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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:
     91   AllocaOffsetRewriter(ScalarEvolution &SE, const Value *AllocaPtr)
     92       : SCEVRewriteVisitor(SE), AllocaPtr(AllocaPtr) {}
     93 
     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.
    189   SafeStack(const TargetMachine *TM)
    190       : FunctionPass(ID), TM(TM), TL(nullptr), DL(nullptr) {
    191     initializeSafeStackPass(*PassRegistry::getPassRegistry());
    192   }
    193   SafeStack() : SafeStack(nullptr) {}
    194 
    195   void getAnalysisUsage(AnalysisUsage &AU) const override {
    196     AU.addRequired<ScalarEvolutionWrapperPass>();
    197   }
    198 
    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 
    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 
    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 
    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).
    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 
    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 
    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 
    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 *
    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 
    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.
    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 
    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 
    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 
    849 FunctionPass *llvm::createSafeStackPass(const llvm::TargetMachine *TM) {
    850   return new SafeStack(TM);
    851 }
    852