lib/CodeGen/ShadowStackGCLowering.cpp

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