src/ld/passes/got.cpp

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  24
  25
  26 #include <stdint.h>
  27 #include <math.h>
  28 #include <unistd.h>
  29 #include <dlfcn.h>
  30
  31 #include <vector>
  32 #include <map>
  33 #include <ext/hash_map>
  34
  35 #include "ld.hpp"
  36 #include "got.h"
  37
  38 namespace ld {
  39 namespace passes {
  40 namespace got {
  41
  42 class File; // forward reference
  43
  44 class GOTEntryAtom : public ld::Atom {
  45 public:
  46                                                                                         GOTEntryAtom(ld::Internal& internal, const ld::Atom* target, bool weakImport)
  47                                 : ld::Atom(_s_section, ld::Atom::definitionRegular, ld::Atom::combineNever,
  48                                                         ld::Atom::scopeLinkageUnit, ld::Atom::typeNonLazyPointer,
  49                                                         symbolTableNotIn, false, false, false, ld::Atom::Alignment(3)),
  50                                 _fixup(0, ld::Fixup::k1of1, ld::Fixup::kindStoreTargetAddressLittleEndian64, target),
  51                                 _target(target)
  52                                         { _fixup.weakImport = weakImport; internal.addAtom(*this); }
  53
  54         virtual const ld::File*                                 file() const                                    { return NULL; }
  55         virtual bool                                                    translationUnitSource(const char** dir, const char**) const
  56                                                                                                                                                         { return false; }
  57         virtual const char*                                             name() const                                    { return _target->name(); }
  58         virtual uint64_t                                                size() const                                    { return 8; }
  59         virtual uint64_t                                                objectAddress() const                   { return 0; }
  60         virtual void                                                    copyRawContent(uint8_t buffer[]) const { }
  61         virtual void                                                    setScope(Scope)                                 { }
  62         virtual ld::Fixup::iterator                             fixupsBegin() const                             { return &_fixup; }
  63         virtual ld::Fixup::iterator                             fixupsEnd()     const                           { return &((ld::Fixup*)&_fixup)[1]; }
  64
  65 private:
  66         mutable ld::Fixup                                               _fixup;
  67         const ld::Atom*                                                 _target;
  68
  69         static ld::Section                                              _s_section;
  70 };
  71
  72 ld::Section GOTEntryAtom::_s_section("__DATA", "__got", ld::Section::typeNonLazyPointer);
  73
  74
  75 static bool gotFixup(const Options& opts, ld::Internal& internal, const ld::Atom* targetOfGOT, const ld::Fixup* fixup, bool* optimizable)
  76 {
  77         switch (fixup->kind) {
  78                 case ld::Fixup::kindStoreTargetAddressX86PCRel32GOTLoad:
  79                         // start by assuming this can be optimized
  80                         *optimizable = true;
  81                         // cannot do LEA optimization if target is in another dylib
  82                         if ( targetOfGOT->definition() == ld::Atom::definitionProxy )
  83                                 *optimizable = false;
  84                         // cannot do LEA optimization if target in __huge section
  85                         if ( internal.usingHugeSections && (targetOfGOT->size() > 1024*1024)
  86                                                                                         && (   (targetOfGOT->section().type() == ld::Section::typeZeroFill)
  87                                                                                                 || (targetOfGOT->section().type() == ld::Section::typeTentativeDefs)) ) {
  88                                 *optimizable = false;
  89                         }
  90                         if ( targetOfGOT->scope() == ld::Atom::scopeGlobal ) {
  91                                 // cannot do LEA optimization if target is weak exported symbol
  92                                 if ( (targetOfGOT->definition() == ld::Atom::definitionRegular) && (targetOfGOT->combine() == ld::Atom::combineByName) )
  93                                         *optimizable = false;
  94                                 // cannot do LEA optimization if target is interposable
  95                                 if ( opts.interposable(targetOfGOT->name()) )
  96                                         *optimizable = false;
  97                                 // cannot do LEA optimization if target is resolver function
  98                                 if ( targetOfGOT->contentType() == ld::Atom::typeResolver )
  99                                         *optimizable = false;
 100                                 // cannot do LEA optimization for flat-namespace
 101                                 if ( opts.nameSpace() != Options::kTwoLevelNameSpace )
 102                                         *optimizable = false;
 103                         }
 104                         return true;
 105                 case ld::Fixup::kindStoreX86PCRel32GOT:
 106                         *optimizable = false;
 107                         return true;
 108                 case ld::Fixup::kindNoneGroupSubordinatePersonality:
 109                         *optimizable = false;
 110                         return true;
 111                 default:
 112                         break;
 113         }
 114
 115         return false;
 116 }
 117
 118 struct AtomByNameSorter
 119 {
 120          bool operator()(const ld::Atom* left, const ld::Atom* right)
 121          {
 122                   return (strcmp(left->name(), right->name()) < 0);
 123          }
 124 };
 125
 126 void doPass(const Options& opts, ld::Internal& internal)
 127 {
 128         const bool log = false;
 129
 130         // only make got section in final linked images
 131         if ( opts.outputKind() == Options::kObjectFile )
 132                 return;
 133
 134         // walk all atoms and fixups looking for GOT-able references
 135         // don't create GOT atoms during this loop because that could invalidate the sections iterator
 136         std::vector<const ld::Atom*> atomsReferencingGOT;
 137         std::map<const ld::Atom*,ld::Atom*> gotMap;
 138         std::map<const ld::Atom*,bool>          weakImportMap;
 139         atomsReferencingGOT.reserve(128);
 140         for (std::vector<ld::Internal::FinalSection*>::iterator sit=internal.sections.begin(); sit != internal.sections.end(); ++sit) {
 141                 ld::Internal::FinalSection* sect = *sit;
 142                 for (std::vector<const ld::Atom*>::iterator ait=sect->atoms.begin();  ait != sect->atoms.end(); ++ait) {
 143                         const ld::Atom* atom = *ait;
 144                         bool atomUsesGOT = false;
 145                         const ld::Atom* targetOfGOT = NULL;
 146                         bool targetIsWeakImport = false;
 147                         for (ld::Fixup::iterator fit = atom->fixupsBegin(), end=atom->fixupsEnd(); fit != end; ++fit) {
 148                                 if ( fit->firstInCluster() )
 149                                         targetOfGOT = NULL;
 150                                 switch ( fit->binding ) {
 151                                         case ld::Fixup::bindingsIndirectlyBound:
 152                                                 targetOfGOT = internal.indirectBindingTable[fit->u.bindingIndex];
 153                                                 targetIsWeakImport = fit->weakImport;
 154                                                 break;
 155                                         case ld::Fixup::bindingDirectlyBound:
 156                                                 targetOfGOT = fit->u.target;
 157                                                 targetIsWeakImport = fit->weakImport;
 158                                                 break;
 159                     default:
 160                         break;
 161                                 }
 162                                 bool optimizable;
 163                                 if ( !gotFixup(opts, internal, targetOfGOT, fit, &optimizable) )
 164                                         continue;
 165                                 if ( optimizable ) {
 166                                         // change from load of GOT entry to lea of target
 167                                         if ( log ) fprintf(stderr, "optimized GOT usage in %s to %s\n", atom->name(), targetOfGOT->name());
 168                                         switch ( fit->binding ) {
 169                                                 case ld::Fixup::bindingsIndirectlyBound:
 170                                                 case ld::Fixup::bindingDirectlyBound:
 171                                                         fit->binding = ld::Fixup::bindingDirectlyBound;
 172                                                         fit->u.target = targetOfGOT;
 173                                                         fit->kind = ld::Fixup::kindStoreTargetAddressX86PCRel32GOTLoadNowLEA;
 174                                                         break;
 175                                                 default:
 176                                                         assert(0 && "unsupported GOT reference");
 177                                                         break;
 178                                         }
 179                                 }
 180                                 else {
 181                                         // remember that we need to use GOT in this function
 182                                         if ( log ) fprintf(stderr, "found GOT use in %s to %s\n", atom->name(), targetOfGOT->name());
 183                                         if ( !atomUsesGOT ) {
 184                                                 atomsReferencingGOT.push_back(atom);
 185                                                 atomUsesGOT = true;
 186                                         }
 187                                         gotMap[targetOfGOT] = NULL;
 188                                         // record weak_import attribute
 189                                         std::map<const ld::Atom*,bool>::iterator pos = weakImportMap.find(targetOfGOT);
 190                                         if ( pos == weakImportMap.end() ) {
 191                                                 // target not in weakImportMap, so add
 192                                                 if ( log ) fprintf(stderr, "weakImportMap[%s] = %d\n", targetOfGOT->name(), targetIsWeakImport);
 193                                                 weakImportMap[targetOfGOT] = targetIsWeakImport;
 194                                         }
 195                                         else {
 196                                                 // target in weakImportMap, check for weakness mismatch
 197                                                 if ( pos->second != targetIsWeakImport ) {
 198                                                         // found mismatch
 199                                                         switch ( opts.weakReferenceMismatchTreatment() ) {
 200                                                                 case Options::kWeakReferenceMismatchError:
 201                                                                         throwf("mismatching weak references for symbol: %s", targetOfGOT->name());
 202                                                                 case Options::kWeakReferenceMismatchWeak:
 203                                                                         pos->second = true;
 204                                                                         break;
 205                                                                 case Options::kWeakReferenceMismatchNonWeak:
 206                                                                         pos->second = false;
 207                                                                         break;
 208                                                         }
 209                                                 }
 210                                         }
 211                                 }
 212                         }
 213                 }
 214         }
 215
 216         // make GOT entries
 217         for (std::map<const ld::Atom*,ld::Atom*>::iterator it = gotMap.begin(); it != gotMap.end(); ++it) {
 218                 it->second = new GOTEntryAtom(internal, it->first, weakImportMap[it->first]);
 219         }
 220
 221         // update atoms to use GOT entries
 222         for (std::vector<const ld::Atom*>::iterator it=atomsReferencingGOT.begin(); it != atomsReferencingGOT.end(); ++it) {
 223                 const ld::Atom* atom = *it;
 224                 const ld::Atom* targetOfGOT = NULL;
 225                 ld::Fixup::iterator fitThatSetTarget = NULL;
 226                 for (ld::Fixup::iterator fit = atom->fixupsBegin(), end=atom->fixupsEnd(); fit != end; ++fit) {
 227                         if ( fit->firstInCluster() ) {
 228                                 targetOfGOT = NULL;
 229                                 fitThatSetTarget = NULL;
 230                         }
 231                         switch ( fit->binding ) {
 232                                 case ld::Fixup::bindingsIndirectlyBound:
 233                                         targetOfGOT = internal.indirectBindingTable[fit->u.bindingIndex];
 234                                         fitThatSetTarget = fit;
 235                                         break;
 236                                 case ld::Fixup::bindingDirectlyBound:
 237                                         targetOfGOT = fit->u.target;
 238                                         fitThatSetTarget = fit;
 239                                         break;
 240                 default:
 241                     break;
 242                         }
 243                         bool optimizable;
 244                         if ( (targetOfGOT == NULL) || !gotFixup(opts, internal, targetOfGOT, fit, &optimizable) )
 245                                 continue;
 246                         if ( !optimizable ) {
 247                                 // GOT use not optimized away, update to bind to GOT entry
 248                                 assert(fitThatSetTarget != NULL);
 249                                 switch ( fitThatSetTarget->binding ) {
 250                                         case ld::Fixup::bindingsIndirectlyBound:
 251                                         case ld::Fixup::bindingDirectlyBound:
 252                                                 fitThatSetTarget->binding = ld::Fixup::bindingDirectlyBound;
 253                                                 fitThatSetTarget->u.target = gotMap[targetOfGOT];
 254                                                 break;
 255                                         default:
 256                                                 assert(0 && "unsupported GOT reference");
 257                                                 break;
 258                                 }
 259                         }
 260                 }
 261         }
 262
 263         // sort new atoms so links are consistent
 264         for (std::vector<ld::Internal::FinalSection*>::iterator sit=internal.sections.begin(); sit != internal.sections.end(); ++sit) {
 265                 ld::Internal::FinalSection* sect = *sit;
 266                 if ( sect->type() == ld::Section::typeNonLazyPointer ) {
 267                         std::sort(sect->atoms.begin(), sect->atoms.end(), AtomByNameSorter());
 268                 }
 269         }
 270 }
 271
 272
 273 } // namespace got
 274 } // namespace passes
 275 } // namespace ld