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 <cstddef>
  43 #include <string>
  44 #include <utility>
  45 #include <vector>
  46
  47 using namespace llvm;
  48
  49 #define DEBUG_TYPE "shadow-stack-gc-lowering"
  50
  51 namespace {
  52
  53 class ShadowStackGCLowering : public FunctionPass {
  54   /// RootChain - This is the global linked-list that contains the chain of GC
  55   /// roots.
  56   GlobalVariable *Head = nullptr;
  57
  58   /// StackEntryTy - Abstract type of a link in the shadow stack.
  59   StructType *StackEntryTy = nullptr;
  60   StructType *FrameMapTy = nullptr;
  61
  62   /// Roots - GC roots in the current function. Each is a pair of the
  63   /// intrinsic call and its corresponding alloca.
  64   std::vector<std::pair<CallInst *, AllocaInst *>> Roots;
  65
  66 public:
  67   static char ID;
  68
  69   ShadowStackGCLowering();
  70
  71   bool doInitialization(Module &M) override;
  72   void getAnalysisUsage(AnalysisUsage &AU) const override;
  73   bool runOnFunction(Function &F) override;
  74
  75 private:
  76   bool IsNullValue(Value *V);
  77   Constant *GetFrameMap(Function &F);
  78   Type *GetConcreteStackEntryType(Function &F);
  79   void CollectRoots(Function &F);
  80
  81   static GetElementPtrInst *CreateGEP(LLVMContext &Context, IRBuilder<> &B,
  82                                       Type *Ty, Value *BasePtr, int Idx1,
  83                                       const char *Name);
  84   static GetElementPtrInst *CreateGEP(LLVMContext &Context, IRBuilder<> &B,
  85                                       Type *Ty, Value *BasePtr, int Idx1, int Idx2,
  86                                       const char *Name);
  87 };
  88
  89 } // end anonymous namespace
  90
  91 char ShadowStackGCLowering::ID = 0;
  92 char &llvm::ShadowStackGCLoweringID = ShadowStackGCLowering::ID;
  93
  94 INITIALIZE_PASS_BEGIN(ShadowStackGCLowering, DEBUG_TYPE,
  95                       "Shadow Stack GC Lowering", false, false)
  96 INITIALIZE_PASS_DEPENDENCY(GCModuleInfo)
  97 INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
  98 INITIALIZE_PASS_END(ShadowStackGCLowering, DEBUG_TYPE,
  99                     "Shadow Stack GC Lowering", false, false)
 100
 101 FunctionPass *llvm::createShadowStackGCLoweringPass() { return new ShadowStackGCLowering(); }
 102
 103 ShadowStackGCLowering::ShadowStackGCLowering() : FunctionPass(ID) {
 104   initializeShadowStackGCLoweringPass(*PassRegistry::getPassRegistry());
 105 }
 106
 107 Constant *ShadowStackGCLowering::GetFrameMap(Function &F) {
 108   // doInitialization creates the abstract type of this value.
 109   Type *VoidPtr = Type::getInt8PtrTy(F.getContext());
 110
 111   // Truncate the ShadowStackDescriptor if some metadata is null.
 112   unsigned NumMeta = 0;
 113   SmallVector<Constant *, 16> Metadata;
 114   for (unsigned I = 0; I != Roots.size(); ++I) {
 115     Constant *C = cast<Constant>(Roots[I].first->getArgOperand(1));
 116     if (!C->isNullValue())
 117       NumMeta = I + 1;
 118     Metadata.push_back(ConstantExpr::getBitCast(C, VoidPtr));
 119   }
 120   Metadata.resize(NumMeta);
 121
 122   Type *Int32Ty = Type::getInt32Ty(F.getContext());
 123
 124   Constant *BaseElts[] = {
 125       ConstantInt::get(Int32Ty, Roots.size(), false),
 126       ConstantInt::get(Int32Ty, NumMeta, false),
 127   };
 128
 129   Constant *DescriptorElts[] = {
 130       ConstantStruct::get(FrameMapTy, BaseElts),
 131       ConstantArray::get(ArrayType::get(VoidPtr, NumMeta), Metadata)};
 132
 133   Type *EltTys[] = {DescriptorElts[0]->getType(), DescriptorElts[1]->getType()};
 134   StructType *STy = StructType::create(EltTys, "gc_map." + utostr(NumMeta));
 135
 136   Constant *FrameMap = ConstantStruct::get(STy, DescriptorElts);
 137
 138   // FIXME: Is this actually dangerous as WritingAnLLVMPass.html claims? Seems
 139   //        that, short of multithreaded LLVM, it should be safe; all that is
 140   //        necessary is that a simple Module::iterator loop not be invalidated.
 141   //        Appending to the GlobalVariable list is safe in that sense.
 142   //
 143   //        All of the output passes emit globals last. The ExecutionEngine
 144   //        explicitly supports adding globals to the module after
 145   //        initialization.
 146   //
 147   //        Still, if it isn't deemed acceptable, then this transformation needs
 148   //        to be a ModulePass (which means it cannot be in the 'llc' pipeline
 149   //        (which uses a FunctionPassManager (which segfaults (not asserts) if
 150   //        provided a ModulePass))).
 151   Constant *GV = new GlobalVariable(*F.getParent(), FrameMap->getType(), true,
 152                                     GlobalVariable::InternalLinkage, FrameMap,
 153                                     "__gc_" + F.getName());
 154
 155   Constant *GEPIndices[2] = {
 156       ConstantInt::get(Type::getInt32Ty(F.getContext()), 0),
 157       ConstantInt::get(Type::getInt32Ty(F.getContext()), 0)};
 158   return ConstantExpr::getGetElementPtr(FrameMap->getType(), GV, GEPIndices);
 159 }
 160
 161 Type *ShadowStackGCLowering::GetConcreteStackEntryType(Function &F) {
 162   // doInitialization creates the generic version of this type.
 163   std::vector<Type *> EltTys;
 164   EltTys.push_back(StackEntryTy);
 165   for (size_t I = 0; I != Roots.size(); I++)
 166     EltTys.push_back(Roots[I].second->getAllocatedType());
 167
 168   return StructType::create(EltTys, ("gc_stackentry." + F.getName()).str());
 169 }
 170
 171 /// doInitialization - If this module uses the GC intrinsics, find them now. If
 172 /// not, exit fast.
 173 bool ShadowStackGCLowering::doInitialization(Module &M) {
 174   bool Active = false;
 175   for (Function &F : M) {
 176     if (F.hasGC() && F.getGC() == std::string("shadow-stack")) {
 177       Active = true;
 178       break;
 179     }
 180   }
 181   if (!Active)
 182     return false;
 183
 184   // struct FrameMap {
 185   //   int32_t NumRoots; // Number of roots in stack frame.
 186   //   int32_t NumMeta;  // Number of metadata descriptors. May be < NumRoots.
 187   //   void *Meta[];     // May be absent for roots without metadata.
 188   // };
 189   std::vector<Type *> EltTys;
 190   // 32 bits is ok up to a 32GB stack frame. :)
 191   EltTys.push_back(Type::getInt32Ty(M.getContext()));
 192   // Specifies length of variable length array.
 193   EltTys.push_back(Type::getInt32Ty(M.getContext()));
 194   FrameMapTy = StructType::create(EltTys, "gc_map");
 195   PointerType *FrameMapPtrTy = PointerType::getUnqual(FrameMapTy);
 196
 197   // struct StackEntry {
 198   //   ShadowStackEntry *Next; // Caller's stack entry.
 199   //   FrameMap *Map;          // Pointer to constant FrameMap.
 200   //   void *Roots[];          // Stack roots (in-place array, so we pretend).
 201   // };
 202
 203   StackEntryTy = StructType::create(M.getContext(), "gc_stackentry");
 204
 205   EltTys.clear();
 206   EltTys.push_back(PointerType::getUnqual(StackEntryTy));
 207   EltTys.push_back(FrameMapPtrTy);
 208   StackEntryTy->setBody(EltTys);
 209   PointerType *StackEntryPtrTy = PointerType::getUnqual(StackEntryTy);
 210
 211   // Get the root chain if it already exists.
 212   Head = M.getGlobalVariable("llvm_gc_root_chain");
 213   if (!Head) {
 214     // If the root chain does not exist, insert a new one with linkonce
 215     // linkage!
 216     Head = new GlobalVariable(
 217         M, StackEntryPtrTy, false, GlobalValue::LinkOnceAnyLinkage,
 218         Constant::getNullValue(StackEntryPtrTy), "llvm_gc_root_chain");
 219   } else if (Head->hasExternalLinkage() && Head->isDeclaration()) {
 220     Head->setInitializer(Constant::getNullValue(StackEntryPtrTy));
 221     Head->setLinkage(GlobalValue::LinkOnceAnyLinkage);
 222   }
 223
 224   return true;
 225 }
 226
 227 bool ShadowStackGCLowering::IsNullValue(Value *V) {
 228   if (Constant *C = dyn_cast<Constant>(V))
 229     return C->isNullValue();
 230   return false;
 231 }
 232
 233 void ShadowStackGCLowering::CollectRoots(Function &F) {
 234   // FIXME: Account for original alignment. Could fragment the root array.
 235   //   Approach 1: Null initialize empty slots at runtime. Yuck.
 236   //   Approach 2: Emit a map of the array instead of just a count.
 237
 238   assert(Roots.empty() && "Not cleaned up?");
 239
 240   SmallVector<std::pair<CallInst *, AllocaInst *>, 16> MetaRoots;
 241
 242   for (BasicBlock &BB : F)
 243     for (BasicBlock::iterator II = BB.begin(), E = BB.end(); II != E;)
 244       if (IntrinsicInst *CI = dyn_cast<IntrinsicInst>(II++))
 245         if (Function *F = CI->getCalledFunction())
 246           if (F->getIntrinsicID() == Intrinsic::gcroot) {
 247             std::pair<CallInst *, AllocaInst *> Pair = std::make_pair(
 248                 CI,
 249                 cast<AllocaInst>(CI->getArgOperand(0)->stripPointerCasts()));
 250             if (IsNullValue(CI->getArgOperand(1)))
 251               Roots.push_back(Pair);
 252             else
 253               MetaRoots.push_back(Pair);
 254           }
 255
 256   // Number roots with metadata (usually empty) at the beginning, so that the
 257   // FrameMap::Meta array can be elided.
 258   Roots.insert(Roots.begin(), MetaRoots.begin(), MetaRoots.end());
 259 }
 260
 261 GetElementPtrInst *ShadowStackGCLowering::CreateGEP(LLVMContext &Context,
 262                                                     IRBuilder<> &B, Type *Ty,
 263                                                     Value *BasePtr, int Idx,
 264                                                     int Idx2,
 265                                                     const char *Name) {
 266   Value *Indices[] = {ConstantInt::get(Type::getInt32Ty(Context), 0),
 267                       ConstantInt::get(Type::getInt32Ty(Context), Idx),
 268                       ConstantInt::get(Type::getInt32Ty(Context), Idx2)};
 269   Value *Val = B.CreateGEP(Ty, BasePtr, Indices, Name);
 270
 271   assert(isa<GetElementPtrInst>(Val) && "Unexpected folded constant");
 272
 273   return dyn_cast<GetElementPtrInst>(Val);
 274 }
 275
 276 GetElementPtrInst *ShadowStackGCLowering::CreateGEP(LLVMContext &Context,
 277                                             IRBuilder<> &B, Type *Ty, Value *BasePtr,
 278                                             int Idx, const char *Name) {
 279   Value *Indices[] = {ConstantInt::get(Type::getInt32Ty(Context), 0),
 280                       ConstantInt::get(Type::getInt32Ty(Context), Idx)};
 281   Value *Val = B.CreateGEP(Ty, BasePtr, Indices, Name);
 282
 283   assert(isa<GetElementPtrInst>(Val) && "Unexpected folded constant");
 284
 285   return dyn_cast<GetElementPtrInst>(Val);
 286 }
 287
 288 void ShadowStackGCLowering::getAnalysisUsage(AnalysisUsage &AU) const {
 289   AU.addPreserved<DominatorTreeWrapperPass>();
 290 }
 291
 292 /// runOnFunction - Insert code to maintain the shadow stack.
 293 bool ShadowStackGCLowering::runOnFunction(Function &F) {
 294   // Quick exit for functions that do not use the shadow stack GC.
 295   if (!F.hasGC() ||
 296       F.getGC() != std::string("shadow-stack"))
 297     return false;
 298
 299   LLVMContext &Context = F.getContext();
 300
 301   // Find calls to llvm.gcroot.
 302   CollectRoots(F);
 303
 304   // If there are no roots in this function, then there is no need to add a
 305   // stack map entry for it.
 306   if (Roots.empty())
 307     return false;
 308
 309   Optional<DomTreeUpdater> DTU;
 310   if (auto *DTWP = getAnalysisIfAvailable<DominatorTreeWrapperPass>())
 311     DTU.emplace(DTWP->getDomTree(), DomTreeUpdater::UpdateStrategy::Lazy);
 312
 313   // Build the constant map and figure the type of the shadow stack entry.
 314   Value *FrameMap = GetFrameMap(F);
 315   Type *ConcreteStackEntryTy = GetConcreteStackEntryType(F);
 316
 317   // Build the shadow stack entry at the very start of the function.
 318   BasicBlock::iterator IP = F.getEntryBlock().begin();
 319   IRBuilder<> AtEntry(IP->getParent(), IP);
 320
 321   Instruction *StackEntry =
 322       AtEntry.CreateAlloca(ConcreteStackEntryTy, nullptr, "gc_frame");
 323
 324   while (isa<AllocaInst>(IP))
 325     ++IP;
 326   AtEntry.SetInsertPoint(IP->getParent(), IP);
 327
 328   // Initialize the map pointer and load the current head of the shadow stack.
 329   Instruction *CurrentHead =
 330       AtEntry.CreateLoad(StackEntryTy->getPointerTo(), Head, "gc_currhead");
 331   Instruction *EntryMapPtr = CreateGEP(Context, AtEntry, ConcreteStackEntryTy,
 332                                        StackEntry, 0, 1, "gc_frame.map");
 333   AtEntry.CreateStore(FrameMap, EntryMapPtr);
 334
 335   // After all the allocas...
 336   for (unsigned I = 0, E = Roots.size(); I != E; ++I) {
 337     // For each root, find the corresponding slot in the aggregate...
 338     Value *SlotPtr = CreateGEP(Context, AtEntry, ConcreteStackEntryTy,
 339                                StackEntry, 1 + I, "gc_root");
 340
 341     // And use it in lieu of the alloca.
 342     AllocaInst *OriginalAlloca = Roots[I].second;
 343     SlotPtr->takeName(OriginalAlloca);
 344     OriginalAlloca->replaceAllUsesWith(SlotPtr);
 345   }
 346
 347   // Move past the original stores inserted by GCStrategy::InitRoots. This isn't
 348   // really necessary (the collector would never see the intermediate state at
 349   // runtime), but it's nicer not to push the half-initialized entry onto the
 350   // shadow stack.
 351   while (isa<StoreInst>(IP))
 352     ++IP;
 353   AtEntry.SetInsertPoint(IP->getParent(), IP);
 354
 355   // Push the entry onto the shadow stack.
 356   Instruction *EntryNextPtr = CreateGEP(Context, AtEntry, ConcreteStackEntryTy,
 357                                         StackEntry, 0, 0, "gc_frame.next");
 358   Instruction *NewHeadVal = CreateGEP(Context, AtEntry, ConcreteStackEntryTy,
 359                                       StackEntry, 0, "gc_newhead");
 360   AtEntry.CreateStore(CurrentHead, EntryNextPtr);
 361   AtEntry.CreateStore(NewHeadVal, Head);
 362
 363   // For each instruction that escapes...
 364   EscapeEnumerator EE(F, "gc_cleanup", /*HandleExceptions=*/true,
 365                       DTU.hasValue() ? DTU.getPointer() : nullptr);
 366   while (IRBuilder<> *AtExit = EE.Next()) {
 367     // Pop the entry from the shadow stack. Don't reuse CurrentHead from
 368     // AtEntry, since that would make the value live for the entire function.
 369     Instruction *EntryNextPtr2 =
 370         CreateGEP(Context, *AtExit, ConcreteStackEntryTy, StackEntry, 0, 0,
 371                   "gc_frame.next");
 372     Value *SavedHead = AtExit->CreateLoad(StackEntryTy->getPointerTo(),
 373                                           EntryNextPtr2, "gc_savedhead");
 374     AtExit->CreateStore(SavedHead, Head);
 375   }
 376
 377   // Delete the original allocas (which are no longer used) and the intrinsic
 378   // calls (which are no longer valid). Doing this last avoids invalidating
 379   // iterators.
 380   for (unsigned I = 0, E = Roots.size(); I != E; ++I) {
 381     Roots[I].first->eraseFromParent();
 382     Roots[I].second->eraseFromParent();
 383   }
 384
 385   Roots.clear();
 386   return true;
 387 }