什么是ABI
ABI 全称 application binary interface，是一个机器语言级别的接口，描述的是二进制代码之间的兼容关系，这也意味着一起工作的二进制组件是ABI兼容的。一个SO库想要调用另一个SO库的函数，就要求它们的ABI兼容。Stack overflow上有一个以API为类比来说明什么是ABI的回答What is Application Binary Interface。
Process的位数下图是app启动的大致流程，图片来自http://gityuan.com/2016/03/26/app-process-create/。
1. 创建出来的进程究竟是32位还是64位呢？
/**
* Start a new process.
*
* If processes are enabled, a new process is created and the
* static main() function of a processClass is executed there.
* The process will continue running after this function returns.
*
*
If processes are not enabled, a new thread in the caller's
* process is created and main() of processClass called there.
*
*
The niceName parameter, if not an empty string, is a custom name to
* give to the process instead of using processClass. This allows you to
* make easily identifyable processes even if you are using the same base
* processClass to start them.
*
* @param processClass The class to use as the process's main entry
* point.
* @param niceName A more readable name to use for the process.
* @param uid The user-id under which the process will run.
* @param gid The group-id under which the process will run.
* @param gids Additional group-ids associated with the process.
* @param debugFlags Additional flags.
* @param targetSdkVersion The target SDK version for the app.
* @param seInfo null-ok SELinux information for the new process.
* @param abi non-null the ABI this app should be started with.
* @param instructionSet null-ok the instruction set to use.
* @param appDataDir null-ok the data directory of the app.
* @param zygoteArgs Additional arguments to supply to the zygote process.
*
* @return An object that describes the result of the attempt to start the process.
* @throws RuntimeException on fatal start failure
*
* {@hide}
*/
public static final ProcessStartResult start(final String processClass,
final String niceName,
int uid, int gid, int[] gids,
int debugFlags, int mountExternal,
int targetSdkVersion,
String seInfo,
String abi,
String instructionSet,
String appDataDir,
String[] zygoteArgs) {
函数start必须要有一个参数String abi。这个参数决定了启动的进程是64位还是32位。
2、这个参数从哪里来呢？
函数start的唯一调用者就是startProcessLocked, 下面看看这个函数的删减版。
private final void startProcessLocked(ProcessRecord app, String hostingType,
String hostingNameStr, String abiOverride, String entryPoint, String[] entryPointArgs) {
...
String requiredAbi = (abiOverride != null) ? abiOverride : app.info.primaryCpuAbi;
if (requiredAbi == null) {
requiredAbi = Build.SUPPORTED_ABIS[0];
}
String instructionSet = null;
if (app.info.primaryCpuAbi != null) {
instructionSet = VMRuntime.getInstructionSet(app.info.primaryCpuAbi);
}
app.gids = gids;
app.requiredAbi = requiredAbi;
app.instructionSet = instructionSet;
// Start the process. It will either succeed and return a result containing
// the PID of the new process, or else throw a RuntimeException.
boolean isActivityProcess = (entryPoint == null);
if (entryPoint == null) entryPoint = "android.app.ActivityThread";
Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "Start proc: " +
app.processName);
checkTime(startTime, "startProcess: asking zygote to start proc");
Process.ProcessStartResult startResult = Process.start(entryPoint,
app.processName, uid, uid, gids, debugFlags, mountExternal,
app.info.targetSdkVersion, app.info.seinfo, requiredAbi, instructionSet,
app.info.dataDir, entryPointArgs);
...
}
正常运行的时候，代码中的abiOverride几乎总是为空，也就是说start函数的abi参数由app.info.primaryCpuAbi决定。如果app.info.primaryCpuAbi为空，就用Build.SUPPORTED_ABIS[0],也就是ro.product.cpu.abilist中，第一个值。
3、app.info.primaryCpuAbi是哪里来的呢？
系统运行起来之后，会将packages的信息保存在/data/system/packages.xml文件中，所以查看该文件可用知道对应App的ABI，在遇到问题时，可用查看该文件，但是也不能回答app.info.primaryCpuAbi是哪里来的。
这个值是在解析apk的时候确定下来的，如果确定不了，app.info.primaryCpuAbi就为空。
不论是安装应用，还是开机，scanPackageLI总是要被执行的一个函数。从Android L开始，引入了新函数scanPackageDirtyLI 。本文关注的ABI就是在scanPackageDirtyLI中被判断出来的。
在PackageManagerService构造函数中会对系统里所有的App执行scanPackageDirtyLI。
private PackageParser.Package scanPackageDirtyLI(PackageParser.Package pkg, int parseFlags,
int scanFlags, long currentTime, UserHandle user)
在进入scanPackageDirtyLI的时候，pkg.applicationInfo.primaryCpuAbi值为空。因为pkg才实例化不久，primaryCpuAbi值还没有初始化。
函数scanPackageDirtyLI很长，经过各种判断，一些pkg.applicationInfo.primaryCpuAbi被确定下来。比如有so的App就根据so确定下来了。
对于高通项目，要注意derivePackageAbi函数。这个函数会调到一个native方法findSupportedAbi。这个方法在文件com_android_internal_content_NativeLibraryHelper.cpp中，被高通修改过。
if(status <= 0) {
// Scan the 'assets' folder only if
// the abi (after scanning the lib folder)
// is not already set to 32-bit (i.e '1' or '2').
int asset_status = NO_NATIVE_LIBRARIES;
int rc = initAssetsVerifierLib();
if (rc == LIB_INITED_AND_SUCCESS) {
asset_status = GetAssetsStatusFunc(zipFile, supportedAbis, numAbis);
} else {
ALOGE("Failed to load assets verifier: %d", rc);
}
if(asset_status >= 0) {
// Override the ABI only if
// 'asset_status' is a valid ABI (64-bit or 32-bit).
// This is to prevent cases where 'lib' folder
// has native libraries, but
// 'assets' folder has none.
status = asset_status;
}
意思就是如果apk的assets文件夹下有so库，系统会根据so库的ABI去决定app的ABI。
还有一个函数需要注意adjustCpuAbisForSharedUserLPw
/**
* Adjusts ABIs for a set of packages belonging to a shared user so that they all match.
* i.e, so that all packages can be run inside a single process if required.
*
* Optionally, callers can pass in a parsed package via {@code newPackage} in which case
* this function will either try and make the ABI for all packages in {@code packagesForUser}
* match {@code scannedPackage} or will update the ABI of {@code scannedPackage} to match
* the ABI selected for {@code packagesForUser}. This variant is used when installing or
* updating a package that belongs to a shared user.
*
* NOTE: We currently only match for the primary CPU abi string. Matching the secondary
* adds unnecessary complexity.
*/
private void adjustCpuAbisForSharedUserLPw(Set packagesForUser,
PackageParser.Package scannedPackage, boolean forceDexOpt, boolean deferDexOpt,
boolean bootComplete) {
这个函数其实是把有相同android:sharedUserId的packages分成一个组，把组里找到的第一个已经确定ABI的package的ABI作为组里其它还没确定ABI的packages的ABI。
在PackageManagerService构造函数执行完毕之后，系统中绝大部分packages的ABI都确定了，还有极少数没有确定的，相关信息都写入文件/data/system/packages.xml。
问题描述
手机重启之后会有应用反复的crash，导致系统不能使用。
如果再次重启，问题可能就消失了。但是再次重启，可能又出现了。
现象看上去很诡异！
问题分析
crash的问题，一般都会有明显的log。
03-01 00:02:40.792 4133 4133 E AndroidRuntime: FATAL EXCEPTION: main
03-01 00:02:40.792 4133 4133 E AndroidRuntime: Process: com.quicinc.cne.CNEService, PID: 4133
03-01 00:02:40.792 4133 4133 E AndroidRuntime: java.lang.RuntimeException: Unable to instantiate application com.quicinc.cne.CNEService.CNEServiceApp: java.lang.ClassNotFoundException: Didn't find class "com.quicinc.cne.CNEService.CNEServiceApp" on path: DexPathList[[zip file "/system/framework/com.quicinc.cne.jar", zip file "/system/priv-app/CNEService/CNEService.apk"],nativeLibraryDirectories=[/system/priv-app/CNEService/lib/arm64, /system/priv-app/CNEService/CNEService.apk!/lib/armeabi-v7a, /vendor/lib, /system/lib]]
03-01 00:02:40.792 4133 4133 E AndroidRuntime: at android.app.LoadedApk.makeApplication(LoadedApk.java:578)
...
从log看，在apk中找不到class文件。
CNEService是一个系统内置应用，编译的是64位系统，user版本，有做odex，apk中的dex文件被删除。
03-01 00:02:36.367 1284 1284 I PackageManager: Adjusting ABI for : com.quicinc.cne.CNEService to armeabi-v7a
只有arm64文件夹且被认为是armeabi-v7a，系统找不到odex，到apk中去找，但是dex已经删除了，故进程崩溃。
正常情况下，CNEService 的primaryCpuAbi应该arm64-v8a，为什么这里有时候就变成了armeabi-v7a？
上面一节已经描述过primaryCpuAbi的来源了。
从log中可以知道CNEService 的ABI在函数adjustCpuAbisForSharedUserLPw中被确定下来的。
还发现有如下log：
03-01 00:02:36.364 1284 1284 W PackageManager: Instruction set mismatch, PackageSetting{43d890 com.qti.smq.qualcommFeedback/1000} requires arm whereas PackageSetting{e6e3c66 com.caf.fmradio/1000} requires arm64
在函数adjustCpuAbisForSharedUserLPw被执行之前，一些package的ABI已经确定出来了。比如log中提到的qualcommFeedback和fmradio。
也就是说qualcommFeedback，fmradio，CNEService 的android:sharedUserId是一样的，且在执行adjustCpuAbisForSharedUserLPw之前qualcommFeedback的ABI已经确定，是armeabi-v7a，而fmradio的ABI是arm64-v8a。但是因为qualcommFeedback是第一个被找到，故认为其它的没有确定ABI的packages的ABI和qualcommFeedback一样，即CNEService的ABI是armeabi-v7a。如果fmradio先被找到，就会认为其它的没有确定ABI的packages的ABI和fmradio一样，即CNEService的ABI是arm64-v8a。
至此，能解释上面诡异现象了。
当CNEService的ABI是arm64-v8a时，由于存在 arm64/CNEService.odex ，程序正常运行。
当CNEService的ABI是armeabi-v7a时，不存在 arm/CNEService.odex ，CNEService.apk 没有dex，程序崩溃。
修改方法有多种：
- 同时生成arm/CNEService.odex 和arm64/CNEService.odex
- 保留apk中的dex文件
- 修改android:sharedUserId
- 把和CNEService的 android:sharedUserId一样的app都变成arm64-v8a。
......
通过对多份log的分析发现，qualcommFeedback的ABI始终是armeabi-v7a。
之前有提到一个native方法findSupportedAbi，这个函数经过高通修改之后会根据assets文件夹下的so决定得ABI。
打开APK之后，确实在assets下发现了一个32位的so库。
再没了所有疑惑之后，解决问题就不写了。
REF
https://read01.com/BDyndB.html
https://my.oschina.net/bugly/blog/738360
https://my.oschina.net/liucundong/blog/653128
http://gityuan.com/2016/03/26/app-process-create/
https://en.wikipedia.org/wiki/Application_binary_interface
https://mssun.me/blog/android-art-runtime-2-dex2oat.html
http://www.iloveandroid.net/2016/06/27/Android_PackageManagerService-9/