1 //===- ShadowStackGCLowering.cpp - Custom lowering for shadow-stack gc ----===//
2 //
3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4 // See https://llvm.org/LICENSE.txt for license information.
5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6 //
7 //===----------------------------------------------------------------------===//
8 //
9 // This file contains the custom lowering code required by the shadow-stack GC
10 // strategy.
11 //
12 // This pass implements the code transformation described in this paper:
13 //   "Accurate Garbage Collection in an Uncooperative Environment"
14 //   Fergus Henderson, ISMM, 2002
15 //
16 //===----------------------------------------------------------------------===//
17 
18 #include "llvm/ADT/SmallVector.h"
19 #include "llvm/ADT/StringExtras.h"
20 #include "llvm/Analysis/DomTreeUpdater.h"
21 #include "llvm/CodeGen/Passes.h"
22 #include "llvm/IR/BasicBlock.h"
23 #include "llvm/IR/Constant.h"
24 #include "llvm/IR/Constants.h"
25 #include "llvm/IR/DerivedTypes.h"
26 #include "llvm/IR/Dominators.h"
27 #include "llvm/IR/Function.h"
28 #include "llvm/IR/GlobalValue.h"
29 #include "llvm/IR/GlobalVariable.h"
30 #include "llvm/IR/IRBuilder.h"
31 #include "llvm/IR/Instructions.h"
32 #include "llvm/IR/IntrinsicInst.h"
33 #include "llvm/IR/Intrinsics.h"
34 #include "llvm/IR/Module.h"
35 #include "llvm/IR/Type.h"
36 #include "llvm/IR/Value.h"
37 #include "llvm/InitializePasses.h"
38 #include "llvm/Pass.h"
39 #include "llvm/Support/Casting.h"
40 #include "llvm/Transforms/Utils/EscapeEnumerator.h"
41 #include <cassert>
42 #include <cstddef>
43 #include <string>
44 #include <utility>
45 #include <vector>
46 
47 using namespace llvm;
48 
49 #define DEBUG_TYPE "shadow-stack-gc-lowering"
50 
51 namespace {
52 
53 class ShadowStackGCLowering : public FunctionPass {
54   /// RootChain - This is the global linked-list that contains the chain of GC
55   /// roots.
56   GlobalVariable *Head = nullptr;
57 
58   /// StackEntryTy - Abstract type of a link in the shadow stack.
59   StructType *StackEntryTy = nullptr;
60   StructType *FrameMapTy = nullptr;
61 
62   /// Roots - GC roots in the current function. Each is a pair of the
63   /// intrinsic call and its corresponding alloca.
64   std::vector<std::pair<CallInst *, AllocaInst *>> Roots;
65 
66 public:
67   static char ID;
68 
69   ShadowStackGCLowering();
70 
71   bool doInitialization(Module &M) override;
72   void getAnalysisUsage(AnalysisUsage &AU) const override;
73   bool runOnFunction(Function &F) override;
74 
75 private:
76   bool IsNullValue(Value *V);
77   Constant *GetFrameMap(Function &F);
78   Type *GetConcreteStackEntryType(Function &F);
79   void CollectRoots(Function &F);
80 
81   static GetElementPtrInst *CreateGEP(LLVMContext &Context, IRBuilder<> &B,
82                                       Type *Ty, Value *BasePtr, int Idx1,
83                                       const char *Name);
84   static GetElementPtrInst *CreateGEP(LLVMContext &Context, IRBuilder<> &B,
85                                       Type *Ty, Value *BasePtr, int Idx1, int Idx2,
86                                       const char *Name);
87 };
88 
89 } // end anonymous namespace
90 
91 char ShadowStackGCLowering::ID = 0;
92 char &llvm::ShadowStackGCLoweringID = ShadowStackGCLowering::ID;
93 
94 INITIALIZE_PASS_BEGIN(ShadowStackGCLowering, DEBUG_TYPE,
95                       "Shadow Stack GC Lowering", false, false)
96 INITIALIZE_PASS_DEPENDENCY(GCModuleInfo)
97 INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
98 INITIALIZE_PASS_END(ShadowStackGCLowering, DEBUG_TYPE,
99                     "Shadow Stack GC Lowering", false, false)
100 
101 FunctionPass *llvm::createShadowStackGCLoweringPass() { return new ShadowStackGCLowering(); }
102 
103 ShadowStackGCLowering::ShadowStackGCLowering() : FunctionPass(ID) {
104   initializeShadowStackGCLoweringPass(*PassRegistry::getPassRegistry());
105 }
106 
107 Constant *ShadowStackGCLowering::GetFrameMap(Function &F) {
108   // doInitialization creates the abstract type of this value.
109   Type *VoidPtr = Type::getInt8PtrTy(F.getContext());
110 
111   // Truncate the ShadowStackDescriptor if some metadata is null.
112   unsigned NumMeta = 0;
113   SmallVector<Constant *, 16> Metadata;
114   for (unsigned I = 0; I != Roots.size(); ++I) {
115     Constant *C = cast<Constant>(Roots[I].first->getArgOperand(1));
116     if (!C->isNullValue())
117       NumMeta = I + 1;
118     Metadata.push_back(ConstantExpr::getBitCast(C, VoidPtr));
119   }
120   Metadata.resize(NumMeta);
121 
122   Type *Int32Ty = Type::getInt32Ty(F.getContext());
123 
124   Constant *BaseElts[] = {
125       ConstantInt::get(Int32Ty, Roots.size(), false),
126       ConstantInt::get(Int32Ty, NumMeta, false),
127   };
128 
129   Constant *DescriptorElts[] = {
130       ConstantStruct::get(FrameMapTy, BaseElts),
131       ConstantArray::get(ArrayType::get(VoidPtr, NumMeta), Metadata)};
132 
133   Type *EltTys[] = {DescriptorElts[0]->getType(), DescriptorElts[1]->getType()};
134   StructType *STy = StructType::create(EltTys, "gc_map." + utostr(NumMeta));
135 
136   Constant *FrameMap = ConstantStruct::get(STy, DescriptorElts);
137 
138   // FIXME: Is this actually dangerous as WritingAnLLVMPass.html claims? Seems
139   //        that, short of multithreaded LLVM, it should be safe; all that is
140   //        necessary is that a simple Module::iterator loop not be invalidated.
141   //        Appending to the GlobalVariable list is safe in that sense.
142   //
143   //        All of the output passes emit globals last. The ExecutionEngine
144   //        explicitly supports adding globals to the module after
145   //        initialization.
146   //
147   //        Still, if it isn't deemed acceptable, then this transformation needs
148   //        to be a ModulePass (which means it cannot be in the 'llc' pipeline
149   //        (which uses a FunctionPassManager (which segfaults (not asserts) if
150   //        provided a ModulePass))).
151   Constant *GV = new GlobalVariable(*F.getParent(), FrameMap->getType(), true,
152                                     GlobalVariable::InternalLinkage, FrameMap,
153                                     "__gc_" + F.getName());
154 
155   Constant *GEPIndices[2] = {
156       ConstantInt::get(Type::getInt32Ty(F.getContext()), 0),
157       ConstantInt::get(Type::getInt32Ty(F.getContext()), 0)};
158   return ConstantExpr::getGetElementPtr(FrameMap->getType(), GV, GEPIndices);
159 }
160 
161 Type *ShadowStackGCLowering::GetConcreteStackEntryType(Function &F) {
162   // doInitialization creates the generic version of this type.
163   std::vector<Type *> EltTys;
164   EltTys.push_back(StackEntryTy);
165   for (const std::pair<CallInst *, AllocaInst *> &Root : Roots)
166     EltTys.push_back(Root.second->getAllocatedType());
167 
168   return StructType::create(EltTys, ("gc_stackentry." + F.getName()).str());
169 }
170 
171 /// doInitialization - If this module uses the GC intrinsics, find them now. If
172 /// not, exit fast.
173 bool ShadowStackGCLowering::doInitialization(Module &M) {
174   bool Active = false;
175   for (Function &F : M) {
176     if (F.hasGC() && F.getGC() == std::string("shadow-stack")) {
177       Active = true;
178       break;
179     }
180   }
181   if (!Active)
182     return false;
183 
184   // struct FrameMap {
185   //   int32_t NumRoots; // Number of roots in stack frame.
186   //   int32_t NumMeta;  // Number of metadata descriptors. May be < NumRoots.
187   //   void *Meta[];     // May be absent for roots without metadata.
188   // };
189   std::vector<Type *> EltTys;
190   // 32 bits is ok up to a 32GB stack frame. :)
191   EltTys.push_back(Type::getInt32Ty(M.getContext()));
192   // Specifies length of variable length array.
193   EltTys.push_back(Type::getInt32Ty(M.getContext()));
194   FrameMapTy = StructType::create(EltTys, "gc_map");
195   PointerType *FrameMapPtrTy = PointerType::getUnqual(FrameMapTy);
196 
197   // struct StackEntry {
198   //   ShadowStackEntry *Next; // Caller's stack entry.
199   //   FrameMap *Map;          // Pointer to constant FrameMap.
200   //   void *Roots[];          // Stack roots (in-place array, so we pretend).
201   // };
202 
203   StackEntryTy = StructType::create(M.getContext(), "gc_stackentry");
204 
205   EltTys.clear();
206   EltTys.push_back(PointerType::getUnqual(StackEntryTy));
207   EltTys.push_back(FrameMapPtrTy);
208   StackEntryTy->setBody(EltTys);
209   PointerType *StackEntryPtrTy = PointerType::getUnqual(StackEntryTy);
210 
211   // Get the root chain if it already exists.
212   Head = M.getGlobalVariable("llvm_gc_root_chain");
213   if (!Head) {
214     // If the root chain does not exist, insert a new one with linkonce
215     // linkage!
216     Head = new GlobalVariable(
217         M, StackEntryPtrTy, false, GlobalValue::LinkOnceAnyLinkage,
218         Constant::getNullValue(StackEntryPtrTy), "llvm_gc_root_chain");
219   } else if (Head->hasExternalLinkage() && Head->isDeclaration()) {
220     Head->setInitializer(Constant::getNullValue(StackEntryPtrTy));
221     Head->setLinkage(GlobalValue::LinkOnceAnyLinkage);
222   }
223 
224   return true;
225 }
226 
227 bool ShadowStackGCLowering::IsNullValue(Value *V) {
228   if (Constant *C = dyn_cast<Constant>(V))
229     return C->isNullValue();
230   return false;
231 }
232 
233 void ShadowStackGCLowering::CollectRoots(Function &F) {
234   // FIXME: Account for original alignment. Could fragment the root array.
235   //   Approach 1: Null initialize empty slots at runtime. Yuck.
236   //   Approach 2: Emit a map of the array instead of just a count.
237 
238   assert(Roots.empty() && "Not cleaned up?");
239 
240   SmallVector<std::pair<CallInst *, AllocaInst *>, 16> MetaRoots;
241 
242   for (BasicBlock &BB : F)
243     for (Instruction &I : BB)
244       if (IntrinsicInst *CI = dyn_cast<IntrinsicInst>(&I))
245         if (Function *F = CI->getCalledFunction())
246           if (F->getIntrinsicID() == Intrinsic::gcroot) {
247             std::pair<CallInst *, AllocaInst *> Pair = std::make_pair(
248                 CI,
249                 cast<AllocaInst>(CI->getArgOperand(0)->stripPointerCasts()));
250             if (IsNullValue(CI->getArgOperand(1)))
251               Roots.push_back(Pair);
252             else
253               MetaRoots.push_back(Pair);
254           }
255 
256   // Number roots with metadata (usually empty) at the beginning, so that the
257   // FrameMap::Meta array can be elided.
258   Roots.insert(Roots.begin(), MetaRoots.begin(), MetaRoots.end());
259 }
260 
261 GetElementPtrInst *ShadowStackGCLowering::CreateGEP(LLVMContext &Context,
262                                                     IRBuilder<> &B, Type *Ty,
263                                                     Value *BasePtr, int Idx,
264                                                     int Idx2,
265                                                     const char *Name) {
266   Value *Indices[] = {ConstantInt::get(Type::getInt32Ty(Context), 0),
267                       ConstantInt::get(Type::getInt32Ty(Context), Idx),
268                       ConstantInt::get(Type::getInt32Ty(Context), Idx2)};
269   Value *Val = B.CreateGEP(Ty, BasePtr, Indices, Name);
270 
271   assert(isa<GetElementPtrInst>(Val) && "Unexpected folded constant");
272 
273   return dyn_cast<GetElementPtrInst>(Val);
274 }
275 
276 GetElementPtrInst *ShadowStackGCLowering::CreateGEP(LLVMContext &Context,
277                                             IRBuilder<> &B, Type *Ty, Value *BasePtr,
278                                             int Idx, const char *Name) {
279   Value *Indices[] = {ConstantInt::get(Type::getInt32Ty(Context), 0),
280                       ConstantInt::get(Type::getInt32Ty(Context), Idx)};
281   Value *Val = B.CreateGEP(Ty, BasePtr, Indices, Name);
282 
283   assert(isa<GetElementPtrInst>(Val) && "Unexpected folded constant");
284 
285   return dyn_cast<GetElementPtrInst>(Val);
286 }
287 
288 void ShadowStackGCLowering::getAnalysisUsage(AnalysisUsage &AU) const {
289   AU.addPreserved<DominatorTreeWrapperPass>();
290 }
291 
292 /// runOnFunction - Insert code to maintain the shadow stack.
293 bool ShadowStackGCLowering::runOnFunction(Function &F) {
294   // Quick exit for functions that do not use the shadow stack GC.
295   if (!F.hasGC() ||
296       F.getGC() != std::string("shadow-stack"))
297     return false;
298 
299   LLVMContext &Context = F.getContext();
300 
301   // Find calls to llvm.gcroot.
302   CollectRoots(F);
303 
304   // If there are no roots in this function, then there is no need to add a
305   // stack map entry for it.
306   if (Roots.empty())
307     return false;
308 
309   Optional<DomTreeUpdater> DTU;
310   if (auto *DTWP = getAnalysisIfAvailable<DominatorTreeWrapperPass>())
311     DTU.emplace(DTWP->getDomTree(), DomTreeUpdater::UpdateStrategy::Lazy);
312 
313   // Build the constant map and figure the type of the shadow stack entry.
314   Value *FrameMap = GetFrameMap(F);
315   Type *ConcreteStackEntryTy = GetConcreteStackEntryType(F);
316 
317   // Build the shadow stack entry at the very start of the function.
318   BasicBlock::iterator IP = F.getEntryBlock().begin();
319   IRBuilder<> AtEntry(IP->getParent(), IP);
320 
321   Instruction *StackEntry =
322       AtEntry.CreateAlloca(ConcreteStackEntryTy, nullptr, "gc_frame");
323 
324   while (isa<AllocaInst>(IP))
325     ++IP;
326   AtEntry.SetInsertPoint(IP->getParent(), IP);
327 
328   // Initialize the map pointer and load the current head of the shadow stack.
329   Instruction *CurrentHead =
330       AtEntry.CreateLoad(StackEntryTy->getPointerTo(), Head, "gc_currhead");
331   Instruction *EntryMapPtr = CreateGEP(Context, AtEntry, ConcreteStackEntryTy,
332                                        StackEntry, 0, 1, "gc_frame.map");
333   AtEntry.CreateStore(FrameMap, EntryMapPtr);
334 
335   // After all the allocas...
336   for (unsigned I = 0, E = Roots.size(); I != E; ++I) {
337     // For each root, find the corresponding slot in the aggregate...
338     Value *SlotPtr = CreateGEP(Context, AtEntry, ConcreteStackEntryTy,
339                                StackEntry, 1 + I, "gc_root");
340 
341     // And use it in lieu of the alloca.
342     AllocaInst *OriginalAlloca = Roots[I].second;
343     SlotPtr->takeName(OriginalAlloca);
344     OriginalAlloca->replaceAllUsesWith(SlotPtr);
345   }
346 
347   // Move past the original stores inserted by GCStrategy::InitRoots. This isn't
348   // really necessary (the collector would never see the intermediate state at
349   // runtime), but it's nicer not to push the half-initialized entry onto the
350   // shadow stack.
351   while (isa<StoreInst>(IP))
352     ++IP;
353   AtEntry.SetInsertPoint(IP->getParent(), IP);
354 
355   // Push the entry onto the shadow stack.
356   Instruction *EntryNextPtr = CreateGEP(Context, AtEntry, ConcreteStackEntryTy,
357                                         StackEntry, 0, 0, "gc_frame.next");
358   Instruction *NewHeadVal = CreateGEP(Context, AtEntry, ConcreteStackEntryTy,
359                                       StackEntry, 0, "gc_newhead");
360   AtEntry.CreateStore(CurrentHead, EntryNextPtr);
361   AtEntry.CreateStore(NewHeadVal, Head);
362 
363   // For each instruction that escapes...
364   EscapeEnumerator EE(F, "gc_cleanup", /*HandleExceptions=*/true,
365                       DTU.hasValue() ? DTU.getPointer() : nullptr);
366   while (IRBuilder<> *AtExit = EE.Next()) {
367     // Pop the entry from the shadow stack. Don't reuse CurrentHead from
368     // AtEntry, since that would make the value live for the entire function.
369     Instruction *EntryNextPtr2 =
370         CreateGEP(Context, *AtExit, ConcreteStackEntryTy, StackEntry, 0, 0,
371                   "gc_frame.next");
372     Value *SavedHead = AtExit->CreateLoad(StackEntryTy->getPointerTo(),
373                                           EntryNextPtr2, "gc_savedhead");
374     AtExit->CreateStore(SavedHead, Head);
375   }
376 
377   // Delete the original allocas (which are no longer used) and the intrinsic
378   // calls (which are no longer valid). Doing this last avoids invalidating
379   // iterators.
380   for (std::pair<CallInst *, AllocaInst *> &Root : Roots) {
381     Root.first->eraseFromParent();
382     Root.second->eraseFromParent();
383   }
384 
385   Roots.clear();
386   return true;
387 }
388