src/dyldInitialization.cpp

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  24
  25 #include <stddef.h>
  26 #include <string.h>
  27 #include <mach/mach.h>
  28 #include <mach-o/loader.h>
  29 #include <mach-o/ldsyms.h>
  30 #include <mach-o/reloc.h>
  31 #if __ppc__ || __ppc64__
  32         #include <mach-o/ppc/reloc.h>
  33 #endif
  34 #include "dyld.h"
  35
  36 #if __LP64__
  37         #define macho_header                    mach_header_64
  38         #define LC_SEGMENT_COMMAND              LC_SEGMENT_64
  39         #define macho_segment_command   segment_command_64
  40         #define macho_section                   section_64
  41         #define RELOC_SIZE                              3
  42 #else
  43         #define macho_header                    mach_header
  44         #define LC_SEGMENT_COMMAND              LC_SEGMENT
  45         #define macho_segment_command   segment_command
  46         #define macho_section                   section
  47         #define RELOC_SIZE                              2
  48 #endif
  49
  50         #define POINTER_RELOC GENERIC_RELOC_VANILLA
  51
  52 //
  53 //  Code to bootstrap dyld into a runnable state
  54 //
  55 //
  56
  57 namespace dyldbootstrap {
  58
  59
  60 typedef void (*Initializer)(int argc, const char* argv[], const char* envp[], const char* apple[]);
  61
  62 //
  63 // For a regular executable, the crt code calls dyld to run the executables initializers.
  64 // For a static executable, crt directly runs the initializers.
  65 // dyld (should be static) but is a dynamic executable and needs this hack to run its own initializers.
  66 // We pass argc, argv, etc in case libc.a uses those arguments
  67 //
  68 static void runDyldInitializers(const struct macho_header* mh, intptr_t slide, int argc, const char* argv[], const char* envp[], const char* apple[])
  69 {
  70         const uint32_t cmd_count = mh->ncmds;
  71         const struct load_command* const cmds = (struct load_command*)(((char*)mh)+sizeof(macho_header));
  72         const struct load_command* cmd = cmds;
  73         for (uint32_t i = 0; i < cmd_count; ++i) {
  74                 switch (cmd->cmd) {
  75                         case LC_SEGMENT_COMMAND:
  76                                 {
  77                                         const struct macho_segment_command* seg = (struct macho_segment_command*)cmd;
  78                                         const struct macho_section* const sectionsStart = (struct macho_section*)((char*)seg + sizeof(struct macho_segment_command));
  79                                         const struct macho_section* const sectionsEnd = &sectionsStart[seg->nsects];
  80                                         for (const struct macho_section* sect=sectionsStart; sect < sectionsEnd; ++sect) {
  81                                                 const uint8_t type = sect->flags & SECTION_TYPE;
  82                                                 if ( type == S_MOD_INIT_FUNC_POINTERS ){
  83                                                         Initializer* inits = (Initializer*)(sect->addr + slide);
  84                                                         const uint32_t count = sect->size / sizeof(uintptr_t);
  85                                                         for (uint32_t i=0; i < count; ++i) {
  86                                                                 Initializer func = inits[i];
  87                                                                 func(argc, argv, envp, apple);
  88                                                         }
  89                                                 }
  90                                         }
  91                                 }
  92                                 break;
  93                 }
  94                 cmd = (const struct load_command*)(((char*)cmd)+cmd->cmdsize);
  95         }
  96 }
  97
  98 //
  99 // If the kernel does not load dyld at its preferred address, we need to apply
 100 // fixups to various initialized parts of the __DATA segment
 101 //
 102 static void rebaseDyld(const struct macho_header* mh, intptr_t slide)
 103 {
 104         // rebase non-lazy pointers (which all point internal to dyld, since dyld uses no shared libraries)
 105         // and get interesting pointers into dyld
 106         const uint32_t cmd_count = mh->ncmds;
 107         const struct load_command* const cmds = (struct load_command*)(((char*)mh)+sizeof(macho_header));
 108         const struct load_command* cmd = cmds;
 109         const struct macho_segment_command* linkEditSeg = NULL;
 110         const struct dysymtab_command* dynamicSymbolTable = NULL;
 111         for (uint32_t i = 0; i < cmd_count; ++i) {
 112                 switch (cmd->cmd) {
 113                         case LC_SEGMENT_COMMAND:
 114                                 {
 115                                         const struct macho_segment_command* seg = (struct macho_segment_command*)cmd;
 116                                         if ( strcmp(seg->segname, "__LINKEDIT") == 0 )
 117                                                 linkEditSeg = seg;
 118                                         const struct macho_section* const sectionsStart = (struct macho_section*)((char*)seg + sizeof(struct macho_segment_command));
 119                                         const struct macho_section* const sectionsEnd = &sectionsStart[seg->nsects];
 120                                         for (const struct macho_section* sect=sectionsStart; sect < sectionsEnd; ++sect) {
 121                                                 const uint8_t type = sect->flags & SECTION_TYPE;
 122                                                 if ( type == S_NON_LAZY_SYMBOL_POINTERS ) {
 123                                                         // rebase non-lazy pointers (which all point internal to dyld, since dyld uses no shared libraries)
 124                                                         const uint32_t pointerCount = sect->size / sizeof(uintptr_t);
 125                                                         uintptr_t* const symbolPointers = (uintptr_t*)(sect->addr + slide);
 126                                                         for (uint32_t j=0; j < pointerCount; ++j) {
 127                                                                 symbolPointers[j] += slide;
 128                                                         }
 129                                                 }
 130                                         }
 131                                 }
 132                                 break;
 133                         case LC_DYSYMTAB:
 134                                 dynamicSymbolTable = (struct dysymtab_command *)cmd;
 135                                 break;
 136                 }
 137                 cmd = (const struct load_command*)(((char*)cmd)+cmd->cmdsize);
 138         }
 139
 140         // use reloc's to rebase all random data pointers
 141         const uintptr_t relocBase = (uintptr_t)mh;
 142         const relocation_info* const relocsStart = (struct relocation_info*)(linkEditSeg->vmaddr + slide + dynamicSymbolTable->locreloff - linkEditSeg->fileoff);
 143         const relocation_info* const relocsEnd = &relocsStart[dynamicSymbolTable->nlocrel];
 144         for (const relocation_info* reloc=relocsStart; reloc < relocsEnd; ++reloc) {
 145         #if __ppc__ || __ppc64__ || __i36__
 146                 if ( (reloc->r_address & R_SCATTERED) != 0 )
 147                         throw "scattered relocation in dyld";
 148         #endif
 149                 if ( reloc->r_length != RELOC_SIZE )
 150                         throw "relocation in dyld has wrong size";
 151
 152                 if ( reloc->r_type != POINTER_RELOC )
 153                         throw "relocation in dyld has wrong type";
 154
 155                 // update pointer by amount dyld slid
 156                 *((uintptr_t*)(reloc->r_address + relocBase)) += slide;
 157         }
 158 }
 159
 160
 161 //
 162 // For some reason the kernel loads dyld with __TEXT and __LINKEDIT writable
 163 // rdar://problem/3702311
 164 //
 165 static void segmentProtectDyld(const struct macho_header* mh, intptr_t slide)
 166 {
 167         const uint32_t cmd_count = mh->ncmds;
 168         const struct load_command* const cmds = (struct load_command*)(((char*)mh)+sizeof(macho_header));
 169         const struct load_command* cmd = cmds;
 170         for (uint32_t i = 0; i < cmd_count; ++i) {
 171                 switch (cmd->cmd) {
 172                         case LC_SEGMENT_COMMAND:
 173                                 {
 174                                         const struct macho_segment_command* seg = (struct macho_segment_command*)cmd;
 175                                         vm_address_t addr = seg->vmaddr + slide;
 176                                         vm_size_t size = seg->vmsize;
 177                                         const bool setCurrentPermissions = false;
 178                                         vm_protect(mach_task_self(), addr, size, setCurrentPermissions, seg->initprot);
 179                                         //fprintf(stderr, "dyld: segment %s, 0x%08X -> 0x%08X, set to %d\n", seg->segname, addr, addr+size-1, seg->initprot);
 180                                 }
 181                                 break;
 182                 }
 183                 cmd = (const struct load_command*)(((char*)cmd)+cmd->cmdsize);
 184         }
 185
 186 }
 187
 188 extern "C" void dyld_exceptions_init(const struct macho_header*, uintptr_t slide); // in dyldExceptions.cpp
 189 extern "C" void mach_init();
 190
 191 //
 192 //  This is code to bootstrap dyld.  This work in normally done for a program by dyld and crt.
 193 //  In dyld we have to do this manually.
 194 //
 195 uintptr_t start(const struct mach_header* appsMachHeader, int argc, const char* argv[], intptr_t slide)
 196 {
 197         // _mh_dylinker_header is magic symbol defined by static linker (ld), see <mach-o/ldsyms.h>
 198         const struct macho_header* dyldsMachHeader =  (const struct macho_header*)(((char*)&_mh_dylinker_header)+slide);
 199
 200         // if kernel had to slide dyld, we need to fix up load sensitive locations
 201         // we have to do this before using any global variables
 202         if ( slide != 0 ) {
 203                 rebaseDyld(dyldsMachHeader, slide);
 204         }
 205
 206         // enable C++ exceptions to work inside dyld
 207         dyld_exceptions_init(dyldsMachHeader, slide);
 208
 209         // allow dyld to use mach messaging
 210         mach_init();
 211
 212         // set protection on segments (has to be done after mach_init)
 213         segmentProtectDyld(dyldsMachHeader, slide);
 214
 215         // kernel sets up env pointer to be just past end of agv array
 216         const char** envp = &argv[argc+1];
 217
 218         // kernel sets up apple pointer to be just past end of envp array
 219         const char** apple = envp;
 220         while(*apple != NULL) { ++apple; }
 221         ++apple;
 222
 223         // run all C++ initializers inside dyld
 224         runDyldInitializers(dyldsMachHeader, slide, argc, argv, envp, apple);
 225
 226         // now that we are done bootstrapping dyld, call dyld's main
 227         return dyld::_main(appsMachHeader, argc, argv, envp, apple);
 228 }
 229
 230
 231
 232
 233 } // end of namespace
 234
 235
 236
 237