fishhook.c

// Copyright (c) 2013, Facebook, Inc.
// All rights reserved.
// Redistribution and use in source and binary forms, with or without
// modification, are permitted provided that the following conditions are met:
//   * Redistributions of source code must retain the above copyright notice,
//     this list of conditions and the following disclaimer.
//   * Redistributions in binary form must reproduce the above copyright notice,
//     this list of conditions and the following disclaimer in the documentation
//     and/or other materials provided with the distribution.
//   * Neither the name Facebook nor the names of its contributors may be used to
//     endorse or promote products derived from this software without specific
//     prior written permission.
// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
// AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
// IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
// DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
// FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
// DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
// SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
// CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
// OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

#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>


struct rebindings_entry {
  struct rebinding *rebindings;
  size_t rebindings_nel;
  struct rebindings_entry *next;
};

static struct rebindings_entry *rebindings_head;

static int prepend_rebindings(struct rebinding rebindings[], size_t nel) {
  struct rebindings_entry *new_entry = malloc(sizeof(struct rebindings_entry));
  if (!new_entry) {
    return -1;
  }
  new_entry->rebindings = malloc(sizeof(struct rebinding) * nel);
  if (!new_entry->rebindings) {
    return -1;
  }
  memcpy(new_entry->rebindings, rebindings, sizeof(struct rebinding) * nel);
  new_entry->rebindings_nel = nel;
  new_entry->next = rebindings_head;
  rebindings_head = new_entry;
  return 0;
}

static void perform_rebinding_with_section(struct section *section,
                                         intptr_t slide,
                                           struct nlist *symtab,
                                           char *strtab,
                                           uint32_t *indirect_symtab) {
  uint32_t *indirect_symbol_indices = indirect_symtab + section->reserved1;
  void **indirect_symbol_bindings = (void **)(slide + section->addr);
  for (int i = 0; i < section->size / sizeof(void *); i++) {
    uint32_t symtab_index = indirect_symbol_indices[i];
    int32_t strtab_offset = symtab[symtab_index].n_un.n_strx;
    char *symbol_name = strtab + strtab_offset;
    struct rebindings_entry *cur = rebindings_head;
    while (cur) {
      for (int j = 0; j < cur->rebindings_nel; j++) {
        if (strlen(symbol_name) > 1 &&
            strcmp(&symbol_name[1], cur->rebindings[j].name) == 0) {
          indirect_symbol_bindings[i] = cur->rebindings[j].replacement;
          goto symbol_loop;
        }
      }
      cur = cur->next;
    }
  symbol_loop:;
  }
}

static void rebind_symbols_for_image(const struct mach_header *header,
									 intptr_t slide) {
  Dl_info info;
  if (dladdr(header, &info) == 0) {
    return;
  }
  intptr_t cur = (intptr_t)header + sizeof(struct mach_header);
  struct segment_command* cur_seg_cmd;
  struct segment_command* linkedit_segment = NULL;
  struct section* lazy_symbols = NULL;
  struct section* non_lazy_symbols = NULL;
  struct symtab_command* symtab_cmd = NULL;
  struct dysymtab_command* dysymtab_cmd = NULL;
  for (int i = 0; i < header->ncmds; i++, cur += cur_seg_cmd->cmdsize) {
    cur_seg_cmd = (struct segment_command*)cur;
    if (cur_seg_cmd->cmd == LC_SEGMENT) {
      if (strcmp(cur_seg_cmd->segname, SEG_LINKEDIT) == 0) {
        linkedit_segment = cur_seg_cmd;
        continue;
      }
      if (strcmp(cur_seg_cmd->segname, SEG_DATA) != 0) {
        continue;
      }
      for (int j = 0; j < cur_seg_cmd->nsects; j++) {
        struct section* sect =
          (struct section *)(cur + sizeof(struct segment_command)) + j;
        if ((sect->flags & SECTION_TYPE) == S_LAZY_SYMBOL_POINTERS) {
          lazy_symbols = sect;
        }
        if ((sect->flags & SECTION_TYPE) == S_NON_LAZY_SYMBOL_POINTERS) {
          non_lazy_symbols = sect;
        }
      }
    } 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
  uint32_t linkedit_base = slide + linkedit_segment->vmaddr - linkedit_segment->fileoff;
  struct nlist* symtab = (struct nlist *)(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);
  if (lazy_symbols) {
    perform_rebinding_with_section(lazy_symbols, slide, symtab, strtab, indirect_symtab);
  }
  if (non_lazy_symbols) {
    perform_rebinding_with_section(non_lazy_symbols, slide, symtab, strtab, indirect_symtab);
  }
}

int rebind_symbols(struct rebinding rebindings[], size_t rebindings_nel) {
  int retval = prepend_rebindings(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;
}