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/CodeGen/ShadowStackGCLowering.h"
  19 #include "llvm/ADT/SmallVector.h"
  20 #include "llvm/ADT/StringExtras.h"
  21 #include "llvm/Analysis/DomTreeUpdater.h"
  22 #include "llvm/CodeGen/GCMetadata.h"
  23 #include "llvm/CodeGen/Passes.h"
  24 #include "llvm/IR/BasicBlock.h"
  25 #include "llvm/IR/Constant.h"
  26 #include "llvm/IR/Constants.h"
  27 #include "llvm/IR/DerivedTypes.h"
  28 #include "llvm/IR/Dominators.h"
  29 #include "llvm/IR/Function.h"
  30 #include "llvm/IR/GlobalValue.h"
  31 #include "llvm/IR/GlobalVariable.h"
  32 #include "llvm/IR/IRBuilder.h"
  33 #include "llvm/IR/Instructions.h"
  34 #include "llvm/IR/IntrinsicInst.h"
  35 #include "llvm/IR/Intrinsics.h"
  36 #include "llvm/IR/Module.h"
  37 #include "llvm/IR/Type.h"
  38 #include "llvm/IR/Value.h"
  39 #include "llvm/InitializePasses.h"
  40 #include "llvm/Pass.h"
  41 #include "llvm/Support/Casting.h"
  42 #include "llvm/Transforms/Utils/EscapeEnumerator.h"
  43 #include <cassert>
  44 #include <optional>
  45 #include <string>
  46 #include <utility>
  47 #include <vector>
  48
  49 using namespace llvm;
  50
  51 #define DEBUG_TYPE "shadow-stack-gc-lowering"
  52
  53 namespace {
  54
  55 class ShadowStackGCLoweringImpl {
  56   /// RootChain - This is the global linked-list that contains the chain of GC
  57   /// roots.
  58   GlobalVariable *Head = nullptr;
  59
  60   /// StackEntryTy - Abstract type of a link in the shadow stack.
  61   StructType *StackEntryTy = nullptr;
  62   StructType *FrameMapTy = nullptr;
  63
  64   /// Roots - GC roots in the current function. Each is a pair of the
  65   /// intrinsic call and its corresponding alloca.
  66   std::vector<std::pair<CallInst *, AllocaInst *>> Roots;
  67
  68 public:
  69   ShadowStackGCLoweringImpl() = default;
  70
  71   bool doInitialization(Module &M);
  72   bool runOnFunction(Function &F, DomTreeUpdater *DTU);
  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 class ShadowStackGCLowering : public FunctionPass {
  89   ShadowStackGCLoweringImpl Impl;
  90
  91 public:
  92   static char ID;
  93
  94   ShadowStackGCLowering();
  95
  96   bool doInitialization(Module &M) override { return Impl.doInitialization(M); }
  97   void getAnalysisUsage(AnalysisUsage &AU) const override {
  98     AU.addPreserved<DominatorTreeWrapperPass>();
  99   }
 100   bool runOnFunction(Function &F) override {
 101     std::optional<DomTreeUpdater> DTU;
 102     if (auto *DTWP = getAnalysisIfAvailable<DominatorTreeWrapperPass>())
 103       DTU.emplace(DTWP->getDomTree(), DomTreeUpdater::UpdateStrategy::Lazy);
 104     return Impl.runOnFunction(F, DTU ? &*DTU : nullptr);
 105   }
 106 };
 107
 108 } // end anonymous namespace
 109
 110 PreservedAnalyses ShadowStackGCLoweringPass::run(Module &M,
 111                                                  ModuleAnalysisManager &MAM) {
 112   auto &Map = MAM.getResult<CollectorMetadataAnalysis>(M);
 113   if (Map.StrategyMap.contains("shadow-stack"))
 114     return PreservedAnalyses::all();
 115
 116   ShadowStackGCLoweringImpl Impl;
 117   bool Changed = Impl.doInitialization(M);
 118   for (auto &F : M) {
 119     auto &FAM =
 120         MAM.getResult<FunctionAnalysisManagerModuleProxy>(M).getManager();
 121     auto *DT = FAM.getCachedResult<DominatorTreeAnalysis>(F);
 122     DomTreeUpdater DTU(DT, DomTreeUpdater::UpdateStrategy::Lazy);
 123     Changed |= Impl.runOnFunction(F, DT ? &DTU : nullptr);
 124   }
 125
 126   if (!Changed)
 127     return PreservedAnalyses::all();
 128   PreservedAnalyses PA;
 129   PA.preserve<DominatorTreeAnalysis>();
 130   return PA;
 131 }
 132
 133 char ShadowStackGCLowering::ID = 0;
 134 char &llvm::ShadowStackGCLoweringID = ShadowStackGCLowering::ID;
 135
 136 INITIALIZE_PASS_BEGIN(ShadowStackGCLowering, DEBUG_TYPE,
 137                       "Shadow Stack GC Lowering", false, false)
 138 INITIALIZE_PASS_DEPENDENCY(GCModuleInfo)
 139 INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
 140 INITIALIZE_PASS_END(ShadowStackGCLowering, DEBUG_TYPE,
 141                     "Shadow Stack GC Lowering", false, false)
 142
 143 FunctionPass *llvm::createShadowStackGCLoweringPass() { return new ShadowStackGCLowering(); }
 144
 145 ShadowStackGCLowering::ShadowStackGCLowering() : FunctionPass(ID) {
 146   initializeShadowStackGCLoweringPass(*PassRegistry::getPassRegistry());
 147 }
 148
 149 Constant *ShadowStackGCLoweringImpl::GetFrameMap(Function &F) {
 150   // doInitialization creates the abstract type of this value.
 151   Type *VoidPtr = PointerType::getUnqual(F.getContext());
 152
 153   // Truncate the ShadowStackDescriptor if some metadata is null.
 154   unsigned NumMeta = 0;
 155   SmallVector<Constant *, 16> Metadata;
 156   for (unsigned I = 0; I != Roots.size(); ++I) {
 157     Constant *C = cast<Constant>(Roots[I].first->getArgOperand(1));
 158     if (!C->isNullValue())
 159       NumMeta = I + 1;
 160     Metadata.push_back(C);
 161   }
 162   Metadata.resize(NumMeta);
 163
 164   Type *Int32Ty = Type::getInt32Ty(F.getContext());
 165
 166   Constant *BaseElts[] = {
 167       ConstantInt::get(Int32Ty, Roots.size(), false),
 168       ConstantInt::get(Int32Ty, NumMeta, false),
 169   };
 170
 171   Constant *DescriptorElts[] = {
 172       ConstantStruct::get(FrameMapTy, BaseElts),
 173       ConstantArray::get(ArrayType::get(VoidPtr, NumMeta), Metadata)};
 174
 175   Type *EltTys[] = {DescriptorElts[0]->getType(), DescriptorElts[1]->getType()};
 176   StructType *STy = StructType::create(EltTys, "gc_map." + utostr(NumMeta));
 177
 178   Constant *FrameMap = ConstantStruct::get(STy, DescriptorElts);
 179
 180   // FIXME: Is this actually dangerous as WritingAnLLVMPass.html claims? Seems
 181   //        that, short of multithreaded LLVM, it should be safe; all that is
 182   //        necessary is that a simple Module::iterator loop not be invalidated.
 183   //        Appending to the GlobalVariable list is safe in that sense.
 184   //
 185   //        All of the output passes emit globals last. The ExecutionEngine
 186   //        explicitly supports adding globals to the module after
 187   //        initialization.
 188   //
 189   //        Still, if it isn't deemed acceptable, then this transformation needs
 190   //        to be a ModulePass (which means it cannot be in the 'llc' pipeline
 191   //        (which uses a FunctionPassManager (which segfaults (not asserts) if
 192   //        provided a ModulePass))).
 193   Constant *GV = new GlobalVariable(*F.getParent(), FrameMap->getType(), true,
 194                                     GlobalVariable::InternalLinkage, FrameMap,
 195                                     "__gc_" + F.getName());
 196
 197   Constant *GEPIndices[2] = {
 198       ConstantInt::get(Type::getInt32Ty(F.getContext()), 0),
 199       ConstantInt::get(Type::getInt32Ty(F.getContext()), 0)};
 200   return ConstantExpr::getGetElementPtr(FrameMap->getType(), GV, GEPIndices);
 201 }
 202
 203 Type *ShadowStackGCLoweringImpl::GetConcreteStackEntryType(Function &F) {
 204   // doInitialization creates the generic version of this type.
 205   std::vector<Type *> EltTys;
 206   EltTys.push_back(StackEntryTy);
 207   for (const std::pair<CallInst *, AllocaInst *> &Root : Roots)
 208     EltTys.push_back(Root.second->getAllocatedType());
 209
 210   return StructType::create(EltTys, ("gc_stackentry." + F.getName()).str());
 211 }
 212
 213 /// doInitialization - If this module uses the GC intrinsics, find them now. If
 214 /// not, exit fast.
 215 bool ShadowStackGCLoweringImpl::doInitialization(Module &M) {
 216   bool Active = false;
 217   for (Function &F : M) {
 218     if (F.hasGC() && F.getGC() == "shadow-stack") {
 219       Active = true;
 220       break;
 221     }
 222   }
 223   if (!Active)
 224     return false;
 225
 226   // struct FrameMap {
 227   //   int32_t NumRoots; // Number of roots in stack frame.
 228   //   int32_t NumMeta;  // Number of metadata descriptors. May be < NumRoots.
 229   //   void *Meta[];     // May be absent for roots without metadata.
 230   // };
 231   std::vector<Type *> EltTys;
 232   // 32 bits is ok up to a 32GB stack frame. :)
 233   EltTys.push_back(Type::getInt32Ty(M.getContext()));
 234   // Specifies length of variable length array.
 235   EltTys.push_back(Type::getInt32Ty(M.getContext()));
 236   FrameMapTy = StructType::create(EltTys, "gc_map");
 237   PointerType *FrameMapPtrTy = PointerType::getUnqual(FrameMapTy);
 238
 239   // struct StackEntry {
 240   //   ShadowStackEntry *Next; // Caller's stack entry.
 241   //   FrameMap *Map;          // Pointer to constant FrameMap.
 242   //   void *Roots[];          // Stack roots (in-place array, so we pretend).
 243   // };
 244
 245   StackEntryTy = StructType::create(M.getContext(), "gc_stackentry");
 246
 247   EltTys.clear();
 248   EltTys.push_back(PointerType::getUnqual(StackEntryTy));
 249   EltTys.push_back(FrameMapPtrTy);
 250   StackEntryTy->setBody(EltTys);
 251   PointerType *StackEntryPtrTy = PointerType::getUnqual(StackEntryTy);
 252
 253   // Get the root chain if it already exists.
 254   Head = M.getGlobalVariable("llvm_gc_root_chain");
 255   if (!Head) {
 256     // If the root chain does not exist, insert a new one with linkonce
 257     // linkage!
 258     Head = new GlobalVariable(
 259         M, StackEntryPtrTy, false, GlobalValue::LinkOnceAnyLinkage,
 260         Constant::getNullValue(StackEntryPtrTy), "llvm_gc_root_chain");
 261   } else if (Head->hasExternalLinkage() && Head->isDeclaration()) {
 262     Head->setInitializer(Constant::getNullValue(StackEntryPtrTy));
 263     Head->setLinkage(GlobalValue::LinkOnceAnyLinkage);
 264   }
 265
 266   return true;
 267 }
 268
 269 bool ShadowStackGCLoweringImpl::IsNullValue(Value *V) {
 270   if (Constant *C = dyn_cast<Constant>(V))
 271     return C->isNullValue();
 272   return false;
 273 }
 274
 275 void ShadowStackGCLoweringImpl::CollectRoots(Function &F) {
 276   // FIXME: Account for original alignment. Could fragment the root array.
 277   //   Approach 1: Null initialize empty slots at runtime. Yuck.
 278   //   Approach 2: Emit a map of the array instead of just a count.
 279
 280   assert(Roots.empty() && "Not cleaned up?");
 281
 282   SmallVector<std::pair<CallInst *, AllocaInst *>, 16> MetaRoots;
 283
 284   for (BasicBlock &BB : F)
 285     for (Instruction &I : BB)
 286       if (IntrinsicInst *CI = dyn_cast<IntrinsicInst>(&I))
 287         if (Function *F = CI->getCalledFunction())
 288           if (F->getIntrinsicID() == Intrinsic::gcroot) {
 289             std::pair<CallInst *, AllocaInst *> Pair = std::make_pair(
 290                 CI,
 291                 cast<AllocaInst>(CI->getArgOperand(0)->stripPointerCasts()));
 292             if (IsNullValue(CI->getArgOperand(1)))
 293               Roots.push_back(Pair);
 294             else
 295               MetaRoots.push_back(Pair);
 296           }
 297
 298   // Number roots with metadata (usually empty) at the beginning, so that the
 299   // FrameMap::Meta array can be elided.
 300   Roots.insert(Roots.begin(), MetaRoots.begin(), MetaRoots.end());
 301 }
 302
 303 GetElementPtrInst *
 304 ShadowStackGCLoweringImpl::CreateGEP(LLVMContext &Context, IRBuilder<> &B,
 305                                      Type *Ty, Value *BasePtr, int Idx,
 306                                      int Idx2, const char *Name) {
 307   Value *Indices[] = {ConstantInt::get(Type::getInt32Ty(Context), 0),
 308                       ConstantInt::get(Type::getInt32Ty(Context), Idx),
 309                       ConstantInt::get(Type::getInt32Ty(Context), Idx2)};
 310   Value *Val = B.CreateGEP(Ty, BasePtr, Indices, Name);
 311
 312   assert(isa<GetElementPtrInst>(Val) && "Unexpected folded constant");
 313
 314   return dyn_cast<GetElementPtrInst>(Val);
 315 }
 316
 317 GetElementPtrInst *ShadowStackGCLoweringImpl::CreateGEP(LLVMContext &Context,
 318                                                         IRBuilder<> &B,
 319                                                         Type *Ty,
 320                                                         Value *BasePtr, int Idx,
 321                                                         const char *Name) {
 322   Value *Indices[] = {ConstantInt::get(Type::getInt32Ty(Context), 0),
 323                       ConstantInt::get(Type::getInt32Ty(Context), Idx)};
 324   Value *Val = B.CreateGEP(Ty, BasePtr, Indices, Name);
 325
 326   assert(isa<GetElementPtrInst>(Val) && "Unexpected folded constant");
 327
 328   return dyn_cast<GetElementPtrInst>(Val);
 329 }
 330
 331 /// runOnFunction - Insert code to maintain the shadow stack.
 332 bool ShadowStackGCLoweringImpl::runOnFunction(Function &F,
 333                                               DomTreeUpdater *DTU) {
 334   // Quick exit for functions that do not use the shadow stack GC.
 335   if (!F.hasGC() || F.getGC() != "shadow-stack")
 336     return false;
 337
 338   LLVMContext &Context = F.getContext();
 339
 340   // Find calls to llvm.gcroot.
 341   CollectRoots(F);
 342
 343   // If there are no roots in this function, then there is no need to add a
 344   // stack map entry for it.
 345   if (Roots.empty())
 346     return false;
 347
 348   // Build the constant map and figure the type of the shadow stack entry.
 349   Value *FrameMap = GetFrameMap(F);
 350   Type *ConcreteStackEntryTy = GetConcreteStackEntryType(F);
 351
 352   // Build the shadow stack entry at the very start of the function.
 353   BasicBlock::iterator IP = F.getEntryBlock().begin();
 354   IRBuilder<> AtEntry(IP->getParent(), IP);
 355
 356   Instruction *StackEntry =
 357       AtEntry.CreateAlloca(ConcreteStackEntryTy, nullptr, "gc_frame");
 358
 359   AtEntry.SetInsertPointPastAllocas(&F);
 360   IP = AtEntry.GetInsertPoint();
 361
 362   // Initialize the map pointer and load the current head of the shadow stack.
 363   Instruction *CurrentHead =
 364       AtEntry.CreateLoad(AtEntry.getPtrTy(), Head, "gc_currhead");
 365   Instruction *EntryMapPtr = CreateGEP(Context, AtEntry, ConcreteStackEntryTy,
 366                                        StackEntry, 0, 1, "gc_frame.map");
 367   AtEntry.CreateStore(FrameMap, EntryMapPtr);
 368
 369   // After all the allocas...
 370   for (unsigned I = 0, E = Roots.size(); I != E; ++I) {
 371     // For each root, find the corresponding slot in the aggregate...
 372     Value *SlotPtr = CreateGEP(Context, AtEntry, ConcreteStackEntryTy,
 373                                StackEntry, 1 + I, "gc_root");
 374
 375     // And use it in lieu of the alloca.
 376     AllocaInst *OriginalAlloca = Roots[I].second;
 377     SlotPtr->takeName(OriginalAlloca);
 378     OriginalAlloca->replaceAllUsesWith(SlotPtr);
 379   }
 380
 381   // Move past the original stores inserted by GCStrategy::InitRoots. This isn't
 382   // really necessary (the collector would never see the intermediate state at
 383   // runtime), but it's nicer not to push the half-initialized entry onto the
 384   // shadow stack.
 385   while (isa<StoreInst>(IP))
 386     ++IP;
 387   AtEntry.SetInsertPoint(IP->getParent(), IP);
 388
 389   // Push the entry onto the shadow stack.
 390   Instruction *EntryNextPtr = CreateGEP(Context, AtEntry, ConcreteStackEntryTy,
 391                                         StackEntry, 0, 0, "gc_frame.next");
 392   Instruction *NewHeadVal = CreateGEP(Context, AtEntry, ConcreteStackEntryTy,
 393                                       StackEntry, 0, "gc_newhead");
 394   AtEntry.CreateStore(CurrentHead, EntryNextPtr);
 395   AtEntry.CreateStore(NewHeadVal, Head);
 396
 397   // For each instruction that escapes...
 398   EscapeEnumerator EE(F, "gc_cleanup", /*HandleExceptions=*/true, DTU);
 399   while (IRBuilder<> *AtExit = EE.Next()) {
 400     // Pop the entry from the shadow stack. Don't reuse CurrentHead from
 401     // AtEntry, since that would make the value live for the entire function.
 402     Instruction *EntryNextPtr2 =
 403         CreateGEP(Context, *AtExit, ConcreteStackEntryTy, StackEntry, 0, 0,
 404                   "gc_frame.next");
 405     Value *SavedHead =
 406         AtExit->CreateLoad(AtExit->getPtrTy(), EntryNextPtr2, "gc_savedhead");
 407     AtExit->CreateStore(SavedHead, Head);
 408   }
 409
 410   // Delete the original allocas (which are no longer used) and the intrinsic
 411   // calls (which are no longer valid). Doing this last avoids invalidating
 412   // iterators.
 413   for (std::pair<CallInst *, AllocaInst *> &Root : Roots) {
 414     Root.first->eraseFromParent();
 415     Root.second->eraseFromParent();
 416   }
 417
 418   Roots.clear();
 419   return true;
 420 }