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