src/dyldInitialization.cpp

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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 __ppc__ || __ppc64__
  35         #include <mach-o/ppc/reloc.h>
  36 #endif
  37 #if __x86_64__
  38         #include <mach-o/x86_64/reloc.h>
  39 #endif
  40 #include "dyld.h"
  41
  42 #ifndef MH_PIE
  43         #define MH_PIE 0x200000
  44 #endif
  45
  46
  47 #if __LP64__
  48         #define LC_SEGMENT_COMMAND              LC_SEGMENT_64
  49         #define macho_segment_command   segment_command_64
  50         #define macho_section                   section_64
  51         #define RELOC_SIZE                              3
  52 #else
  53         #define LC_SEGMENT_COMMAND              LC_SEGMENT
  54         #define macho_segment_command   segment_command
  55         #define macho_section                   section
  56         #define RELOC_SIZE                              2
  57 #endif
  58
  59 #if __x86_64__
  60         #define POINTER_RELOC X86_64_RELOC_UNSIGNED
  61 #else
  62         #define POINTER_RELOC GENERIC_RELOC_VANILLA
  63 #endif
  64
  65 // from dyld.cpp
  66 namespace dyld { extern bool isRosetta(); };
  67
  68
  69 //
  70 //  Code to bootstrap dyld into a runnable state
  71 //
  72 //
  73
  74 namespace dyldbootstrap {
  75
  76
  77 typedef void (*Initializer)(int argc, const char* argv[], const char* envp[], const char* apple[]);
  78
  79 //
  80 // For a regular executable, the crt code calls dyld to run the executables initializers.
  81 // For a static executable, crt directly runs the initializers.
  82 // dyld (should be static) but is a dynamic executable and needs this hack to run its own initializers.
  83 // We pass argc, argv, etc in case libc.a uses those arguments
  84 //
  85 static void runDyldInitializers(const struct macho_header* mh, intptr_t slide, int argc, const char* argv[], const char* envp[], const char* apple[])
  86 {
  87         const uint32_t cmd_count = mh->ncmds;
  88         const struct load_command* const cmds = (struct load_command*)(((char*)mh)+sizeof(macho_header));
  89         const struct load_command* cmd = cmds;
  90         for (uint32_t i = 0; i < cmd_count; ++i) {
  91                 switch (cmd->cmd) {
  92                         case LC_SEGMENT_COMMAND:
  93                                 {
  94                                         const struct macho_segment_command* seg = (struct macho_segment_command*)cmd;
  95                                         const struct macho_section* const sectionsStart = (struct macho_section*)((char*)seg + sizeof(struct macho_segment_command));
  96                                         const struct macho_section* const sectionsEnd = &sectionsStart[seg->nsects];
  97                                         for (const struct macho_section* sect=sectionsStart; sect < sectionsEnd; ++sect) {
  98                                                 const uint8_t type = sect->flags & SECTION_TYPE;
  99                                                 if ( type == S_MOD_INIT_FUNC_POINTERS ){
 100                                                         Initializer* inits = (Initializer*)(sect->addr + slide);
 101                                                         const uint32_t count = sect->size / sizeof(uintptr_t);
 102                                                         for (uint32_t i=0; i < count; ++i) {
 103                                                                 Initializer func = inits[i];
 104                                                                 func(argc, argv, envp, apple);
 105                                                         }
 106                                                 }
 107                                         }
 108                                 }
 109                                 break;
 110                 }
 111                 cmd = (const struct load_command*)(((char*)cmd)+cmd->cmdsize);
 112         }
 113 }
 114
 115
 116 //
 117 //  The kernel may have slid a Position Independent Executable
 118 //
 119 static uintptr_t slideOfMainExecutable(const struct macho_header* mh)
 120 {
 121         const uint32_t cmd_count = mh->ncmds;
 122         const struct load_command* const cmds = (struct load_command*)(((char*)mh)+sizeof(macho_header));
 123         const struct load_command* cmd = cmds;
 124         for (uint32_t i = 0; i < cmd_count; ++i) {
 125                 if ( cmd->cmd == LC_SEGMENT_COMMAND ) {
 126                         const struct macho_segment_command* segCmd = (struct macho_segment_command*)cmd;
 127                         if ( strcmp(segCmd->segname, "__TEXT") == 0 ) {
 128                                 return (uintptr_t)mh - segCmd->vmaddr;
 129                         }
 130                 }
 131                 cmd = (const struct load_command*)(((char*)cmd)+cmd->cmdsize);
 132         }
 133         return 0;
 134 }
 135
 136
 137 //
 138 // If the kernel does not load dyld at its preferred address, we need to apply
 139 // fixups to various initialized parts of the __DATA segment
 140 //
 141 static void rebaseDyld(const struct macho_header* mh, intptr_t slide)
 142 {
 143         // rebase non-lazy pointers (which all point internal to dyld, since dyld uses no shared libraries)
 144         // and get interesting pointers into dyld
 145         const uint32_t cmd_count = mh->ncmds;
 146         const struct load_command* const cmds = (struct load_command*)(((char*)mh)+sizeof(macho_header));
 147         const struct load_command* cmd = cmds;
 148         const struct macho_segment_command* linkEditSeg = NULL;
 149 #if __x86_64__
 150         const struct macho_segment_command* firstWritableSeg = NULL;
 151 #endif
 152         const struct dysymtab_command* dynamicSymbolTable = NULL;
 153         for (uint32_t i = 0; i < cmd_count; ++i) {
 154                 switch (cmd->cmd) {
 155                         case LC_SEGMENT_COMMAND:
 156                                 {
 157                                         const struct macho_segment_command* seg = (struct macho_segment_command*)cmd;
 158                                         if ( strcmp(seg->segname, "__LINKEDIT") == 0 )
 159                                                 linkEditSeg = seg;
 160                                         const struct macho_section* const sectionsStart = (struct macho_section*)((char*)seg + sizeof(struct macho_segment_command));
 161                                         const struct macho_section* const sectionsEnd = &sectionsStart[seg->nsects];
 162                                         for (const struct macho_section* sect=sectionsStart; sect < sectionsEnd; ++sect) {
 163                                                 const uint8_t type = sect->flags & SECTION_TYPE;
 164                                                 if ( type == S_NON_LAZY_SYMBOL_POINTERS ) {
 165                                                         // rebase non-lazy pointers (which all point internal to dyld, since dyld uses no shared libraries)
 166                                                         const uint32_t pointerCount = sect->size / sizeof(uintptr_t);
 167                                                         uintptr_t* const symbolPointers = (uintptr_t*)(sect->addr + slide);
 168                                                         for (uint32_t j=0; j < pointerCount; ++j) {
 169                                                                 symbolPointers[j] += slide;
 170                                                         }
 171                                                 }
 172                                         }
 173 #if __x86_64__
 174                                         if ( (firstWritableSeg == NULL) && (seg->initprot & VM_PROT_WRITE) )
 175                                                 firstWritableSeg = seg;
 176 #endif
 177                                 }
 178                                 break;
 179                         case LC_DYSYMTAB:
 180                                 dynamicSymbolTable = (struct dysymtab_command *)cmd;
 181                                 break;
 182                 }
 183                 cmd = (const struct load_command*)(((char*)cmd)+cmd->cmdsize);
 184         }
 185
 186         // use reloc's to rebase all random data pointers
 187 #if __x86_64__
 188         const uintptr_t relocBase = firstWritableSeg->vmaddr + slide;
 189 #else
 190         const uintptr_t relocBase = (uintptr_t)mh;
 191 #endif
 192         const relocation_info* const relocsStart = (struct relocation_info*)(linkEditSeg->vmaddr + slide + dynamicSymbolTable->locreloff - linkEditSeg->fileoff);
 193         const relocation_info* const relocsEnd = &relocsStart[dynamicSymbolTable->nlocrel];
 194         for (const relocation_info* reloc=relocsStart; reloc < relocsEnd; ++reloc) {
 195         #if __ppc__ || __ppc64__ || __i36__
 196                 if ( (reloc->r_address & R_SCATTERED) != 0 )
 197                         throw "scattered relocation in dyld";
 198         #endif
 199                 if ( reloc->r_length != RELOC_SIZE )
 200                         throw "relocation in dyld has wrong size";
 201
 202                 if ( reloc->r_type != POINTER_RELOC )
 203                         throw "relocation in dyld has wrong type";
 204
 205                 // update pointer by amount dyld slid
 206                 *((uintptr_t*)(reloc->r_address + relocBase)) += slide;
 207         }
 208 }
 209
 210
 211 extern "C" void dyld_exceptions_init(const struct macho_header*, uintptr_t slide); // in dyldExceptions.cpp
 212 extern "C" void mach_init();
 213
 214 //
 215 // _pthread_keys is partitioned in a lower part that dyld will use; libSystem
 216 // will use the upper part.  We set __pthread_tsd_first to 1 as the start of
 217 // the lower part.  Libc will take #1 and c++ exceptions will take #2.  There
 218 // is one free key=3 left.
 219 //
 220 extern "C" {
 221         extern int __pthread_tsd_first;
 222         extern void _pthread_keys_init();
 223 }
 224
 225
 226 //
 227 //  This is code to bootstrap dyld.  This work in normally done for a program by dyld and crt.
 228 //  In dyld we have to do this manually.
 229 //
 230 uintptr_t start(const struct macho_header* appsMachHeader, int argc, const char* argv[],
 231                                                                 intptr_t slide, const struct macho_header* dyldsMachHeader)
 232 {
 233         // if kernel had to slide dyld, we need to fix up load sensitive locations
 234         // we have to do this before using any global variables
 235         if ( slide != 0 ) {
 236                 rebaseDyld(dyldsMachHeader, slide);
 237         }
 238
 239 #if __IPHONE_OS_VERSION_MIN_REQUIRED
 240         // set pthread keys to dyld range
 241         __pthread_tsd_first = 1;
 242         _pthread_keys_init();
 243 #endif
 244
 245         // enable C++ exceptions to work inside dyld
 246         dyld_exceptions_init(dyldsMachHeader, slide);
 247
 248         // allow dyld to use mach messaging
 249         mach_init();
 250
 251         // kernel sets up env pointer to be just past end of agv array
 252         const char** envp = &argv[argc+1];
 253
 254         // kernel sets up apple pointer to be just past end of envp array
 255         const char** apple = envp;
 256         while(*apple != NULL) { ++apple; }
 257         ++apple;
 258
 259         // run all C++ initializers inside dyld
 260         runDyldInitializers(dyldsMachHeader, slide, argc, argv, envp, apple);
 261
 262         // now that we are done bootstrapping dyld, call dyld's main
 263         uintptr_t appsSlide = slideOfMainExecutable(appsMachHeader);
 264         return dyld::_main(appsMachHeader, appsSlide, argc, argv, envp, apple);
 265 }
 266
 267
 268
 269
 270 } // end of namespace
 271
 272
 273
 274