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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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16 //
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17 //===----------------------------------------------------------------------===//
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18
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19 #include "llvm/ADT/SmallVector.h"
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20 #include "llvm/ADT/StringExtras.h"
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21 #include "llvm/CodeGen/Passes.h"
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22 #include "llvm/IR/BasicBlock.h"
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23 #include "llvm/IR/Constant.h"
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24 #include "llvm/IR/Constants.h"
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25 #include "llvm/IR/DerivedTypes.h"
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26 #include "llvm/IR/Function.h"
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27 #include "llvm/IR/GlobalValue.h"
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28 #include "llvm/IR/GlobalVariable.h"
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29 #include "llvm/IR/IRBuilder.h"
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121
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30 #include "llvm/IR/Instructions.h"
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31 #include "llvm/IR/IntrinsicInst.h"
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32 #include "llvm/IR/Intrinsics.h"
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33 #include "llvm/IR/Module.h"
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34 #include "llvm/IR/Type.h"
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35 #include "llvm/IR/Value.h"
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36 #include "llvm/Pass.h"
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37 #include "llvm/Support/Casting.h"
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120
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38 #include "llvm/Transforms/Utils/EscapeEnumerator.h"
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121
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39 #include <cassert>
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40 #include <cstddef>
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41 #include <string>
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42 #include <utility>
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43 #include <vector>
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44
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45 using namespace llvm;
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46
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121
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47 #define DEBUG_TYPE "shadow-stack-gc-lowering"
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48
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49 namespace {
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50
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51 class ShadowStackGCLowering : public FunctionPass {
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52 /// RootChain - This is the global linked-list that contains the chain of GC
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53 /// roots.
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54 GlobalVariable *Head = nullptr;
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55
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56 /// StackEntryTy - Abstract type of a link in the shadow stack.
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57 StructType *StackEntryTy = nullptr;
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58 StructType *FrameMapTy = nullptr;
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59
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60 /// Roots - GC roots in the current function. Each is a pair of the
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61 /// intrinsic call and its corresponding alloca.
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62 std::vector<std::pair<CallInst *, AllocaInst *>> Roots;
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63
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64 public:
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65 static char ID;
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66
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67 ShadowStackGCLowering();
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68
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69 bool doInitialization(Module &M) override;
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70 bool runOnFunction(Function &F) override;
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71
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72 private:
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73 bool IsNullValue(Value *V);
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74 Constant *GetFrameMap(Function &F);
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75 Type *GetConcreteStackEntryType(Function &F);
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76 void CollectRoots(Function &F);
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77
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78 static GetElementPtrInst *CreateGEP(LLVMContext &Context, IRBuilder<> &B,
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79 Type *Ty, Value *BasePtr, int Idx1,
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80 const char *Name);
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81 static GetElementPtrInst *CreateGEP(LLVMContext &Context, IRBuilder<> &B,
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82 Type *Ty, Value *BasePtr, int Idx1, int Idx2,
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83 const char *Name);
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84 };
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85
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86 } // end anonymous namespace
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87
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88 char ShadowStackGCLowering::ID = 0;
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89
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90 INITIALIZE_PASS_BEGIN(ShadowStackGCLowering, DEBUG_TYPE,
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91 "Shadow Stack GC Lowering", false, false)
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92 INITIALIZE_PASS_DEPENDENCY(GCModuleInfo)
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93 INITIALIZE_PASS_END(ShadowStackGCLowering, DEBUG_TYPE,
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94 "Shadow Stack GC Lowering", false, false)
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95
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96 FunctionPass *llvm::createShadowStackGCLoweringPass() { return new ShadowStackGCLowering(); }
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97
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98 ShadowStackGCLowering::ShadowStackGCLowering() : FunctionPass(ID) {
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99 initializeShadowStackGCLoweringPass(*PassRegistry::getPassRegistry());
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100 }
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101
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102 Constant *ShadowStackGCLowering::GetFrameMap(Function &F) {
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103 // doInitialization creates the abstract type of this value.
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104 Type *VoidPtr = Type::getInt8PtrTy(F.getContext());
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105
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106 // Truncate the ShadowStackDescriptor if some metadata is null.
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107 unsigned NumMeta = 0;
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108 SmallVector<Constant *, 16> Metadata;
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109 for (unsigned I = 0; I != Roots.size(); ++I) {
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110 Constant *C = cast<Constant>(Roots[I].first->getArgOperand(1));
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111 if (!C->isNullValue())
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112 NumMeta = I + 1;
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113 Metadata.push_back(ConstantExpr::getBitCast(C, VoidPtr));
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114 }
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115 Metadata.resize(NumMeta);
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116
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117 Type *Int32Ty = Type::getInt32Ty(F.getContext());
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118
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119 Constant *BaseElts[] = {
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120 ConstantInt::get(Int32Ty, Roots.size(), false),
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121 ConstantInt::get(Int32Ty, NumMeta, false),
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122 };
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123
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124 Constant *DescriptorElts[] = {
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125 ConstantStruct::get(FrameMapTy, BaseElts),
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126 ConstantArray::get(ArrayType::get(VoidPtr, NumMeta), Metadata)};
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127
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128 Type *EltTys[] = {DescriptorElts[0]->getType(), DescriptorElts[1]->getType()};
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129 StructType *STy = StructType::create(EltTys, "gc_map." + utostr(NumMeta));
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130
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131 Constant *FrameMap = ConstantStruct::get(STy, DescriptorElts);
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132
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133 // FIXME: Is this actually dangerous as WritingAnLLVMPass.html claims? Seems
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134 // that, short of multithreaded LLVM, it should be safe; all that is
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135 // necessary is that a simple Module::iterator loop not be invalidated.
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136 // Appending to the GlobalVariable list is safe in that sense.
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137 //
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138 // All of the output passes emit globals last. The ExecutionEngine
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139 // explicitly supports adding globals to the module after
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140 // initialization.
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141 //
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142 // Still, if it isn't deemed acceptable, then this transformation needs
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143 // to be a ModulePass (which means it cannot be in the 'llc' pipeline
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144 // (which uses a FunctionPassManager (which segfaults (not asserts) if
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145 // provided a ModulePass))).
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146 Constant *GV = new GlobalVariable(*F.getParent(), FrameMap->getType(), true,
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147 GlobalVariable::InternalLinkage, FrameMap,
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148 "__gc_" + F.getName());
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149
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150 Constant *GEPIndices[2] = {
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151 ConstantInt::get(Type::getInt32Ty(F.getContext()), 0),
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152 ConstantInt::get(Type::getInt32Ty(F.getContext()), 0)};
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153 return ConstantExpr::getGetElementPtr(FrameMap->getType(), GV, GEPIndices);
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154 }
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155
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156 Type *ShadowStackGCLowering::GetConcreteStackEntryType(Function &F) {
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157 // doInitialization creates the generic version of this type.
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158 std::vector<Type *> EltTys;
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159 EltTys.push_back(StackEntryTy);
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160 for (size_t I = 0; I != Roots.size(); I++)
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161 EltTys.push_back(Roots[I].second->getAllocatedType());
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162
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163 return StructType::create(EltTys, ("gc_stackentry." + F.getName()).str());
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164 }
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165
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166 /// doInitialization - If this module uses the GC intrinsics, find them now. If
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167 /// not, exit fast.
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168 bool ShadowStackGCLowering::doInitialization(Module &M) {
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169 bool Active = false;
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170 for (Function &F : M) {
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171 if (F.hasGC() && F.getGC() == std::string("shadow-stack")) {
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172 Active = true;
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173 break;
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174 }
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175 }
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176 if (!Active)
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177 return false;
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178
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179 // struct FrameMap {
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180 // int32_t NumRoots; // Number of roots in stack frame.
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181 // int32_t NumMeta; // Number of metadata descriptors. May be < NumRoots.
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182 // void *Meta[]; // May be absent for roots without metadata.
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183 // };
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184 std::vector<Type *> EltTys;
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185 // 32 bits is ok up to a 32GB stack frame. :)
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186 EltTys.push_back(Type::getInt32Ty(M.getContext()));
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187 // Specifies length of variable length array.
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188 EltTys.push_back(Type::getInt32Ty(M.getContext()));
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189 FrameMapTy = StructType::create(EltTys, "gc_map");
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190 PointerType *FrameMapPtrTy = PointerType::getUnqual(FrameMapTy);
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191
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192 // struct StackEntry {
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193 // ShadowStackEntry *Next; // Caller's stack entry.
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194 // FrameMap *Map; // Pointer to constant FrameMap.
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195 // void *Roots[]; // Stack roots (in-place array, so we pretend).
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196 // };
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197
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198 StackEntryTy = StructType::create(M.getContext(), "gc_stackentry");
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199
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200 EltTys.clear();
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201 EltTys.push_back(PointerType::getUnqual(StackEntryTy));
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202 EltTys.push_back(FrameMapPtrTy);
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203 StackEntryTy->setBody(EltTys);
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204 PointerType *StackEntryPtrTy = PointerType::getUnqual(StackEntryTy);
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205
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206 // Get the root chain if it already exists.
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207 Head = M.getGlobalVariable("llvm_gc_root_chain");
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208 if (!Head) {
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209 // If the root chain does not exist, insert a new one with linkonce
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210 // linkage!
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211 Head = new GlobalVariable(
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212 M, StackEntryPtrTy, false, GlobalValue::LinkOnceAnyLinkage,
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213 Constant::getNullValue(StackEntryPtrTy), "llvm_gc_root_chain");
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214 } else if (Head->hasExternalLinkage() && Head->isDeclaration()) {
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215 Head->setInitializer(Constant::getNullValue(StackEntryPtrTy));
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216 Head->setLinkage(GlobalValue::LinkOnceAnyLinkage);
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217 }
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218
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219 return true;
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220 }
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221
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222 bool ShadowStackGCLowering::IsNullValue(Value *V) {
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223 if (Constant *C = dyn_cast<Constant>(V))
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224 return C->isNullValue();
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225 return false;
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226 }
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227
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228 void ShadowStackGCLowering::CollectRoots(Function &F) {
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229 // FIXME: Account for original alignment. Could fragment the root array.
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230 // Approach 1: Null initialize empty slots at runtime. Yuck.
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231 // Approach 2: Emit a map of the array instead of just a count.
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232
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233 assert(Roots.empty() && "Not cleaned up?");
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234
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235 SmallVector<std::pair<CallInst *, AllocaInst *>, 16> MetaRoots;
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236
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237 for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB)
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238 for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E;)
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239 if (IntrinsicInst *CI = dyn_cast<IntrinsicInst>(II++))
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240 if (Function *F = CI->getCalledFunction())
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241 if (F->getIntrinsicID() == Intrinsic::gcroot) {
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242 std::pair<CallInst *, AllocaInst *> Pair = std::make_pair(
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243 CI,
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244 cast<AllocaInst>(CI->getArgOperand(0)->stripPointerCasts()));
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245 if (IsNullValue(CI->getArgOperand(1)))
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246 Roots.push_back(Pair);
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247 else
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248 MetaRoots.push_back(Pair);
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249 }
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250
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251 // Number roots with metadata (usually empty) at the beginning, so that the
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252 // FrameMap::Meta array can be elided.
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253 Roots.insert(Roots.begin(), MetaRoots.begin(), MetaRoots.end());
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254 }
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255
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256 GetElementPtrInst *ShadowStackGCLowering::CreateGEP(LLVMContext &Context,
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257 IRBuilder<> &B, Type *Ty,
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258 Value *BasePtr, int Idx,
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259 int Idx2,
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260 const char *Name) {
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261 Value *Indices[] = {ConstantInt::get(Type::getInt32Ty(Context), 0),
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262 ConstantInt::get(Type::getInt32Ty(Context), Idx),
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263 ConstantInt::get(Type::getInt32Ty(Context), Idx2)};
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264 Value *Val = B.CreateGEP(Ty, BasePtr, Indices, Name);
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265
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266 assert(isa<GetElementPtrInst>(Val) && "Unexpected folded constant");
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267
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268 return dyn_cast<GetElementPtrInst>(Val);
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269 }
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270
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271 GetElementPtrInst *ShadowStackGCLowering::CreateGEP(LLVMContext &Context,
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272 IRBuilder<> &B, Type *Ty, Value *BasePtr,
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273 int Idx, const char *Name) {
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274 Value *Indices[] = {ConstantInt::get(Type::getInt32Ty(Context), 0),
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275 ConstantInt::get(Type::getInt32Ty(Context), Idx)};
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276 Value *Val = B.CreateGEP(Ty, BasePtr, Indices, Name);
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277
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278 assert(isa<GetElementPtrInst>(Val) && "Unexpected folded constant");
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279
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280 return dyn_cast<GetElementPtrInst>(Val);
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281 }
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282
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283 /// runOnFunction - Insert code to maintain the shadow stack.
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284 bool ShadowStackGCLowering::runOnFunction(Function &F) {
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285 // Quick exit for functions that do not use the shadow stack GC.
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286 if (!F.hasGC() ||
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287 F.getGC() != std::string("shadow-stack"))
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288 return false;
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289
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290 LLVMContext &Context = F.getContext();
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291
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292 // Find calls to llvm.gcroot.
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293 CollectRoots(F);
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294
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295 // If there are no roots in this function, then there is no need to add a
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296 // stack map entry for it.
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297 if (Roots.empty())
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298 return false;
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299
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300 // Build the constant map and figure the type of the shadow stack entry.
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301 Value *FrameMap = GetFrameMap(F);
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302 Type *ConcreteStackEntryTy = GetConcreteStackEntryType(F);
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303
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304 // Build the shadow stack entry at the very start of the function.
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305 BasicBlock::iterator IP = F.getEntryBlock().begin();
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306 IRBuilder<> AtEntry(IP->getParent(), IP);
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307
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308 Instruction *StackEntry =
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309 AtEntry.CreateAlloca(ConcreteStackEntryTy, nullptr, "gc_frame");
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310
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311 while (isa<AllocaInst>(IP))
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312 ++IP;
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313 AtEntry.SetInsertPoint(IP->getParent(), IP);
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314
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315 // Initialize the map pointer and load the current head of the shadow stack.
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316 Instruction *CurrentHead = AtEntry.CreateLoad(Head, "gc_currhead");
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317 Instruction *EntryMapPtr = CreateGEP(Context, AtEntry, ConcreteStackEntryTy,
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318 StackEntry, 0, 1, "gc_frame.map");
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319 AtEntry.CreateStore(FrameMap, EntryMapPtr);
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320
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321 // After all the allocas...
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322 for (unsigned I = 0, E = Roots.size(); I != E; ++I) {
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323 // For each root, find the corresponding slot in the aggregate...
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324 Value *SlotPtr = CreateGEP(Context, AtEntry, ConcreteStackEntryTy,
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325 StackEntry, 1 + I, "gc_root");
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326
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327 // And use it in lieu of the alloca.
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328 AllocaInst *OriginalAlloca = Roots[I].second;
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329 SlotPtr->takeName(OriginalAlloca);
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330 OriginalAlloca->replaceAllUsesWith(SlotPtr);
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331 }
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332
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333 // Move past the original stores inserted by GCStrategy::InitRoots. This isn't
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334 // really necessary (the collector would never see the intermediate state at
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335 // runtime), but it's nicer not to push the half-initialized entry onto the
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336 // shadow stack.
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337 while (isa<StoreInst>(IP))
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338 ++IP;
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339 AtEntry.SetInsertPoint(IP->getParent(), IP);
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340
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341 // Push the entry onto the shadow stack.
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342 Instruction *EntryNextPtr = CreateGEP(Context, AtEntry, ConcreteStackEntryTy,
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343 StackEntry, 0, 0, "gc_frame.next");
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344 Instruction *NewHeadVal = CreateGEP(Context, AtEntry, ConcreteStackEntryTy,
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345 StackEntry, 0, "gc_newhead");
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346 AtEntry.CreateStore(CurrentHead, EntryNextPtr);
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347 AtEntry.CreateStore(NewHeadVal, Head);
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348
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349 // For each instruction that escapes...
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350 EscapeEnumerator EE(F, "gc_cleanup");
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351 while (IRBuilder<> *AtExit = EE.Next()) {
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352 // Pop the entry from the shadow stack. Don't reuse CurrentHead from
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353 // AtEntry, since that would make the value live for the entire function.
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354 Instruction *EntryNextPtr2 =
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355 CreateGEP(Context, *AtExit, ConcreteStackEntryTy, StackEntry, 0, 0,
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356 "gc_frame.next");
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357 Value *SavedHead = AtExit->CreateLoad(EntryNextPtr2, "gc_savedhead");
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358 AtExit->CreateStore(SavedHead, Head);
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359 }
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360
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361 // Delete the original allocas (which are no longer used) and the intrinsic
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362 // calls (which are no longer valid). Doing this last avoids invalidating
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363 // iterators.
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364 for (unsigned I = 0, E = Roots.size(); I != E; ++I) {
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365 Roots[I].first->eraseFromParent();
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366 Roots[I].second->eraseFromParent();
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367 }
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368
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369 Roots.clear();
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370 return true;
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371 }
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