ShadowStackGC.cpp revision 667d4b8de6dea70195ff12ef39a4deebffa2f5c7 
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
37using namespace llvm;
38
39namespace {
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(IRBuilder<> &B, Value *BasePtr,
66                                        int Idx1, const char *Name);
67    static GetElementPtrInst *CreateGEP(IRBuilder<> &B, Value *BasePtr,
68                                        int Idx1, int Idx2, const char *Name);
69  };
70
71}
72
73static GCRegistry::Add<ShadowStackGC>
74X("shadow-stack", "Very portable GC for uncooperative code generators");
75
76namespace {
77  /// EscapeEnumerator - This is a little algorithm to find all escape points
78  /// from a function so that "finally"-style code can be inserted. In addition
79  /// to finding the existing return and unwind instructions, it also (if
80  /// necessary) transforms any call instructions into invokes and sends them to
81  /// a landing pad.
82  ///
83  /// It's wrapped up in a state machine using the same transform C# uses for
84  /// 'yield return' enumerators, This transform allows it to be non-allocating.
85  class VISIBILITY_HIDDEN EscapeEnumerator {
86    Function &F;
87    const char *CleanupBBName;
88
89    // State.
90    int State;
91    Function::iterator StateBB, StateE;
92    IRBuilder<> Builder;
93
94  public:
95    EscapeEnumerator(Function &F, const char *N = "cleanup")
96      : F(F), CleanupBBName(N), State(0) {}
97
98    IRBuilder<> *Next() {
99      switch (State) {
100      default:
101        return 0;
102
103      case 0:
104        StateBB = F.begin();
105        StateE = F.end();
106        State = 1;
107
108      case 1:
109        // Find all 'return' and 'unwind' instructions.
110        while (StateBB != StateE) {
111          BasicBlock *CurBB = StateBB++;
112
113          // Branches and invokes do not escape, only unwind and return do.
114          TerminatorInst *TI = CurBB->getTerminator();
115          if (!isa<UnwindInst>(TI) && !isa<ReturnInst>(TI))
116            continue;
117
118          Builder.SetInsertPoint(TI->getParent(), TI);
119          return &Builder;
120        }
121
122        State = 2;
123
124        // Find all 'call' instructions.
125        SmallVector<Instruction*,16> Calls;
126        for (Function::iterator BB = F.begin(),
127                                E = F.end(); BB != E; ++BB)
128          for (BasicBlock::iterator II = BB->begin(),
129                                    EE = BB->end(); II != EE; ++II)
130            if (CallInst *CI = dyn_cast<CallInst>(II))
131              if (!CI->getCalledFunction() ||
132                  !CI->getCalledFunction()->getIntrinsicID())
133                Calls.push_back(CI);
134
135        if (Calls.empty())
136          return 0;
137
138        // Create a cleanup block.
139        BasicBlock *CleanupBB = BasicBlock::Create(CleanupBBName, &F);
140        UnwindInst *UI = new UnwindInst(CleanupBB);
141
142        // Transform the 'call' instructions into 'invoke's branching to the
143        // cleanup block. Go in reverse order to make prettier BB names.
144        SmallVector<Value*,16> Args;
145        for (unsigned I = Calls.size(); I != 0; ) {
146          CallInst *CI = cast<CallInst>(Calls[--I]);
147
148          // Split the basic block containing the function call.
149          BasicBlock *CallBB = CI->getParent();
150          BasicBlock *NewBB =
151            CallBB->splitBasicBlock(CI, CallBB->getName() + ".cont");
152
153          // Remove the unconditional branch inserted at the end of CallBB.
154          CallBB->getInstList().pop_back();
155          NewBB->getInstList().remove(CI);
156
157          // Create a new invoke instruction.
158          Args.clear();
159          Args.append(CI->op_begin() + 1, CI->op_end());
160
161          InvokeInst *II = InvokeInst::Create(CI->getOperand(0),
162                                              NewBB, CleanupBB,
163                                              Args.begin(), Args.end(),
164                                              CI->getName(), CallBB);
165          II->setCallingConv(CI->getCallingConv());
166          II->setAttributes(CI->getAttributes());
167          CI->replaceAllUsesWith(II);
168          delete CI;
169        }
170
171        Builder.SetInsertPoint(UI->getParent(), UI);
172        return &Builder;
173      }
174    }
175  };
176}
177
178// -----------------------------------------------------------------------------
179
180void llvm::linkShadowStackGC() { }
181
182ShadowStackGC::ShadowStackGC() : Head(0), StackEntryTy(0) {
183  InitRoots = true;
184  CustomRoots = true;
185}
186
187Constant *ShadowStackGC::GetFrameMap(Function &F) {
188  // doInitialization creates the abstract type of this value.
189
190  Type *VoidPtr = PointerType::getUnqual(Type::Int8Ty);
191
192  // Truncate the ShadowStackDescriptor if some metadata is null.
193  unsigned NumMeta = 0;
194  SmallVector<Constant*,16> Metadata;
195  for (unsigned I = 0; I != Roots.size(); ++I) {
196    Constant *C = cast<Constant>(Roots[I].first->getOperand(2));
197    if (!C->isNullValue())
198      NumMeta = I + 1;
199    Metadata.push_back(ConstantExpr::getBitCast(C, VoidPtr));
200  }
201
202  Constant *BaseElts[] = {
203    ConstantInt::get(Type::Int32Ty, Roots.size(), false),
204    ConstantInt::get(Type::Int32Ty, NumMeta, false),
205  };
206
207  Constant *DescriptorElts[] = {
208    ConstantStruct::get(BaseElts, 2),
209    ConstantArray::get(ArrayType::get(VoidPtr, NumMeta),
210                       Metadata.begin(), NumMeta)
211  };
212
213  Constant *FrameMap = ConstantStruct::get(DescriptorElts, 2);
214
215  std::string TypeName("gc_map.");
216  TypeName += utostr(NumMeta);
217  F.getParent()->addTypeName(TypeName, FrameMap->getType());
218
219  // FIXME: Is this actually dangerous as WritingAnLLVMPass.html claims? Seems
220  //        that, short of multithreaded LLVM, it should be safe; all that is
221  //        necessary is that a simple Module::iterator loop not be invalidated.
222  //        Appending to the GlobalVariable list is safe in that sense.
223  //
224  //        All of the output passes emit globals last. The ExecutionEngine
225  //        explicitly supports adding globals to the module after
226  //        initialization.
227  //
228  //        Still, if it isn't deemed acceptable, then this transformation needs
229  //        to be a ModulePass (which means it cannot be in the 'llc' pipeline
230  //        (which uses a FunctionPassManager (which segfaults (not asserts) if
231  //        provided a ModulePass))).
232  Constant *GV = new GlobalVariable(FrameMap->getType(), true,
233                                    GlobalVariable::InternalLinkage,
234                                    FrameMap, "__gc_" + F.getName(),
235                                    F.getParent());
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
242const 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(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.
259bool 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(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(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(StackEntryPtrTy, false,
296                              GlobalValue::LinkOnceAnyLinkage,
297                              Constant::getNullValue(StackEntryPtrTy),
298                              "llvm_gc_root_chain", &M);
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
307bool ShadowStackGC::IsNullValue(Value *V) {
308  if (Constant *C = dyn_cast<Constant>(V))
309    return C->isNullValue();
310  return false;
311}
312
313void 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
340GetElementPtrInst *
341ShadowStackGC::CreateGEP(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
353GetElementPtrInst *
354ShadowStackGC::CreateGEP(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.
366bool ShadowStackGC::performCustomLowering(Function &F) {
367  // Find calls to llvm.gcroot.
368  CollectRoots(F);
369
370  // If there are no roots in this function, then there is no need to add a
371  // stack map entry for it.
372  if (Roots.empty())
373    return false;
374
375  // Build the constant map and figure the type of the shadow stack entry.
376  Value *FrameMap = GetFrameMap(F);
377  const Type *ConcreteStackEntryTy = GetConcreteStackEntryType(F);
378
379  // Build the shadow stack entry at the very start of the function.
380  BasicBlock::iterator IP = F.getEntryBlock().begin();
381  IRBuilder<> AtEntry(IP->getParent(), IP);
382
383  Instruction *StackEntry   = AtEntry.CreateAlloca(ConcreteStackEntryTy, 0,
384                                                   "gc_frame");
385
386  while (isa<AllocaInst>(IP)) ++IP;
387  AtEntry.SetInsertPoint(IP->getParent(), IP);
388
389  // Initialize the map pointer and load the current head of the shadow stack.
390  Instruction *CurrentHead  = AtEntry.CreateLoad(Head, "gc_currhead");
391  Instruction *EntryMapPtr  = CreateGEP(AtEntry, StackEntry,0,1,"gc_frame.map");
392                              AtEntry.CreateStore(FrameMap, EntryMapPtr);
393
394  // After all the allocas...
395  for (unsigned I = 0, E = Roots.size(); I != E; ++I) {
396    // For each root, find the corresponding slot in the aggregate...
397    Value *SlotPtr = CreateGEP(AtEntry, StackEntry, 1 + I, "gc_root");
398
399    // And use it in lieu of the alloca.
400    AllocaInst *OriginalAlloca = Roots[I].second;
401    SlotPtr->takeName(OriginalAlloca);
402    OriginalAlloca->replaceAllUsesWith(SlotPtr);
403  }
404
405  // Move past the original stores inserted by GCStrategy::InitRoots. This isn't
406  // really necessary (the collector would never see the intermediate state at
407  // runtime), but it's nicer not to push the half-initialized entry onto the
408  // shadow stack.
409  while (isa<StoreInst>(IP)) ++IP;
410  AtEntry.SetInsertPoint(IP->getParent(), IP);
411
412  // Push the entry onto the shadow stack.
413  Instruction *EntryNextPtr = CreateGEP(AtEntry,StackEntry,0,0,"gc_frame.next");
414  Instruction *NewHeadVal   = CreateGEP(AtEntry,StackEntry, 0, "gc_newhead");
415                              AtEntry.CreateStore(CurrentHead, EntryNextPtr);
416                              AtEntry.CreateStore(NewHeadVal, Head);
417
418  // For each instruction that escapes...
419  EscapeEnumerator EE(F, "gc_cleanup");
420  while (IRBuilder<> *AtExit = EE.Next()) {
421    // Pop the entry from the shadow stack. Don't reuse CurrentHead from
422    // AtEntry, since that would make the value live for the entire function.
423    Instruction *EntryNextPtr2 = CreateGEP(*AtExit, StackEntry, 0, 0,
424                                           "gc_frame.next");
425    Value *SavedHead = AtExit->CreateLoad(EntryNextPtr2, "gc_savedhead");
426                       AtExit->CreateStore(SavedHead, Head);
427  }
428
429  // Delete the original allocas (which are no longer used) and the intrinsic
430  // calls (which are no longer valid). Doing this last avoids invalidating
431  // iterators.
432  for (unsigned I = 0, E = Roots.size(); I != E; ++I) {
433    Roots[I].first->eraseFromParent();
434    Roots[I].second->eraseFromParent();
435  }
436
437  Roots.clear();
438  return true;
439}
440