src/dyldInitialization.cpp

   1 /* -*- mode: C++; c-basic-offset: 4; tab-width: 4 -*-
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  24
  25 #define __STDC_LIMIT_MACROS
  26 #include <stdint.h>
  27 #include <stddef.h>
  28 #include <string.h>
  29 #include <stdlib.h>
  30 #include <mach/mach.h>
  31 #include <mach-o/loader.h>
  32 #include <mach-o/ldsyms.h>
  33 #include <mach-o/reloc.h>
  34 #if __x86_64__
  35         #include <mach-o/x86_64/reloc.h>
  36 #endif
  37 #include "dyld.h"
  38
  39 #ifndef MH_PIE
  40         #define MH_PIE 0x200000
  41 #endif
  42
  43
  44 #if __LP64__
  45         #define LC_SEGMENT_COMMAND              LC_SEGMENT_64
  46         #define macho_segment_command   segment_command_64
  47         #define macho_section                   section_64
  48         #define RELOC_SIZE                              3
  49 #else
  50         #define LC_SEGMENT_COMMAND              LC_SEGMENT
  51         #define macho_segment_command   segment_command
  52         #define macho_section                   section
  53         #define RELOC_SIZE                              2
  54 #endif
  55
  56 #if __x86_64__
  57         #define POINTER_RELOC X86_64_RELOC_UNSIGNED
  58 #else
  59         #define POINTER_RELOC GENERIC_RELOC_VANILLA
  60 #endif
  61
  62 // from dyld.cpp
  63 namespace dyld { extern bool isRosetta(); };
  64
  65
  66 //
  67 //  Code to bootstrap dyld into a runnable state
  68 //
  69 //
  70
  71 namespace dyldbootstrap {
  72
  73
  74 typedef void (*Initializer)(int argc, const char* argv[], const char* envp[], const char* apple[]);
  75
  76 //
  77 // For a regular executable, the crt code calls dyld to run the executables initializers.
  78 // For a static executable, crt directly runs the initializers.
  79 // dyld (should be static) but is a dynamic executable and needs this hack to run its own initializers.
  80 // We pass argc, argv, etc in case libc.a uses those arguments
  81 //
  82 static void runDyldInitializers(const struct macho_header* mh, intptr_t slide, int argc, const char* argv[], const char* envp[], const char* apple[])
  83 {
  84         const uint32_t cmd_count = mh->ncmds;
  85         const struct load_command* const cmds = (struct load_command*)(((char*)mh)+sizeof(macho_header));
  86         const struct load_command* cmd = cmds;
  87         for (uint32_t i = 0; i < cmd_count; ++i) {
  88                 switch (cmd->cmd) {
  89                         case LC_SEGMENT_COMMAND:
  90                                 {
  91                                         const struct macho_segment_command* seg = (struct macho_segment_command*)cmd;
  92                                         const struct macho_section* const sectionsStart = (struct macho_section*)((char*)seg + sizeof(struct macho_segment_command));
  93                                         const struct macho_section* const sectionsEnd = &sectionsStart[seg->nsects];
  94                                         for (const struct macho_section* sect=sectionsStart; sect < sectionsEnd; ++sect) {
  95                                                 const uint8_t type = sect->flags & SECTION_TYPE;
  96                                                 if ( type == S_MOD_INIT_FUNC_POINTERS ){
  97                                                         Initializer* inits = (Initializer*)(sect->addr + slide);
  98                                                         const uint32_t count = sect->size / sizeof(uintptr_t);
  99                                                         for (uint32_t i=0; i < count; ++i) {
 100                                                                 Initializer func = inits[i];
 101                                                                 func(argc, argv, envp, apple);
 102                                                         }
 103                                                 }
 104                                         }
 105                                 }
 106                                 break;
 107                 }
 108                 cmd = (const struct load_command*)(((char*)cmd)+cmd->cmdsize);
 109         }
 110 }
 111
 112
 113 //
 114 //  The kernel may have slid a Position Independent Executable
 115 //
 116 static uintptr_t slideOfMainExecutable(const struct macho_header* mh)
 117 {
 118         const uint32_t cmd_count = mh->ncmds;
 119         const struct load_command* const cmds = (struct load_command*)(((char*)mh)+sizeof(macho_header));
 120         const struct load_command* cmd = cmds;
 121         for (uint32_t i = 0; i < cmd_count; ++i) {
 122                 if ( cmd->cmd == LC_SEGMENT_COMMAND ) {
 123                         const struct macho_segment_command* segCmd = (struct macho_segment_command*)cmd;
 124                         if ( strcmp(segCmd->segname, "__TEXT") == 0 ) {
 125                                 return (uintptr_t)mh - segCmd->vmaddr;
 126                         }
 127                 }
 128                 cmd = (const struct load_command*)(((char*)cmd)+cmd->cmdsize);
 129         }
 130         return 0;
 131 }
 132
 133
 134 //
 135 // If the kernel does not load dyld at its preferred address, we need to apply
 136 // fixups to various initialized parts of the __DATA segment
 137 //
 138 static void rebaseDyld(const struct macho_header* mh, intptr_t slide)
 139 {
 140         // rebase non-lazy pointers (which all point internal to dyld, since dyld uses no shared libraries)
 141         // and get interesting pointers into dyld
 142         const uint32_t cmd_count = mh->ncmds;
 143         const struct load_command* const cmds = (struct load_command*)(((char*)mh)+sizeof(macho_header));
 144         const struct load_command* cmd = cmds;
 145         const struct macho_segment_command* linkEditSeg = NULL;
 146 #if __x86_64__
 147         const struct macho_segment_command* firstWritableSeg = NULL;
 148 #endif
 149         const struct dysymtab_command* dynamicSymbolTable = NULL;
 150         for (uint32_t i = 0; i < cmd_count; ++i) {
 151                 switch (cmd->cmd) {
 152                         case LC_SEGMENT_COMMAND:
 153                                 {
 154                                         const struct macho_segment_command* seg = (struct macho_segment_command*)cmd;
 155                                         if ( strcmp(seg->segname, "__LINKEDIT") == 0 )
 156                                                 linkEditSeg = seg;
 157                                         const struct macho_section* const sectionsStart = (struct macho_section*)((char*)seg + sizeof(struct macho_segment_command));
 158                                         const struct macho_section* const sectionsEnd = &sectionsStart[seg->nsects];
 159                                         for (const struct macho_section* sect=sectionsStart; sect < sectionsEnd; ++sect) {
 160                                                 const uint8_t type = sect->flags & SECTION_TYPE;
 161                                                 if ( type == S_NON_LAZY_SYMBOL_POINTERS ) {
 162                                                         // rebase non-lazy pointers (which all point internal to dyld, since dyld uses no shared libraries)
 163                                                         const uint32_t pointerCount = sect->size / sizeof(uintptr_t);
 164                                                         uintptr_t* const symbolPointers = (uintptr_t*)(sect->addr + slide);
 165                                                         for (uint32_t j=0; j < pointerCount; ++j) {
 166                                                                 symbolPointers[j] += slide;
 167                                                         }
 168                                                 }
 169                                         }
 170 #if __x86_64__
 171                                         if ( (firstWritableSeg == NULL) && (seg->initprot & VM_PROT_WRITE) )
 172                                                 firstWritableSeg = seg;
 173 #endif
 174                                 }
 175                                 break;
 176                         case LC_DYSYMTAB:
 177                                 dynamicSymbolTable = (struct dysymtab_command *)cmd;
 178                                 break;
 179                 }
 180                 cmd = (const struct load_command*)(((char*)cmd)+cmd->cmdsize);
 181         }
 182
 183         // use reloc's to rebase all random data pointers
 184 #if __x86_64__
 185         const uintptr_t relocBase = firstWritableSeg->vmaddr + slide;
 186 #else
 187         const uintptr_t relocBase = (uintptr_t)mh;
 188 #endif
 189         const relocation_info* const relocsStart = (struct relocation_info*)(linkEditSeg->vmaddr + slide + dynamicSymbolTable->locreloff - linkEditSeg->fileoff);
 190         const relocation_info* const relocsEnd = &relocsStart[dynamicSymbolTable->nlocrel];
 191         for (const relocation_info* reloc=relocsStart; reloc < relocsEnd; ++reloc) {
 192                 if ( reloc->r_length != RELOC_SIZE )
 193                         throw "relocation in dyld has wrong size";
 194
 195                 if ( reloc->r_type != POINTER_RELOC )
 196                         throw "relocation in dyld has wrong type";
 197
 198                 // update pointer by amount dyld slid
 199                 *((uintptr_t*)(reloc->r_address + relocBase)) += slide;
 200         }
 201 }
 202
 203
 204 extern "C" void mach_init();
 205
 206 //
 207 // _pthread_keys is partitioned in a lower part that dyld will use; libSystem
 208 // will use the upper part.  We set __pthread_tsd_first to 1 as the start of
 209 // the lower part.  Libc will take #1 and c++ exceptions will take #2.  There
 210 // is one free key=3 left.
 211 //
 212 extern "C" {
 213         extern int __pthread_tsd_first;
 214         extern void _pthread_keys_init();
 215 }
 216
 217
 218 //
 219 //  This is code to bootstrap dyld.  This work in normally done for a program by dyld and crt.
 220 //  In dyld we have to do this manually.
 221 //
 222 uintptr_t start(const struct macho_header* appsMachHeader, int argc, const char* argv[],
 223                                 intptr_t slide, const struct macho_header* dyldsMachHeader,
 224                                 uintptr_t* startGlue)
 225 {
 226         // if kernel had to slide dyld, we need to fix up load sensitive locations
 227         // we have to do this before using any global variables
 228         if ( slide != 0 ) {
 229                 rebaseDyld(dyldsMachHeader, slide);
 230         }
 231
 232 #if __IPHONE_OS_VERSION_MIN_REQUIRED
 233         // set pthread keys to dyld range
 234         __pthread_tsd_first = 1;
 235         _pthread_keys_init();
 236 #endif
 237
 238         // allow dyld to use mach messaging
 239         mach_init();
 240
 241         // kernel sets up env pointer to be just past end of agv array
 242         const char** envp = &argv[argc+1];
 243
 244         // kernel sets up apple pointer to be just past end of envp array
 245         const char** apple = envp;
 246         while(*apple != NULL) { ++apple; }
 247         ++apple;
 248
 249         // run all C++ initializers inside dyld
 250         runDyldInitializers(dyldsMachHeader, slide, argc, argv, envp, apple);
 251
 252         // now that we are done bootstrapping dyld, call dyld's main
 253         uintptr_t appsSlide = slideOfMainExecutable(appsMachHeader);
 254         return dyld::_main(appsMachHeader, appsSlide, argc, argv, envp, apple, startGlue);
 255 }
 256
 257
 258
 259
 260 } // end of namespace
 261
 262
 263
 264