text: //
  //  Example
  //  man
  //
  //  Created by man 11/11/2018.
  //  Copyright © 2020 man. All rights reserved.
  //
  
  #import "_fishhook.h"
  
  #import <dlfcn.h>
  #import <stdlib.h>
  #import <string.h>
  #import <sys/types.h>
  #import <mach-o/dyld.h>
  #import <mach-o/loader.h>
  #import <mach-o/nlist.h>
  
  #ifdef __LP64__
  typedef struct mach_header_64 mach_header_t;
  typedef struct segment_command_64 segment_command_t;
  typedef struct section_64 section_t;
  typedef struct nlist_64 nlist_t;
  #define LC_SEGMENT_ARCH_DEPENDENT LC_SEGMENT_64
  #else
  typedef struct mach_header mach_header_t;
  typedef struct segment_command segment_command_t;
  typedef struct section section_t;
  typedef struct nlist nlist_t;
  #define LC_SEGMENT_ARCH_DEPENDENT LC_SEGMENT
  #endif
  
  #ifndef SEG_DATA_CONST
  #define SEG_DATA_CONST  "__DATA_CONST"
  #endif
  
  struct rcd_rebindings_entry {
    struct rcd_rebinding *rebindings;
    size_t rebindings_nel;
    struct rcd_rebindings_entry *next;
  };
  
  static struct rcd_rebindings_entry *_rebindings_head;
  
  static int rcd_prepend_rebindings(struct rcd_rebindings_entry **rebindings_head,
                                    struct rcd_rebinding rebindings[],
                                    size_t nel) {
    struct rcd_rebindings_entry *new_entry = malloc(sizeof(struct rcd_rebindings_entry));
    if (!new_entry) {
      return -1;
    }
    new_entry->rebindings = malloc(sizeof(struct rcd_rebinding) * nel);
    if (!new_entry->rebindings) {
      free(new_entry);
      return -1;
    }
    memcpy(new_entry->rebindings, rebindings, sizeof(struct rcd_rebinding) * nel);
    new_entry->rebindings_nel = nel;
    new_entry->next = *rebindings_head;
    *rebindings_head = new_entry;
    return 0;
  }
  
  static void rcd_perform_rebinding_with_section(struct rcd_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;
      struct rcd_rebindings_entry *cur = rebindings;
      while (cur) {
        for (uint j = 0; j < cur->rebindings_nel; j++) {
          if (strlen(symbol_name) > 1 &&
              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:;
    }
  }
  
  static void rebind_symbols_for_image(struct rcd_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) {
            rcd_perform_rebinding_with_section(rebindings, sect, slide, symtab, strtab, indirect_symtab);
          }
          if ((sect->flags & SECTION_TYPE) == S_NON_LAZY_SYMBOL_POINTERS) {
            rcd_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 rcd_rebind_symbols_image(void *header,
                               intptr_t slide,
                               struct rcd_rebinding rebindings[],
                               size_t rebindings_nel) {
      struct rcd_rebindings_entry *rebindings_head = NULL;
      int retval = rcd_prepend_rebindings(&rebindings_head, rebindings, rebindings_nel);
      rebind_symbols_for_image(rebindings_head, header, slide);
      free(rebindings_head);
      return retval;
  }
  
  int rcd_rebind_symbols(struct rcd_rebinding rebindings[], size_t rebindings_nel) {
    int retval = rcd_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;
  }
  