1 //===-- ShadowStackGCLowering.cpp - Custom lowering for shadow-stack gc ---===//
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 contains the custom lowering code required by the shadow-stack GC
11 // strategy.
12 //
13 // This pass implements the code transformation described in this paper:
14 // "Accurate Garbage Collection in an Uncooperative Environment"
15 // Fergus Henderson, ISMM, 2002
16 //
17 //===----------------------------------------------------------------------===//
18
19 #include "llvm/CodeGen/Passes.h"
20 #include "llvm/ADT/StringExtras.h"
21 #include "llvm/CodeGen/GCStrategy.h"
22 #include "llvm/IR/CallSite.h"
23 #include "llvm/IR/IRBuilder.h"
24 #include "llvm/IR/IntrinsicInst.h"
25 #include "llvm/IR/Module.h"
26
27 using namespace llvm;
28
29 #define DEBUG_TYPE "shadowstackgclowering"
30
31 namespace {
32
33 class ShadowStackGCLowering : public FunctionPass {
34 /// RootChain - This is the global linked-list that contains the chain of GC
35 /// roots.
36 GlobalVariable *Head;
37
38 /// StackEntryTy - Abstract type of a link in the shadow stack.
39 ///
40 StructType *StackEntryTy;
41 StructType *FrameMapTy;
42
43 /// Roots - GC roots in the current function. Each is a pair of the
44 /// intrinsic call and its corresponding alloca.
45 std::vector<std::pair<CallInst *, AllocaInst *>> Roots;
46
47 public:
48 static char ID;
49 ShadowStackGCLowering();
50
51 bool doInitialization(Module &M) override;
52 bool runOnFunction(Function &F) override;
53
54 private:
55 bool IsNullValue(Value *V);
56 Constant *GetFrameMap(Function &F);
57 Type *GetConcreteStackEntryType(Function &F);
58 void CollectRoots(Function &F);
59 static GetElementPtrInst *CreateGEP(LLVMContext &Context, IRBuilder<> &B,
60 Type *Ty, Value *BasePtr, int Idx1,
61 const char *Name);
62 static GetElementPtrInst *CreateGEP(LLVMContext &Context, IRBuilder<> &B,
63 Type *Ty, Value *BasePtr, int Idx1, int Idx2,
64 const char *Name);
65 };
66 }
67
68 INITIALIZE_PASS_BEGIN(ShadowStackGCLowering, "shadow-stack-gc-lowering",
69 "Shadow Stack GC Lowering", false, false)
INITIALIZE_PASS_DEPENDENCY(GCModuleInfo)70 INITIALIZE_PASS_DEPENDENCY(GCModuleInfo)
71 INITIALIZE_PASS_END(ShadowStackGCLowering, "shadow-stack-gc-lowering",
72 "Shadow Stack GC Lowering", false, false)
73
74 FunctionPass *llvm::createShadowStackGCLoweringPass() { return new ShadowStackGCLowering(); }
75
76 char ShadowStackGCLowering::ID = 0;
77
ShadowStackGCLowering()78 ShadowStackGCLowering::ShadowStackGCLowering()
79 : FunctionPass(ID), Head(nullptr), StackEntryTy(nullptr),
80 FrameMapTy(nullptr) {
81 initializeShadowStackGCLoweringPass(*PassRegistry::getPassRegistry());
82 }
83
84 namespace {
85 /// EscapeEnumerator - This is a little algorithm to find all escape points
86 /// from a function so that "finally"-style code can be inserted. In addition
87 /// to finding the existing return and unwind instructions, it also (if
88 /// necessary) transforms any call instructions into invokes and sends them to
89 /// a landing pad.
90 ///
91 /// It's wrapped up in a state machine using the same transform C# uses for
92 /// 'yield return' enumerators, This transform allows it to be non-allocating.
93 class EscapeEnumerator {
94 Function &F;
95 const char *CleanupBBName;
96
97 // State.
98 int State;
99 Function::iterator StateBB, StateE;
100 IRBuilder<> Builder;
101
102 public:
EscapeEnumerator(Function & F,const char * N="cleanup")103 EscapeEnumerator(Function &F, const char *N = "cleanup")
104 : F(F), CleanupBBName(N), State(0), Builder(F.getContext()) {}
105
Next()106 IRBuilder<> *Next() {
107 switch (State) {
108 default:
109 return nullptr;
110
111 case 0:
112 StateBB = F.begin();
113 StateE = F.end();
114 State = 1;
115
116 case 1:
117 // Find all 'return', 'resume', and 'unwind' instructions.
118 while (StateBB != StateE) {
119 BasicBlock *CurBB = &*StateBB++;
120
121 // Branches and invokes do not escape, only unwind, resume, and return
122 // do.
123 TerminatorInst *TI = CurBB->getTerminator();
124 if (!isa<ReturnInst>(TI) && !isa<ResumeInst>(TI))
125 continue;
126
127 Builder.SetInsertPoint(TI);
128 return &Builder;
129 }
130
131 State = 2;
132
133 // Find all 'call' instructions.
134 SmallVector<Instruction *, 16> Calls;
135 for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB)
136 for (BasicBlock::iterator II = BB->begin(), EE = BB->end(); II != EE;
137 ++II)
138 if (CallInst *CI = dyn_cast<CallInst>(II))
139 if (!CI->getCalledFunction() ||
140 !CI->getCalledFunction()->getIntrinsicID())
141 Calls.push_back(CI);
142
143 if (Calls.empty())
144 return nullptr;
145
146 // Create a cleanup block.
147 LLVMContext &C = F.getContext();
148 BasicBlock *CleanupBB = BasicBlock::Create(C, CleanupBBName, &F);
149 Type *ExnTy =
150 StructType::get(Type::getInt8PtrTy(C), Type::getInt32Ty(C), nullptr);
151 if (!F.hasPersonalityFn()) {
152 Constant *PersFn = F.getParent()->getOrInsertFunction(
153 "__gcc_personality_v0",
154 FunctionType::get(Type::getInt32Ty(C), true));
155 F.setPersonalityFn(PersFn);
156 }
157 LandingPadInst *LPad =
158 LandingPadInst::Create(ExnTy, 1, "cleanup.lpad", CleanupBB);
159 LPad->setCleanup(true);
160 ResumeInst *RI = ResumeInst::Create(LPad, CleanupBB);
161
162 // Transform the 'call' instructions into 'invoke's branching to the
163 // cleanup block. Go in reverse order to make prettier BB names.
164 SmallVector<Value *, 16> Args;
165 for (unsigned I = Calls.size(); I != 0;) {
166 CallInst *CI = cast<CallInst>(Calls[--I]);
167
168 // Split the basic block containing the function call.
169 BasicBlock *CallBB = CI->getParent();
170 BasicBlock *NewBB = CallBB->splitBasicBlock(
171 CI->getIterator(), CallBB->getName() + ".cont");
172
173 // Remove the unconditional branch inserted at the end of CallBB.
174 CallBB->getInstList().pop_back();
175 NewBB->getInstList().remove(CI);
176
177 // Create a new invoke instruction.
178 Args.clear();
179 CallSite CS(CI);
180 Args.append(CS.arg_begin(), CS.arg_end());
181
182 InvokeInst *II =
183 InvokeInst::Create(CI->getCalledValue(), NewBB, CleanupBB, Args,
184 CI->getName(), CallBB);
185 II->setCallingConv(CI->getCallingConv());
186 II->setAttributes(CI->getAttributes());
187 CI->replaceAllUsesWith(II);
188 delete CI;
189 }
190
191 Builder.SetInsertPoint(RI);
192 return &Builder;
193 }
194 }
195 };
196 }
197
198
GetFrameMap(Function & F)199 Constant *ShadowStackGCLowering::GetFrameMap(Function &F) {
200 // doInitialization creates the abstract type of this value.
201 Type *VoidPtr = Type::getInt8PtrTy(F.getContext());
202
203 // Truncate the ShadowStackDescriptor if some metadata is null.
204 unsigned NumMeta = 0;
205 SmallVector<Constant *, 16> Metadata;
206 for (unsigned I = 0; I != Roots.size(); ++I) {
207 Constant *C = cast<Constant>(Roots[I].first->getArgOperand(1));
208 if (!C->isNullValue())
209 NumMeta = I + 1;
210 Metadata.push_back(ConstantExpr::getBitCast(C, VoidPtr));
211 }
212 Metadata.resize(NumMeta);
213
214 Type *Int32Ty = Type::getInt32Ty(F.getContext());
215
216 Constant *BaseElts[] = {
217 ConstantInt::get(Int32Ty, Roots.size(), false),
218 ConstantInt::get(Int32Ty, NumMeta, false),
219 };
220
221 Constant *DescriptorElts[] = {
222 ConstantStruct::get(FrameMapTy, BaseElts),
223 ConstantArray::get(ArrayType::get(VoidPtr, NumMeta), Metadata)};
224
225 Type *EltTys[] = {DescriptorElts[0]->getType(), DescriptorElts[1]->getType()};
226 StructType *STy = StructType::create(EltTys, "gc_map." + utostr(NumMeta));
227
228 Constant *FrameMap = ConstantStruct::get(STy, DescriptorElts);
229
230 // FIXME: Is this actually dangerous as WritingAnLLVMPass.html claims? Seems
231 // that, short of multithreaded LLVM, it should be safe; all that is
232 // necessary is that a simple Module::iterator loop not be invalidated.
233 // Appending to the GlobalVariable list is safe in that sense.
234 //
235 // All of the output passes emit globals last. The ExecutionEngine
236 // explicitly supports adding globals to the module after
237 // initialization.
238 //
239 // Still, if it isn't deemed acceptable, then this transformation needs
240 // to be a ModulePass (which means it cannot be in the 'llc' pipeline
241 // (which uses a FunctionPassManager (which segfaults (not asserts) if
242 // provided a ModulePass))).
243 Constant *GV = new GlobalVariable(*F.getParent(), FrameMap->getType(), true,
244 GlobalVariable::InternalLinkage, FrameMap,
245 "__gc_" + F.getName());
246
247 Constant *GEPIndices[2] = {
248 ConstantInt::get(Type::getInt32Ty(F.getContext()), 0),
249 ConstantInt::get(Type::getInt32Ty(F.getContext()), 0)};
250 return ConstantExpr::getGetElementPtr(FrameMap->getType(), GV, GEPIndices);
251 }
252
GetConcreteStackEntryType(Function & F)253 Type *ShadowStackGCLowering::GetConcreteStackEntryType(Function &F) {
254 // doInitialization creates the generic version of this type.
255 std::vector<Type *> EltTys;
256 EltTys.push_back(StackEntryTy);
257 for (size_t I = 0; I != Roots.size(); I++)
258 EltTys.push_back(Roots[I].second->getAllocatedType());
259
260 return StructType::create(EltTys, ("gc_stackentry." + F.getName()).str());
261 }
262
263 /// doInitialization - If this module uses the GC intrinsics, find them now. If
264 /// not, exit fast.
doInitialization(Module & M)265 bool ShadowStackGCLowering::doInitialization(Module &M) {
266 bool Active = false;
267 for (Function &F : M) {
268 if (F.hasGC() && F.getGC() == std::string("shadow-stack")) {
269 Active = true;
270 break;
271 }
272 }
273 if (!Active)
274 return false;
275
276 // struct FrameMap {
277 // int32_t NumRoots; // Number of roots in stack frame.
278 // int32_t NumMeta; // Number of metadata descriptors. May be < NumRoots.
279 // void *Meta[]; // May be absent for roots without metadata.
280 // };
281 std::vector<Type *> EltTys;
282 // 32 bits is ok up to a 32GB stack frame. :)
283 EltTys.push_back(Type::getInt32Ty(M.getContext()));
284 // Specifies length of variable length array.
285 EltTys.push_back(Type::getInt32Ty(M.getContext()));
286 FrameMapTy = StructType::create(EltTys, "gc_map");
287 PointerType *FrameMapPtrTy = PointerType::getUnqual(FrameMapTy);
288
289 // struct StackEntry {
290 // ShadowStackEntry *Next; // Caller's stack entry.
291 // FrameMap *Map; // Pointer to constant FrameMap.
292 // void *Roots[]; // Stack roots (in-place array, so we pretend).
293 // };
294
295 StackEntryTy = StructType::create(M.getContext(), "gc_stackentry");
296
297 EltTys.clear();
298 EltTys.push_back(PointerType::getUnqual(StackEntryTy));
299 EltTys.push_back(FrameMapPtrTy);
300 StackEntryTy->setBody(EltTys);
301 PointerType *StackEntryPtrTy = PointerType::getUnqual(StackEntryTy);
302
303 // Get the root chain if it already exists.
304 Head = M.getGlobalVariable("llvm_gc_root_chain");
305 if (!Head) {
306 // If the root chain does not exist, insert a new one with linkonce
307 // linkage!
308 Head = new GlobalVariable(
309 M, StackEntryPtrTy, false, GlobalValue::LinkOnceAnyLinkage,
310 Constant::getNullValue(StackEntryPtrTy), "llvm_gc_root_chain");
311 } else if (Head->hasExternalLinkage() && Head->isDeclaration()) {
312 Head->setInitializer(Constant::getNullValue(StackEntryPtrTy));
313 Head->setLinkage(GlobalValue::LinkOnceAnyLinkage);
314 }
315
316 return true;
317 }
318
IsNullValue(Value * V)319 bool ShadowStackGCLowering::IsNullValue(Value *V) {
320 if (Constant *C = dyn_cast<Constant>(V))
321 return C->isNullValue();
322 return false;
323 }
324
CollectRoots(Function & F)325 void ShadowStackGCLowering::CollectRoots(Function &F) {
326 // FIXME: Account for original alignment. Could fragment the root array.
327 // Approach 1: Null initialize empty slots at runtime. Yuck.
328 // Approach 2: Emit a map of the array instead of just a count.
329
330 assert(Roots.empty() && "Not cleaned up?");
331
332 SmallVector<std::pair<CallInst *, AllocaInst *>, 16> MetaRoots;
333
334 for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB)
335 for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E;)
336 if (IntrinsicInst *CI = dyn_cast<IntrinsicInst>(II++))
337 if (Function *F = CI->getCalledFunction())
338 if (F->getIntrinsicID() == Intrinsic::gcroot) {
339 std::pair<CallInst *, AllocaInst *> Pair = std::make_pair(
340 CI,
341 cast<AllocaInst>(CI->getArgOperand(0)->stripPointerCasts()));
342 if (IsNullValue(CI->getArgOperand(1)))
343 Roots.push_back(Pair);
344 else
345 MetaRoots.push_back(Pair);
346 }
347
348 // Number roots with metadata (usually empty) at the beginning, so that the
349 // FrameMap::Meta array can be elided.
350 Roots.insert(Roots.begin(), MetaRoots.begin(), MetaRoots.end());
351 }
352
CreateGEP(LLVMContext & Context,IRBuilder<> & B,Type * Ty,Value * BasePtr,int Idx,int Idx2,const char * Name)353 GetElementPtrInst *ShadowStackGCLowering::CreateGEP(LLVMContext &Context,
354 IRBuilder<> &B, Type *Ty,
355 Value *BasePtr, int Idx,
356 int Idx2,
357 const char *Name) {
358 Value *Indices[] = {ConstantInt::get(Type::getInt32Ty(Context), 0),
359 ConstantInt::get(Type::getInt32Ty(Context), Idx),
360 ConstantInt::get(Type::getInt32Ty(Context), Idx2)};
361 Value *Val = B.CreateGEP(Ty, BasePtr, Indices, Name);
362
363 assert(isa<GetElementPtrInst>(Val) && "Unexpected folded constant");
364
365 return dyn_cast<GetElementPtrInst>(Val);
366 }
367
CreateGEP(LLVMContext & Context,IRBuilder<> & B,Type * Ty,Value * BasePtr,int Idx,const char * Name)368 GetElementPtrInst *ShadowStackGCLowering::CreateGEP(LLVMContext &Context,
369 IRBuilder<> &B, Type *Ty, Value *BasePtr,
370 int Idx, const char *Name) {
371 Value *Indices[] = {ConstantInt::get(Type::getInt32Ty(Context), 0),
372 ConstantInt::get(Type::getInt32Ty(Context), Idx)};
373 Value *Val = B.CreateGEP(Ty, BasePtr, Indices, Name);
374
375 assert(isa<GetElementPtrInst>(Val) && "Unexpected folded constant");
376
377 return dyn_cast<GetElementPtrInst>(Val);
378 }
379
380 /// runOnFunction - Insert code to maintain the shadow stack.
runOnFunction(Function & F)381 bool ShadowStackGCLowering::runOnFunction(Function &F) {
382 // Quick exit for functions that do not use the shadow stack GC.
383 if (!F.hasGC() ||
384 F.getGC() != std::string("shadow-stack"))
385 return false;
386
387 LLVMContext &Context = F.getContext();
388
389 // Find calls to llvm.gcroot.
390 CollectRoots(F);
391
392 // If there are no roots in this function, then there is no need to add a
393 // stack map entry for it.
394 if (Roots.empty())
395 return false;
396
397 // Build the constant map and figure the type of the shadow stack entry.
398 Value *FrameMap = GetFrameMap(F);
399 Type *ConcreteStackEntryTy = GetConcreteStackEntryType(F);
400
401 // Build the shadow stack entry at the very start of the function.
402 BasicBlock::iterator IP = F.getEntryBlock().begin();
403 IRBuilder<> AtEntry(IP->getParent(), IP);
404
405 Instruction *StackEntry =
406 AtEntry.CreateAlloca(ConcreteStackEntryTy, nullptr, "gc_frame");
407
408 while (isa<AllocaInst>(IP))
409 ++IP;
410 AtEntry.SetInsertPoint(IP->getParent(), IP);
411
412 // Initialize the map pointer and load the current head of the shadow stack.
413 Instruction *CurrentHead = AtEntry.CreateLoad(Head, "gc_currhead");
414 Instruction *EntryMapPtr = CreateGEP(Context, AtEntry, ConcreteStackEntryTy,
415 StackEntry, 0, 1, "gc_frame.map");
416 AtEntry.CreateStore(FrameMap, EntryMapPtr);
417
418 // After all the allocas...
419 for (unsigned I = 0, E = Roots.size(); I != E; ++I) {
420 // For each root, find the corresponding slot in the aggregate...
421 Value *SlotPtr = CreateGEP(Context, AtEntry, ConcreteStackEntryTy,
422 StackEntry, 1 + I, "gc_root");
423
424 // And use it in lieu of the alloca.
425 AllocaInst *OriginalAlloca = Roots[I].second;
426 SlotPtr->takeName(OriginalAlloca);
427 OriginalAlloca->replaceAllUsesWith(SlotPtr);
428 }
429
430 // Move past the original stores inserted by GCStrategy::InitRoots. This isn't
431 // really necessary (the collector would never see the intermediate state at
432 // runtime), but it's nicer not to push the half-initialized entry onto the
433 // shadow stack.
434 while (isa<StoreInst>(IP))
435 ++IP;
436 AtEntry.SetInsertPoint(IP->getParent(), IP);
437
438 // Push the entry onto the shadow stack.
439 Instruction *EntryNextPtr = CreateGEP(Context, AtEntry, ConcreteStackEntryTy,
440 StackEntry, 0, 0, "gc_frame.next");
441 Instruction *NewHeadVal = CreateGEP(Context, AtEntry, ConcreteStackEntryTy,
442 StackEntry, 0, "gc_newhead");
443 AtEntry.CreateStore(CurrentHead, EntryNextPtr);
444 AtEntry.CreateStore(NewHeadVal, Head);
445
446 // For each instruction that escapes...
447 EscapeEnumerator EE(F, "gc_cleanup");
448 while (IRBuilder<> *AtExit = EE.Next()) {
449 // Pop the entry from the shadow stack. Don't reuse CurrentHead from
450 // AtEntry, since that would make the value live for the entire function.
451 Instruction *EntryNextPtr2 =
452 CreateGEP(Context, *AtExit, ConcreteStackEntryTy, StackEntry, 0, 0,
453 "gc_frame.next");
454 Value *SavedHead = AtExit->CreateLoad(EntryNextPtr2, "gc_savedhead");
455 AtExit->CreateStore(SavedHead, Head);
456 }
457
458 // Delete the original allocas (which are no longer used) and the intrinsic
459 // calls (which are no longer valid). Doing this last avoids invalidating
460 // iterators.
461 for (unsigned I = 0, E = Roots.size(); I != E; ++I) {
462 Roots[I].first->eraseFromParent();
463 Roots[I].second->eraseFromParent();
464 }
465
466 Roots.clear();
467 return true;
468 }
469