ShadowStackGC.cpp revision 10c6d12a9fd4dab411091f64db4db69670b88850 
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    StructType *StackEntryTy;
49    StructType *FrameMapTy;
50
51    /// Roots - GC roots in the current function. Each is a pair of the
52    /// intrinsic call and its corresponding alloca.
53    std::vector<std::pair<CallInst*,AllocaInst*> > Roots;
54
55  public:
56    ShadowStackGC();
57
58    bool initializeCustomLowering(Module &M);
59    bool performCustomLowering(Function &F);
60
61  private:
62    bool IsNullValue(Value *V);
63    Constant *GetFrameMap(Function &F);
64    Type* GetConcreteStackEntryType(Function &F);
65    void CollectRoots(Function &F);
66    static GetElementPtrInst *CreateGEP(LLVMContext &Context,
67                                        IRBuilder<> &B, Value *BasePtr,
68                                        int Idx1, const char *Name);
69    static GetElementPtrInst *CreateGEP(LLVMContext &Context,
70                                        IRBuilder<> &B, Value *BasePtr,
71                                        int Idx1, int Idx2, const char *Name);
72  };
73
74}
75
76static GCRegistry::Add<ShadowStackGC>
77X("shadow-stack", "Very portable GC for uncooperative code generators");
78
79namespace {
80  /// EscapeEnumerator - This is a little algorithm to find all escape points
81  /// from a function so that "finally"-style code can be inserted. In addition
82  /// to finding the existing return and unwind instructions, it also (if
83  /// necessary) transforms any call instructions into invokes and sends them to
84  /// a landing pad.
85  ///
86  /// It's wrapped up in a state machine using the same transform C# uses for
87  /// 'yield return' enumerators, This transform allows it to be non-allocating.
88  class EscapeEnumerator {
89    Function &F;
90    const char *CleanupBBName;
91
92    // State.
93    int State;
94    Function::iterator StateBB, StateE;
95    IRBuilder<> Builder;
96
97  public:
98    EscapeEnumerator(Function &F, const char *N = "cleanup")
99      : F(F), CleanupBBName(N), State(0), Builder(F.getContext()) {}
100
101    IRBuilder<> *Next() {
102      switch (State) {
103      default:
104        return 0;
105
106      case 0:
107        StateBB = F.begin();
108        StateE = F.end();
109        State = 1;
110
111      case 1:
112        // Find all 'return' and 'unwind' instructions.
113        while (StateBB != StateE) {
114          BasicBlock *CurBB = StateBB++;
115
116          // Branches and invokes do not escape, only unwind and return do.
117          TerminatorInst *TI = CurBB->getTerminator();
118          if (!isa<UnwindInst>(TI) && !isa<ReturnInst>(TI))
119            continue;
120
121          Builder.SetInsertPoint(TI->getParent(), TI);
122          return &Builder;
123        }
124
125        State = 2;
126
127        // Find all 'call' instructions.
128        SmallVector<Instruction*,16> Calls;
129        for (Function::iterator BB = F.begin(),
130                                E = F.end(); BB != E; ++BB)
131          for (BasicBlock::iterator II = BB->begin(),
132                                    EE = BB->end(); II != EE; ++II)
133            if (CallInst *CI = dyn_cast<CallInst>(II))
134              if (!CI->getCalledFunction() ||
135                  !CI->getCalledFunction()->getIntrinsicID())
136                Calls.push_back(CI);
137
138        if (Calls.empty())
139          return 0;
140
141        // Create a cleanup block.
142        BasicBlock *CleanupBB = BasicBlock::Create(F.getContext(),
143                                                   CleanupBBName, &F);
144        UnwindInst *UI = new UnwindInst(F.getContext(), CleanupBB);
145
146        // Transform the 'call' instructions into 'invoke's branching to the
147        // cleanup block. Go in reverse order to make prettier BB names.
148        SmallVector<Value*,16> Args;
149        for (unsigned I = Calls.size(); I != 0; ) {
150          CallInst *CI = cast<CallInst>(Calls[--I]);
151
152          // Split the basic block containing the function call.
153          BasicBlock *CallBB = CI->getParent();
154          BasicBlock *NewBB =
155            CallBB->splitBasicBlock(CI, CallBB->getName() + ".cont");
156
157          // Remove the unconditional branch inserted at the end of CallBB.
158          CallBB->getInstList().pop_back();
159          NewBB->getInstList().remove(CI);
160
161          // Create a new invoke instruction.
162          Args.clear();
163          CallSite CS(CI);
164          Args.append(CS.arg_begin(), CS.arg_end());
165
166          InvokeInst *II = InvokeInst::Create(CI->getCalledValue(),
167                                              NewBB, CleanupBB,
168                                              Args, 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  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  Metadata.resize(NumMeta);
205
206  Type *Int32Ty = Type::getInt32Ty(F.getContext());
207
208  Constant *BaseElts[] = {
209    ConstantInt::get(Int32Ty, Roots.size(), false),
210    ConstantInt::get(Int32Ty, NumMeta, false),
211  };
212
213  Constant *DescriptorElts[] = {
214    ConstantStruct::get(FrameMapTy, BaseElts),
215    ConstantArray::get(ArrayType::get(VoidPtr, NumMeta), Metadata)
216  };
217
218  Type *EltTys[] = { DescriptorElts[0]->getType(),DescriptorElts[1]->getType()};
219  StructType *STy = StructType::createNamed("gc_map."+utostr(NumMeta), EltTys);
220
221  Constant *FrameMap = ConstantStruct::get(STy, DescriptorElts);
222
223  // FIXME: Is this actually dangerous as WritingAnLLVMPass.html claims? Seems
224  //        that, short of multithreaded LLVM, it should be safe; all that is
225  //        necessary is that a simple Module::iterator loop not be invalidated.
226  //        Appending to the GlobalVariable list is safe in that sense.
227  //
228  //        All of the output passes emit globals last. The ExecutionEngine
229  //        explicitly supports adding globals to the module after
230  //        initialization.
231  //
232  //        Still, if it isn't deemed acceptable, then this transformation needs
233  //        to be a ModulePass (which means it cannot be in the 'llc' pipeline
234  //        (which uses a FunctionPassManager (which segfaults (not asserts) if
235  //        provided a ModulePass))).
236  Constant *GV = new GlobalVariable(*F.getParent(), FrameMap->getType(), true,
237                                    GlobalVariable::InternalLinkage,
238                                    FrameMap, "__gc_" + F.getName());
239
240  Constant *GEPIndices[2] = {
241                          ConstantInt::get(Type::getInt32Ty(F.getContext()), 0),
242                          ConstantInt::get(Type::getInt32Ty(F.getContext()), 0)
243                          };
244  return ConstantExpr::getGetElementPtr(GV, GEPIndices);
245}
246
247Type* ShadowStackGC::GetConcreteStackEntryType(Function &F) {
248  // doInitialization creates the generic version of this type.
249  std::vector<Type*> EltTys;
250  EltTys.push_back(StackEntryTy);
251  for (size_t I = 0; I != Roots.size(); I++)
252    EltTys.push_back(Roots[I].second->getAllocatedType());
253
254  return StructType::createNamed("gc_stackentry."+F.getName().str(), EltTys);
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<Type*> EltTys;
266  // 32 bits is ok up to a 32GB stack frame. :)
267  EltTys.push_back(Type::getInt32Ty(M.getContext()));
268  // Specifies length of variable length array.
269  EltTys.push_back(Type::getInt32Ty(M.getContext()));
270  FrameMapTy = StructType::createNamed("gc_map", EltTys);
271  PointerType *FrameMapPtrTy = PointerType::getUnqual(FrameMapTy);
272
273  // struct StackEntry {
274  //   ShadowStackEntry *Next; // Caller's stack entry.
275  //   FrameMap *Map;          // Pointer to constant FrameMap.
276  //   void *Roots[];          // Stack roots (in-place array, so we pretend).
277  // };
278
279  StackEntryTy = StructType::createNamed(M.getContext(), "gc_stackentry");
280
281  EltTys.clear();
282  EltTys.push_back(PointerType::getUnqual(StackEntryTy));
283  EltTys.push_back(FrameMapPtrTy);
284  StackEntryTy->setBody(EltTys);
285  PointerType *StackEntryPtrTy = PointerType::getUnqual(StackEntryTy);
286
287  // Get the root chain if it already exists.
288  Head = M.getGlobalVariable("llvm_gc_root_chain");
289  if (!Head) {
290    // If the root chain does not exist, insert a new one with linkonce
291    // linkage!
292    Head = new GlobalVariable(M, StackEntryPtrTy, false,
293                              GlobalValue::LinkOnceAnyLinkage,
294                              Constant::getNullValue(StackEntryPtrTy),
295                              "llvm_gc_root_chain");
296  } else if (Head->hasExternalLinkage() && Head->isDeclaration()) {
297    Head->setInitializer(Constant::getNullValue(StackEntryPtrTy));
298    Head->setLinkage(GlobalValue::LinkOnceAnyLinkage);
299  }
300
301  return true;
302}
303
304bool ShadowStackGC::IsNullValue(Value *V) {
305  if (Constant *C = dyn_cast<Constant>(V))
306    return C->isNullValue();
307  return false;
308}
309
310void ShadowStackGC::CollectRoots(Function &F) {
311  // FIXME: Account for original alignment. Could fragment the root array.
312  //   Approach 1: Null initialize empty slots at runtime. Yuck.
313  //   Approach 2: Emit a map of the array instead of just a count.
314
315  assert(Roots.empty() && "Not cleaned up?");
316
317  SmallVector<std::pair<CallInst*, AllocaInst*>, 16> MetaRoots;
318
319  for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB)
320    for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E;)
321      if (IntrinsicInst *CI = dyn_cast<IntrinsicInst>(II++))
322        if (Function *F = CI->getCalledFunction())
323          if (F->getIntrinsicID() == Intrinsic::gcroot) {
324            std::pair<CallInst*, AllocaInst*> Pair = std::make_pair(
325              CI, cast<AllocaInst>(CI->getArgOperand(0)->stripPointerCasts()));
326            if (IsNullValue(CI->getArgOperand(1)))
327              Roots.push_back(Pair);
328            else
329              MetaRoots.push_back(Pair);
330          }
331
332  // Number roots with metadata (usually empty) at the beginning, so that the
333  // FrameMap::Meta array can be elided.
334  Roots.insert(Roots.begin(), MetaRoots.begin(), MetaRoots.end());
335}
336
337GetElementPtrInst *
338ShadowStackGC::CreateGEP(LLVMContext &Context, IRBuilder<> &B, Value *BasePtr,
339                         int Idx, int Idx2, const char *Name) {
340  Value *Indices[] = { ConstantInt::get(Type::getInt32Ty(Context), 0),
341                       ConstantInt::get(Type::getInt32Ty(Context), Idx),
342                       ConstantInt::get(Type::getInt32Ty(Context), Idx2) };
343  Value* Val = B.CreateGEP(BasePtr, Indices, Name);
344
345  assert(isa<GetElementPtrInst>(Val) && "Unexpected folded constant");
346
347  return dyn_cast<GetElementPtrInst>(Val);
348}
349
350GetElementPtrInst *
351ShadowStackGC::CreateGEP(LLVMContext &Context, IRBuilder<> &B, Value *BasePtr,
352                         int Idx, const char *Name) {
353  Value *Indices[] = { ConstantInt::get(Type::getInt32Ty(Context), 0),
354                       ConstantInt::get(Type::getInt32Ty(Context), Idx) };
355  Value *Val = B.CreateGEP(BasePtr, Indices, Name);
356
357  assert(isa<GetElementPtrInst>(Val) && "Unexpected folded constant");
358
359  return dyn_cast<GetElementPtrInst>(Val);
360}
361
362/// runOnFunction - Insert code to maintain the shadow stack.
363bool ShadowStackGC::performCustomLowering(Function &F) {
364  LLVMContext &Context = F.getContext();
365
366  // Find calls to llvm.gcroot.
367  CollectRoots(F);
368
369  // If there are no roots in this function, then there is no need to add a
370  // stack map entry for it.
371  if (Roots.empty())
372    return false;
373
374  // Build the constant map and figure the type of the shadow stack entry.
375  Value *FrameMap = GetFrameMap(F);
376  Type *ConcreteStackEntryTy = GetConcreteStackEntryType(F);
377
378  // Build the shadow stack entry at the very start of the function.
379  BasicBlock::iterator IP = F.getEntryBlock().begin();
380  IRBuilder<> AtEntry(IP->getParent(), IP);
381
382  Instruction *StackEntry   = AtEntry.CreateAlloca(ConcreteStackEntryTy, 0,
383                                                   "gc_frame");
384
385  while (isa<AllocaInst>(IP)) ++IP;
386  AtEntry.SetInsertPoint(IP->getParent(), IP);
387
388  // Initialize the map pointer and load the current head of the shadow stack.
389  Instruction *CurrentHead  = AtEntry.CreateLoad(Head, "gc_currhead");
390  Instruction *EntryMapPtr  = CreateGEP(Context, AtEntry, StackEntry,
391                                        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(Context, 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(Context, AtEntry,
414                                        StackEntry,0,0,"gc_frame.next");
415  Instruction *NewHeadVal   = CreateGEP(Context, AtEntry,
416                                        StackEntry, 0, "gc_newhead");
417  AtEntry.CreateStore(CurrentHead, EntryNextPtr);
418  AtEntry.CreateStore(NewHeadVal, Head);
419
420  // For each instruction that escapes...
421  EscapeEnumerator EE(F, "gc_cleanup");
422  while (IRBuilder<> *AtExit = EE.Next()) {
423    // Pop the entry from the shadow stack. Don't reuse CurrentHead from
424    // AtEntry, since that would make the value live for the entire function.
425    Instruction *EntryNextPtr2 = CreateGEP(Context, *AtExit, StackEntry, 0, 0,
426                                           "gc_frame.next");
427    Value *SavedHead = AtExit->CreateLoad(EntryNextPtr2, "gc_savedhead");
428                       AtExit->CreateStore(SavedHead, Head);
429  }
430
431  // Delete the original allocas (which are no longer used) and the intrinsic
432  // calls (which are no longer valid). Doing this last avoids invalidating
433  // iterators.
434  for (unsigned I = 0, E = Roots.size(); I != E; ++I) {
435    Roots[I].first->eraseFromParent();
436    Roots[I].second->eraseFromParent();
437  }
438
439  Roots.clear();
440  return true;
441}
442