llvm/lib/CodeGen/ShadowStackGCLowering.cpp

   1 //===- ShadowStackGCLowering.cpp - Custom lowering for shadow-stack gc ----===//
   2 //
   3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
   4 // See https://llvm.org/LICENSE.txt for license information.
   5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
   6 //
   7 //===----------------------------------------------------------------------===//
   8 //
   9 // This file contains the custom lowering code required by the shadow-stack GC
  10 // strategy.
  11 //
  12 // This pass implements the code transformation described in this paper:
  13 //   "Accurate Garbage Collection in an Uncooperative Environment"
  14 //   Fergus Henderson, ISMM, 2002
  15 //
  16 //===----------------------------------------------------------------------===//
  17
  18 #include "llvm/ADT/SmallVector.h"
  19 #include "llvm/ADT/StringExtras.h"
  20 #include "llvm/Analysis/DomTreeUpdater.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/Dominators.h"
  27 #include "llvm/IR/Function.h"
  28 #include "llvm/IR/GlobalValue.h"
  29 #include "llvm/IR/GlobalVariable.h"
  30 #include "llvm/IR/IRBuilder.h"
  31 #include "llvm/IR/Instructions.h"
  32 #include "llvm/IR/IntrinsicInst.h"
  33 #include "llvm/IR/Intrinsics.h"
  34 #include "llvm/IR/Module.h"
  35 #include "llvm/IR/Type.h"
  36 #include "llvm/IR/Value.h"
  37 #include "llvm/InitializePasses.h"
  38 #include "llvm/Pass.h"
  39 #include "llvm/Support/Casting.h"
  40 #include "llvm/Transforms/Utils/EscapeEnumerator.h"
  41 #include <cassert>
  42 #include <string>
  43 #include <utility>
  44 #include <vector>
  45
  46 using namespace llvm;
  47
  48 #define DEBUG_TYPE "shadow-stack-gc-lowering"
  49
  50 namespace {
  51
  52 class ShadowStackGCLowering : public FunctionPass {
  53   /// RootChain - This is the global linked-list that contains the chain of GC
  54   /// roots.
  55   GlobalVariable *Head = nullptr;
  56
  57   /// StackEntryTy - Abstract type of a link in the shadow stack.
  58   StructType *StackEntryTy = nullptr;
  59   StructType *FrameMapTy = nullptr;
  60
  61   /// Roots - GC roots in the current function. Each is a pair of the
  62   /// intrinsic call and its corresponding alloca.
  63   std::vector<std::pair<CallInst *, AllocaInst *>> Roots;
  64
  65 public:
  66   static char ID;
  67
  68   ShadowStackGCLowering();
  69
  70   bool doInitialization(Module &M) override;
  71   void getAnalysisUsage(AnalysisUsage &AU) const override;
  72   bool runOnFunction(Function &F) override;
  73
  74 private:
  75   bool IsNullValue(Value *V);
  76   Constant *GetFrameMap(Function &F);
  77   Type *GetConcreteStackEntryType(Function &F);
  78   void CollectRoots(Function &F);
  79
  80   static GetElementPtrInst *CreateGEP(LLVMContext &Context, IRBuilder<> &B,
  81                                       Type *Ty, Value *BasePtr, int Idx1,
  82                                       const char *Name);
  83   static GetElementPtrInst *CreateGEP(LLVMContext &Context, IRBuilder<> &B,
  84                                       Type *Ty, Value *BasePtr, int Idx1, int Idx2,
  85                                       const char *Name);
  86 };
  87
  88 } // end anonymous namespace
  89
  90 char ShadowStackGCLowering::ID = 0;
  91 char &llvm::ShadowStackGCLoweringID = ShadowStackGCLowering::ID;
  92
  93 INITIALIZE_PASS_BEGIN(ShadowStackGCLowering, DEBUG_TYPE,
  94                       "Shadow Stack GC Lowering", false, false)
  95 INITIALIZE_PASS_DEPENDENCY(GCModuleInfo)
  96 INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
  97 INITIALIZE_PASS_END(ShadowStackGCLowering, DEBUG_TYPE,
  98                     "Shadow Stack GC Lowering", false, false)
  99
 100 FunctionPass *llvm::createShadowStackGCLoweringPass() { return new ShadowStackGCLowering(); }
 101
 102 ShadowStackGCLowering::ShadowStackGCLowering() : FunctionPass(ID) {
 103   initializeShadowStackGCLoweringPass(*PassRegistry::getPassRegistry());
 104 }
 105
 106 Constant *ShadowStackGCLowering::GetFrameMap(Function &F) {
 107   // doInitialization creates the abstract type of this value.
 108   Type *VoidPtr = Type::getInt8PtrTy(F.getContext());
 109
 110   // Truncate the ShadowStackDescriptor if some metadata is null.
 111   unsigned NumMeta = 0;
 112   SmallVector<Constant *, 16> Metadata;
 113   for (unsigned I = 0; I != Roots.size(); ++I) {
 114     Constant *C = cast<Constant>(Roots[I].first->getArgOperand(1));
 115     if (!C->isNullValue())
 116       NumMeta = I + 1;
 117     Metadata.push_back(ConstantExpr::getBitCast(C, VoidPtr));
 118   }
 119   Metadata.resize(NumMeta);
 120
 121   Type *Int32Ty = Type::getInt32Ty(F.getContext());
 122
 123   Constant *BaseElts[] = {
 124       ConstantInt::get(Int32Ty, Roots.size(), false),
 125       ConstantInt::get(Int32Ty, NumMeta, false),
 126   };
 127
 128   Constant *DescriptorElts[] = {
 129       ConstantStruct::get(FrameMapTy, BaseElts),
 130       ConstantArray::get(ArrayType::get(VoidPtr, NumMeta), Metadata)};
 131
 132   Type *EltTys[] = {DescriptorElts[0]->getType(), DescriptorElts[1]->getType()};
 133   StructType *STy = StructType::create(EltTys, "gc_map." + utostr(NumMeta));
 134
 135   Constant *FrameMap = ConstantStruct::get(STy, DescriptorElts);
 136
 137   // FIXME: Is this actually dangerous as WritingAnLLVMPass.html claims? Seems
 138   //        that, short of multithreaded LLVM, it should be safe; all that is
 139   //        necessary is that a simple Module::iterator loop not be invalidated.
 140   //        Appending to the GlobalVariable list is safe in that sense.
 141   //
 142   //        All of the output passes emit globals last. The ExecutionEngine
 143   //        explicitly supports adding globals to the module after
 144   //        initialization.
 145   //
 146   //        Still, if it isn't deemed acceptable, then this transformation needs
 147   //        to be a ModulePass (which means it cannot be in the 'llc' pipeline
 148   //        (which uses a FunctionPassManager (which segfaults (not asserts) if
 149   //        provided a ModulePass))).
 150   Constant *GV = new GlobalVariable(*F.getParent(), FrameMap->getType(), true,
 151                                     GlobalVariable::InternalLinkage, FrameMap,
 152                                     "__gc_" + F.getName());
 153
 154   Constant *GEPIndices[2] = {
 155       ConstantInt::get(Type::getInt32Ty(F.getContext()), 0),
 156       ConstantInt::get(Type::getInt32Ty(F.getContext()), 0)};
 157   return ConstantExpr::getGetElementPtr(FrameMap->getType(), GV, GEPIndices);
 158 }
 159
 160 Type *ShadowStackGCLowering::GetConcreteStackEntryType(Function &F) {
 161   // doInitialization creates the generic version of this type.
 162   std::vector<Type *> EltTys;
 163   EltTys.push_back(StackEntryTy);
 164   for (const std::pair<CallInst *, AllocaInst *> &Root : Roots)
 165     EltTys.push_back(Root.second->getAllocatedType());
 166
 167   return StructType::create(EltTys, ("gc_stackentry." + F.getName()).str());
 168 }
 169
 170 /// doInitialization - If this module uses the GC intrinsics, find them now. If
 171 /// not, exit fast.
 172 bool ShadowStackGCLowering::doInitialization(Module &M) {
 173   bool Active = false;
 174   for (Function &F : M) {
 175     if (F.hasGC() && F.getGC() == std::string("shadow-stack")) {
 176       Active = true;
 177       break;
 178     }
 179   }
 180   if (!Active)
 181     return false;
 182
 183   // struct FrameMap {
 184   //   int32_t NumRoots; // Number of roots in stack frame.
 185   //   int32_t NumMeta;  // Number of metadata descriptors. May be < NumRoots.
 186   //   void *Meta[];     // May be absent for roots without metadata.
 187   // };
 188   std::vector<Type *> EltTys;
 189   // 32 bits is ok up to a 32GB stack frame. :)
 190   EltTys.push_back(Type::getInt32Ty(M.getContext()));
 191   // Specifies length of variable length array.
 192   EltTys.push_back(Type::getInt32Ty(M.getContext()));
 193   FrameMapTy = StructType::create(EltTys, "gc_map");
 194   PointerType *FrameMapPtrTy = PointerType::getUnqual(FrameMapTy);
 195
 196   // struct StackEntry {
 197   //   ShadowStackEntry *Next; // Caller's stack entry.
 198   //   FrameMap *Map;          // Pointer to constant FrameMap.
 199   //   void *Roots[];          // Stack roots (in-place array, so we pretend).
 200   // };
 201
 202   StackEntryTy = StructType::create(M.getContext(), "gc_stackentry");
 203
 204   EltTys.clear();
 205   EltTys.push_back(PointerType::getUnqual(StackEntryTy));
 206   EltTys.push_back(FrameMapPtrTy);
 207   StackEntryTy->setBody(EltTys);
 208   PointerType *StackEntryPtrTy = PointerType::getUnqual(StackEntryTy);
 209
 210   // Get the root chain if it already exists.
 211   Head = M.getGlobalVariable("llvm_gc_root_chain");
 212   if (!Head) {
 213     // If the root chain does not exist, insert a new one with linkonce
 214     // linkage!
 215     Head = new GlobalVariable(
 216         M, StackEntryPtrTy, false, GlobalValue::LinkOnceAnyLinkage,
 217         Constant::getNullValue(StackEntryPtrTy), "llvm_gc_root_chain");
 218   } else if (Head->hasExternalLinkage() && Head->isDeclaration()) {
 219     Head->setInitializer(Constant::getNullValue(StackEntryPtrTy));
 220     Head->setLinkage(GlobalValue::LinkOnceAnyLinkage);
 221   }
 222
 223   return true;
 224 }
 225
 226 bool ShadowStackGCLowering::IsNullValue(Value *V) {
 227   if (Constant *C = dyn_cast<Constant>(V))
 228     return C->isNullValue();
 229   return false;
 230 }
 231
 232 void ShadowStackGCLowering::CollectRoots(Function &F) {
 233   // FIXME: Account for original alignment. Could fragment the root array.
 234   //   Approach 1: Null initialize empty slots at runtime. Yuck.
 235   //   Approach 2: Emit a map of the array instead of just a count.
 236
 237   assert(Roots.empty() && "Not cleaned up?");
 238
 239   SmallVector<std::pair<CallInst *, AllocaInst *>, 16> MetaRoots;
 240
 241   for (BasicBlock &BB : F)
 242     for (Instruction &I : BB)
 243       if (IntrinsicInst *CI = dyn_cast<IntrinsicInst>(&I))
 244         if (Function *F = CI->getCalledFunction())
 245           if (F->getIntrinsicID() == Intrinsic::gcroot) {
 246             std::pair<CallInst *, AllocaInst *> Pair = std::make_pair(
 247                 CI,
 248                 cast<AllocaInst>(CI->getArgOperand(0)->stripPointerCasts()));
 249             if (IsNullValue(CI->getArgOperand(1)))
 250               Roots.push_back(Pair);
 251             else
 252               MetaRoots.push_back(Pair);
 253           }
 254
 255   // Number roots with metadata (usually empty) at the beginning, so that the
 256   // FrameMap::Meta array can be elided.
 257   Roots.insert(Roots.begin(), MetaRoots.begin(), MetaRoots.end());
 258 }
 259
 260 GetElementPtrInst *ShadowStackGCLowering::CreateGEP(LLVMContext &Context,
 261                                                     IRBuilder<> &B, Type *Ty,
 262                                                     Value *BasePtr, int Idx,
 263                                                     int Idx2,
 264                                                     const char *Name) {
 265   Value *Indices[] = {ConstantInt::get(Type::getInt32Ty(Context), 0),
 266                       ConstantInt::get(Type::getInt32Ty(Context), Idx),
 267                       ConstantInt::get(Type::getInt32Ty(Context), Idx2)};
 268   Value *Val = B.CreateGEP(Ty, BasePtr, Indices, Name);
 269
 270   assert(isa<GetElementPtrInst>(Val) && "Unexpected folded constant");
 271
 272   return dyn_cast<GetElementPtrInst>(Val);
 273 }
 274
 275 GetElementPtrInst *ShadowStackGCLowering::CreateGEP(LLVMContext &Context,
 276                                             IRBuilder<> &B, Type *Ty, Value *BasePtr,
 277                                             int Idx, const char *Name) {
 278   Value *Indices[] = {ConstantInt::get(Type::getInt32Ty(Context), 0),
 279                       ConstantInt::get(Type::getInt32Ty(Context), Idx)};
 280   Value *Val = B.CreateGEP(Ty, BasePtr, Indices, Name);
 281
 282   assert(isa<GetElementPtrInst>(Val) && "Unexpected folded constant");
 283
 284   return dyn_cast<GetElementPtrInst>(Val);
 285 }
 286
 287 void ShadowStackGCLowering::getAnalysisUsage(AnalysisUsage &AU) const {
 288   AU.addPreserved<DominatorTreeWrapperPass>();
 289 }
 290
 291 /// runOnFunction - Insert code to maintain the shadow stack.
 292 bool ShadowStackGCLowering::runOnFunction(Function &F) {
 293   // Quick exit for functions that do not use the shadow stack GC.
 294   if (!F.hasGC() ||
 295       F.getGC() != std::string("shadow-stack"))
 296     return false;
 297
 298   LLVMContext &Context = F.getContext();
 299
 300   // Find calls to llvm.gcroot.
 301   CollectRoots(F);
 302
 303   // If there are no roots in this function, then there is no need to add a
 304   // stack map entry for it.
 305   if (Roots.empty())
 306     return false;
 307
 308   Optional<DomTreeUpdater> DTU;
 309   if (auto *DTWP = getAnalysisIfAvailable<DominatorTreeWrapperPass>())
 310     DTU.emplace(DTWP->getDomTree(), DomTreeUpdater::UpdateStrategy::Lazy);
 311
 312   // Build the constant map and figure the type of the shadow stack entry.
 313   Value *FrameMap = GetFrameMap(F);
 314   Type *ConcreteStackEntryTy = GetConcreteStackEntryType(F);
 315
 316   // Build the shadow stack entry at the very start of the function.
 317   BasicBlock::iterator IP = F.getEntryBlock().begin();
 318   IRBuilder<> AtEntry(IP->getParent(), IP);
 319
 320   Instruction *StackEntry =
 321       AtEntry.CreateAlloca(ConcreteStackEntryTy, nullptr, "gc_frame");
 322
 323   while (isa<AllocaInst>(IP))
 324     ++IP;
 325   AtEntry.SetInsertPoint(IP->getParent(), IP);
 326
 327   // Initialize the map pointer and load the current head of the shadow stack.
 328   Instruction *CurrentHead =
 329       AtEntry.CreateLoad(StackEntryTy->getPointerTo(), Head, "gc_currhead");
 330   Instruction *EntryMapPtr = CreateGEP(Context, AtEntry, ConcreteStackEntryTy,
 331                                        StackEntry, 0, 1, "gc_frame.map");
 332   AtEntry.CreateStore(FrameMap, EntryMapPtr);
 333
 334   // After all the allocas...
 335   for (unsigned I = 0, E = Roots.size(); I != E; ++I) {
 336     // For each root, find the corresponding slot in the aggregate...
 337     Value *SlotPtr = CreateGEP(Context, AtEntry, ConcreteStackEntryTy,
 338                                StackEntry, 1 + I, "gc_root");
 339
 340     // And use it in lieu of the alloca.
 341     AllocaInst *OriginalAlloca = Roots[I].second;
 342     SlotPtr->takeName(OriginalAlloca);
 343     OriginalAlloca->replaceAllUsesWith(SlotPtr);
 344   }
 345
 346   // Move past the original stores inserted by GCStrategy::InitRoots. This isn't
 347   // really necessary (the collector would never see the intermediate state at
 348   // runtime), but it's nicer not to push the half-initialized entry onto the
 349   // shadow stack.
 350   while (isa<StoreInst>(IP))
 351     ++IP;
 352   AtEntry.SetInsertPoint(IP->getParent(), IP);
 353
 354   // Push the entry onto the shadow stack.
 355   Instruction *EntryNextPtr = CreateGEP(Context, AtEntry, ConcreteStackEntryTy,
 356                                         StackEntry, 0, 0, "gc_frame.next");
 357   Instruction *NewHeadVal = CreateGEP(Context, AtEntry, ConcreteStackEntryTy,
 358                                       StackEntry, 0, "gc_newhead");
 359   AtEntry.CreateStore(CurrentHead, EntryNextPtr);
 360   AtEntry.CreateStore(NewHeadVal, Head);
 361
 362   // For each instruction that escapes...
 363   EscapeEnumerator EE(F, "gc_cleanup", /*HandleExceptions=*/true,
 364                       DTU ? DTU.getPointer() : nullptr);
 365   while (IRBuilder<> *AtExit = EE.Next()) {
 366     // Pop the entry from the shadow stack. Don't reuse CurrentHead from
 367     // AtEntry, since that would make the value live for the entire function.
 368     Instruction *EntryNextPtr2 =
 369         CreateGEP(Context, *AtExit, ConcreteStackEntryTy, StackEntry, 0, 0,
 370                   "gc_frame.next");
 371     Value *SavedHead = AtExit->CreateLoad(StackEntryTy->getPointerTo(),
 372                                           EntryNextPtr2, "gc_savedhead");
 373     AtExit->CreateStore(SavedHead, Head);
 374   }
 375
 376   // Delete the original allocas (which are no longer used) and the intrinsic
 377   // calls (which are no longer valid). Doing this last avoids invalidating
 378   // iterators.
 379   for (std::pair<CallInst *, AllocaInst *> &Root : Roots) {
 380     Root.first->eraseFromParent();
 381     Root.second->eraseFromParent();
 382   }
 383
 384   Roots.clear();
 385   return true;
 386 }