ShadowStackGC.cpp revision b065b06c12dba6001b8140df2744d0c856ef6ea1 
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/CallSite.h"
35#include "llvm/Support/IRBuilder.h"
36
37using namespace llvm;
38
39namespace {
40
41  class 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
75static GCRegistry::Add<ShadowStackGC>
76X("shadow-stack", "Very portable GC for uncooperative code generators");
77
78namespace {
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 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(F.getContext(),
142                                                   CleanupBBName, &F);
143        UnwindInst *UI = new UnwindInst(F.getContext(), CleanupBB);
144
145        // Transform the 'call' instructions into 'invoke's branching to the
146        // cleanup block. Go in reverse order to make prettier BB names.
147        SmallVector<Value*,16> Args;
148        for (unsigned I = Calls.size(); I != 0; ) {
149          CallInst *CI = cast<CallInst>(Calls[--I]);
150
151          // Split the basic block containing the function call.
152          BasicBlock *CallBB = CI->getParent();
153          BasicBlock *NewBB =
154            CallBB->splitBasicBlock(CI, CallBB->getName() + ".cont");
155
156          // Remove the unconditional branch inserted at the end of CallBB.
157          CallBB->getInstList().pop_back();
158          NewBB->getInstList().remove(CI);
159
160          // Create a new invoke instruction.
161          Args.clear();
162          CallSite CS(CI);
163          Args.append(CS.arg_begin(), CS.arg_end());
164
165          InvokeInst *II = InvokeInst::Create(CI->getCalledValue(),
166                                              NewBB, CleanupBB,
167                                              Args.begin(), Args.end(),
168                                              CI->getName(), CallBB);
169          II->setCallingConv(CI->getCallingConv());
170          II->setAttributes(CI->getAttributes());
171          CI->replaceAllUsesWith(II);
172          delete CI;
173        }
174
175        Builder.SetInsertPoint(UI->getParent(), UI);
176        return &Builder;
177      }
178    }
179  };
180}
181
182// -----------------------------------------------------------------------------
183
184void llvm::linkShadowStackGC() { }
185
186ShadowStackGC::ShadowStackGC() : Head(0), StackEntryTy(0) {
187  InitRoots = true;
188  CustomRoots = true;
189}
190
191Constant *ShadowStackGC::GetFrameMap(Function &F) {
192  // doInitialization creates the abstract type of this value.
193  const Type *VoidPtr = Type::getInt8PtrTy(F.getContext());
194
195  // Truncate the ShadowStackDescriptor if some metadata is null.
196  unsigned NumMeta = 0;
197  SmallVector<Constant*, 16> Metadata;
198  for (unsigned I = 0; I != Roots.size(); ++I) {
199    Constant *C = cast<Constant>(Roots[I].first->getArgOperand(1));
200    if (!C->isNullValue())
201      NumMeta = I + 1;
202    Metadata.push_back(ConstantExpr::getBitCast(C, VoidPtr));
203  }
204
205  const Type *Int32Ty = Type::getInt32Ty(F.getContext());
206
207  Constant *BaseElts[] = {
208    ConstantInt::get(Int32Ty, Roots.size(), false),
209    ConstantInt::get(Int32Ty, NumMeta, false),
210  };
211
212  Constant *DescriptorElts[] = {
213    ConstantStruct::get(StructType::get(Int32Ty, Int32Ty, NULL), BaseElts),
214    ConstantArray::get(ArrayType::get(VoidPtr, NumMeta),
215                       Metadata.begin(), NumMeta)
216  };
217
218  Constant *FrameMap =
219    ConstantStruct::get(StructType::get(DescriptorElts[0]->getType(),
220                                        DescriptorElts[1]->getType(), NULL),
221                        DescriptorElts);
222
223  std::string TypeName("gc_map.");
224  TypeName += utostr(NumMeta);
225  F.getParent()->addTypeName(TypeName, FrameMap->getType());
226
227  // FIXME: Is this actually dangerous as WritingAnLLVMPass.html claims? Seems
228  //        that, short of multithreaded LLVM, it should be safe; all that is
229  //        necessary is that a simple Module::iterator loop not be invalidated.
230  //        Appending to the GlobalVariable list is safe in that sense.
231  //
232  //        All of the output passes emit globals last. The ExecutionEngine
233  //        explicitly supports adding globals to the module after
234  //        initialization.
235  //
236  //        Still, if it isn't deemed acceptable, then this transformation needs
237  //        to be a ModulePass (which means it cannot be in the 'llc' pipeline
238  //        (which uses a FunctionPassManager (which segfaults (not asserts) if
239  //        provided a ModulePass))).
240  Constant *GV = new GlobalVariable(*F.getParent(), FrameMap->getType(), true,
241                                    GlobalVariable::InternalLinkage,
242                                    FrameMap, "__gc_" + F.getName());
243
244  Constant *GEPIndices[2] = {
245                          ConstantInt::get(Type::getInt32Ty(F.getContext()), 0),
246                          ConstantInt::get(Type::getInt32Ty(F.getContext()), 0)
247                          };
248  return ConstantExpr::getGetElementPtr(GV, GEPIndices, 2);
249}
250
251const Type* ShadowStackGC::GetConcreteStackEntryType(Function &F) {
252  // doInitialization creates the generic version of this type.
253  std::vector<const Type*> EltTys;
254  EltTys.push_back(StackEntryTy);
255  for (size_t I = 0; I != Roots.size(); I++)
256    EltTys.push_back(Roots[I].second->getAllocatedType());
257  Type *Ty = StructType::get(F.getContext(), EltTys);
258
259  std::string TypeName("gc_stackentry.");
260  TypeName += F.getName();
261  F.getParent()->addTypeName(TypeName, Ty);
262
263  return Ty;
264}
265
266/// doInitialization - If this module uses the GC intrinsics, find them now. If
267/// not, exit fast.
268bool ShadowStackGC::initializeCustomLowering(Module &M) {
269  // struct FrameMap {
270  //   int32_t NumRoots; // Number of roots in stack frame.
271  //   int32_t NumMeta;  // Number of metadata descriptors. May be < NumRoots.
272  //   void *Meta[];     // May be absent for roots without metadata.
273  // };
274  std::vector<const Type*> EltTys;
275  // 32 bits is ok up to a 32GB stack frame. :)
276  EltTys.push_back(Type::getInt32Ty(M.getContext()));
277  // Specifies length of variable length array.
278  EltTys.push_back(Type::getInt32Ty(M.getContext()));
279  StructType *FrameMapTy = StructType::get(M.getContext(), EltTys);
280  M.addTypeName("gc_map", FrameMapTy);
281  PointerType *FrameMapPtrTy = PointerType::getUnqual(FrameMapTy);
282
283  // struct StackEntry {
284  //   ShadowStackEntry *Next; // Caller's stack entry.
285  //   FrameMap *Map;          // Pointer to constant FrameMap.
286  //   void *Roots[];          // Stack roots (in-place array, so we pretend).
287  // };
288  OpaqueType *RecursiveTy = OpaqueType::get(M.getContext());
289
290  EltTys.clear();
291  EltTys.push_back(PointerType::getUnqual(RecursiveTy));
292  EltTys.push_back(FrameMapPtrTy);
293  PATypeHolder LinkTyH = StructType::get(M.getContext(), EltTys);
294
295  RecursiveTy->refineAbstractTypeTo(LinkTyH.get());
296  StackEntryTy = cast<StructType>(LinkTyH.get());
297  const PointerType *StackEntryPtrTy = PointerType::getUnqual(StackEntryTy);
298  M.addTypeName("gc_stackentry", LinkTyH.get());  // FIXME: Is this safe from
299                                                  //        a FunctionPass?
300
301  // Get the root chain if it already exists.
302  Head = M.getGlobalVariable("llvm_gc_root_chain");
303  if (!Head) {
304    // If the root chain does not exist, insert a new one with linkonce
305    // linkage!
306    Head = new GlobalVariable(M, StackEntryPtrTy, false,
307                              GlobalValue::LinkOnceAnyLinkage,
308                              Constant::getNullValue(StackEntryPtrTy),
309                              "llvm_gc_root_chain");
310  } else if (Head->hasExternalLinkage() && Head->isDeclaration()) {
311    Head->setInitializer(Constant::getNullValue(StackEntryPtrTy));
312    Head->setLinkage(GlobalValue::LinkOnceAnyLinkage);
313  }
314
315  return true;
316}
317
318bool ShadowStackGC::IsNullValue(Value *V) {
319  if (Constant *C = dyn_cast<Constant>(V))
320    return C->isNullValue();
321  return false;
322}
323
324void ShadowStackGC::CollectRoots(Function &F) {
325  // FIXME: Account for original alignment. Could fragment the root array.
326  //   Approach 1: Null initialize empty slots at runtime. Yuck.
327  //   Approach 2: Emit a map of the array instead of just a count.
328
329  assert(Roots.empty() && "Not cleaned up?");
330
331  SmallVector<std::pair<CallInst*, AllocaInst*>, 16> MetaRoots;
332
333  for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB)
334    for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E;)
335      if (IntrinsicInst *CI = dyn_cast<IntrinsicInst>(II++))
336        if (Function *F = CI->getCalledFunction())
337          if (F->getIntrinsicID() == Intrinsic::gcroot) {
338            std::pair<CallInst*, AllocaInst*> Pair = std::make_pair(
339              CI, cast<AllocaInst>(CI->getArgOperand(0)->stripPointerCasts()));
340            if (IsNullValue(CI->getArgOperand(1)))
341              Roots.push_back(Pair);
342            else
343              MetaRoots.push_back(Pair);
344          }
345
346  // Number roots with metadata (usually empty) at the beginning, so that the
347  // FrameMap::Meta array can be elided.
348  Roots.insert(Roots.begin(), MetaRoots.begin(), MetaRoots.end());
349}
350
351GetElementPtrInst *
352ShadowStackGC::CreateGEP(LLVMContext &Context, IRBuilder<> &B, Value *BasePtr,
353                         int Idx, int Idx2, const char *Name) {
354  Value *Indices[] = { ConstantInt::get(Type::getInt32Ty(Context), 0),
355                       ConstantInt::get(Type::getInt32Ty(Context), Idx),
356                       ConstantInt::get(Type::getInt32Ty(Context), Idx2) };
357  Value* Val = B.CreateGEP(BasePtr, Indices, Indices + 3, Name);
358
359  assert(isa<GetElementPtrInst>(Val) && "Unexpected folded constant");
360
361  return dyn_cast<GetElementPtrInst>(Val);
362}
363
364GetElementPtrInst *
365ShadowStackGC::CreateGEP(LLVMContext &Context, IRBuilder<> &B, Value *BasePtr,
366                         int Idx, const char *Name) {
367  Value *Indices[] = { ConstantInt::get(Type::getInt32Ty(Context), 0),
368                       ConstantInt::get(Type::getInt32Ty(Context), Idx) };
369  Value *Val = B.CreateGEP(BasePtr, Indices, Indices + 2, Name);
370
371  assert(isa<GetElementPtrInst>(Val) && "Unexpected folded constant");
372
373  return dyn_cast<GetElementPtrInst>(Val);
374}
375
376/// runOnFunction - Insert code to maintain the shadow stack.
377bool ShadowStackGC::performCustomLowering(Function &F) {
378  LLVMContext &Context = F.getContext();
379
380  // Find calls to llvm.gcroot.
381  CollectRoots(F);
382
383  // If there are no roots in this function, then there is no need to add a
384  // stack map entry for it.
385  if (Roots.empty())
386    return false;
387
388  // Build the constant map and figure the type of the shadow stack entry.
389  Value *FrameMap = GetFrameMap(F);
390  const Type *ConcreteStackEntryTy = GetConcreteStackEntryType(F);
391
392  // Build the shadow stack entry at the very start of the function.
393  BasicBlock::iterator IP = F.getEntryBlock().begin();
394  IRBuilder<> AtEntry(IP->getParent(), IP);
395
396  Instruction *StackEntry   = AtEntry.CreateAlloca(ConcreteStackEntryTy, 0,
397                                                   "gc_frame");
398
399  while (isa<AllocaInst>(IP)) ++IP;
400  AtEntry.SetInsertPoint(IP->getParent(), IP);
401
402  // Initialize the map pointer and load the current head of the shadow stack.
403  Instruction *CurrentHead  = AtEntry.CreateLoad(Head, "gc_currhead");
404  Instruction *EntryMapPtr  = CreateGEP(Context, AtEntry, StackEntry,
405                                        0,1,"gc_frame.map");
406                              AtEntry.CreateStore(FrameMap, EntryMapPtr);
407
408  // After all the allocas...
409  for (unsigned I = 0, E = Roots.size(); I != E; ++I) {
410    // For each root, find the corresponding slot in the aggregate...
411    Value *SlotPtr = CreateGEP(Context, AtEntry, StackEntry, 1 + I, "gc_root");
412
413    // And use it in lieu of the alloca.
414    AllocaInst *OriginalAlloca = Roots[I].second;
415    SlotPtr->takeName(OriginalAlloca);
416    OriginalAlloca->replaceAllUsesWith(SlotPtr);
417  }
418
419  // Move past the original stores inserted by GCStrategy::InitRoots. This isn't
420  // really necessary (the collector would never see the intermediate state at
421  // runtime), but it's nicer not to push the half-initialized entry onto the
422  // shadow stack.
423  while (isa<StoreInst>(IP)) ++IP;
424  AtEntry.SetInsertPoint(IP->getParent(), IP);
425
426  // Push the entry onto the shadow stack.
427  Instruction *EntryNextPtr = CreateGEP(Context, AtEntry,
428                                        StackEntry,0,0,"gc_frame.next");
429  Instruction *NewHeadVal   = CreateGEP(Context, AtEntry,
430                                        StackEntry, 0, "gc_newhead");
431  AtEntry.CreateStore(CurrentHead, EntryNextPtr);
432  AtEntry.CreateStore(NewHeadVal, Head);
433
434  // For each instruction that escapes...
435  EscapeEnumerator EE(F, "gc_cleanup");
436  while (IRBuilder<> *AtExit = EE.Next()) {
437    // Pop the entry from the shadow stack. Don't reuse CurrentHead from
438    // AtEntry, since that would make the value live for the entire function.
439    Instruction *EntryNextPtr2 = CreateGEP(Context, *AtExit, StackEntry, 0, 0,
440                                           "gc_frame.next");
441    Value *SavedHead = AtExit->CreateLoad(EntryNextPtr2, "gc_savedhead");
442                       AtExit->CreateStore(SavedHead, Head);
443  }
444
445  // Delete the original allocas (which are no longer used) and the intrinsic
446  // calls (which are no longer valid). Doing this last avoids invalidating
447  // iterators.
448  for (unsigned I = 0, E = Roots.size(); I != E; ++I) {
449    Roots[I].first->eraseFromParent();
450    Roots[I].second->eraseFromParent();
451  }
452
453  Roots.clear();
454  return true;
455}
456