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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/ADT/SmallVector.h"
20 : #include "llvm/ADT/StringExtras.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/Function.h"
27 : #include "llvm/IR/GlobalValue.h"
28 : #include "llvm/IR/GlobalVariable.h"
29 : #include "llvm/IR/IRBuilder.h"
30 : #include "llvm/IR/Instructions.h"
31 : #include "llvm/IR/IntrinsicInst.h"
32 : #include "llvm/IR/Intrinsics.h"
33 : #include "llvm/IR/Module.h"
34 : #include "llvm/IR/Type.h"
35 : #include "llvm/IR/Value.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 27129 : INITIALIZE_PASS_BEGIN(ShadowStackGCLowering, DEBUG_TYPE,
91 : "Shadow Stack GC Lowering", false, false)
92 27129 : INITIALIZE_PASS_DEPENDENCY(GCModuleInfo)
93 54582 : INITIALIZE_PASS_END(ShadowStackGCLowering, DEBUG_TYPE,
94 : "Shadow Stack GC Lowering", false, false)
95 :
96 27453 : FunctionPass *llvm::createShadowStackGCLoweringPass() { return new ShadowStackGCLowering(); }
97 :
98 54906 : ShadowStackGCLowering::ShadowStackGCLowering() : FunctionPass(ID) {
99 27453 : initializeShadowStackGCLoweringPass(*PassRegistry::getPassRegistry());
100 27453 : }
101 :
102 2 : Constant *ShadowStackGCLowering::GetFrameMap(Function &F) {
103 : // doInitialization creates the abstract type of this value.
104 2 : 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 10 : for (unsigned I = 0; I != Roots.size(); ++I) {
110 3 : Constant *C = cast<Constant>(Roots[I].first->getArgOperand(1));
111 3 : if (!C->isNullValue())
112 0 : NumMeta = I + 1;
113 3 : Metadata.push_back(ConstantExpr::getBitCast(C, VoidPtr));
114 : }
115 2 : Metadata.resize(NumMeta);
116 :
117 2 : Type *Int32Ty = Type::getInt32Ty(F.getContext());
118 :
119 : Constant *BaseElts[] = {
120 4 : ConstantInt::get(Int32Ty, Roots.size(), false),
121 2 : ConstantInt::get(Int32Ty, NumMeta, false),
122 2 : };
123 :
124 : Constant *DescriptorElts[] = {
125 2 : ConstantStruct::get(FrameMapTy, BaseElts),
126 4 : ConstantArray::get(ArrayType::get(VoidPtr, NumMeta), Metadata)};
127 :
128 2 : Type *EltTys[] = {DescriptorElts[0]->getType(), DescriptorElts[1]->getType()};
129 2 : StructType *STy = StructType::create(EltTys, "gc_map." + utostr(NumMeta));
130 :
131 2 : 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 2 : Constant *GV = new GlobalVariable(*F.getParent(), FrameMap->getType(), true,
147 : GlobalVariable::InternalLinkage, FrameMap,
148 4 : "__gc_" + F.getName());
149 :
150 : Constant *GEPIndices[2] = {
151 2 : ConstantInt::get(Type::getInt32Ty(F.getContext()), 0),
152 2 : ConstantInt::get(Type::getInt32Ty(F.getContext()), 0)};
153 2 : return ConstantExpr::getGetElementPtr(FrameMap->getType(), GV, GEPIndices);
154 : }
155 :
156 2 : Type *ShadowStackGCLowering::GetConcreteStackEntryType(Function &F) {
157 : // doInitialization creates the generic version of this type.
158 : std::vector<Type *> EltTys;
159 2 : EltTys.push_back(StackEntryTy);
160 10 : for (size_t I = 0; I != Roots.size(); I++)
161 3 : EltTys.push_back(Roots[I].second->getAllocatedType());
162 :
163 4 : 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 27319 : bool ShadowStackGCLowering::doInitialization(Module &M) {
169 : bool Active = false;
170 553123 : for (Function &F : M) {
171 525936 : if (F.hasGC() && F.getGC() == std::string("shadow-stack")) {
172 : Active = true;
173 : break;
174 : }
175 : }
176 27319 : 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 2 : EltTys.push_back(Type::getInt32Ty(M.getContext()));
187 : // Specifies length of variable length array.
188 2 : EltTys.push_back(Type::getInt32Ty(M.getContext()));
189 2 : 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 2 : StackEntryTy = StructType::create(M.getContext(), "gc_stackentry");
199 :
200 : EltTys.clear();
201 2 : EltTys.push_back(PointerType::getUnqual(StackEntryTy));
202 2 : EltTys.push_back(FrameMapPtrTy);
203 4 : StackEntryTy->setBody(EltTys);
204 2 : PointerType *StackEntryPtrTy = PointerType::getUnqual(StackEntryTy);
205 :
206 : // Get the root chain if it already exists.
207 2 : Head = M.getGlobalVariable("llvm_gc_root_chain");
208 2 : if (!Head) {
209 : // If the root chain does not exist, insert a new one with linkonce
210 : // linkage!
211 2 : Head = new GlobalVariable(
212 : M, StackEntryPtrTy, false, GlobalValue::LinkOnceAnyLinkage,
213 4 : Constant::getNullValue(StackEntryPtrTy), "llvm_gc_root_chain");
214 0 : } else if (Head->hasExternalLinkage() && Head->isDeclaration()) {
215 0 : Head->setInitializer(Constant::getNullValue(StackEntryPtrTy));
216 0 : Head->setLinkage(GlobalValue::LinkOnceAnyLinkage);
217 : }
218 :
219 : return true;
220 : }
221 :
222 0 : bool ShadowStackGCLowering::IsNullValue(Value *V) {
223 : if (Constant *C = dyn_cast<Constant>(V))
224 3 : return C->isNullValue();
225 : return false;
226 : }
227 :
228 2 : 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 6 : for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB)
238 31 : 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 3 : if (F->getIntrinsicID() == Intrinsic::gcroot) {
242 : std::pair<CallInst *, AllocaInst *> Pair = std::make_pair(
243 : CI,
244 3 : cast<AllocaInst>(CI->getArgOperand(0)->stripPointerCasts()));
245 3 : if (IsNullValue(CI->getArgOperand(1)))
246 3 : Roots.push_back(Pair);
247 : else
248 0 : 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 2 : Roots.insert(Roots.begin(), MetaRoots.begin(), MetaRoots.end());
254 2 : }
255 :
256 6 : GetElementPtrInst *ShadowStackGCLowering::CreateGEP(LLVMContext &Context,
257 : IRBuilder<> &B, Type *Ty,
258 : Value *BasePtr, int Idx,
259 : int Idx2,
260 : const char *Name) {
261 6 : Value *Indices[] = {ConstantInt::get(Type::getInt32Ty(Context), 0),
262 6 : ConstantInt::get(Type::getInt32Ty(Context), Idx),
263 12 : ConstantInt::get(Type::getInt32Ty(Context), Idx2)};
264 6 : Value *Val = B.CreateGEP(Ty, BasePtr, Indices, Name);
265 :
266 : assert(isa<GetElementPtrInst>(Val) && "Unexpected folded constant");
267 :
268 6 : return dyn_cast<GetElementPtrInst>(Val);
269 : }
270 :
271 5 : GetElementPtrInst *ShadowStackGCLowering::CreateGEP(LLVMContext &Context,
272 : IRBuilder<> &B, Type *Ty, Value *BasePtr,
273 : int Idx, const char *Name) {
274 5 : Value *Indices[] = {ConstantInt::get(Type::getInt32Ty(Context), 0),
275 5 : ConstantInt::get(Type::getInt32Ty(Context), Idx)};
276 5 : Value *Val = B.CreateGEP(Ty, BasePtr, Indices, Name);
277 :
278 : assert(isa<GetElementPtrInst>(Val) && "Unexpected folded constant");
279 :
280 5 : return dyn_cast<GetElementPtrInst>(Val);
281 : }
282 :
283 : /// runOnFunction - Insert code to maintain the shadow stack.
284 406524 : bool ShadowStackGCLowering::runOnFunction(Function &F) {
285 : // Quick exit for functions that do not use the shadow stack GC.
286 406590 : if (!F.hasGC() ||
287 406656 : F.getGC() != std::string("shadow-stack"))
288 : return false;
289 :
290 2 : LLVMContext &Context = F.getContext();
291 :
292 : // Find calls to llvm.gcroot.
293 2 : 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 2 : if (Roots.empty())
298 : return false;
299 :
300 : // Build the constant map and figure the type of the shadow stack entry.
301 2 : Value *FrameMap = GetFrameMap(F);
302 2 : 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 2 : IRBuilder<> AtEntry(IP->getParent(), IP);
307 :
308 : Instruction *StackEntry =
309 2 : AtEntry.CreateAlloca(ConcreteStackEntryTy, nullptr, "gc_frame");
310 :
311 4 : while (isa<AllocaInst>(IP))
312 : ++IP;
313 2 : AtEntry.SetInsertPoint(IP->getParent(), IP);
314 :
315 : // Initialize the map pointer and load the current head of the shadow stack.
316 2 : Instruction *CurrentHead = AtEntry.CreateLoad(Head, "gc_currhead");
317 2 : Instruction *EntryMapPtr = CreateGEP(Context, AtEntry, ConcreteStackEntryTy,
318 : StackEntry, 0, 1, "gc_frame.map");
319 2 : AtEntry.CreateStore(FrameMap, EntryMapPtr);
320 :
321 : // After all the allocas...
322 7 : for (unsigned I = 0, E = Roots.size(); I != E; ++I) {
323 : // For each root, find the corresponding slot in the aggregate...
324 3 : Value *SlotPtr = CreateGEP(Context, AtEntry, ConcreteStackEntryTy,
325 3 : StackEntry, 1 + I, "gc_root");
326 :
327 : // And use it in lieu of the alloca.
328 3 : AllocaInst *OriginalAlloca = Roots[I].second;
329 3 : SlotPtr->takeName(OriginalAlloca);
330 3 : OriginalAlloca->replaceAllUsesWith(SlotPtr);
331 : }
332 :
333 : // Move past the original stores inserted by GCStrategy::InitRoots. This isn't
334 : // really necessary (the collector would never see the intermediate state at
335 : // runtime), but it's nicer not to push the half-initialized entry onto the
336 : // shadow stack.
337 4 : while (isa<StoreInst>(IP))
338 : ++IP;
339 2 : AtEntry.SetInsertPoint(IP->getParent(), IP);
340 :
341 : // Push the entry onto the shadow stack.
342 2 : Instruction *EntryNextPtr = CreateGEP(Context, AtEntry, ConcreteStackEntryTy,
343 : StackEntry, 0, 0, "gc_frame.next");
344 2 : Instruction *NewHeadVal = CreateGEP(Context, AtEntry, ConcreteStackEntryTy,
345 : StackEntry, 0, "gc_newhead");
346 2 : AtEntry.CreateStore(CurrentHead, EntryNextPtr);
347 2 : AtEntry.CreateStore(NewHeadVal, Head);
348 :
349 : // For each instruction that escapes...
350 : EscapeEnumerator EE(F, "gc_cleanup");
351 4 : while (IRBuilder<> *AtExit = EE.Next()) {
352 : // Pop the entry from the shadow stack. Don't reuse CurrentHead from
353 : // AtEntry, since that would make the value live for the entire function.
354 : Instruction *EntryNextPtr2 =
355 2 : CreateGEP(Context, *AtExit, ConcreteStackEntryTy, StackEntry, 0, 0,
356 : "gc_frame.next");
357 2 : Value *SavedHead = AtExit->CreateLoad(EntryNextPtr2, "gc_savedhead");
358 2 : AtExit->CreateStore(SavedHead, Head);
359 2 : }
360 :
361 : // Delete the original allocas (which are no longer used) and the intrinsic
362 : // calls (which are no longer valid). Doing this last avoids invalidating
363 : // iterators.
364 7 : for (unsigned I = 0, E = Roots.size(); I != E; ++I) {
365 6 : Roots[I].first->eraseFromParent();
366 6 : Roots[I].second->eraseFromParent();
367 : }
368 :
369 : Roots.clear();
370 : return true;
371 : }
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