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