apk安装和优化原理
0x00
apk安装的方式有：
1、开机启动时安装
2、通过adb install 或者在手机中点击apk，进行界面安装。
0x01
开机启动后在system_server中调用PackageManagerService.main，随着调用的深入，循环对每个apk都调用scanPackageLI方法，这个函数提取apk的AndroidManifest.xml里面的内容放在PackagemanagerService中，并且安装了apk，还有优化了dex。
安装apk的代码：
int ret = mInstaller.install(pkgName, useEncryptedFSDir, pkg.applicationInfo.uid,
pkg.applicationInfo.uid); 优化dex的代码：
if (performDexOptLI(pkg, forceDex) == DEX_OPT_FAILED) {
mLastScanError = PackageManager.INSTALL_FAILED_DEXOPT;
return null;
} private int performDexOptLI(PackageParser.Package pkg, boolean forceDex) {
boolean performed = false;
if ((pkg.applicationInfo.flags&ApplicationInfo.FLAG_HAS_CODE) != 0 && mInstaller != null) {
String path = pkg.mScanPath;
int ret = 0;
try {
if (forceDex || dalvik.system.DexFile.isDexOptNeeded(path)) {
ret = mInstaller.dexopt(path, pkg.applicationInfo.uid,
!isForwardLocked(pkg));
pkg.mDidDexOpt = true;
performed = true;
}
} catch (FileNotFoundException e) {
Slog.w(TAG, "Apk not found for dexopt: " + path);
ret = -1;
} catch (IOException e) {
Slog.w(TAG, "IOException reading apk: " + path, e);
ret = -1;
} catch (dalvik.system.StaleDexCacheError e) {
Slog.w(TAG, "StaleDexCacheError when reading apk: " + path, e);
ret = -1;
} catch (Exception e) {
Slog.w(TAG, "Exception when doing dexopt : ", e);
ret = -1;
}
if (ret < 0) {
//error from installer
return DEX_OPT_FAILED;
}
}
return performed ? DEX_OPT_PERFORMED : DEX_OPT_SKIPPED;
}
run_dexopt代码如下：
static void run_dexopt(int zip_fd, int odex_fd, const char* input_file_name,
const char* dexopt_flags)
{
//input_file_name为apk的路径
static const char* DEX_OPT_BIN = "/system/bin/dexopt";
static const int MAX_INT_LEN = 12;
char zip_num[MAX_INT_LEN];
char odex_num[MAX_INT_LEN];
sprintf(zip_num, "%d", zip_fd);//apk文件句柄
sprintf(odex_num, "%d", odex_fd);//dex文件句柄
//调用/system/bin/dexopt工具来优化apk文件
execl(DEX_OPT_BIN, DEX_OPT_BIN, "--zip", zip_num, odex_num, input_file_name,
dexopt_flags, (char*) NULL);
ALOGE("execl(%s) failed: %s\n", DEX_OPT_BIN, strerror(errno));
}fork出的子线程执行的是/system/bin/dexopt，代码位于dalvik\dexopt\OptMain.c
0x02
执行的是/system/bin/dexopt，实际上就是OptMain.c的main函数。
/*
* Main entry point. Decide where to go.
*/
int main(int argc, char* const argv[])
{
set_process_name("dexopt");
setvbuf(stdout, NULL, _IONBF, 0);
if (argc > 1) {
if (strcmp(argv[1], "--zip") == 0)
return fromZip(argc, argv);
else if (strcmp(argv[1], "--dex") == 0)
return fromDex(argc, argv);
else if (strcmp(argv[1], "--preopt") == 0)
return preopt(argc, argv);
}
......
return 1;
}
由于执行时传入的参数是--zip，所以这里执行fromZip。
static int fromZip(int argc, char* const argv[])
{
......
result = processZipFile(zipFd, cacheFd, zipName, dexoptFlags);
bail:
return result;
}然后经过processZipFile->extractAndProcessZip->dvmContinueOptimization。
bool dvmContinueOptimization(int fd, off_t dexOffset, long dexLength,
const char* fileName, u4 modWhen, u4 crc, bool isBootstrap)
{
DexClassLookup* pClassLookup = NULL;
RegisterMapBuilder* pRegMapBuilder = NULL;
u4 headerFlags = 0;
......
{
/*
* Map the entire file (so we don't have to worry about page
* alignment). The expectation is that the output file contains
* our DEX data plus room for a small header.
*/
bool success;
void* mapAddr;
mapAddr = mmap(NULL, dexOffset + dexLength, PROT_READ|PROT_WRITE,
MAP_SHARED, fd, 0);
if (mapAddr == MAP_FAILED) {
LOGE("unable to mmap DEX cache: %s\n", strerror(errno));
goto bail;
}
......
success = rewriteDex(((u1*) mapAddr) + dexOffset, dexLength,
&headerFlags, &pClassLookup);
if (success) {
DvmDex* pDvmDex = NULL;
u1* dexAddr = ((u1*) mapAddr) + dexOffset;
if (dvmDexFileOpenPartial(dexAddr, dexLength, &pDvmDex) != 0) {
LOGE("Unable to create DexFile\n");
success = false;
} else {
......
}
}
......
if (!success)
goto bail;
}
......
if (writeDependencies(fd, modWhen, crc) != 0) {
LOGW("Failed writing dependencies\n");
goto bail;
}
......
if (!writeOptData(fd, pClassLookup, pRegMapBuilder)) {
LOGW("Failed writing opt data\n");
goto bail;
}
......
DexOptHeader optHdr;
memset(&optHdr, 0xff, sizeof(optHdr));
memcpy(optHdr.magic, DEX_OPT_MAGIC, 4);
memcpy(optHdr.magic+4, DEX_OPT_MAGIC_VERS, 4);
optHdr.dexOffset = (u4) dexOffset;
optHdr.dexLength = (u4) dexLength;
optHdr.depsOffset = (u4) depsOffset;
optHdr.depsLength = (u4) depsLength;
optHdr.optOffset = (u4) optOffset;
optHdr.optLength = (u4) optLength;
optHdr.flags = headerFlags;
optHdr.checksum = optChecksum;
fsync(fd); /* ensure previous writes go before header is written */
lseek(fd, 0, SEEK_SET);
if (sysWriteFully(fd, &optHdr, sizeof(optHdr), "DexOpt opt header") != 0)
goto bail;
LOGV("Successfully wrote DEX header\n");
result = true;
//dvmRegisterMapDumpStats();
bail:
dvmFreeRegisterMapBuilder(pRegMapBuilder);
free(pClassLookup);
return result;
}
代码位于dalvik\vm\analysis\DexPrepare.c
此时fd所代表的文件是/data/dalvik-cache/[email protected]，包括一个odex文件头部+从apk中提取的原始的classes.dex。
dexOffset为odex文件头部大小，dexLength为dex文件长度。首先调用mmap把要优化的dex加载到内存虚拟地址mapAddr，dexOffset+dexLength是目前整个文件的大小，这个dex其实就是位于/data/dalvik-cache/[email protected]。
然后调用rewriteDex函数对目标文件进行优化验证，其主要内容包括：字符顺序调整、字节码替换、字节码验证以及文件结构重新对齐。
然后通过writeDependencies写入依赖库信息，writeOptData写入其他优化信息，包括类索引信息以及寄存器映射关系。
最后修改odex文件的头部内容。
生成odex更为详细的流程请参考Android系统ODEX文件格式解析。
此时生成的odex其实就是位于/data/dalvik-cache/[email protected]。
odex结构图如下：
0x03
adb install的安装流程请参考深入理解PackageManagerService。整个安装流程，首先把apk拷贝到/data/local/tmp目录下，在安装的过程中把apk拷贝到/data/app中，最后调用了PackageManagerService的InstallPackagtLI，这个函数调用了installNewPackageLI，installNewPackageLI调用了scanPackageLI，在这个函数里面完成了apk的优化和安装，优化和安装的流程和上面一样。
0x04
本文中讲解了用于PathClassLoader加载/data/dalvik-cache/[email protected]的生成流程。
那么DexClassLoader加载apk的流程是什么呢？
注意PathClassLoader和DexClassLoader的构造函数有不同：
PathClassLoader：
public PathClassLoader(String path, String libPath, ClassLoader parent) {
super(parent);
if (path == null)
throw new NullPointerException();
this.path = path;
this.libPath = libPath;
mPaths = path.split(":");
int length = mPaths.length;
//System.out.println("PathClassLoader: " + mPaths);
mFiles = new File[length];
mZips = new ZipFile[length];
mDexs = new DexFile[length];
boolean wantDex =
System.getProperty("android.vm.dexfile", "").equals("true");
/* open all Zip and DEX files up front */
for (int i = 0; i < length; i++) {
//System.out.println("My path is: " + mPaths[i]);
File pathFile = new File(mPaths[i]);
mFiles[i] = pathFile;
if (pathFile.isFile()) {
try {
mZips[i] = new ZipFile(pathFile);
}
catch (IOException ioex) {
// expecting IOException and ZipException
//System.out.println("Failed opening '" + pathFile + "': " + ioex);
//ioex.printStackTrace();
}
if (wantDex) {
/* we need both DEX and Zip, because dex has no resources */
try {
mDexs[i] = new DexFile(pathFile);
}
catch (IOException ioex) {}
}
}
}
......
}最终调用的是new DexFile(pathFile)。
而DexClassLoader：
public DexClassLoader(String dexPath, String dexOutputDir, String libPath,
ClassLoader parent) {
super(parent);
if (dexPath == null || dexOutputDir == null)
throw new NullPointerException();
mRawDexPath = dexPath;
mDexOutputPath = dexOutputDir;
mRawLibPath = libPath;
String[] dexPathList = mRawDexPath.split(":");
int length = dexPathList.length;
//System.out.println("DexClassLoader: " + dexPathList);
mFiles = new File[length];
mZips = new ZipFile[length];
mDexs = new DexFile[length];
/* open all Zip and DEX files up front */
for (int i = 0; i < length; i++) {
//System.out.println("My path is: " + dexPathList[i]);
File pathFile = new File(dexPathList[i]);
mFiles[i] = pathFile;
if (pathFile.isFile()) {
try {
mZips[i] = new ZipFile(pathFile);
} catch (IOException ioex) {
// expecting IOException and ZipException
System.out.println("Failed opening '" + pathFile
+ "': " + ioex);
//ioex.printStackTrace();
}
/* we need both DEX and Zip, because dex has no resources */
try {
String outputName =
generateOutputName(dexPathList[i], mDexOutputPath);
mDexs[i] = DexFile.loadDex(dexPathList[i], outputName, 0);
} catch (IOException ioex) {
// might be a resource-only zip
System.out.println("Failed loadDex '" + pathFile
+ "': " + ioex);
}
} else {
if (VERBOSE_DEBUG)
System.out.println("Not found: " + pathFile.getPath());
}
}
.......
}
说明DexClassLoader还需要指定一个生成优化后的apk的路径。而PathClassLoader则不需要，因为在安装阶段已经生成了/data/dalvik-cache/[email protected]。