iOS 逆向开发18：fishhook源码分析

iOS 逆向开发 文章汇总

源码

fishhook.c

static void perform_rebinding_with_section(struct rebindings_entry *rebindings,
                                           section_t *section,
                                           intptr_t slide,
                                           nlist_t *symtab,
                                           char *strtab,
                                           uint32_t *indirect_symtab) {
    //nl_symbol_ptr和la_symbol_ptrsection中的reserved1字段指明对应的indirect symbol table起始的index
  uint32_t *indirect_symbol_indices = indirect_symtab + section->reserved1;
    //slide+section->addr 就是符号对应的存放函数实现的数组也就是我相应的__nl_symbol_ptr和__la_symbol_ptr相应的函数指针都在这里面了，所以可以去寻找到函数的地址
  void **indirect_symbol_bindings = (void **)((uintptr_t)slide + section->addr);
    //遍历section里面的每一个符号
  for (uint i = 0; i < section->size / sizeof(void *); i++) {
      //找到符号在Indrect Symbol Table表中的值
      //读取indirect table中的数据
    uint32_t symtab_index = indirect_symbol_indices[I];
    if (symtab_index == INDIRECT_SYMBOL_ABS || symtab_index == INDIRECT_SYMBOL_LOCAL ||
        symtab_index == (INDIRECT_SYMBOL_LOCAL   | INDIRECT_SYMBOL_ABS)) {
      continue;
    }
      //以symtab_index作为下标，访问symbol table
      uint32_t strtab_offset = symtab[symtab_index].n_un.n_strx;
      //获取到symbol_name
      char *symbol_name = strtab + strtab_offset;
      //判断是否函数的名称是否有两个字符，为啥是两个，因为函数前面有个_，所以方法的名称最少要1个
      bool symbol_name_longer_than_1 = symbol_name[0] && symbol_name[1];
      //遍历最初的链表，来进行hook
      struct rebindings_entry *cur = rebindings;
      while (cur) {
          for (uint j = 0; j < cur->rebindings_nel; j++) {
              //这里if的条件就是判断从symbol_name[1]两个函数的名字是否都是一致的，以及判断两个
              if (symbol_name_longer_than_1 &&
                  strcmp(&symbol_name[1], cur->rebindings[j].name) == 0) {
                  //判断replaced的地址不为NULL以及我方法的实现和rebindings[j].replacement的方法不一致
                  if (cur->rebindings[j].replaced != NULL &&
                      indirect_symbol_bindings[i] != cur->rebindings[j].replacement) {
                      //让rebindings[j].replaced保存indirect_symbol_bindings[i]的函数地址
                      *(cur->rebindings[j].replaced) = indirect_symbol_bindings[I];
                  }
                  //将替换后的方法给原先的方法，也就是替换内容为自定义函数地址
                  indirect_symbol_bindings[i] = cur->rebindings[j].replacement;
                  goto symbol_loop;
        }
      }
      cur = cur->next;
    }
  symbol_loop:;
  }
}

//回调的最终就是这个函数！ 三个参数：要交换的数组  、 image的头 、 ASLR的偏移
static void rebind_symbols_for_image(struct rebindings_entry *rebindings,
                                     const struct mach_header *header,
                                     intptr_t slide) {
    
    /*dladdr() 可确定指定的address 是否位于构成进程的进址空间的其中一个加载模块（可执行库或共享库）内，如果某个地址位于在其上面映射加载模块的基址和为该加载模块映射的最高虚拟地址之间（包括两端），则认为该地址在加载模块的范围内。如果某个加载模块符合这个条件，则会搜索其动态符号表，以查找与指定的address 最接近的符号。最接近的符号是指其值等于，或最为接近但小于指定的address 的符号。
     */
    /*
     如果指定的address 不在其中一个加载模块的范围内，则返回0 ；且不修改Dl_info 结构的内容。否则，将返回一个非零值，同时设置Dl_info 结构的字段。
     如果在包含address 的加载模块内，找不到其值小于或等于address 的符号，则dli_sname 、dli_saddr 和dli_size字段将设置为0 ； dli_bind 字段设置为STB_LOCAL ， dli_type 字段设置为STT_NOTYPE 。
     */
    //这个dladdr函数就是在程序里面找header
  Dl_info info;
  if (dladdr(header, &info) == 0) {
    return;
  }
    //下面就是定义好几个变量，准备从MachO里面去找！
  segment_command_t *cur_seg_cmd;
  segment_command_t *linkedit_segment = NULL;
  struct symtab_command* symtab_cmd = NULL;
  struct dysymtab_command* dysymtab_cmd = NULL;
    //跳过header的大小，找loadCommand
  uintptr_t cur = (uintptr_t)header + sizeof(mach_header_t);
  for (uint i = 0; i < header->ncmds; i++, cur += cur_seg_cmd->cmdsize) {
    cur_seg_cmd = (segment_command_t *)cur;
    if (cur_seg_cmd->cmd == LC_SEGMENT_ARCH_DEPENDENT) {
      if (strcmp(cur_seg_cmd->segname, SEG_LINKEDIT) == 0) {
        linkedit_segment = cur_seg_cmd;
      }
    } else if (cur_seg_cmd->cmd == LC_SYMTAB) {
      symtab_cmd = (struct symtab_command*)cur_seg_cmd;
    } else if (cur_seg_cmd->cmd == LC_DYSYMTAB) {
      dysymtab_cmd = (struct dysymtab_command*)cur_seg_cmd;
    }
  }
   //如果刚才获取的，有一项为空就直接返回
  if (!symtab_cmd || !dysymtab_cmd || !linkedit_segment ||
      !dysymtab_cmd->nindirectsyms) {
    return;
  }

  // Find base symbol/string table addresses
//链接时程序的基址 = __LINKEDIT.VM_Address -__LINKEDIT.File_Offset + silde的改变值
  uintptr_t linkedit_base = (uintptr_t)slide + linkedit_segment->vmaddr - linkedit_segment->fileoff;
//    printf("地址:%p\n",linkedit_base);
    //符号表的地址 = 基址 + 符号表偏移量
  nlist_t *symtab = (nlist_t *)(linkedit_base + symtab_cmd->symoff);
     //字符串表的地址 = 基址 + 字符串表偏移量
  char *strtab = (char *)(linkedit_base + symtab_cmd->stroff);

  // Get indirect symbol table (array of uint32_t indices into symbol table)
    //动态符号表地址 = 基址 + 动态符号表偏移量
  uint32_t *indirect_symtab = (uint32_t *)(linkedit_base + dysymtab_cmd->indirectsymoff);

  cur = (uintptr_t)header + sizeof(mach_header_t);
  for (uint i = 0; i < header->ncmds; i++, cur += cur_seg_cmd->cmdsize) {
    cur_seg_cmd = (segment_command_t *)cur;
    if (cur_seg_cmd->cmd == LC_SEGMENT_ARCH_DEPENDENT) {
        //寻找到data段
      if (strcmp(cur_seg_cmd->segname, SEG_DATA) != 0 &&
          strcmp(cur_seg_cmd->segname, SEG_DATA_CONST) != 0) {
        continue;
      }
        
      for (uint j = 0; j < cur_seg_cmd->nsects; j++) {
        section_t *sect =
          (section_t *)(cur + sizeof(segment_command_t)) + j;
          //找懒加载表
        if ((sect->flags & SECTION_TYPE) == S_LAZY_SYMBOL_POINTERS) {
          perform_rebinding_with_section(rebindings, sect, slide, symtab, strtab, indirect_symtab);
        }
          //非懒加载表
        if ((sect->flags & SECTION_TYPE) == S_NON_LAZY_SYMBOL_POINTERS) {
          perform_rebinding_with_section(rebindings, sect, slide, symtab, strtab, indirect_symtab);
        }
      }
    }
  }
}

static void _rebind_symbols_for_image(const struct mach_header *header,
                                      intptr_t slide) {
    rebind_symbols_for_image(_rebindings_head, header, slide);
}

int rebind_symbols_image(void *header,
                         intptr_t slide,
                         struct rebinding rebindings[],
                         size_t rebindings_nel) {
    struct rebindings_entry *rebindings_head = NULL;
    int retval = prepend_rebindings(&rebindings_head, rebindings, rebindings_nel);
    rebind_symbols_for_image(rebindings_head, (const struct mach_header *) header, slide);
    if (rebindings_head) {
      free(rebindings_head->rebindings);
    }
    free(rebindings_head);
    return retval;
}

int rebind_symbols(struct rebinding rebindings[], size_t rebindings_nel) {
    //prepend_rebindings的函数会将整个 rebindings 数组添加到 _rebindings_head 这个链表的头部
    //Fishhook采用链表的方式来存储每一次调用rebind_symbols传入的参数，每次调用，就会在链表的头部插入一个节点，链表的头部是：_rebindings_head
    int retval = prepend_rebindings(&_rebindings_head, rebindings, rebindings_nel);
    //根据上面的prepend_rebinding来做判断，如果小于0的话，直接返回一个错误码回去
    if (retval < 0) {
    return retval;
  }
    //根据_rebindings_head->next是否为空判断是不是第一次调用。
  if (!_rebindings_head->next) {
      //第一次调用的话，调用_dyld_register_func_for_add_image注册监听方法.
      //已经被dyld加载的image会立刻进入回调。
      //之后的image会在dyld装载的时候触发回调。
    _dyld_register_func_for_add_image(_rebind_symbols_for_image);
  } else {
      //遍历已经加载的image，进行的hook
    uint32_t c = _dyld_image_count();
    for (uint32_t i = 0; i < c; i++) {
      _rebind_symbols_for_image(_dyld_get_image_header(i), _dyld_get_image_vmaddr_slide(i));
    }
  }
  return retval;
}

总结

1. rebind_symbols

• rebindings 数组添加到链表
• 根据链表判断是否第一次调用。 这么做的目的是保证注册方法只会调用一次。 两种情况都是为了回调_rebind_symbols_for_image
• 第一次：利用_dyld_register_func_for_add_image注册监听方法
• 如果不是第一次：循环遍历已经加装的image。进行_rebind_symbols_for_image回调。

2. _rebind_symbols_for_image

• 第一步、拿到那三张表在内存中的地址
  • 符号表的地址:symtab
  • 字符串表的地址:strtab
  • 动态(间接)符号表地址:indirect_symtab
• 第二步、找懒加载和非懒加载表
• 第三步、调用perform_rebinding_with_section:要交换的数组、懒加载和非懒加载表、三个表地址!

3. perform_rebinding_with_section

• 得到indirect_symbol_bindings
• 遍历间接符号表最终找到符号的过程。
• 判断是否是需要HOOK的函数!
• 保存函数指针，然后替换懒加载符号表里面的函数地址!完成HOOK!