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1 //===-- SafeStack.cpp - Safe Stack Insertion ------------------------------===//
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 pass splits the stack into the safe stack (kept as-is for LLVM backend)
11 // and the unsafe stack (explicitly allocated and managed through the runtime
12 // support library).
13 //
14 // http://clang.llvm.org/docs/SafeStack.html
15 //
16 //===----------------------------------------------------------------------===//
17 
18 #include "SafeStackColoring.h"
19 #include "SafeStackLayout.h"
20 #include "llvm/ADT/Statistic.h"
21 #include "llvm/ADT/Triple.h"
22 #include "llvm/Analysis/BranchProbabilityInfo.h"
23 #include "llvm/Analysis/ScalarEvolution.h"
24 #include "llvm/Analysis/ScalarEvolutionExpressions.h"
25 #include "llvm/CodeGen/Passes.h"
26 #include "llvm/IR/Constants.h"
27 #include "llvm/IR/DIBuilder.h"
28 #include "llvm/IR/DataLayout.h"
29 #include "llvm/IR/DerivedTypes.h"
30 #include "llvm/IR/Function.h"
31 #include "llvm/IR/IRBuilder.h"
32 #include "llvm/IR/InstIterator.h"
33 #include "llvm/IR/Instructions.h"
34 #include "llvm/IR/IntrinsicInst.h"
35 #include "llvm/IR/Intrinsics.h"
36 #include "llvm/IR/MDBuilder.h"
37 #include "llvm/IR/Module.h"
38 #include "llvm/Pass.h"
39 #include "llvm/Support/CommandLine.h"
40 #include "llvm/Support/Debug.h"
41 #include "llvm/Support/Format.h"
42 #include "llvm/Support/MathExtras.h"
43 #include "llvm/Support/raw_os_ostream.h"
44 #include "llvm/Target/TargetLowering.h"
45 #include "llvm/Target/TargetSubtargetInfo.h"
46 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
47 #include "llvm/Transforms/Utils/Local.h"
48 #include "llvm/Transforms/Utils/ModuleUtils.h"
49 
50 using namespace llvm;
51 using namespace llvm::safestack;
52 
53 #define DEBUG_TYPE "safestack"
54 
55 enum UnsafeStackPtrStorageVal { ThreadLocalUSP, SingleThreadUSP };
56 
57 static cl::opt<UnsafeStackPtrStorageVal> USPStorage("safe-stack-usp-storage",
58     cl::Hidden, cl::init(ThreadLocalUSP),
59     cl::desc("Type of storage for the unsafe stack pointer"),
60     cl::values(clEnumValN(ThreadLocalUSP, "thread-local",
61                           "Thread-local storage"),
62                clEnumValN(SingleThreadUSP, "single-thread",
63                           "Non-thread-local storage"),
64                clEnumValEnd));
65 
66 namespace llvm {
67 
68 STATISTIC(NumFunctions, "Total number of functions");
69 STATISTIC(NumUnsafeStackFunctions, "Number of functions with unsafe stack");
70 STATISTIC(NumUnsafeStackRestorePointsFunctions,
71           "Number of functions that use setjmp or exceptions");
72 
73 STATISTIC(NumAllocas, "Total number of allocas");
74 STATISTIC(NumUnsafeStaticAllocas, "Number of unsafe static allocas");
75 STATISTIC(NumUnsafeDynamicAllocas, "Number of unsafe dynamic allocas");
76 STATISTIC(NumUnsafeByValArguments, "Number of unsafe byval arguments");
77 STATISTIC(NumUnsafeStackRestorePoints, "Number of setjmps and landingpads");
78 
79 } // namespace llvm
80 
81 namespace {
82 
83 /// Rewrite an SCEV expression for a memory access address to an expression that
84 /// represents offset from the given alloca.
85 ///
86 /// The implementation simply replaces all mentions of the alloca with zero.
87 class AllocaOffsetRewriter : public SCEVRewriteVisitor<AllocaOffsetRewriter> {
88   const Value *AllocaPtr;
89 
90 public:
AllocaOffsetRewriter(ScalarEvolution & SE,const Value * AllocaPtr)91   AllocaOffsetRewriter(ScalarEvolution &SE, const Value *AllocaPtr)
92       : SCEVRewriteVisitor(SE), AllocaPtr(AllocaPtr) {}
93 
visitUnknown(const SCEVUnknown * Expr)94   const SCEV *visitUnknown(const SCEVUnknown *Expr) {
95     if (Expr->getValue() == AllocaPtr)
96       return SE.getZero(Expr->getType());
97     return Expr;
98   }
99 };
100 
101 /// The SafeStack pass splits the stack of each function into the safe
102 /// stack, which is only accessed through memory safe dereferences (as
103 /// determined statically), and the unsafe stack, which contains all
104 /// local variables that are accessed in ways that we can't prove to
105 /// be safe.
106 class SafeStack : public FunctionPass {
107   const TargetMachine *TM;
108   const TargetLoweringBase *TL;
109   const DataLayout *DL;
110   ScalarEvolution *SE;
111 
112   Type *StackPtrTy;
113   Type *IntPtrTy;
114   Type *Int32Ty;
115   Type *Int8Ty;
116 
117   Value *UnsafeStackPtr = nullptr;
118 
119   /// Unsafe stack alignment. Each stack frame must ensure that the stack is
120   /// aligned to this value. We need to re-align the unsafe stack if the
121   /// alignment of any object on the stack exceeds this value.
122   ///
123   /// 16 seems like a reasonable upper bound on the alignment of objects that we
124   /// might expect to appear on the stack on most common targets.
125   enum { StackAlignment = 16 };
126 
127   /// \brief Build a value representing a pointer to the unsafe stack pointer.
128   Value *getOrCreateUnsafeStackPtr(IRBuilder<> &IRB, Function &F);
129 
130   /// \brief Return the value of the stack canary.
131   Value *getStackGuard(IRBuilder<> &IRB, Function &F);
132 
133   /// \brief Load stack guard from the frame and check if it has changed.
134   void checkStackGuard(IRBuilder<> &IRB, Function &F, ReturnInst &RI,
135                        AllocaInst *StackGuardSlot, Value *StackGuard);
136 
137   /// \brief Find all static allocas, dynamic allocas, return instructions and
138   /// stack restore points (exception unwind blocks and setjmp calls) in the
139   /// given function and append them to the respective vectors.
140   void findInsts(Function &F, SmallVectorImpl<AllocaInst *> &StaticAllocas,
141                  SmallVectorImpl<AllocaInst *> &DynamicAllocas,
142                  SmallVectorImpl<Argument *> &ByValArguments,
143                  SmallVectorImpl<ReturnInst *> &Returns,
144                  SmallVectorImpl<Instruction *> &StackRestorePoints);
145 
146   /// \brief Calculate the allocation size of a given alloca. Returns 0 if the
147   /// size can not be statically determined.
148   uint64_t getStaticAllocaAllocationSize(const AllocaInst* AI);
149 
150   /// \brief Allocate space for all static allocas in \p StaticAllocas,
151   /// replace allocas with pointers into the unsafe stack and generate code to
152   /// restore the stack pointer before all return instructions in \p Returns.
153   ///
154   /// \returns A pointer to the top of the unsafe stack after all unsafe static
155   /// allocas are allocated.
156   Value *moveStaticAllocasToUnsafeStack(IRBuilder<> &IRB, Function &F,
157                                         ArrayRef<AllocaInst *> StaticAllocas,
158                                         ArrayRef<Argument *> ByValArguments,
159                                         ArrayRef<ReturnInst *> Returns,
160                                         Instruction *BasePointer,
161                                         AllocaInst *StackGuardSlot);
162 
163   /// \brief Generate code to restore the stack after all stack restore points
164   /// in \p StackRestorePoints.
165   ///
166   /// \returns A local variable in which to maintain the dynamic top of the
167   /// unsafe stack if needed.
168   AllocaInst *
169   createStackRestorePoints(IRBuilder<> &IRB, Function &F,
170                            ArrayRef<Instruction *> StackRestorePoints,
171                            Value *StaticTop, bool NeedDynamicTop);
172 
173   /// \brief Replace all allocas in \p DynamicAllocas with code to allocate
174   /// space dynamically on the unsafe stack and store the dynamic unsafe stack
175   /// top to \p DynamicTop if non-null.
176   void moveDynamicAllocasToUnsafeStack(Function &F, Value *UnsafeStackPtr,
177                                        AllocaInst *DynamicTop,
178                                        ArrayRef<AllocaInst *> DynamicAllocas);
179 
180   bool IsSafeStackAlloca(const Value *AllocaPtr, uint64_t AllocaSize);
181 
182   bool IsMemIntrinsicSafe(const MemIntrinsic *MI, const Use &U,
183                           const Value *AllocaPtr, uint64_t AllocaSize);
184   bool IsAccessSafe(Value *Addr, uint64_t Size, const Value *AllocaPtr,
185                     uint64_t AllocaSize);
186 
187 public:
188   static char ID; // Pass identification, replacement for typeid.
SafeStack(const TargetMachine * TM)189   SafeStack(const TargetMachine *TM)
190       : FunctionPass(ID), TM(TM), TL(nullptr), DL(nullptr) {
191     initializeSafeStackPass(*PassRegistry::getPassRegistry());
192   }
SafeStack()193   SafeStack() : SafeStack(nullptr) {}
194 
getAnalysisUsage(AnalysisUsage & AU) const195   void getAnalysisUsage(AnalysisUsage &AU) const override {
196     AU.addRequired<ScalarEvolutionWrapperPass>();
197   }
198 
doInitialization(Module & M)199   bool doInitialization(Module &M) override {
200     DL = &M.getDataLayout();
201 
202     StackPtrTy = Type::getInt8PtrTy(M.getContext());
203     IntPtrTy = DL->getIntPtrType(M.getContext());
204     Int32Ty = Type::getInt32Ty(M.getContext());
205     Int8Ty = Type::getInt8Ty(M.getContext());
206 
207     return false;
208   }
209 
210   bool runOnFunction(Function &F) override;
211 }; // class SafeStack
212 
getStaticAllocaAllocationSize(const AllocaInst * AI)213 uint64_t SafeStack::getStaticAllocaAllocationSize(const AllocaInst* AI) {
214   uint64_t Size = DL->getTypeAllocSize(AI->getAllocatedType());
215   if (AI->isArrayAllocation()) {
216     auto C = dyn_cast<ConstantInt>(AI->getArraySize());
217     if (!C)
218       return 0;
219     Size *= C->getZExtValue();
220   }
221   return Size;
222 }
223 
IsAccessSafe(Value * Addr,uint64_t AccessSize,const Value * AllocaPtr,uint64_t AllocaSize)224 bool SafeStack::IsAccessSafe(Value *Addr, uint64_t AccessSize,
225                              const Value *AllocaPtr, uint64_t AllocaSize) {
226   AllocaOffsetRewriter Rewriter(*SE, AllocaPtr);
227   const SCEV *Expr = Rewriter.visit(SE->getSCEV(Addr));
228 
229   uint64_t BitWidth = SE->getTypeSizeInBits(Expr->getType());
230   ConstantRange AccessStartRange = SE->getUnsignedRange(Expr);
231   ConstantRange SizeRange =
232       ConstantRange(APInt(BitWidth, 0), APInt(BitWidth, AccessSize));
233   ConstantRange AccessRange = AccessStartRange.add(SizeRange);
234   ConstantRange AllocaRange =
235       ConstantRange(APInt(BitWidth, 0), APInt(BitWidth, AllocaSize));
236   bool Safe = AllocaRange.contains(AccessRange);
237 
238   DEBUG(dbgs() << "[SafeStack] "
239                << (isa<AllocaInst>(AllocaPtr) ? "Alloca " : "ByValArgument ")
240                << *AllocaPtr << "\n"
241                << "            Access " << *Addr << "\n"
242                << "            SCEV " << *Expr
243                << " U: " << SE->getUnsignedRange(Expr)
244                << ", S: " << SE->getSignedRange(Expr) << "\n"
245                << "            Range " << AccessRange << "\n"
246                << "            AllocaRange " << AllocaRange << "\n"
247                << "            " << (Safe ? "safe" : "unsafe") << "\n");
248 
249   return Safe;
250 }
251 
IsMemIntrinsicSafe(const MemIntrinsic * MI,const Use & U,const Value * AllocaPtr,uint64_t AllocaSize)252 bool SafeStack::IsMemIntrinsicSafe(const MemIntrinsic *MI, const Use &U,
253                                    const Value *AllocaPtr,
254                                    uint64_t AllocaSize) {
255   // All MemIntrinsics have destination address in Arg0 and size in Arg2.
256   if (MI->getRawDest() != U) return true;
257   const auto *Len = dyn_cast<ConstantInt>(MI->getLength());
258   // Non-constant size => unsafe. FIXME: try SCEV getRange.
259   if (!Len) return false;
260   return IsAccessSafe(U, Len->getZExtValue(), AllocaPtr, AllocaSize);
261 }
262 
263 /// Check whether a given allocation must be put on the safe
264 /// stack or not. The function analyzes all uses of AI and checks whether it is
265 /// only accessed in a memory safe way (as decided statically).
IsSafeStackAlloca(const Value * AllocaPtr,uint64_t AllocaSize)266 bool SafeStack::IsSafeStackAlloca(const Value *AllocaPtr, uint64_t AllocaSize) {
267   // Go through all uses of this alloca and check whether all accesses to the
268   // allocated object are statically known to be memory safe and, hence, the
269   // object can be placed on the safe stack.
270   SmallPtrSet<const Value *, 16> Visited;
271   SmallVector<const Value *, 8> WorkList;
272   WorkList.push_back(AllocaPtr);
273 
274   // A DFS search through all uses of the alloca in bitcasts/PHI/GEPs/etc.
275   while (!WorkList.empty()) {
276     const Value *V = WorkList.pop_back_val();
277     for (const Use &UI : V->uses()) {
278       auto I = cast<const Instruction>(UI.getUser());
279       assert(V == UI.get());
280 
281       switch (I->getOpcode()) {
282       case Instruction::Load: {
283         if (!IsAccessSafe(UI, DL->getTypeStoreSize(I->getType()), AllocaPtr,
284                           AllocaSize))
285           return false;
286         break;
287       }
288       case Instruction::VAArg:
289         // "va-arg" from a pointer is safe.
290         break;
291       case Instruction::Store: {
292         if (V == I->getOperand(0)) {
293           // Stored the pointer - conservatively assume it may be unsafe.
294           DEBUG(dbgs() << "[SafeStack] Unsafe alloca: " << *AllocaPtr
295                        << "\n            store of address: " << *I << "\n");
296           return false;
297         }
298 
299         if (!IsAccessSafe(UI, DL->getTypeStoreSize(I->getOperand(0)->getType()),
300                           AllocaPtr, AllocaSize))
301           return false;
302         break;
303       }
304       case Instruction::Ret: {
305         // Information leak.
306         return false;
307       }
308 
309       case Instruction::Call:
310       case Instruction::Invoke: {
311         ImmutableCallSite CS(I);
312 
313         if (const IntrinsicInst *II = dyn_cast<IntrinsicInst>(I)) {
314           if (II->getIntrinsicID() == Intrinsic::lifetime_start ||
315               II->getIntrinsicID() == Intrinsic::lifetime_end)
316             continue;
317         }
318 
319         if (const MemIntrinsic *MI = dyn_cast<MemIntrinsic>(I)) {
320           if (!IsMemIntrinsicSafe(MI, UI, AllocaPtr, AllocaSize)) {
321             DEBUG(dbgs() << "[SafeStack] Unsafe alloca: " << *AllocaPtr
322                          << "\n            unsafe memintrinsic: " << *I
323                          << "\n");
324             return false;
325           }
326           continue;
327         }
328 
329         // LLVM 'nocapture' attribute is only set for arguments whose address
330         // is not stored, passed around, or used in any other non-trivial way.
331         // We assume that passing a pointer to an object as a 'nocapture
332         // readnone' argument is safe.
333         // FIXME: a more precise solution would require an interprocedural
334         // analysis here, which would look at all uses of an argument inside
335         // the function being called.
336         ImmutableCallSite::arg_iterator B = CS.arg_begin(), E = CS.arg_end();
337         for (ImmutableCallSite::arg_iterator A = B; A != E; ++A)
338           if (A->get() == V)
339             if (!(CS.doesNotCapture(A - B) && (CS.doesNotAccessMemory(A - B) ||
340                                                CS.doesNotAccessMemory()))) {
341               DEBUG(dbgs() << "[SafeStack] Unsafe alloca: " << *AllocaPtr
342                            << "\n            unsafe call: " << *I << "\n");
343               return false;
344             }
345         continue;
346       }
347 
348       default:
349         if (Visited.insert(I).second)
350           WorkList.push_back(cast<const Instruction>(I));
351       }
352     }
353   }
354 
355   // All uses of the alloca are safe, we can place it on the safe stack.
356   return true;
357 }
358 
getOrCreateUnsafeStackPtr(IRBuilder<> & IRB,Function & F)359 Value *SafeStack::getOrCreateUnsafeStackPtr(IRBuilder<> &IRB, Function &F) {
360   // Check if there is a target-specific location for the unsafe stack pointer.
361   if (TL)
362     if (Value *V = TL->getSafeStackPointerLocation(IRB))
363       return V;
364 
365   // Otherwise, assume the target links with compiler-rt, which provides a
366   // thread-local variable with a magic name.
367   Module &M = *F.getParent();
368   const char *UnsafeStackPtrVar = "__safestack_unsafe_stack_ptr";
369   auto UnsafeStackPtr =
370       dyn_cast_or_null<GlobalVariable>(M.getNamedValue(UnsafeStackPtrVar));
371 
372   bool UseTLS = USPStorage == ThreadLocalUSP;
373 
374   if (!UnsafeStackPtr) {
375     auto TLSModel = UseTLS ?
376         GlobalValue::InitialExecTLSModel :
377         GlobalValue::NotThreadLocal;
378     // The global variable is not defined yet, define it ourselves.
379     // We use the initial-exec TLS model because we do not support the
380     // variable living anywhere other than in the main executable.
381     UnsafeStackPtr = new GlobalVariable(
382         M, StackPtrTy, false, GlobalValue::ExternalLinkage, nullptr,
383         UnsafeStackPtrVar, nullptr, TLSModel);
384   } else {
385     // The variable exists, check its type and attributes.
386     if (UnsafeStackPtr->getValueType() != StackPtrTy)
387       report_fatal_error(Twine(UnsafeStackPtrVar) + " must have void* type");
388     if (UseTLS != UnsafeStackPtr->isThreadLocal())
389       report_fatal_error(Twine(UnsafeStackPtrVar) + " must " +
390                          (UseTLS ? "" : "not ") + "be thread-local");
391   }
392   return UnsafeStackPtr;
393 }
394 
getStackGuard(IRBuilder<> & IRB,Function & F)395 Value *SafeStack::getStackGuard(IRBuilder<> &IRB, Function &F) {
396   Value *StackGuardVar = nullptr;
397   if (TL)
398     StackGuardVar = TL->getIRStackGuard(IRB);
399   if (!StackGuardVar)
400     StackGuardVar =
401         F.getParent()->getOrInsertGlobal("__stack_chk_guard", StackPtrTy);
402   return IRB.CreateLoad(StackGuardVar, "StackGuard");
403 }
404 
findInsts(Function & F,SmallVectorImpl<AllocaInst * > & StaticAllocas,SmallVectorImpl<AllocaInst * > & DynamicAllocas,SmallVectorImpl<Argument * > & ByValArguments,SmallVectorImpl<ReturnInst * > & Returns,SmallVectorImpl<Instruction * > & StackRestorePoints)405 void SafeStack::findInsts(Function &F,
406                           SmallVectorImpl<AllocaInst *> &StaticAllocas,
407                           SmallVectorImpl<AllocaInst *> &DynamicAllocas,
408                           SmallVectorImpl<Argument *> &ByValArguments,
409                           SmallVectorImpl<ReturnInst *> &Returns,
410                           SmallVectorImpl<Instruction *> &StackRestorePoints) {
411   for (Instruction &I : instructions(&F)) {
412     if (auto AI = dyn_cast<AllocaInst>(&I)) {
413       ++NumAllocas;
414 
415       uint64_t Size = getStaticAllocaAllocationSize(AI);
416       if (IsSafeStackAlloca(AI, Size))
417         continue;
418 
419       if (AI->isStaticAlloca()) {
420         ++NumUnsafeStaticAllocas;
421         StaticAllocas.push_back(AI);
422       } else {
423         ++NumUnsafeDynamicAllocas;
424         DynamicAllocas.push_back(AI);
425       }
426     } else if (auto RI = dyn_cast<ReturnInst>(&I)) {
427       Returns.push_back(RI);
428     } else if (auto CI = dyn_cast<CallInst>(&I)) {
429       // setjmps require stack restore.
430       if (CI->getCalledFunction() && CI->canReturnTwice())
431         StackRestorePoints.push_back(CI);
432     } else if (auto LP = dyn_cast<LandingPadInst>(&I)) {
433       // Exception landing pads require stack restore.
434       StackRestorePoints.push_back(LP);
435     } else if (auto II = dyn_cast<IntrinsicInst>(&I)) {
436       if (II->getIntrinsicID() == Intrinsic::gcroot)
437         llvm::report_fatal_error(
438             "gcroot intrinsic not compatible with safestack attribute");
439     }
440   }
441   for (Argument &Arg : F.args()) {
442     if (!Arg.hasByValAttr())
443       continue;
444     uint64_t Size =
445         DL->getTypeStoreSize(Arg.getType()->getPointerElementType());
446     if (IsSafeStackAlloca(&Arg, Size))
447       continue;
448 
449     ++NumUnsafeByValArguments;
450     ByValArguments.push_back(&Arg);
451   }
452 }
453 
454 AllocaInst *
createStackRestorePoints(IRBuilder<> & IRB,Function & F,ArrayRef<Instruction * > StackRestorePoints,Value * StaticTop,bool NeedDynamicTop)455 SafeStack::createStackRestorePoints(IRBuilder<> &IRB, Function &F,
456                                     ArrayRef<Instruction *> StackRestorePoints,
457                                     Value *StaticTop, bool NeedDynamicTop) {
458   assert(StaticTop && "The stack top isn't set.");
459 
460   if (StackRestorePoints.empty())
461     return nullptr;
462 
463   // We need the current value of the shadow stack pointer to restore
464   // after longjmp or exception catching.
465 
466   // FIXME: On some platforms this could be handled by the longjmp/exception
467   // runtime itself.
468 
469   AllocaInst *DynamicTop = nullptr;
470   if (NeedDynamicTop) {
471     // If we also have dynamic alloca's, the stack pointer value changes
472     // throughout the function. For now we store it in an alloca.
473     DynamicTop = IRB.CreateAlloca(StackPtrTy, /*ArraySize=*/nullptr,
474                                   "unsafe_stack_dynamic_ptr");
475     IRB.CreateStore(StaticTop, DynamicTop);
476   }
477 
478   // Restore current stack pointer after longjmp/exception catch.
479   for (Instruction *I : StackRestorePoints) {
480     ++NumUnsafeStackRestorePoints;
481 
482     IRB.SetInsertPoint(I->getNextNode());
483     Value *CurrentTop = DynamicTop ? IRB.CreateLoad(DynamicTop) : StaticTop;
484     IRB.CreateStore(CurrentTop, UnsafeStackPtr);
485   }
486 
487   return DynamicTop;
488 }
489 
checkStackGuard(IRBuilder<> & IRB,Function & F,ReturnInst & RI,AllocaInst * StackGuardSlot,Value * StackGuard)490 void SafeStack::checkStackGuard(IRBuilder<> &IRB, Function &F, ReturnInst &RI,
491                                 AllocaInst *StackGuardSlot, Value *StackGuard) {
492   Value *V = IRB.CreateLoad(StackGuardSlot);
493   Value *Cmp = IRB.CreateICmpNE(StackGuard, V);
494 
495   auto SuccessProb = BranchProbabilityInfo::getBranchProbStackProtector(true);
496   auto FailureProb = BranchProbabilityInfo::getBranchProbStackProtector(false);
497   MDNode *Weights = MDBuilder(F.getContext())
498                         .createBranchWeights(SuccessProb.getNumerator(),
499                                              FailureProb.getNumerator());
500   Instruction *CheckTerm =
501       SplitBlockAndInsertIfThen(Cmp, &RI,
502                                 /* Unreachable */ true, Weights);
503   IRBuilder<> IRBFail(CheckTerm);
504   // FIXME: respect -fsanitize-trap / -ftrap-function here?
505   Constant *StackChkFail = F.getParent()->getOrInsertFunction(
506       "__stack_chk_fail", IRB.getVoidTy(), nullptr);
507   IRBFail.CreateCall(StackChkFail, {});
508 }
509 
510 /// We explicitly compute and set the unsafe stack layout for all unsafe
511 /// static alloca instructions. We save the unsafe "base pointer" in the
512 /// prologue into a local variable and restore it in the epilogue.
moveStaticAllocasToUnsafeStack(IRBuilder<> & IRB,Function & F,ArrayRef<AllocaInst * > StaticAllocas,ArrayRef<Argument * > ByValArguments,ArrayRef<ReturnInst * > Returns,Instruction * BasePointer,AllocaInst * StackGuardSlot)513 Value *SafeStack::moveStaticAllocasToUnsafeStack(
514     IRBuilder<> &IRB, Function &F, ArrayRef<AllocaInst *> StaticAllocas,
515     ArrayRef<Argument *> ByValArguments, ArrayRef<ReturnInst *> Returns,
516     Instruction *BasePointer, AllocaInst *StackGuardSlot) {
517   if (StaticAllocas.empty() && ByValArguments.empty())
518     return BasePointer;
519 
520   DIBuilder DIB(*F.getParent());
521 
522   StackColoring SSC(F, StaticAllocas);
523   SSC.run();
524   SSC.removeAllMarkers();
525 
526   // Unsafe stack always grows down.
527   StackLayout SSL(StackAlignment);
528   if (StackGuardSlot) {
529     Type *Ty = StackGuardSlot->getAllocatedType();
530     unsigned Align =
531         std::max(DL->getPrefTypeAlignment(Ty), StackGuardSlot->getAlignment());
532     SSL.addObject(StackGuardSlot, getStaticAllocaAllocationSize(StackGuardSlot),
533                   Align, SSC.getFullLiveRange());
534   }
535 
536   for (Argument *Arg : ByValArguments) {
537     Type *Ty = Arg->getType()->getPointerElementType();
538     uint64_t Size = DL->getTypeStoreSize(Ty);
539     if (Size == 0)
540       Size = 1; // Don't create zero-sized stack objects.
541 
542     // Ensure the object is properly aligned.
543     unsigned Align = std::max((unsigned)DL->getPrefTypeAlignment(Ty),
544                               Arg->getParamAlignment());
545     SSL.addObject(Arg, Size, Align, SSC.getFullLiveRange());
546   }
547 
548   for (AllocaInst *AI : StaticAllocas) {
549     Type *Ty = AI->getAllocatedType();
550     uint64_t Size = getStaticAllocaAllocationSize(AI);
551     if (Size == 0)
552       Size = 1; // Don't create zero-sized stack objects.
553 
554     // Ensure the object is properly aligned.
555     unsigned Align =
556         std::max((unsigned)DL->getPrefTypeAlignment(Ty), AI->getAlignment());
557 
558     SSL.addObject(AI, Size, Align, SSC.getLiveRange(AI));
559   }
560 
561   SSL.computeLayout();
562   unsigned FrameAlignment = SSL.getFrameAlignment();
563 
564   // FIXME: tell SSL that we start at a less-then-MaxAlignment aligned location
565   // (AlignmentSkew).
566   if (FrameAlignment > StackAlignment) {
567     // Re-align the base pointer according to the max requested alignment.
568     assert(isPowerOf2_32(FrameAlignment));
569     IRB.SetInsertPoint(BasePointer->getNextNode());
570     BasePointer = cast<Instruction>(IRB.CreateIntToPtr(
571         IRB.CreateAnd(IRB.CreatePtrToInt(BasePointer, IntPtrTy),
572                       ConstantInt::get(IntPtrTy, ~uint64_t(FrameAlignment - 1))),
573         StackPtrTy));
574   }
575 
576   IRB.SetInsertPoint(BasePointer->getNextNode());
577 
578   if (StackGuardSlot) {
579     unsigned Offset = SSL.getObjectOffset(StackGuardSlot);
580     Value *Off = IRB.CreateGEP(BasePointer, // BasePointer is i8*
581                                ConstantInt::get(Int32Ty, -Offset));
582     Value *NewAI =
583         IRB.CreateBitCast(Off, StackGuardSlot->getType(), "StackGuardSlot");
584 
585     // Replace alloc with the new location.
586     StackGuardSlot->replaceAllUsesWith(NewAI);
587     StackGuardSlot->eraseFromParent();
588   }
589 
590   for (Argument *Arg : ByValArguments) {
591     unsigned Offset = SSL.getObjectOffset(Arg);
592     Type *Ty = Arg->getType()->getPointerElementType();
593 
594     uint64_t Size = DL->getTypeStoreSize(Ty);
595     if (Size == 0)
596       Size = 1; // Don't create zero-sized stack objects.
597 
598     Value *Off = IRB.CreateGEP(BasePointer, // BasePointer is i8*
599                                ConstantInt::get(Int32Ty, -Offset));
600     Value *NewArg = IRB.CreateBitCast(Off, Arg->getType(),
601                                      Arg->getName() + ".unsafe-byval");
602 
603     // Replace alloc with the new location.
604     replaceDbgDeclare(Arg, BasePointer, BasePointer->getNextNode(), DIB,
605                       /*Deref=*/true, -Offset);
606     Arg->replaceAllUsesWith(NewArg);
607     IRB.SetInsertPoint(cast<Instruction>(NewArg)->getNextNode());
608     IRB.CreateMemCpy(Off, Arg, Size, Arg->getParamAlignment());
609   }
610 
611   // Allocate space for every unsafe static AllocaInst on the unsafe stack.
612   for (AllocaInst *AI : StaticAllocas) {
613     IRB.SetInsertPoint(AI);
614     unsigned Offset = SSL.getObjectOffset(AI);
615 
616     uint64_t Size = getStaticAllocaAllocationSize(AI);
617     if (Size == 0)
618       Size = 1; // Don't create zero-sized stack objects.
619 
620     replaceDbgDeclareForAlloca(AI, BasePointer, DIB, /*Deref=*/true, -Offset);
621     replaceDbgValueForAlloca(AI, BasePointer, DIB, -Offset);
622 
623     // Replace uses of the alloca with the new location.
624     // Insert address calculation close to each use to work around PR27844.
625     std::string Name = std::string(AI->getName()) + ".unsafe";
626     while (!AI->use_empty()) {
627       Use &U = *AI->use_begin();
628       Instruction *User = cast<Instruction>(U.getUser());
629 
630       Instruction *InsertBefore;
631       if (auto *PHI = dyn_cast<PHINode>(User))
632         InsertBefore = PHI->getIncomingBlock(U)->getTerminator();
633       else
634         InsertBefore = User;
635 
636       IRBuilder<> IRBUser(InsertBefore);
637       Value *Off = IRBUser.CreateGEP(BasePointer, // BasePointer is i8*
638                                      ConstantInt::get(Int32Ty, -Offset));
639       Value *Replacement = IRBUser.CreateBitCast(Off, AI->getType(), Name);
640 
641       if (auto *PHI = dyn_cast<PHINode>(User)) {
642         // PHI nodes may have multiple incoming edges from the same BB (why??),
643         // all must be updated at once with the same incoming value.
644         auto *BB = PHI->getIncomingBlock(U);
645         for (unsigned I = 0; I < PHI->getNumIncomingValues(); ++I)
646           if (PHI->getIncomingBlock(I) == BB)
647             PHI->setIncomingValue(I, Replacement);
648       } else {
649         U.set(Replacement);
650       }
651     }
652 
653     AI->eraseFromParent();
654   }
655 
656   // Re-align BasePointer so that our callees would see it aligned as
657   // expected.
658   // FIXME: no need to update BasePointer in leaf functions.
659   unsigned FrameSize = alignTo(SSL.getFrameSize(), StackAlignment);
660 
661   // Update shadow stack pointer in the function epilogue.
662   IRB.SetInsertPoint(BasePointer->getNextNode());
663 
664   Value *StaticTop =
665       IRB.CreateGEP(BasePointer, ConstantInt::get(Int32Ty, -FrameSize),
666                     "unsafe_stack_static_top");
667   IRB.CreateStore(StaticTop, UnsafeStackPtr);
668   return StaticTop;
669 }
670 
moveDynamicAllocasToUnsafeStack(Function & F,Value * UnsafeStackPtr,AllocaInst * DynamicTop,ArrayRef<AllocaInst * > DynamicAllocas)671 void SafeStack::moveDynamicAllocasToUnsafeStack(
672     Function &F, Value *UnsafeStackPtr, AllocaInst *DynamicTop,
673     ArrayRef<AllocaInst *> DynamicAllocas) {
674   DIBuilder DIB(*F.getParent());
675 
676   for (AllocaInst *AI : DynamicAllocas) {
677     IRBuilder<> IRB(AI);
678 
679     // Compute the new SP value (after AI).
680     Value *ArraySize = AI->getArraySize();
681     if (ArraySize->getType() != IntPtrTy)
682       ArraySize = IRB.CreateIntCast(ArraySize, IntPtrTy, false);
683 
684     Type *Ty = AI->getAllocatedType();
685     uint64_t TySize = DL->getTypeAllocSize(Ty);
686     Value *Size = IRB.CreateMul(ArraySize, ConstantInt::get(IntPtrTy, TySize));
687 
688     Value *SP = IRB.CreatePtrToInt(IRB.CreateLoad(UnsafeStackPtr), IntPtrTy);
689     SP = IRB.CreateSub(SP, Size);
690 
691     // Align the SP value to satisfy the AllocaInst, type and stack alignments.
692     unsigned Align = std::max(
693         std::max((unsigned)DL->getPrefTypeAlignment(Ty), AI->getAlignment()),
694         (unsigned)StackAlignment);
695 
696     assert(isPowerOf2_32(Align));
697     Value *NewTop = IRB.CreateIntToPtr(
698         IRB.CreateAnd(SP, ConstantInt::get(IntPtrTy, ~uint64_t(Align - 1))),
699         StackPtrTy);
700 
701     // Save the stack pointer.
702     IRB.CreateStore(NewTop, UnsafeStackPtr);
703     if (DynamicTop)
704       IRB.CreateStore(NewTop, DynamicTop);
705 
706     Value *NewAI = IRB.CreatePointerCast(NewTop, AI->getType());
707     if (AI->hasName() && isa<Instruction>(NewAI))
708       NewAI->takeName(AI);
709 
710     replaceDbgDeclareForAlloca(AI, NewAI, DIB, /*Deref=*/true);
711     AI->replaceAllUsesWith(NewAI);
712     AI->eraseFromParent();
713   }
714 
715   if (!DynamicAllocas.empty()) {
716     // Now go through the instructions again, replacing stacksave/stackrestore.
717     for (inst_iterator It = inst_begin(&F), Ie = inst_end(&F); It != Ie;) {
718       Instruction *I = &*(It++);
719       auto II = dyn_cast<IntrinsicInst>(I);
720       if (!II)
721         continue;
722 
723       if (II->getIntrinsicID() == Intrinsic::stacksave) {
724         IRBuilder<> IRB(II);
725         Instruction *LI = IRB.CreateLoad(UnsafeStackPtr);
726         LI->takeName(II);
727         II->replaceAllUsesWith(LI);
728         II->eraseFromParent();
729       } else if (II->getIntrinsicID() == Intrinsic::stackrestore) {
730         IRBuilder<> IRB(II);
731         Instruction *SI = IRB.CreateStore(II->getArgOperand(0), UnsafeStackPtr);
732         SI->takeName(II);
733         assert(II->use_empty());
734         II->eraseFromParent();
735       }
736     }
737   }
738 }
739 
runOnFunction(Function & F)740 bool SafeStack::runOnFunction(Function &F) {
741   DEBUG(dbgs() << "[SafeStack] Function: " << F.getName() << "\n");
742 
743   if (!F.hasFnAttribute(Attribute::SafeStack)) {
744     DEBUG(dbgs() << "[SafeStack]     safestack is not requested"
745                     " for this function\n");
746     return false;
747   }
748 
749   if (F.isDeclaration()) {
750     DEBUG(dbgs() << "[SafeStack]     function definition"
751                     " is not available\n");
752     return false;
753   }
754 
755   TL = TM ? TM->getSubtargetImpl(F)->getTargetLowering() : nullptr;
756   SE = &getAnalysis<ScalarEvolutionWrapperPass>().getSE();
757 
758   ++NumFunctions;
759 
760   SmallVector<AllocaInst *, 16> StaticAllocas;
761   SmallVector<AllocaInst *, 4> DynamicAllocas;
762   SmallVector<Argument *, 4> ByValArguments;
763   SmallVector<ReturnInst *, 4> Returns;
764 
765   // Collect all points where stack gets unwound and needs to be restored
766   // This is only necessary because the runtime (setjmp and unwind code) is
767   // not aware of the unsafe stack and won't unwind/restore it prorerly.
768   // To work around this problem without changing the runtime, we insert
769   // instrumentation to restore the unsafe stack pointer when necessary.
770   SmallVector<Instruction *, 4> StackRestorePoints;
771 
772   // Find all static and dynamic alloca instructions that must be moved to the
773   // unsafe stack, all return instructions and stack restore points.
774   findInsts(F, StaticAllocas, DynamicAllocas, ByValArguments, Returns,
775             StackRestorePoints);
776 
777   if (StaticAllocas.empty() && DynamicAllocas.empty() &&
778       ByValArguments.empty() && StackRestorePoints.empty())
779     return false; // Nothing to do in this function.
780 
781   if (!StaticAllocas.empty() || !DynamicAllocas.empty() ||
782       !ByValArguments.empty())
783     ++NumUnsafeStackFunctions; // This function has the unsafe stack.
784 
785   if (!StackRestorePoints.empty())
786     ++NumUnsafeStackRestorePointsFunctions;
787 
788   IRBuilder<> IRB(&F.front(), F.begin()->getFirstInsertionPt());
789   UnsafeStackPtr = getOrCreateUnsafeStackPtr(IRB, F);
790 
791   // Load the current stack pointer (we'll also use it as a base pointer).
792   // FIXME: use a dedicated register for it ?
793   Instruction *BasePointer =
794       IRB.CreateLoad(UnsafeStackPtr, false, "unsafe_stack_ptr");
795   assert(BasePointer->getType() == StackPtrTy);
796 
797   AllocaInst *StackGuardSlot = nullptr;
798   // FIXME: implement weaker forms of stack protector.
799   if (F.hasFnAttribute(Attribute::StackProtect) ||
800       F.hasFnAttribute(Attribute::StackProtectStrong) ||
801       F.hasFnAttribute(Attribute::StackProtectReq)) {
802     Value *StackGuard = getStackGuard(IRB, F);
803     StackGuardSlot = IRB.CreateAlloca(StackPtrTy, nullptr);
804     IRB.CreateStore(StackGuard, StackGuardSlot);
805 
806     for (ReturnInst *RI : Returns) {
807       IRBuilder<> IRBRet(RI);
808       checkStackGuard(IRBRet, F, *RI, StackGuardSlot, StackGuard);
809     }
810   }
811 
812   // The top of the unsafe stack after all unsafe static allocas are
813   // allocated.
814   Value *StaticTop =
815       moveStaticAllocasToUnsafeStack(IRB, F, StaticAllocas, ByValArguments,
816                                      Returns, BasePointer, StackGuardSlot);
817 
818   // Safe stack object that stores the current unsafe stack top. It is updated
819   // as unsafe dynamic (non-constant-sized) allocas are allocated and freed.
820   // This is only needed if we need to restore stack pointer after longjmp
821   // or exceptions, and we have dynamic allocations.
822   // FIXME: a better alternative might be to store the unsafe stack pointer
823   // before setjmp / invoke instructions.
824   AllocaInst *DynamicTop = createStackRestorePoints(
825       IRB, F, StackRestorePoints, StaticTop, !DynamicAllocas.empty());
826 
827   // Handle dynamic allocas.
828   moveDynamicAllocasToUnsafeStack(F, UnsafeStackPtr, DynamicTop,
829                                   DynamicAllocas);
830 
831   // Restore the unsafe stack pointer before each return.
832   for (ReturnInst *RI : Returns) {
833     IRB.SetInsertPoint(RI);
834     IRB.CreateStore(BasePointer, UnsafeStackPtr);
835   }
836 
837   DEBUG(dbgs() << "[SafeStack]     safestack applied\n");
838   return true;
839 }
840 
841 } // anonymous namespace
842 
843 char SafeStack::ID = 0;
844 INITIALIZE_TM_PASS_BEGIN(SafeStack, "safe-stack",
845                          "Safe Stack instrumentation pass", false, false)
846 INITIALIZE_TM_PASS_END(SafeStack, "safe-stack",
847                        "Safe Stack instrumentation pass", false, false)
848 
createSafeStackPass(const llvm::TargetMachine * TM)849 FunctionPass *llvm::createSafeStackPass(const llvm::TargetMachine *TM) {
850   return new SafeStack(TM);
851 }
852