深入 fishhook 的内心世界

fishhook 用于替换 iOS 程序中动态库的符号，常被用来 hook 系统中的 C 函数。

fishhook 的实现得益于 iOS 的动态链接机制，相关信息可以参考：iOS 系统的延迟绑定机制，而在此之前，你还需要了解 Mach-O 文件，详情可参考这篇文章：Mach-O 文件探索。

对于程序引用的动态库中的函数，链接器会将它的地址放在 __la_symbol_ptr 中，而对于动态库的全局数据，则是放在 __nl_symbol_ptr 中，所以整个 fishhook 的核心工作就是替换 __la_symbol_ptr 以及 __nl_symbol_ptr 的内容。

下面是一段官方示例代码，它告诉我们如何用正确的姿势来使用 fishhook：

#import <UIKit/UIKit.h>
#import "AppDelegate.h"
#import "fishhook.h"

static int (*orig_close)(int);
static int (*orig_open)(const char *, int, ...);

int my_close(int fd) {
    printf("Calling real close(%d)\n", fd);
    return orig_close(fd);
}

int my_open(const char *path, int oflag, ...) {
    va_list ap = {0};
    mode_t mode = 0;
    
    if ((oflag & O_CREAT) != 0) {
        // mode only applies to O_CREAT
        va_start(ap, oflag);
        mode = va_arg(ap, int);
        va_end(ap);
        printf("Calling real open('%s', %d, %d)\n", path, oflag, mode);
        return orig_open(path, oflag, mode);
    } else {
        printf("Calling real open('%s', %d)\n", path, oflag);
        return orig_open(path, oflag, mode);
    }
}

int main(int argc, char * argv[]) {
    @autoreleasepool {
        rebind_symbols((struct rebinding[2]){{"close", my_close, (void *)&orig_close}, {"open", my_open, (void *)&orig_open}}, 2);
        
        // Open our own binary and print out first 4 bytes (which is the same
        // for all Mach-O binaries on a given architecture)
        int fd = open(argv[0], O_RDONLY);
        uint32_t magic_number = 0;
        read(fd, &magic_number, 4);
        printf("Mach-O Magic Number: %x \n", magic_number);
        close(fd);
        
        return UIApplicationMain(argc, argv, nil, NSStringFromClass([AppDelegate class]));
    }
}

上述代码的作用就是将系统的 open 函数以及 close 函数替换成我们自己的实现，整个过程由 rebind_symbols 函数完成。

int rebind_symbols(struct rebinding rebindings[], size_t rebindings_nel) {
  int retval = prepend_rebindings(&_rebindings_head, rebindings, rebindings_nel);
  if (retval < 0) {
    return retval;
  }
  // If this was the first call, register callback for image additions (which is also invoked for
  // existing images, otherwise, just run on existing images
  if (!_rebindings_head->next) {
    _dyld_register_func_for_add_image(_rebind_symbols_for_image);
  } else {
    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;
}

rebind_symbols 函数的处理流程如下：

使用 prepend_rebindings 函数来处理我们传入的绑定信息，fishhook 会将它们组织成一个链表，并用 _rebindings_head 指向链表的头部，接下来会判断用户是否是第一次进行符号替换。
如果是第一次替换符号，则调用 _dyld_register_func_for_add_image 注册回调函数 _rebind_symbols_for_image，之后程序每次加载动态库时都会去调用 _rebind_symbols_for_image。当注册成功时，对于那些已经加载的动态库也会触发回调，所以不用担心注册的时机。
如果不是第一次替换符号，那么就遍历程序已经加载的动态库，对其调用 _rebind_symbols_for_image 函数。

_rebind_symbols_for_image 函数如下，它实际上是调用 rebind_symbols_for_image ：

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

rebind_symbols_for_image 的代码：

static void rebind_symbols_for_image(struct rebindings_entry *rebindings,
                                     const struct mach_header *header,
                                     intptr_t slide) {
  Dl_info info;
  if (dladdr(header, &info) == 0) {
    return;
  }

  segment_command_t *cur_seg_cmd;
  segment_command_t *linkedit_segment = NULL;
  struct symtab_command* symtab_cmd = NULL;
  struct dysymtab_command* dysymtab_cmd = NULL;

  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;
    }
  }

  if (!symtab_cmd || !dysymtab_cmd || !linkedit_segment ||
      !dysymtab_cmd->nindirectsyms) {
    return;
  }

  // Find base symbol/string table addresses
  uintptr_t linkedit_base = (uintptr_t)slide + linkedit_segment->vmaddr - linkedit_segment->fileoff;
  nlist_t *symtab = (nlist_t *)(linkedit_base + symtab_cmd->symoff);
  char *strtab = (char *)(linkedit_base + symtab_cmd->stroff);

  // Get indirect symbol table (array of uint32_t indices into symbol table)
  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) {
      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;
        if ((sect->flags & SECTION_TYPE) == S_LAZY_SYMBOL_POINTERS) {
          perform_rebinding_with_section(rebindings, sect, slide, symtab, strtab, indirect_symtab);
        }
        if ((sect->flags & SECTION_TYPE) == S_NON_LAZY_SYMBOL_POINTERS) {
          perform_rebinding_with_section(rebindings, sect, slide, symtab, strtab, indirect_symtab);
        }
      }
    }
  }
}

rebind_symbols_for_image 函数看上去有点长，但是如果你静下心来观察的话，发现它还是没有超过80行。。。。。。不过不要紧，我们慢慢往下看。

函数中经常出现的变量类型如 mach_header、segment_command、symtab_command 等都可以在系统的 loader.h 文件中找到相应的定义，所以这里不做过多的解释，想要了解更多的话请参考：Mach-O 文件探索。

rebind_symbols_for_image 首先会在程序的 load commands 中寻找符号表、动态符号表以及 LINKEDIT 段所对应的加载命令，它们包含程序动态链接过程的关键信息：

Dl_info info;
  if (dladdr(header, &info) == 0) {
    return;
  }

  segment_command_t *cur_seg_cmd;
  segment_command_t *linkedit_segment = NULL;
  struct symtab_command* symtab_cmd = NULL;
  struct dysymtab_command* dysymtab_cmd = NULL;

  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;
    }
  }

  if (!symtab_cmd || !dysymtab_cmd || !linkedit_segment ||
      !dysymtab_cmd->nindirectsyms) {
    return;
  }

在确认上述信息齐全之后，会去计算它们的虚拟地址：

  // Find base symbol/string table addresses
  uintptr_t linkedit_base = (uintptr_t)slide + linkedit_segment->vmaddr - linkedit_segment->fileoff;
  nlist_t *symtab = (nlist_t *)(linkedit_base + symtab_cmd->symoff);
  char *strtab = (char *)(linkedit_base + symtab_cmd->stroff);

  // Get indirect symbol table (array of uint32_t indices into symbol table)
  uint32_t *indirect_symtab = (uint32_t *)(linkedit_base + dysymtab_cmd->indirectsymoff);

值得一提的是这行代码：

uintptr_t linkedit_base = (uintptr_t)slide + linkedit_segment->vmaddr - linkedit_segment->fileoff;

因为地址空间加载随机化的缘故，系统在加载程序时，会在其原有的地址空间上进行偏移操作，而这个 slide 正是偏移的大小，所以 linkedit_base 代表的是程序被加载后的基地址。

接下来，rebind_symbols_for_image 会在 __DATA 段中寻找类型为 S_LAZY_SYMBOL_POINTERS 以及 S_NON_LAZY_SYMBOL_POINTERS 的节，二者分别包含我们要替换的 __la_symbol_ptr 以及 __nl_symbol_ptr，然后调用 perform_rebinding_with_section 函数：

 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_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;
        if ((sect->flags & SECTION_TYPE) == S_LAZY_SYMBOL_POINTERS) {
          perform_rebinding_with_section(rebindings, sect, slide, symtab, strtab, indirect_symtab);
        }
        if ((sect->flags & SECTION_TYPE) == S_NON_LAZY_SYMBOL_POINTERS) {
          perform_rebinding_with_section(rebindings, sect, slide, symtab, strtab, indirect_symtab);
        }
      }
    }
  }

perform_rebinding_with_section 函数也是整个 fishhook 的核心代码，负责完成符号的替换：

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) {
  uint32_t *indirect_symbol_indices = indirect_symtab + section->reserved1;
  void **indirect_symbol_bindings = (void **)((uintptr_t)slide + section->addr);
  for (uint i = 0; i < section->size / sizeof(void *); i++) {
    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;
    }
    uint32_t strtab_offset = symtab[symtab_index].n_un.n_strx;
    char *symbol_name = strtab + strtab_offset;
    if (strnlen(symbol_name, 2) < 2) {
      continue;
    }
    struct rebindings_entry *cur = rebindings;
    while (cur) {
      for (uint j = 0; j < cur->rebindings_nel; j++) {
        if (strcmp(&symbol_name[1], cur->rebindings[j].name) == 0) {
          if (cur->rebindings[j].replaced != NULL &&
              indirect_symbol_bindings[i] != cur->rebindings[j].replacement) {
            *(cur->rebindings[j].replaced) = indirect_symbol_bindings[i];
          }
          indirect_symbol_bindings[i] = cur->rebindings[j].replacement;
          goto symbol_loop;
        }
      }
      cur = cur->next;
    }
  symbol_loop:;
  }
}

这里最关键的就是下面两行代码的解读：

uint32_t *indirect_symbol_indices = indirect_symtab + section->reserved1;
void **indirect_symbol_bindings = (void **)((uintptr_t)slide + section->addr);

indirect_symtab 是动态符号表的地址，表中包含动态符号在符号表中的索引。那么 section->reserved1 又是代表什么呢？这里的 section 实际上是指 __DATA 段中包含 __la_symbol_ptr 以及 __nl_symbol_ptr 的 section，它们会在 reserved1 字段中记录自身所包含的动态符号在 indirect_symtab 的起始索引，因此通过 indirect_symbol_indices 便可以得到 section 所包含的动态符号在符号表中的索引信息。

indirect_symbol_bindings 则是代表程序偏移后的相关 section 的虚拟地址，fishhook 会在其中寻找指向目标动态符号的指针，然后将其指向我们自己的符号。

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;
    }
    uint32_t strtab_offset = symtab[symtab_index].n_un.n_strx;
    char *symbol_name = strtab + strtab_offset;
    if (strnlen(symbol_name, 2) < 2) {
      continue;
    }

symtab_index 表示动态符号在符号表中的索引，据此我们可以得到动态符号的名字，接下来就是遍历用户传来的绑定信息，若是和动态符号相匹配，则进行替换：

 while (cur) {
      for (uint j = 0; j < cur->rebindings_nel; j++) {
        if (strcmp(&symbol_name[1], cur->rebindings[j].name) == 0) {
          if (cur->rebindings[j].replaced != NULL &&
              indirect_symbol_bindings[i] != cur->rebindings[j].replacement) {
            *(cur->rebindings[j].replaced) = indirect_symbol_bindings[i];
          }
          indirect_symbol_bindings[i] = cur->rebindings[j].replacement;
          goto symbol_loop;
        }
      }
      cur = cur->next;
    }
  symbol_loop:;

这里引用下 fishhook 官方的流程图：

最后的感慨：fishhook 真是有创意！

最后编辑于：2017.12.08 01:22:05

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