作者：郭嘉
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【Android SDK程序逆向分析与破解系列】章节索引

一 DEX文件数据结构

DEX使用的数据类型如下表所示：

u1~u8：表示1~8字节的无符号数。
sleb128、uled128和uled128pl：DEX文件特有的LEB128数据类型。每个LEB128由1~5个字节组成，所有的字节组合在一起表示一个32位的数据。

LEB128数据类型如下表所示：

每个字节只有7位有效位，如果第一个字节的最高位为1，则表明LEB128需要用到第2个字节，如果第2个字节最高位为1，则表明LEB128需要用到第3个字节，依次类推，知道最后的字节最高位为0.LEB128最多只会使用5个字节，如果读取5个字节后下一个字节的最高位仍为1，则表明该DEX文件无效，Dalvik虚拟机在验证DEX会失败返回。

Android系统源码中读取无符号LEB128的实现如下所示:

源码位置：dalvik\libdex\Leb128.h

/* * Reads an unsigned LEB128 value, updating the given pointer to point * just past the end of the read value. This function tolerates * non-zero high-order bits in the fifth encoded byte. */DEX_INLINE int readUnsignedLeb128(const u1** pStream) {    const u1* ptr = *pStream;    int result = *(ptr++);    if (result > 0x7f) {    //大于0x77f表示第1个字节最高位为1        int cur = *(ptr++); //第2个字节        result = (result & 0x7f) | ((cur & 0x7f) << 7);//前2个字节组合        if (cur > 0x7f) {   //大于0x77f表示第2个字节最高位为1            cur = *(ptr++); //第3个字节            result |= (cur & 0x7f) << 14;    //前3个字节的组合            if (cur > 0x7f) {                cur = *(ptr++);              //第4个字节                result |= (cur & 0x7f) << 21;//前4个字节的组合                if (cur > 0x7f) {                    /*                     * Note: We don't check to see if cur is out of                     * range here, meaning we tolerate garbage in the                     * high four-order bits.                     */                    cur = *(ptr++);          //第5个字节                    result |= cur << 28;     //前5个字节的组合                }            }        }    }    *pStream = ptr;    return result;}

Android系统源码中读取有符号LEB128的实现如下所示(有符号与无符号的计算方法是一样的，只是对有符号的LEB128最后一个字节的最高有效位进行了符号扩展):

源码位置：dalvik\libdex\Leb128.h

/* * Reads a signed LEB128 value, updating the given pointer to point * just past the end of the read value. This function tolerates * non-zero high-order bits in the fifth encoded byte. */DEX_INLINE int readSignedLeb128(const u1** pStream) {    const u1* ptr = *pStream;    int result = *(ptr++);    if (result <= 0x7f) {        result = (result << 25) >> 25;    } else {        int cur = *(ptr++);        result = (result & 0x7f) | ((cur & 0x7f) << 7);        if (cur <= 0x7f) {            result = (result << 18) >> 18;        } else {            cur = *(ptr++);            result |= (cur & 0x7f) << 14;            if (cur <= 0x7f) {                result = (result << 11) >> 11;            } else {                cur = *(ptr++);                result |= (cur & 0x7f) << 21;                if (cur <= 0x7f) {                    result = (result << 4) >> 4;                } else {                    /*                     * Note: We don't check to see if cur is out of                     * range here, meaning we tolerate garbage in the                     * high four-order bits.                     */                    cur = *(ptr++);                    result |= cur << 28;                }            }        }    }    *pStream = ptr;    return result;}

二 DEX文件整体结构

DEX文件由多个结构体组合而成，具体结构如下图所示：

- dex header：DEX文件头，指定了dex文件的一些属性，并记录了其他6部分在DEX文件中的物理偏移。
- string_ids
- type_ids
- proto_ids
- field_ids
- method_ids
- class_def
- data：真实的数据存放区。
- link_data：静态链接数据区。

Android源码中DexFile结构体的定义如下所示：

DexFile结构为DEX文件被映射到内存中结构，保存各个结构的指针，还包括了DexOptHeader与DexFile尾部附加的数据。

源码位置：dalvik\libdex\DexFile.h

/* * Structure representing a DEX file. * * Code should regard DexFile as opaque, using the API calls provided here * to access specific structures. */struct DexFile {    /* directly-mapped "opt" header */    const DexOptHeader* pOptHeader;    /* pointers to directly-mapped structs and arrays in base DEX */    const DexHeader*    pHeader;    const DexStringId*  pStringIds;    const DexTypeId*    pTypeIds;    const DexFieldId*   pFieldIds;    const DexMethodId*  pMethodIds;    const DexProtoId*   pProtoIds;    const DexClassDef*  pClassDefs;    const DexLink*      pLinkData;    /*     * These are mapped out of the "auxillary" section, and may not be     * included in the file.     */    const DexClassLookup* pClassLookup;    const void*         pRegisterMapPool;       // RegisterMapClassPool    /* points to start of DEX file data */    const u1*           baseAddr;    /* track memory overhead for auxillary structures */    int                 overhead;    /* additional app-specific data structures associated with the DEX */    //void*               auxData;};