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1 //===-- ShadowStackGCLowering.cpp - Custom lowering for shadow-stack gc ---===//
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2 //
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3 // The LLVM Compiler Infrastructure
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4 //
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5 // This file is distributed under the University of Illinois Open Source
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6 // License. See LICENSE.TXT for details.
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7 //
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8 //===----------------------------------------------------------------------===//
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9 //
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10 // This file contains the custom lowering code required by the shadow-stack GC
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100
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11 // strategy.
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12 //
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13 // This pass implements the code transformation described in this paper:
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14 // "Accurate Garbage Collection in an Uncooperative Environment"
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15 // Fergus Henderson, ISMM, 2002
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83
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16 //
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17 //===----------------------------------------------------------------------===//
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18
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19 #include "llvm/CodeGen/Passes.h"
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20 #include "llvm/ADT/StringExtras.h"
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21 #include "llvm/CodeGen/GCStrategy.h"
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22 #include "llvm/IR/CallSite.h"
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23 #include "llvm/IR/IRBuilder.h"
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24 #include "llvm/IR/IntrinsicInst.h"
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25 #include "llvm/IR/Module.h"
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26
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27 using namespace llvm;
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28
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29 #define DEBUG_TYPE "shadowstackgclowering"
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30
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31 namespace {
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32
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33 class ShadowStackGCLowering : public FunctionPass {
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34 /// RootChain - This is the global linked-list that contains the chain of GC
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35 /// roots.
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36 GlobalVariable *Head;
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37
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38 /// StackEntryTy - Abstract type of a link in the shadow stack.
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39 ///
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40 StructType *StackEntryTy;
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41 StructType *FrameMapTy;
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42
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43 /// Roots - GC roots in the current function. Each is a pair of the
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44 /// intrinsic call and its corresponding alloca.
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45 std::vector<std::pair<CallInst *, AllocaInst *>> Roots;
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46
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47 public:
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48 static char ID;
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49 ShadowStackGCLowering();
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50
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51 bool doInitialization(Module &M) override;
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52 bool runOnFunction(Function &F) override;
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53
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54 private:
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55 bool IsNullValue(Value *V);
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56 Constant *GetFrameMap(Function &F);
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57 Type *GetConcreteStackEntryType(Function &F);
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58 void CollectRoots(Function &F);
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59 static GetElementPtrInst *CreateGEP(LLVMContext &Context, IRBuilder<> &B,
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60 Type *Ty, Value *BasePtr, int Idx1,
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61 const char *Name);
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62 static GetElementPtrInst *CreateGEP(LLVMContext &Context, IRBuilder<> &B,
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63 Type *Ty, Value *BasePtr, int Idx1, int Idx2,
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64 const char *Name);
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65 };
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66 }
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67
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68 INITIALIZE_PASS_BEGIN(ShadowStackGCLowering, "shadow-stack-gc-lowering",
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69 "Shadow Stack GC Lowering", false, false)
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70 INITIALIZE_PASS_DEPENDENCY(GCModuleInfo)
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71 INITIALIZE_PASS_END(ShadowStackGCLowering, "shadow-stack-gc-lowering",
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72 "Shadow Stack GC Lowering", false, false)
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73
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74 FunctionPass *llvm::createShadowStackGCLoweringPass() { return new ShadowStackGCLowering(); }
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75
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76 char ShadowStackGCLowering::ID = 0;
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77
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78 ShadowStackGCLowering::ShadowStackGCLowering()
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79 : FunctionPass(ID), Head(nullptr), StackEntryTy(nullptr),
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80 FrameMapTy(nullptr) {
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81 initializeShadowStackGCLoweringPass(*PassRegistry::getPassRegistry());
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82 }
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83
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84 namespace {
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85 /// EscapeEnumerator - This is a little algorithm to find all escape points
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86 /// from a function so that "finally"-style code can be inserted. In addition
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87 /// to finding the existing return and unwind instructions, it also (if
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88 /// necessary) transforms any call instructions into invokes and sends them to
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89 /// a landing pad.
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90 ///
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91 /// It's wrapped up in a state machine using the same transform C# uses for
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92 /// 'yield return' enumerators, This transform allows it to be non-allocating.
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93 class EscapeEnumerator {
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94 Function &F;
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95 const char *CleanupBBName;
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96
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97 // State.
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98 int State;
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99 Function::iterator StateBB, StateE;
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100 IRBuilder<> Builder;
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101
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102 public:
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103 EscapeEnumerator(Function &F, const char *N = "cleanup")
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104 : F(F), CleanupBBName(N), State(0), Builder(F.getContext()) {}
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105
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106 IRBuilder<> *Next() {
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107 switch (State) {
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108 default:
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109 return nullptr;
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110
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111 case 0:
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112 StateBB = F.begin();
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113 StateE = F.end();
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114 State = 1;
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115
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116 case 1:
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117 // Find all 'return', 'resume', and 'unwind' instructions.
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118 while (StateBB != StateE) {
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119 BasicBlock *CurBB = &*StateBB++;
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120
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121 // Branches and invokes do not escape, only unwind, resume, and return
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122 // do.
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123 TerminatorInst *TI = CurBB->getTerminator();
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124 if (!isa<ReturnInst>(TI) && !isa<ResumeInst>(TI))
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125 continue;
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126
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127 Builder.SetInsertPoint(TI);
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128 return &Builder;
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129 }
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130
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131 State = 2;
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132
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133 // Find all 'call' instructions.
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134 SmallVector<Instruction *, 16> Calls;
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135 for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB)
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136 for (BasicBlock::iterator II = BB->begin(), EE = BB->end(); II != EE;
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137 ++II)
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138 if (CallInst *CI = dyn_cast<CallInst>(II))
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139 if (!CI->getCalledFunction() ||
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140 !CI->getCalledFunction()->getIntrinsicID())
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141 Calls.push_back(CI);
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142
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143 if (Calls.empty())
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144 return nullptr;
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145
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146 // Create a cleanup block.
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147 LLVMContext &C = F.getContext();
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148 BasicBlock *CleanupBB = BasicBlock::Create(C, CleanupBBName, &F);
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149 Type *ExnTy =
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150 StructType::get(Type::getInt8PtrTy(C), Type::getInt32Ty(C), nullptr);
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151 if (!F.hasPersonalityFn()) {
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152 Constant *PersFn = F.getParent()->getOrInsertFunction(
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153 "__gcc_personality_v0",
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154 FunctionType::get(Type::getInt32Ty(C), true));
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155 F.setPersonalityFn(PersFn);
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156 }
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157 LandingPadInst *LPad =
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158 LandingPadInst::Create(ExnTy, 1, "cleanup.lpad", CleanupBB);
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159 LPad->setCleanup(true);
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160 ResumeInst *RI = ResumeInst::Create(LPad, CleanupBB);
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161
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162 // Transform the 'call' instructions into 'invoke's branching to the
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163 // cleanup block. Go in reverse order to make prettier BB names.
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164 SmallVector<Value *, 16> Args;
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165 for (unsigned I = Calls.size(); I != 0;) {
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166 CallInst *CI = cast<CallInst>(Calls[--I]);
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167
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168 // Split the basic block containing the function call.
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169 BasicBlock *CallBB = CI->getParent();
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170 BasicBlock *NewBB = CallBB->splitBasicBlock(
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171 CI->getIterator(), CallBB->getName() + ".cont");
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172
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173 // Remove the unconditional branch inserted at the end of CallBB.
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174 CallBB->getInstList().pop_back();
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175 NewBB->getInstList().remove(CI);
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176
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177 // Create a new invoke instruction.
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178 Args.clear();
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179 CallSite CS(CI);
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180 Args.append(CS.arg_begin(), CS.arg_end());
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181
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182 InvokeInst *II =
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183 InvokeInst::Create(CI->getCalledValue(), NewBB, CleanupBB, Args,
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184 CI->getName(), CallBB);
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185 II->setCallingConv(CI->getCallingConv());
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186 II->setAttributes(CI->getAttributes());
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187 CI->replaceAllUsesWith(II);
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188 delete CI;
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189 }
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190
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191 Builder.SetInsertPoint(RI);
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192 return &Builder;
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193 }
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194 }
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195 };
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196 }
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197
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198
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199 Constant *ShadowStackGCLowering::GetFrameMap(Function &F) {
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200 // doInitialization creates the abstract type of this value.
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201 Type *VoidPtr = Type::getInt8PtrTy(F.getContext());
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202
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203 // Truncate the ShadowStackDescriptor if some metadata is null.
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204 unsigned NumMeta = 0;
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205 SmallVector<Constant *, 16> Metadata;
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206 for (unsigned I = 0; I != Roots.size(); ++I) {
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207 Constant *C = cast<Constant>(Roots[I].first->getArgOperand(1));
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208 if (!C->isNullValue())
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209 NumMeta = I + 1;
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210 Metadata.push_back(ConstantExpr::getBitCast(C, VoidPtr));
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211 }
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212 Metadata.resize(NumMeta);
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213
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214 Type *Int32Ty = Type::getInt32Ty(F.getContext());
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215
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216 Constant *BaseElts[] = {
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217 ConstantInt::get(Int32Ty, Roots.size(), false),
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218 ConstantInt::get(Int32Ty, NumMeta, false),
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219 };
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220
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221 Constant *DescriptorElts[] = {
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222 ConstantStruct::get(FrameMapTy, BaseElts),
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223 ConstantArray::get(ArrayType::get(VoidPtr, NumMeta), Metadata)};
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224
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225 Type *EltTys[] = {DescriptorElts[0]->getType(), DescriptorElts[1]->getType()};
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226 StructType *STy = StructType::create(EltTys, "gc_map." + utostr(NumMeta));
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227
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228 Constant *FrameMap = ConstantStruct::get(STy, DescriptorElts);
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229
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230 // FIXME: Is this actually dangerous as WritingAnLLVMPass.html claims? Seems
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231 // that, short of multithreaded LLVM, it should be safe; all that is
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232 // necessary is that a simple Module::iterator loop not be invalidated.
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233 // Appending to the GlobalVariable list is safe in that sense.
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234 //
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235 // All of the output passes emit globals last. The ExecutionEngine
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236 // explicitly supports adding globals to the module after
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237 // initialization.
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238 //
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239 // Still, if it isn't deemed acceptable, then this transformation needs
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240 // to be a ModulePass (which means it cannot be in the 'llc' pipeline
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241 // (which uses a FunctionPassManager (which segfaults (not asserts) if
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242 // provided a ModulePass))).
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243 Constant *GV = new GlobalVariable(*F.getParent(), FrameMap->getType(), true,
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244 GlobalVariable::InternalLinkage, FrameMap,
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245 "__gc_" + F.getName());
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246
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247 Constant *GEPIndices[2] = {
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248 ConstantInt::get(Type::getInt32Ty(F.getContext()), 0),
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249 ConstantInt::get(Type::getInt32Ty(F.getContext()), 0)};
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250 return ConstantExpr::getGetElementPtr(FrameMap->getType(), GV, GEPIndices);
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251 }
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252
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253 Type *ShadowStackGCLowering::GetConcreteStackEntryType(Function &F) {
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254 // doInitialization creates the generic version of this type.
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255 std::vector<Type *> EltTys;
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256 EltTys.push_back(StackEntryTy);
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257 for (size_t I = 0; I != Roots.size(); I++)
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258 EltTys.push_back(Roots[I].second->getAllocatedType());
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259
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260 return StructType::create(EltTys, ("gc_stackentry." + F.getName()).str());
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261 }
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262
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263 /// doInitialization - If this module uses the GC intrinsics, find them now. If
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264 /// not, exit fast.
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265 bool ShadowStackGCLowering::doInitialization(Module &M) {
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266 bool Active = false;
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267 for (Function &F : M) {
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268 if (F.hasGC() && F.getGC() == std::string("shadow-stack")) {
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269 Active = true;
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270 break;
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271 }
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272 }
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273 if (!Active)
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274 return false;
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275
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276 // struct FrameMap {
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277 // int32_t NumRoots; // Number of roots in stack frame.
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278 // int32_t NumMeta; // Number of metadata descriptors. May be < NumRoots.
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279 // void *Meta[]; // May be absent for roots without metadata.
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280 // };
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281 std::vector<Type *> EltTys;
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282 // 32 bits is ok up to a 32GB stack frame. :)
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283 EltTys.push_back(Type::getInt32Ty(M.getContext()));
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284 // Specifies length of variable length array.
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285 EltTys.push_back(Type::getInt32Ty(M.getContext()));
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286 FrameMapTy = StructType::create(EltTys, "gc_map");
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287 PointerType *FrameMapPtrTy = PointerType::getUnqual(FrameMapTy);
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288
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289 // struct StackEntry {
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290 // ShadowStackEntry *Next; // Caller's stack entry.
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291 // FrameMap *Map; // Pointer to constant FrameMap.
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292 // void *Roots[]; // Stack roots (in-place array, so we pretend).
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293 // };
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294
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295 StackEntryTy = StructType::create(M.getContext(), "gc_stackentry");
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296
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297 EltTys.clear();
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298 EltTys.push_back(PointerType::getUnqual(StackEntryTy));
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299 EltTys.push_back(FrameMapPtrTy);
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300 StackEntryTy->setBody(EltTys);
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301 PointerType *StackEntryPtrTy = PointerType::getUnqual(StackEntryTy);
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302
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303 // Get the root chain if it already exists.
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304 Head = M.getGlobalVariable("llvm_gc_root_chain");
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305 if (!Head) {
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306 // If the root chain does not exist, insert a new one with linkonce
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307 // linkage!
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308 Head = new GlobalVariable(
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309 M, StackEntryPtrTy, false, GlobalValue::LinkOnceAnyLinkage,
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310 Constant::getNullValue(StackEntryPtrTy), "llvm_gc_root_chain");
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311 } else if (Head->hasExternalLinkage() && Head->isDeclaration()) {
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312 Head->setInitializer(Constant::getNullValue(StackEntryPtrTy));
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313 Head->setLinkage(GlobalValue::LinkOnceAnyLinkage);
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314 }
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315
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316 return true;
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317 }
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318
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319 bool ShadowStackGCLowering::IsNullValue(Value *V) {
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320 if (Constant *C = dyn_cast<Constant>(V))
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321 return C->isNullValue();
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322 return false;
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323 }
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324
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325 void ShadowStackGCLowering::CollectRoots(Function &F) {
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326 // FIXME: Account for original alignment. Could fragment the root array.
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327 // Approach 1: Null initialize empty slots at runtime. Yuck.
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328 // Approach 2: Emit a map of the array instead of just a count.
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329
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330 assert(Roots.empty() && "Not cleaned up?");
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331
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332 SmallVector<std::pair<CallInst *, AllocaInst *>, 16> MetaRoots;
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333
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334 for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB)
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335 for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E;)
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336 if (IntrinsicInst *CI = dyn_cast<IntrinsicInst>(II++))
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337 if (Function *F = CI->getCalledFunction())
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338 if (F->getIntrinsicID() == Intrinsic::gcroot) {
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339 std::pair<CallInst *, AllocaInst *> Pair = std::make_pair(
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340 CI,
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341 cast<AllocaInst>(CI->getArgOperand(0)->stripPointerCasts()));
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342 if (IsNullValue(CI->getArgOperand(1)))
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343 Roots.push_back(Pair);
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344 else
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345 MetaRoots.push_back(Pair);
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346 }
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347
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348 // Number roots with metadata (usually empty) at the beginning, so that the
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349 // FrameMap::Meta array can be elided.
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350 Roots.insert(Roots.begin(), MetaRoots.begin(), MetaRoots.end());
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351 }
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352
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353 GetElementPtrInst *ShadowStackGCLowering::CreateGEP(LLVMContext &Context,
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354 IRBuilder<> &B, Type *Ty,
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355 Value *BasePtr, int Idx,
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356 int Idx2,
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357 const char *Name) {
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358 Value *Indices[] = {ConstantInt::get(Type::getInt32Ty(Context), 0),
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359 ConstantInt::get(Type::getInt32Ty(Context), Idx),
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360 ConstantInt::get(Type::getInt32Ty(Context), Idx2)};
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361 Value *Val = B.CreateGEP(Ty, BasePtr, Indices, Name);
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362
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363 assert(isa<GetElementPtrInst>(Val) && "Unexpected folded constant");
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364
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365 return dyn_cast<GetElementPtrInst>(Val);
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366 }
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367
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368 GetElementPtrInst *ShadowStackGCLowering::CreateGEP(LLVMContext &Context,
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369 IRBuilder<> &B, Type *Ty, Value *BasePtr,
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370 int Idx, const char *Name) {
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371 Value *Indices[] = {ConstantInt::get(Type::getInt32Ty(Context), 0),
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372 ConstantInt::get(Type::getInt32Ty(Context), Idx)};
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95
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373 Value *Val = B.CreateGEP(Ty, BasePtr, Indices, Name);
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374
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375 assert(isa<GetElementPtrInst>(Val) && "Unexpected folded constant");
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376
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377 return dyn_cast<GetElementPtrInst>(Val);
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378 }
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379
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380 /// runOnFunction - Insert code to maintain the shadow stack.
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381 bool ShadowStackGCLowering::runOnFunction(Function &F) {
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382 // Quick exit for functions that do not use the shadow stack GC.
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383 if (!F.hasGC() ||
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384 F.getGC() != std::string("shadow-stack"))
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385 return false;
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386
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387 LLVMContext &Context = F.getContext();
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388
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389 // Find calls to llvm.gcroot.
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390 CollectRoots(F);
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391
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392 // If there are no roots in this function, then there is no need to add a
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393 // stack map entry for it.
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394 if (Roots.empty())
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395 return false;
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396
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397 // Build the constant map and figure the type of the shadow stack entry.
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398 Value *FrameMap = GetFrameMap(F);
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399 Type *ConcreteStackEntryTy = GetConcreteStackEntryType(F);
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400
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401 // Build the shadow stack entry at the very start of the function.
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402 BasicBlock::iterator IP = F.getEntryBlock().begin();
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403 IRBuilder<> AtEntry(IP->getParent(), IP);
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404
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405 Instruction *StackEntry =
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406 AtEntry.CreateAlloca(ConcreteStackEntryTy, nullptr, "gc_frame");
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407
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408 while (isa<AllocaInst>(IP))
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409 ++IP;
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410 AtEntry.SetInsertPoint(IP->getParent(), IP);
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411
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412 // Initialize the map pointer and load the current head of the shadow stack.
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413 Instruction *CurrentHead = AtEntry.CreateLoad(Head, "gc_currhead");
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95
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414 Instruction *EntryMapPtr = CreateGEP(Context, AtEntry, ConcreteStackEntryTy,
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415 StackEntry, 0, 1, "gc_frame.map");
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83
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416 AtEntry.CreateStore(FrameMap, EntryMapPtr);
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417
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418 // After all the allocas...
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419 for (unsigned I = 0, E = Roots.size(); I != E; ++I) {
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420 // For each root, find the corresponding slot in the aggregate...
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95
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421 Value *SlotPtr = CreateGEP(Context, AtEntry, ConcreteStackEntryTy,
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422 StackEntry, 1 + I, "gc_root");
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83
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423
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424 // And use it in lieu of the alloca.
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425 AllocaInst *OriginalAlloca = Roots[I].second;
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426 SlotPtr->takeName(OriginalAlloca);
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427 OriginalAlloca->replaceAllUsesWith(SlotPtr);
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428 }
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429
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430 // Move past the original stores inserted by GCStrategy::InitRoots. This isn't
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431 // really necessary (the collector would never see the intermediate state at
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432 // runtime), but it's nicer not to push the half-initialized entry onto the
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433 // shadow stack.
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434 while (isa<StoreInst>(IP))
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435 ++IP;
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436 AtEntry.SetInsertPoint(IP->getParent(), IP);
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437
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438 // Push the entry onto the shadow stack.
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95
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439 Instruction *EntryNextPtr = CreateGEP(Context, AtEntry, ConcreteStackEntryTy,
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440 StackEntry, 0, 0, "gc_frame.next");
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441 Instruction *NewHeadVal = CreateGEP(Context, AtEntry, ConcreteStackEntryTy,
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442 StackEntry, 0, "gc_newhead");
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83
|
443 AtEntry.CreateStore(CurrentHead, EntryNextPtr);
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444 AtEntry.CreateStore(NewHeadVal, Head);
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445
|
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446 // For each instruction that escapes...
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447 EscapeEnumerator EE(F, "gc_cleanup");
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|
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448 while (IRBuilder<> *AtExit = EE.Next()) {
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449 // Pop the entry from the shadow stack. Don't reuse CurrentHead from
|
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|
450 // AtEntry, since that would make the value live for the entire function.
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|
|
451 Instruction *EntryNextPtr2 =
|
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95
|
452 CreateGEP(Context, *AtExit, ConcreteStackEntryTy, StackEntry, 0, 0,
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453 "gc_frame.next");
|
|
83
|
454 Value *SavedHead = AtExit->CreateLoad(EntryNextPtr2, "gc_savedhead");
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455 AtExit->CreateStore(SavedHead, Head);
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456 }
|
|
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457
|
|
|
458 // Delete the original allocas (which are no longer used) and the intrinsic
|
|
|
459 // calls (which are no longer valid). Doing this last avoids invalidating
|
|
|
460 // iterators.
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|
461 for (unsigned I = 0, E = Roots.size(); I != E; ++I) {
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|
|
462 Roots[I].first->eraseFromParent();
|
|
|
463 Roots[I].second->eraseFromParent();
|
|
|
464 }
|
|
|
465
|
|
|
466 Roots.clear();
|
|
|
467 return true;
|
|
|
468 }
|
