1. 追根溯源dvmDexFileOpenPartial脫殼點

首先,我們需要知道,Dex載入最後的邏輯實現幾乎都是在BaseDexClassLoader

//  http://androidxref.com/4.4.2_r2/xref/libcore/dalvik/src/main/java/dalvik/system/BaseDexClassLoader.java#29
public class BaseDexClassLoader extends ClassLoader {
    private final DexPathList pathList;
    
    ...
    
    public BaseDexClassLoader(String dexPath, File optimizedDirectory,
            String libraryPath, ClassLoader parent) {
        //這裡就是設定parent的地方
        super(parent);
        this.pathList = new DexPathList(this, dexPath, libraryPath, optimizedDirectory);
    }
    
    ...
}

然後進入到DexPathList

// http://androidxref.com/4.4.2_r2/xref/libcore/dalvik/src/main/java/dalvik/system/DexPathList.java#85
public DexPathList(ClassLoader definingContext, String dexPath,
           String libraryPath, File optimizedDirectory) {
        
        ...
        
        this.definingContext = definingContext;
        ArrayList<IOException> suppressedExceptions = new ArrayList<IOException>();
        this.dexElements = makeDexElements(splitDexPath(dexPath), optimizedDirectory,
                                           suppressedExceptions);
        ...
}

再到 makeDexElements，它是建立Dex檔案的真正方法.

//http://androidxref.com/4.4.2_r2/xref/libcore/dalvik/src/main/java/dalvik/system/DexPathList.java#makeDexElements
private static Element[] makeDexElements(ArrayList<File> files, File optimizedDirectory,
                                             ArrayList<IOException> suppressedExceptions) {
        ArrayList<Element> elements = new ArrayList<Element>();
        /*
         * Open all files and load the (direct or contained) dex files
         * up front.
         */
        for (File file : files) {
            File zip = null;
            DexFile dex = null;
            String name = file.getName();

            if (name.endsWith(DEX_SUFFIX)) {
                // Raw dex file (not inside a zip/jar).
                try {
                    dex = loadDexFile(file, optimizedDirectory);
                } catch (IOException ex) {
                    System.logE("Unable to load dex file: " + file, ex);
                }
            } else if (name.endsWith(APK_SUFFIX) || name.endsWith(JAR_SUFFIX)
                ...
            } else if (file.isDirectory()) {
                ...
            } else {
                System.logW("Unknown file type for: " + file);
            }

            if ((zip != null) || (dex != null)) {
                elements.add(new Element(file, false, zip, dex));
            }
        }

        return elements.toArray(new Element[elements.size()]);
    }

makeDexElements其實就是不斷的去files中取出file,然後構建Element新增到elements中我們的dex檔案，緊接著執行loadDexFile。去看看 loadDexFile方法.

// http://androidxref.com/4.4.2_r2/xref/libcore/dalvik/src/main/java/dalvik/system/DexPathList.java#loadDexFile

private static DexFile loadDexFile(File file, File optimizedDirectory)
            throws IOException {
        if (optimizedDirectory == null) {
            return new DexFile(file);
        } else {
            String optimizedPath = optimizedPathFor(file, optimizedDirectory);
            return DexFile.loadDex(file.getPath(), optimizedPath, 0);
        }
    }

繼續跟蹤loadDex方法.

// http://androidxref.com/4.4.2_r2/xref/libcore/dalvik/src/main/java/dalvik/system/DexFile.java#141

 static public DexFile loadDex(String sourcePathName, String outputPathName,
        int flags) throws IOException {

        /*
         * TODO: we may want to cache previously-opened DexFile objects.
         * The cache would be synchronized with close().  This would help
         * us avoid mapping the same DEX more than once when an app
         * decided to open it multiple times.  In practice this may not
         * be a real issue.
         */
        return new DexFile(sourcePathName, outputPathName, flags);
    }

// http://androidxref.com/4.4.2_r2/xref/libcore/dalvik/src/main/java/dalvik/system/DexFile.java#97

private DexFile(String sourceName, String outputName, int flags) throws IOException {
        ...
		
        mCookie = openDexFile(sourceName, outputName, flags);
        mFileName = sourceName;
        guard.open("close");
       
    }

看到openDexFile就猜得到，這是開啟dex檔案的方法.

// http://androidxref.com/4.4.2_r2/xref/libcore/dalvik/src/main/java/dalvik/system/DexFile.java#openDexFile
// sourceName : 載入路徑
// outputName : 輸出路徑
// flags : 0
private static int openDexFile(String sourceName, String outputName,
        int flags) throws IOException {
        return openDexFileNative(new File(sourceName).getCanonicalPath(),
                                 (outputName == null) ? null : new File(outputName).getCanonicalPath(),
                                 flags);
    }

native private static int openDexFileNative(String sourceName, String outputName,
        int flags) throws IOException;

然後看到openDexFileNative也就知道是個native方法了. 所以我們需要去看c++程式碼。Android原始碼中native檔案命名就是以native方法所在路徑命名的

//http://androidxref.com/4.4.4_r1/xref/dalvik/vm/native/dalvik_system_DexFile.cpp

static void Dalvik_dalvik_system_DexFile_openDexFileNative(const u4* args,
    JValue* pResult)
{
    ...

    /*
     * Try to open it directly as a DEX if the name ends with ".dex".
     * If that fails (or isn't tried in the first place), try it as a
     * Zip with a "classes.dex" inside.
     */
    if (hasDexExtension(sourceName)
            && dvmRawDexFileOpen(sourceName, outputName, &pRawDexFile, false) == 0) {
        ALOGV("Opening DEX file '%s' (DEX)", sourceName);

        pDexOrJar = (DexOrJar*) malloc(sizeof(DexOrJar));
        pDexOrJar->isDex = true;
        pDexOrJar->pRawDexFile = pRawDexFile;
        pDexOrJar->pDexMemory = NULL;
    } else if (dvmJarFileOpen(sourceName, outputName, &pJarFile, false) == 0) {
        ...
    } else {
       ...
    }

    ...
}

hasDexExtension判斷檔案是否是dex字尾，dvmRawDexFileOpen就是去開啟dex檔案了

//http://androidxref.com/4.4.2_r2/xref/dalvik/vm/RawDexFile.cpp#109

int dvmRawDexFileOpen(const char* fileName, const char* odexOutputName,
    RawDexFile** ppRawDexFile, bool isBootstrap)
{
    ...

    dexFd = open(fileName, O_RDONLY);
    ...
    //生成優化後的dex檔案路徑
    if (odexOutputName == NULL) {
        cachedName = dexOptGenerateCacheFileName(fileName, NULL);
        if (cachedName == NULL)
            goto bail;
    } else {
        cachedName = strdup(odexOutputName);
    }

    ...
    if (newFile) {
        u8 startWhen, copyWhen, endWhen;
        bool result;
        off_t dexOffset;

        dexOffset = lseek(optFd, 0, SEEK_CUR);
        result = (dexOffset > 0);

        if (result) {
            startWhen = dvmGetRelativeTimeUsec();
            result = copyFileToFile(optFd, dexFd, fileSize) == 0;
            copyWhen = dvmGetRelativeTimeUsec();
        }

        if (result) {
            //進入對dex檔案的優化流程中
            result = dvmOptimizeDexFile(optFd, dexOffset, fileSize,
                fileName, modTime, adler32, isBootstrap);
        }

        if (!result) {
            ALOGE("Unable to extract+optimize DEX from '%s'", fileName);
            goto bail;
        }

        endWhen = dvmGetRelativeTimeUsec();
        ALOGD("DEX prep '%s': copy in %dms, rewrite %dms",
            fileName,
            (int) (copyWhen - startWhen) / 1000,
            (int) (endWhen - copyWhen) / 1000);
    }

    ...
    return result;
}

來關注一下優化dex的函式dvmOptimizeDexFile

// http://androidxref.com/4.4.2_r2/xref/dalvik/vm/analysis/DexPrepare.cpp#351

bool dvmOptimizeDexFile(int fd, off_t dexOffset, long dexLength,
    const char* fileName, u4 modWhen, u4 crc, bool isBootstrap)
{
    const char* lastPart = strrchr(fileName, '/');
    if (lastPart != NULL)
        lastPart++;
    else
        lastPart = fileName;

    ALOGD("DexOpt: --- BEGIN '%s' (bootstrap=%d) ---", lastPart, isBootstrap);

    pid_t pid;

    /*
     * This could happen if something in our bootclasspath, which we thought
     * was all optimized, got rejected.
     */
    if (gDvm.optimizing) {
        ALOGW("Rejecting recursive optimization attempt on '%s'", fileName);
        return false;
    }

    pid = fork();
    if (pid == 0) {
        static const int kUseValgrind = 0;
        //需要呼叫優化的程式dexopt
        static const char* kDexOptBin = "/bin/dexopt";
        static const char* kValgrinder = "/usr/bin/valgrind";
        static const int kFixedArgCount = 10;
        static const int kValgrindArgCount = 5;
        static const int kMaxIntLen = 12;   // '-'+10dig+'\0' -OR- 0x+8dig
        int bcpSize = dvmGetBootPathSize();
        int argc = kFixedArgCount + bcpSize
            + (kValgrindArgCount * kUseValgrind);
        const char* argv[argc+1];             // last entry is NULL
        char values[argc][kMaxIntLen];
        char* execFile;
        const char* androidRoot;
        int flags;

        /* change process groups, so we don't clash with ProcessManager */
        setpgid(0, 0);

        /* full path to optimizer */
        androidRoot = getenv("ANDROID_ROOT");
        if (androidRoot == NULL) {
            ALOGW("ANDROID_ROOT not set, defaulting to /system");
            androidRoot = "/system";
        }
        execFile = (char*)alloca(strlen(androidRoot) + strlen(kDexOptBin) + 1);
        strcpy(execFile, androidRoot);
        strcat(execFile, kDexOptBin);

        /*
         * Create arg vector.
         */
        int curArg = 0;

        if (kUseValgrind) {
            /* probably shouldn't ship the hard-coded path */
            argv[curArg++] = (char*)kValgrinder;
            argv[curArg++] = "--tool=memcheck";
            argv[curArg++] = "--leak-check=yes";        // check for leaks too
            argv[curArg++] = "--leak-resolution=med";   // increase from 2 to 4
            argv[curArg++] = "--num-callers=16";        // default is 12
            assert(curArg == kValgrindArgCount);
        }
        //拼接命令
        argv[curArg++] = execFile;

        argv[curArg++] = "--dex";

        sprintf(values[2], "%d", DALVIK_VM_BUILD);
        argv[curArg++] = values[2];

        sprintf(values[3], "%d", fd);
        argv[curArg++] = values[3];

        sprintf(values[4], "%d", (int) dexOffset);
        argv[curArg++] = values[4];

        sprintf(values[5], "%d", (int) dexLength);
        argv[curArg++] = values[5];

        argv[curArg++] = (char*)fileName;

        sprintf(values[7], "%d", (int) modWhen);
        argv[curArg++] = values[7];

        sprintf(values[8], "%d", (int) crc);
        argv[curArg++] = values[8];

        ...

        if (kUseValgrind)
            execv(kValgrinder, const_cast<char**>(argv));
        else
            //這裡執行了/bin/dexopt程式
            execv(execFile, const_cast<char**>(argv));

        ALOGE("execv '%s'%s failed: %s", execFile,
            kUseValgrind ? " [valgrind]" : "", strerror(errno));
        exit(1);
    } else {
       ...
    }
}

此函式中會拼接命令列，然後呼叫execv是執行/bin/dexopt程式等待此程式去優化dex檔案
它的原始碼就在/dalvik/dexopt/OptMain.cpp下
它的main函式如下

// http://androidxref.com/4.4.2_r2/xref/dalvik/dexopt/OptMain.cpp

int main(int argc, char* const argv[])
{
    ...

    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;
}

此處肯定是呼叫了fromDex函式了

 // http://androidxref.com/4.4.2_r2/xref/dalvik/dexopt/OptMain.cpp#fromDex
 
static int fromDex(int argc, char* const argv[])
{
    ...

    /* do the optimization */
    if (!dvmContinueOptimization(fd, offset, length, debugFileName,
            modWhen, crc, (flags & DEXOPT_IS_BOOTSTRAP) != 0))
    {
        ALOGE("Optimization failed");
        goto bail;
    }

    result = 0;

    ...
}

關注dvmContinueOptimization

bool dvmContinueOptimization(int fd, off_t dexOffset, long dexLength,
    const char* fileName, u4 modWhen, u4 crc, bool isBootstrap)
{
    ...

    {
        /*
         * 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;
        //對當前dex進行記憶體對映
        mapAddr = mmap(NULL, dexOffset + dexLength, PROT_READ|PROT_WRITE,
                    MAP_SHARED, fd, 0);
        ...
        //重寫dex
        success = rewriteDex(((u1*) mapAddr) + dexOffset, dexLength,
                    doVerify, doOpt, &pClassLookup, NULL);

        if (success) {
            DvmDex* pDvmDex = NULL;
            u1* dexAddr = ((u1*) mapAddr) + dexOffset;

            if (dvmDexFileOpenPartial(dexAddr, dexLength, &pDvmDex) != 0) {
                ALOGE("Unable to create DexFile");
                success = false;
            } else {
                /*
                 * If configured to do so, generate register map output
                 * for all verified classes.  The register maps were
                 * generated during verification, and will now be serialized.
                 */
                if (gDvm.generateRegisterMaps) {
                    pRegMapBuilder = dvmGenerateRegisterMaps(pDvmDex);
                    if (pRegMapBuilder == NULL) {
                        ALOGE("Failed generating register maps");
                        success = false;
                    }
                }

                DexHeader* pHeader = (DexHeader*)pDvmDex->pHeader;
                updateChecksum(dexAddr, dexLength, pHeader);

                dvmDexFileFree(pDvmDex);
            }
        }

       ...
    }

    ...
    return result;
}

發揮一目十行的能力， 看到rewriteDex

// http://androidxref.com/4.4.2_r2/xref/dalvik/vm/analysis/DexPrepare.cpp#rewriteDex

/**
第一個引數：dex的起始地址
第二個引數：dex的位元組數
**/
static bool rewriteDex(u1* addr, int len, bool doVerify, bool doOpt,
    DexClassLookup** ppClassLookup, DvmDex** ppDvmDex)
{
    DexClassLookup* pClassLookup = NULL;
    u8 prepWhen, loadWhen, verifyOptWhen;
    DvmDex* pDvmDex = NULL;
    bool result = false;
    const char* msgStr = "???";

    /* if the DEX is in the wrong byte order, swap it now */
    if (dexSwapAndVerify(addr, len) != 0)
        goto bail;

    /*
     * Now that the DEX file can be read directly, create a DexFile struct
     * for it.
     */
     //此函式也有我們需要的dex的起始地址和位元組數
    if (dvmDexFileOpenPartial(addr, len, &pDvmDex) != 0) {
        ALOGE("Unable to create DexFile");
        goto bail;
    }

    ...

    result = true;

bail:
   ...
    return result;
}

這個函式就有我們想要的東西了，就是dex的起始地址和dex的位元組數
其中dvmDexFileOpenPartial方法就是一個脫殼點，其程式碼如下

// http://androidxref.com/4.4.2_r2/xref/dalvik/vm/DvmDex.cpp#146

int dvmDexFileOpenPartial(const void* addr, int len, DvmDex** ppDvmDex)
{
   ...

    pDexFile = dexFileParse((u1*)addr, len, parseFlags);
   ...
    result = 0;

bail:
    return result;
}

於是乎我們可以修改這邊的程式碼為:

#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>


int dvmDexFileOpenPartial(const void* addr, int len, DvmDex** ppDvmDex)
{
	char dexfilepath[100] = {0};
	int pid = getpid();
	sprintf(dexfilepath,"/sdcard/%d_%d_dvmDexFileOpenPartial.dex",len,pid);
	//fopen
	int fd = open(dexfilepath,O_CREAT|O_RDWR,0666);
	if (fd>0){
		write(fd,addr,len);
		close(fd);
		
	}

	
    DvmDex* pDvmDex;
    DexFile* pDexFile;
    int parseFlags = kDexParseDefault;
    int result = -1;

    /* -- file is incomplete, new checksum has not yet been calculated
    if (gDvm.verifyDexChecksum)
        parseFlags |= kDexParseVerifyChecksum;
    */

    pDexFile = dexFileParse((u1*)addr, len, parseFlags);
    if (pDexFile == NULL) {
        ALOGE("DEX parse failed");
        goto bail;
    }
    pDvmDex = allocateAuxStructures(pDexFile);
    if (pDvmDex == NULL) {
        dexFileFree(pDexFile);
        goto bail;
    }

    pDvmDex->isMappedReadOnly = false;
    *ppDvmDex = pDvmDex;
    result = 0;

bail:
    return result;
}

2. 追根溯源dexFileParse脫殼點

// http://androidxref.com/4.4.2_r2/xref/dalvik/vm/DvmDex.cpp#146

int dvmDexFileOpenPartial(const void* addr, int len, DvmDex** ppDvmDex)
{
   ...

    pDexFile = dexFileParse((u1*)addr, len, parseFlags);
   ...
    result = 0;

bail:
    return result;
}

關注dexFileParse， 它也能作為一個脫殼點.

// http://androidxref.com/4.4.2_r2/xref/dalvik/libdex/DexFile.cpp#289

DexFile* dexFileParse(const u1* data, size_t length, int flags)
{
    DexFile* pDexFile = NULL;
    const DexHeader* pHeader;
    const u1* magic;
    int result = -1;

    if (length < sizeof(DexHeader)) {
        ALOGE("too short to be a valid .dex");
        goto bail;      /* bad file format */
    }
    ...
}

修改這邊的程式碼為：

DexFile* dexFileParse(const u1* data, size_t length, int flags)
{
	char dexfilepath[100] = {0};
	int pid = getpid();
	sprintf(dexfilepath,"/sdcard/%d_%d_dexFileParse.dex",length,pid);
	//fopen
	int fd = open(dexfilepath,O_CREAT|O_RDWR,0666);
	if (fd>0){
		write(fd,data,length);
		close(fd);
		
	}
	
	
	
    DexFile* pDexFile = NULL;
    const DexHeader* pHeader;
    const u1* magic;
    int result = -1;

    if (length < sizeof(DexHeader)) {
        ALOGE("too short to be a valid .dex");
        goto bail;      /* bad file format */
    }

    pDexFile = (DexFile*) malloc(sizeof(DexFile));
    if (pDexFile == NULL)
        goto bail;      /* alloc failure */
    memset(pDexFile, 0, sizeof(DexFile));

    /*
     * Peel off the optimized header.
     */
    if (memcmp(data, DEX_OPT_MAGIC, 4) == 0) {
        magic = data;
        if (memcmp(magic+4, DEX_OPT_MAGIC_VERS, 4) != 0) {
            ALOGE("bad opt version (0x%02x %02x %02x %02x)",
                 magic[4], magic[5], magic[6], magic[7]);
            goto bail;
        }

        pDexFile->pOptHeader = (const DexOptHeader*) data;
        ALOGV("Good opt header, DEX offset is %d, flags=0x%02x",
            pDexFile->pOptHeader->dexOffset, pDexFile->pOptHeader->flags);

        /* parse the optimized dex file tables */
        if (!dexParseOptData(data, length, pDexFile))
            goto bail;

        /* ignore the opt header and appended data from here on out */
        data += pDexFile->pOptHeader->dexOffset;
        length -= pDexFile->pOptHeader->dexOffset;
        if (pDexFile->pOptHeader->dexLength > length) {
            ALOGE("File truncated? stored len=%d, rem len=%d",
                pDexFile->pOptHeader->dexLength, (int) length);
            goto bail;
        }
        length = pDexFile->pOptHeader->dexLength;
    }

    dexFileSetupBasicPointers(pDexFile, data);
    pHeader = pDexFile->pHeader;

    if (!dexHasValidMagic(pHeader)) {
        goto bail;
    }

    /*
     * Verify the checksum(s).  This is reasonably quick, but does require
     * touching every byte in the DEX file.  The base checksum changes after
     * byte-swapping and DEX optimization.
     */
    if (flags & kDexParseVerifyChecksum) {
        u4 adler = dexComputeChecksum(pHeader);
        if (adler != pHeader->checksum) {
            ALOGE("ERROR: bad checksum (%08x vs %08x)",
                adler, pHeader->checksum);
            if (!(flags & kDexParseContinueOnError))
                goto bail;
        } else {
            ALOGV("+++ adler32 checksum (%08x) verified", adler);
        }

        const DexOptHeader* pOptHeader = pDexFile->pOptHeader;
        if (pOptHeader != NULL) {
            adler = dexComputeOptChecksum(pOptHeader);
            if (adler != pOptHeader->checksum) {
                ALOGE("ERROR: bad opt checksum (%08x vs %08x)",
                    adler, pOptHeader->checksum);
                if (!(flags & kDexParseContinueOnError))
                    goto bail;
            } else {
                ALOGV("+++ adler32 opt checksum (%08x) verified", adler);
            }
        }
    }

    /*
     * Verify the SHA-1 digest.  (Normally we don't want to do this --
     * the digest is used to uniquely identify the original DEX file, and
     * can't be computed for verification after the DEX is byte-swapped
     * and optimized.)
     */
    if (kVerifySignature) {
        unsigned char sha1Digest[kSHA1DigestLen];
        const int nonSum = sizeof(pHeader->magic) + sizeof(pHeader->checksum) +
                            kSHA1DigestLen;

        dexComputeSHA1Digest(data + nonSum, length - nonSum, sha1Digest);
        if (memcmp(sha1Digest, pHeader->signature, kSHA1DigestLen) != 0) {
            char tmpBuf1[kSHA1DigestOutputLen];
            char tmpBuf2[kSHA1DigestOutputLen];
            ALOGE("ERROR: bad SHA1 digest (%s vs %s)",
                dexSHA1DigestToStr(sha1Digest, tmpBuf1),
                dexSHA1DigestToStr(pHeader->signature, tmpBuf2));
            if (!(flags & kDexParseContinueOnError))
                goto bail;
        } else {
            ALOGV("+++ sha1 digest verified");
        }
    }

    if (pHeader->fileSize != length) {
        ALOGE("ERROR: stored file size (%d) != expected (%d)",
            (int) pHeader->fileSize, (int) length);
        if (!(flags & kDexParseContinueOnError))
            goto bail;
    }

    if (pHeader->classDefsSize == 0) {
        ALOGE("ERROR: DEX file has no classes in it, failing");
        goto bail;
    }

    /*
     * Success!
     */
    result = 0;

bail:
    if (result != 0 && pDexFile != NULL) {
        dexFileFree(pDexFile);
        pDexFile = NULL;
    }
    return pDexFile;
}

3.編譯系統.

這邊我使用的是ubuntu16.04的系統. 具體其他配置就不多說了.

操作步驟:

1. 設定環境 [source build/envsetup.sh]
2. 選擇目標 [lunch]
3. 這裡編譯的是nexus5， 所以選擇 7 （aosp_hammerhead-userdebug） [7]
4. 編譯程式碼 -j4 意味著4執行緒， 根據自己cpu而定. [time make -j4]

4. 刷機

編譯出來的檔案在 ~/SourceCode/Android-4.4.4_r1/out/target/product/hammerhead
拿出 boot.img/cache.img/system.img/userdata.img

然後執行命令

fastboot flash boot boot.img
fastboot flash cache cache.img
fastboot flash system system.img
fastboot flash userdata userdata.img