lib/CodeGen/ShadowStackGC.cpp

   1 //===-- ShadowStackGC.cpp - GC support for uncooperative targets ----------===//
   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 implements lowering for the llvm.gc* intrinsics for targets that do
  11 // not natively support them (which includes the C backend). Note that the code
  12 // generated is not quite as efficient as algorithms which generate stack maps
  13 // to identify roots.
  14 //
  15 // This pass implements the code transformation described in this paper:
  16 //   "Accurate Garbage Collection in an Uncooperative Environment"
  17 //   Fergus Henderson, ISMM, 2002
  18 //
  19 // In runtime/GC/SemiSpace.cpp is a prototype runtime which is compatible with
  20 // ShadowStackGC.
  21 //
  22 // In order to support this particular transformation, all stack roots are
  23 // coallocated in the stack. This allows a fully target-independent stack map
  24 // while introducing only minor runtime overhead.
  25 //
  26 //===----------------------------------------------------------------------===//
  27
  28 #define DEBUG_TYPE "shadowstackgc"
  29 #include "llvm/CodeGen/GCs.h"
  30 #include "llvm/ADT/StringExtras.h"
  31 #include "llvm/CodeGen/GCStrategy.h"
  32 #include "llvm/IntrinsicInst.h"
  33 #include "llvm/Module.h"
  34 #include "llvm/Support/Compiler.h"
  35 #include "llvm/Support/IRBuilder.h"
  36
  37 using namespace llvm;
  38
  39 namespace {
  40
  41   class VISIBILITY_HIDDEN ShadowStackGC : public GCStrategy {
  42     /// RootChain - This is the global linked-list that contains the chain of GC
  43     /// roots.
  44     GlobalVariable *Head;
  45
  46     /// StackEntryTy - Abstract type of a link in the shadow stack.
  47     ///
  48     const StructType *StackEntryTy;
  49
  50     /// Roots - GC roots in the current function. Each is a pair of the
  51     /// intrinsic call and its corresponding alloca.
  52     std::vector<std::pair<CallInst*,AllocaInst*> > Roots;
  53
  54   public:
  55     ShadowStackGC();
  56
  57     bool initializeCustomLowering(Module &M);
  58     bool performCustomLowering(Function &F);
  59
  60   private:
  61     bool IsNullValue(Value *V);
  62     Constant *GetFrameMap(Function &F);
  63     const Type* GetConcreteStackEntryType(Function &F);
  64     void CollectRoots(Function &F);
  65     static GetElementPtrInst *CreateGEP(LLVMContext &Context,
  66                                         IRBuilder<> &B, Value *BasePtr,
  67                                         int Idx1, const char *Name);
  68     static GetElementPtrInst *CreateGEP(LLVMContext &Context,
  69                                         IRBuilder<> &B, Value *BasePtr,
  70                                         int Idx1, int Idx2, const char *Name);
  71   };
  72
  73 }
  74
  75 static GCRegistry::Add<ShadowStackGC>
  76 X("shadow-stack", "Very portable GC for uncooperative code generators");
  77
  78 namespace {
  79   /// EscapeEnumerator - This is a little algorithm to find all escape points
  80   /// from a function so that "finally"-style code can be inserted. In addition
  81   /// to finding the existing return and unwind instructions, it also (if
  82   /// necessary) transforms any call instructions into invokes and sends them to
  83   /// a landing pad.
  84   ///
  85   /// It's wrapped up in a state machine using the same transform C# uses for
  86   /// 'yield return' enumerators, This transform allows it to be non-allocating.
  87   class VISIBILITY_HIDDEN EscapeEnumerator {
  88     Function &F;
  89     const char *CleanupBBName;
  90
  91     // State.
  92     int State;
  93     Function::iterator StateBB, StateE;
  94     IRBuilder<> Builder;
  95
  96   public:
  97     EscapeEnumerator(Function &F, const char *N = "cleanup")
  98       : F(F), CleanupBBName(N), State(0), Builder(F.getContext()) {}
  99
 100     IRBuilder<> *Next() {
 101       switch (State) {
 102       default:
 103         return 0;
 104
 105       case 0:
 106         StateBB = F.begin();
 107         StateE = F.end();
 108         State = 1;
 109
 110       case 1:
 111         // Find all 'return' and 'unwind' instructions.
 112         while (StateBB != StateE) {
 113           BasicBlock *CurBB = StateBB++;
 114
 115           // Branches and invokes do not escape, only unwind and return do.
 116           TerminatorInst *TI = CurBB->getTerminator();
 117           if (!isa<UnwindInst>(TI) && !isa<ReturnInst>(TI))
 118             continue;
 119
 120           Builder.SetInsertPoint(TI->getParent(), TI);
 121           return &Builder;
 122         }
 123
 124         State = 2;
 125
 126         // Find all 'call' instructions.
 127         SmallVector<Instruction*,16> Calls;
 128         for (Function::iterator BB = F.begin(),
 129                                 E = F.end(); BB != E; ++BB)
 130           for (BasicBlock::iterator II = BB->begin(),
 131                                     EE = BB->end(); II != EE; ++II)
 132             if (CallInst *CI = dyn_cast<CallInst>(II))
 133               if (!CI->getCalledFunction() ||
 134                   !CI->getCalledFunction()->getIntrinsicID())
 135                 Calls.push_back(CI);
 136
 137         if (Calls.empty())
 138           return 0;
 139
 140         // Create a cleanup block.
 141         BasicBlock *CleanupBB = BasicBlock::Create(CleanupBBName, &F);
 142         UnwindInst *UI = new UnwindInst(CleanupBB);
 143
 144         // Transform the 'call' instructions into 'invoke's branching to the
 145         // cleanup block. Go in reverse order to make prettier BB names.
 146         SmallVector<Value*,16> Args;
 147         for (unsigned I = Calls.size(); I != 0; ) {
 148           CallInst *CI = cast<CallInst>(Calls[--I]);
 149
 150           // Split the basic block containing the function call.
 151           BasicBlock *CallBB = CI->getParent();
 152           BasicBlock *NewBB =
 153             CallBB->splitBasicBlock(CI, CallBB->getName() + ".cont");
 154
 155           // Remove the unconditional branch inserted at the end of CallBB.
 156           CallBB->getInstList().pop_back();
 157           NewBB->getInstList().remove(CI);
 158
 159           // Create a new invoke instruction.
 160           Args.clear();
 161           Args.append(CI->op_begin() + 1, CI->op_end());
 162
 163           InvokeInst *II = InvokeInst::Create(CI->getOperand(0),
 164                                               NewBB, CleanupBB,
 165                                               Args.begin(), Args.end(),
 166                                               CI->getName(), CallBB);
 167           II->setCallingConv(CI->getCallingConv());
 168           II->setAttributes(CI->getAttributes());
 169           CI->replaceAllUsesWith(II);
 170           delete CI;
 171         }
 172
 173         Builder.SetInsertPoint(UI->getParent(), UI);
 174         return &Builder;
 175       }
 176     }
 177   };
 178 }
 179
 180 // -----------------------------------------------------------------------------
 181
 182 void llvm::linkShadowStackGC() { }
 183
 184 ShadowStackGC::ShadowStackGC() : Head(0), StackEntryTy(0) {
 185   InitRoots = true;
 186   CustomRoots = true;
 187 }
 188
 189 Constant *ShadowStackGC::GetFrameMap(Function &F) {
 190   // doInitialization creates the abstract type of this value.
 191   Type *VoidPtr = PointerType::getUnqual(Type::Int8Ty);
 192
 193   // Truncate the ShadowStackDescriptor if some metadata is null.
 194   unsigned NumMeta = 0;
 195   SmallVector<Constant*,16> Metadata;
 196   for (unsigned I = 0; I != Roots.size(); ++I) {
 197     Constant *C = cast<Constant>(Roots[I].first->getOperand(2));
 198     if (!C->isNullValue())
 199       NumMeta = I + 1;
 200     Metadata.push_back(ConstantExpr::getBitCast(C, VoidPtr));
 201   }
 202
 203   Constant *BaseElts[] = {
 204     ConstantInt::get(Type::Int32Ty, Roots.size(), false),
 205     ConstantInt::get(Type::Int32Ty, NumMeta, false),
 206   };
 207
 208   Constant *DescriptorElts[] = {
 209     ConstantStruct::get(F.getContext(), BaseElts, 2),
 210     ConstantArray::get(ArrayType::get(VoidPtr, NumMeta),
 211                        Metadata.begin(), NumMeta)
 212   };
 213
 214   Constant *FrameMap = ConstantStruct::get(F.getContext(), DescriptorElts, 2);
 215
 216   std::string TypeName("gc_map.");
 217   TypeName += utostr(NumMeta);
 218   F.getParent()->addTypeName(TypeName, FrameMap->getType());
 219
 220   // FIXME: Is this actually dangerous as WritingAnLLVMPass.html claims? Seems
 221   //        that, short of multithreaded LLVM, it should be safe; all that is
 222   //        necessary is that a simple Module::iterator loop not be invalidated.
 223   //        Appending to the GlobalVariable list is safe in that sense.
 224   //
 225   //        All of the output passes emit globals last. The ExecutionEngine
 226   //        explicitly supports adding globals to the module after
 227   //        initialization.
 228   //
 229   //        Still, if it isn't deemed acceptable, then this transformation needs
 230   //        to be a ModulePass (which means it cannot be in the 'llc' pipeline
 231   //        (which uses a FunctionPassManager (which segfaults (not asserts) if
 232   //        provided a ModulePass))).
 233   Constant *GV = new GlobalVariable(*F.getParent(), FrameMap->getType(), true,
 234                                     GlobalVariable::InternalLinkage,
 235                                     FrameMap, "__gc_" + F.getName());
 236
 237   Constant *GEPIndices[2] = { ConstantInt::get(Type::Int32Ty, 0),
 238                               ConstantInt::get(Type::Int32Ty, 0) };
 239   return ConstantExpr::getGetElementPtr(GV, GEPIndices, 2);
 240 }
 241
 242 const Type* ShadowStackGC::GetConcreteStackEntryType(Function &F) {
 243   // doInitialization creates the generic version of this type.
 244   std::vector<const Type*> EltTys;
 245   EltTys.push_back(StackEntryTy);
 246   for (size_t I = 0; I != Roots.size(); I++)
 247     EltTys.push_back(Roots[I].second->getAllocatedType());
 248   Type *Ty = StructType::get(F.getContext(), EltTys);
 249
 250   std::string TypeName("gc_stackentry.");
 251   TypeName += F.getName();
 252   F.getParent()->addTypeName(TypeName, Ty);
 253
 254   return Ty;
 255 }
 256
 257 /// doInitialization - If this module uses the GC intrinsics, find them now. If
 258 /// not, exit fast.
 259 bool ShadowStackGC::initializeCustomLowering(Module &M) {
 260   // struct FrameMap {
 261   //   int32_t NumRoots; // Number of roots in stack frame.
 262   //   int32_t NumMeta;  // Number of metadata descriptors. May be < NumRoots.
 263   //   void *Meta[];     // May be absent for roots without metadata.
 264   // };
 265   std::vector<const Type*> EltTys;
 266   EltTys.push_back(Type::Int32Ty); // 32 bits is ok up to a 32GB stack frame. :)
 267   EltTys.push_back(Type::Int32Ty); // Specifies length of variable length array.
 268   StructType *FrameMapTy = StructType::get(M.getContext(), EltTys);
 269   M.addTypeName("gc_map", FrameMapTy);
 270   PointerType *FrameMapPtrTy = PointerType::getUnqual(FrameMapTy);
 271
 272   // struct StackEntry {
 273   //   ShadowStackEntry *Next; // Caller's stack entry.
 274   //   FrameMap *Map;          // Pointer to constant FrameMap.
 275   //   void *Roots[];          // Stack roots (in-place array, so we pretend).
 276   // };
 277   OpaqueType *RecursiveTy = OpaqueType::get();
 278
 279   EltTys.clear();
 280   EltTys.push_back(PointerType::getUnqual(RecursiveTy));
 281   EltTys.push_back(FrameMapPtrTy);
 282   PATypeHolder LinkTyH = StructType::get(M.getContext(), EltTys);
 283
 284   RecursiveTy->refineAbstractTypeTo(LinkTyH.get());
 285   StackEntryTy = cast<StructType>(LinkTyH.get());
 286   const PointerType *StackEntryPtrTy = PointerType::getUnqual(StackEntryTy);
 287   M.addTypeName("gc_stackentry", LinkTyH.get());  // FIXME: Is this safe from
 288                                                   //        a FunctionPass?
 289
 290   // Get the root chain if it already exists.
 291   Head = M.getGlobalVariable("llvm_gc_root_chain");
 292   if (!Head) {
 293     // If the root chain does not exist, insert a new one with linkonce
 294     // linkage!
 295     Head = new GlobalVariable(M, StackEntryPtrTy, false,
 296                               GlobalValue::LinkOnceAnyLinkage,
 297                               Constant::getNullValue(StackEntryPtrTy),
 298                               "llvm_gc_root_chain");
 299   } else if (Head->hasExternalLinkage() && Head->isDeclaration()) {
 300     Head->setInitializer(Constant::getNullValue(StackEntryPtrTy));
 301     Head->setLinkage(GlobalValue::LinkOnceAnyLinkage);
 302   }
 303
 304   return true;
 305 }
 306
 307 bool ShadowStackGC::IsNullValue(Value *V) {
 308   if (Constant *C = dyn_cast<Constant>(V))
 309     return C->isNullValue();
 310   return false;
 311 }
 312
 313 void ShadowStackGC::CollectRoots(Function &F) {
 314   // FIXME: Account for original alignment. Could fragment the root array.
 315   //   Approach 1: Null initialize empty slots at runtime. Yuck.
 316   //   Approach 2: Emit a map of the array instead of just a count.
 317
 318   assert(Roots.empty() && "Not cleaned up?");
 319
 320   SmallVector<std::pair<CallInst*,AllocaInst*>,16> MetaRoots;
 321
 322   for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB)
 323     for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E;)
 324       if (IntrinsicInst *CI = dyn_cast<IntrinsicInst>(II++))
 325         if (Function *F = CI->getCalledFunction())
 326           if (F->getIntrinsicID() == Intrinsic::gcroot) {
 327             std::pair<CallInst*,AllocaInst*> Pair = std::make_pair(
 328               CI, cast<AllocaInst>(CI->getOperand(1)->stripPointerCasts()));
 329             if (IsNullValue(CI->getOperand(2)))
 330               Roots.push_back(Pair);
 331             else
 332               MetaRoots.push_back(Pair);
 333           }
 334
 335   // Number roots with metadata (usually empty) at the beginning, so that the
 336   // FrameMap::Meta array can be elided.
 337   Roots.insert(Roots.begin(), MetaRoots.begin(), MetaRoots.end());
 338 }
 339
 340 GetElementPtrInst *
 341 ShadowStackGC::CreateGEP(LLVMContext &Context, IRBuilder<> &B, Value *BasePtr,
 342                          int Idx, int Idx2, const char *Name) {
 343   Value *Indices[] = { ConstantInt::get(Type::Int32Ty, 0),
 344                        ConstantInt::get(Type::Int32Ty, Idx),
 345                        ConstantInt::get(Type::Int32Ty, Idx2) };
 346   Value* Val = B.CreateGEP(BasePtr, Indices, Indices + 3, Name);
 347
 348   assert(isa<GetElementPtrInst>(Val) && "Unexpected folded constant");
 349
 350   return dyn_cast<GetElementPtrInst>(Val);
 351 }
 352
 353 GetElementPtrInst *
 354 ShadowStackGC::CreateGEP(LLVMContext &Context, IRBuilder<> &B, Value *BasePtr,
 355                          int Idx, const char *Name) {
 356   Value *Indices[] = { ConstantInt::get(Type::Int32Ty, 0),
 357                        ConstantInt::get(Type::Int32Ty, Idx) };
 358   Value *Val = B.CreateGEP(BasePtr, Indices, Indices + 2, Name);
 359
 360   assert(isa<GetElementPtrInst>(Val) && "Unexpected folded constant");
 361
 362   return dyn_cast<GetElementPtrInst>(Val);
 363 }
 364
 365 /// runOnFunction - Insert code to maintain the shadow stack.
 366 bool ShadowStackGC::performCustomLowering(Function &F) {
 367   LLVMContext &Context = F.getContext();
 368
 369   // Find calls to llvm.gcroot.
 370   CollectRoots(F);
 371
 372   // If there are no roots in this function, then there is no need to add a
 373   // stack map entry for it.
 374   if (Roots.empty())
 375     return false;
 376
 377   // Build the constant map and figure the type of the shadow stack entry.
 378   Value *FrameMap = GetFrameMap(F);
 379   const Type *ConcreteStackEntryTy = GetConcreteStackEntryType(F);
 380
 381   // Build the shadow stack entry at the very start of the function.
 382   BasicBlock::iterator IP = F.getEntryBlock().begin();
 383   IRBuilder<> AtEntry(IP->getParent(), IP);
 384
 385   Instruction *StackEntry   = AtEntry.CreateAlloca(ConcreteStackEntryTy, 0,
 386                                                    "gc_frame");
 387
 388   while (isa<AllocaInst>(IP)) ++IP;
 389   AtEntry.SetInsertPoint(IP->getParent(), IP);
 390
 391   // Initialize the map pointer and load the current head of the shadow stack.
 392   Instruction *CurrentHead  = AtEntry.CreateLoad(Head, "gc_currhead");
 393   Instruction *EntryMapPtr  = CreateGEP(Context, AtEntry, StackEntry,
 394                                         0,1,"gc_frame.map");
 395                               AtEntry.CreateStore(FrameMap, EntryMapPtr);
 396
 397   // After all the allocas...
 398   for (unsigned I = 0, E = Roots.size(); I != E; ++I) {
 399     // For each root, find the corresponding slot in the aggregate...
 400     Value *SlotPtr = CreateGEP(Context, AtEntry, StackEntry, 1 + I, "gc_root");
 401
 402     // And use it in lieu of the alloca.
 403     AllocaInst *OriginalAlloca = Roots[I].second;
 404     SlotPtr->takeName(OriginalAlloca);
 405     OriginalAlloca->replaceAllUsesWith(SlotPtr);
 406   }
 407
 408   // Move past the original stores inserted by GCStrategy::InitRoots. This isn't
 409   // really necessary (the collector would never see the intermediate state at
 410   // runtime), but it's nicer not to push the half-initialized entry onto the
 411   // shadow stack.
 412   while (isa<StoreInst>(IP)) ++IP;
 413   AtEntry.SetInsertPoint(IP->getParent(), IP);
 414
 415   // Push the entry onto the shadow stack.
 416   Instruction *EntryNextPtr = CreateGEP(Context, AtEntry,
 417                                         StackEntry,0,0,"gc_frame.next");
 418   Instruction *NewHeadVal   = CreateGEP(Context, AtEntry,
 419                                         StackEntry, 0, "gc_newhead");
 420   AtEntry.CreateStore(CurrentHead, EntryNextPtr);
 421   AtEntry.CreateStore(NewHeadVal, Head);
 422
 423   // For each instruction that escapes...
 424   EscapeEnumerator EE(F, "gc_cleanup");
 425   while (IRBuilder<> *AtExit = EE.Next()) {
 426     // Pop the entry from the shadow stack. Don't reuse CurrentHead from
 427     // AtEntry, since that would make the value live for the entire function.
 428     Instruction *EntryNextPtr2 = CreateGEP(Context, *AtExit, StackEntry, 0, 0,
 429                                            "gc_frame.next");
 430     Value *SavedHead = AtExit->CreateLoad(EntryNextPtr2, "gc_savedhead");
 431                        AtExit->CreateStore(SavedHead, Head);
 432   }
 433
 434   // Delete the original allocas (which are no longer used) and the intrinsic
 435   // calls (which are no longer valid). Doing this last avoids invalidating
 436   // iterators.
 437   for (unsigned I = 0, E = Roots.size(); I != E; ++I) {
 438     Roots[I].first->eraseFromParent();
 439     Roots[I].second->eraseFromParent();
 440   }
 441
 442   Roots.clear();
 443   return true;
 444 }