its absence indicates either a misconfigured alternative toolchain or a misconfiguration in your build system. A missing SONAME may lead to runtime issues such as the wrong library being loaded: the filename is used instead when this attribute is missing.

$ readelf --dynamic libWithSoName.so | grep SONAME
 0x0000000e (SONAME)                     Library soname: [libWithSoName.so]

Potential problems: namespace conflicts may lead to the wrong library being loaded at runtime, which leads to crashes when required symbols are not found, or you try to use an ABI-incompatible library that isn't the library you were expecting.

Resolution: the current NDK generates the correct SONAME by default. Ensure you're using the current NDK and that you haven't configured your build system to generate incorrect SONAME entries (using the -soname linker option).

Writable and Executable Segments (Enforced for API level >= 26)

Each segment in an ELF file has associated flags that tell the dynamic linker what permissions to give the corresponding page in memory. For security, data shouldn't be executable and code shouldn't be writable. This means that the W (for Writable) and E (for Executable) flags should be mutually exclusive. This wasn't historically enforced, but is now.

$ readelf --program-headers -W libBadFlags.so | grep WE
  LOAD           0x000000 0x00000000 0x00000000 0x4c01d 0x4c01d RWE 0x1000

Resolution: we're aware of one middleware product that introduces these into your app. The middleware vendor is aware of the problem and has a fix available.

Invalid ELF header/section headers (Enforced for API level >= 26)

In API level 26 and above the dynamic linker checks more values in the ELF header and section headers and fails if they are invalid.

Example error

dlopen failed: "/data/data/com.example.bad/lib.so" has unsupported e_shentsize: 0x0 (expected 0x28)

Resolution: don't use tools that produce invalid/malformed ELF files. Note that using them puts application under high risk of being incompatible with future versions of Android.