llvm/lib/Transforms/Instrumentation/BoundsChecking.cpp

   1 //===- BoundsChecking.cpp - Instrumentation for run-time bounds checking --===//
   2 //
   3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
   4 // See https://llvm.org/LICENSE.txt for license information.
   5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
   6 //
   7 //===----------------------------------------------------------------------===//
   8
   9 #include "llvm/Transforms/Instrumentation/BoundsChecking.h"
  10 #include "llvm/ADT/Statistic.h"
  11 #include "llvm/ADT/Twine.h"
  12 #include "llvm/Analysis/MemoryBuiltins.h"
  13 #include "llvm/Analysis/ScalarEvolution.h"
  14 #include "llvm/Analysis/TargetFolder.h"
  15 #include "llvm/Analysis/TargetLibraryInfo.h"
  16 #include "llvm/IR/BasicBlock.h"
  17 #include "llvm/IR/Constants.h"
  18 #include "llvm/IR/DataLayout.h"
  19 #include "llvm/IR/Function.h"
  20 #include "llvm/IR/IRBuilder.h"
  21 #include "llvm/IR/InstIterator.h"
  22 #include "llvm/IR/InstrTypes.h"
  23 #include "llvm/IR/Instruction.h"
  24 #include "llvm/IR/Instructions.h"
  25 #include "llvm/IR/Intrinsics.h"
  26 #include "llvm/IR/Value.h"
  27 #include "llvm/InitializePasses.h"
  28 #include "llvm/Pass.h"
  29 #include "llvm/Support/Casting.h"
  30 #include "llvm/Support/CommandLine.h"
  31 #include "llvm/Support/Debug.h"
  32 #include "llvm/Support/ErrorHandling.h"
  33 #include "llvm/Support/raw_ostream.h"
  34 #include <cstdint>
  35 #include <utility>
  36
  37 using namespace llvm;
  38
  39 #define DEBUG_TYPE "bounds-checking"
  40
  41 static cl::opt<bool> SingleTrapBB("bounds-checking-single-trap",
  42                                   cl::desc("Use one trap block per function"));
  43
  44 STATISTIC(ChecksAdded, "Bounds checks added");
  45 STATISTIC(ChecksSkipped, "Bounds checks skipped");
  46 STATISTIC(ChecksUnable, "Bounds checks unable to add");
  47
  48 using BuilderTy = IRBuilder<TargetFolder>;
  49
  50 /// Gets the conditions under which memory accessing instructions will overflow.
  51 ///
  52 /// \p Ptr is the pointer that will be read/written, and \p InstVal is either
  53 /// the result from the load or the value being stored. It is used to determine
  54 /// the size of memory block that is touched.
  55 ///
  56 /// Returns the condition under which the access will overflow.
  57 static Value *getBoundsCheckCond(Value *Ptr, Value *InstVal,
  58                                  const DataLayout &DL, TargetLibraryInfo &TLI,
  59                                  ObjectSizeOffsetEvaluator &ObjSizeEval,
  60                                  BuilderTy &IRB, ScalarEvolution &SE) {
  61   uint64_t NeededSize = DL.getTypeStoreSize(InstVal->getType());
  62   LLVM_DEBUG(dbgs() << "Instrument " << *Ptr << " for " << Twine(NeededSize)
  63                     << " bytes\n");
  64
  65   SizeOffsetEvalType SizeOffset = ObjSizeEval.compute(Ptr);
  66
  67   if (!ObjSizeEval.bothKnown(SizeOffset)) {
  68     ++ChecksUnable;
  69     return nullptr;
  70   }
  71
  72   Value *Size   = SizeOffset.first;
  73   Value *Offset = SizeOffset.second;
  74   ConstantInt *SizeCI = dyn_cast<ConstantInt>(Size);
  75
  76   Type *IntTy = DL.getIntPtrType(Ptr->getType());
  77   Value *NeededSizeVal = ConstantInt::get(IntTy, NeededSize);
  78
  79   auto SizeRange = SE.getUnsignedRange(SE.getSCEV(Size));
  80   auto OffsetRange = SE.getUnsignedRange(SE.getSCEV(Offset));
  81   auto NeededSizeRange = SE.getUnsignedRange(SE.getSCEV(NeededSizeVal));
  82
  83   // three checks are required to ensure safety:
  84   // . Offset >= 0  (since the offset is given from the base ptr)
  85   // . Size >= Offset  (unsigned)
  86   // . Size - Offset >= NeededSize  (unsigned)
  87   //
  88   // optimization: if Size >= 0 (signed), skip 1st check
  89   // FIXME: add NSW/NUW here?  -- we dont care if the subtraction overflows
  90   Value *ObjSize = IRB.CreateSub(Size, Offset);
  91   Value *Cmp2 = SizeRange.getUnsignedMin().uge(OffsetRange.getUnsignedMax())
  92                     ? ConstantInt::getFalse(Ptr->getContext())
  93                     : IRB.CreateICmpULT(Size, Offset);
  94   Value *Cmp3 = SizeRange.sub(OffsetRange)
  95                         .getUnsignedMin()
  96                         .uge(NeededSizeRange.getUnsignedMax())
  97                     ? ConstantInt::getFalse(Ptr->getContext())
  98                     : IRB.CreateICmpULT(ObjSize, NeededSizeVal);
  99   Value *Or = IRB.CreateOr(Cmp2, Cmp3);
 100   if ((!SizeCI || SizeCI->getValue().slt(0)) &&
 101       !SizeRange.getSignedMin().isNonNegative()) {
 102     Value *Cmp1 = IRB.CreateICmpSLT(Offset, ConstantInt::get(IntTy, 0));
 103     Or = IRB.CreateOr(Cmp1, Or);
 104   }
 105
 106   return Or;
 107 }
 108
 109 /// Adds run-time bounds checks to memory accessing instructions.
 110 ///
 111 /// \p Or is the condition that should guard the trap.
 112 ///
 113 /// \p GetTrapBB is a callable that returns the trap BB to use on failure.
 114 template <typename GetTrapBBT>
 115 static void insertBoundsCheck(Value *Or, BuilderTy &IRB, GetTrapBBT GetTrapBB) {
 116   // check if the comparison is always false
 117   ConstantInt *C = dyn_cast_or_null<ConstantInt>(Or);
 118   if (C) {
 119     ++ChecksSkipped;
 120     // If non-zero, nothing to do.
 121     if (!C->getZExtValue())
 122       return;
 123   }
 124   ++ChecksAdded;
 125
 126   BasicBlock::iterator SplitI = IRB.GetInsertPoint();
 127   BasicBlock *OldBB = SplitI->getParent();
 128   BasicBlock *Cont = OldBB->splitBasicBlock(SplitI);
 129   OldBB->getTerminator()->eraseFromParent();
 130
 131   if (C) {
 132     // If we have a constant zero, unconditionally branch.
 133     // FIXME: We should really handle this differently to bypass the splitting
 134     // the block.
 135     BranchInst::Create(GetTrapBB(IRB), OldBB);
 136     return;
 137   }
 138
 139   // Create the conditional branch.
 140   BranchInst::Create(GetTrapBB(IRB), Cont, Or, OldBB);
 141 }
 142
 143 static bool addBoundsChecking(Function &F, TargetLibraryInfo &TLI,
 144                               ScalarEvolution &SE) {
 145   const DataLayout &DL = F.getParent()->getDataLayout();
 146   ObjectSizeOpts EvalOpts;
 147   EvalOpts.RoundToAlign = true;
 148   ObjectSizeOffsetEvaluator ObjSizeEval(DL, &TLI, F.getContext(), EvalOpts);
 149
 150   // check HANDLE_MEMORY_INST in include/llvm/Instruction.def for memory
 151   // touching instructions
 152   SmallVector<std::pair<Instruction *, Value *>, 4> TrapInfo;
 153   for (Instruction &I : instructions(F)) {
 154     Value *Or = nullptr;
 155     BuilderTy IRB(I.getParent(), BasicBlock::iterator(&I), TargetFolder(DL));
 156     if (LoadInst *LI = dyn_cast<LoadInst>(&I)) {
 157       if (!LI->isVolatile())
 158         Or = getBoundsCheckCond(LI->getPointerOperand(), LI, DL, TLI,
 159                                 ObjSizeEval, IRB, SE);
 160     } else if (StoreInst *SI = dyn_cast<StoreInst>(&I)) {
 161       if (!SI->isVolatile())
 162         Or = getBoundsCheckCond(SI->getPointerOperand(), SI->getValueOperand(),
 163                                 DL, TLI, ObjSizeEval, IRB, SE);
 164     } else if (AtomicCmpXchgInst *AI = dyn_cast<AtomicCmpXchgInst>(&I)) {
 165       if (!AI->isVolatile())
 166         Or =
 167             getBoundsCheckCond(AI->getPointerOperand(), AI->getCompareOperand(),
 168                                DL, TLI, ObjSizeEval, IRB, SE);
 169     } else if (AtomicRMWInst *AI = dyn_cast<AtomicRMWInst>(&I)) {
 170       if (!AI->isVolatile())
 171         Or = getBoundsCheckCond(AI->getPointerOperand(), AI->getValOperand(),
 172                                 DL, TLI, ObjSizeEval, IRB, SE);
 173     }
 174     if (Or)
 175       TrapInfo.push_back(std::make_pair(&I, Or));
 176   }
 177
 178   // Create a trapping basic block on demand using a callback. Depending on
 179   // flags, this will either create a single block for the entire function or
 180   // will create a fresh block every time it is called.
 181   BasicBlock *TrapBB = nullptr;
 182   auto GetTrapBB = [&TrapBB](BuilderTy &IRB) {
 183     if (TrapBB && SingleTrapBB)
 184       return TrapBB;
 185
 186     Function *Fn = IRB.GetInsertBlock()->getParent();
 187     // FIXME: This debug location doesn't make a lot of sense in the
 188     // `SingleTrapBB` case.
 189     auto DebugLoc = IRB.getCurrentDebugLocation();
 190     IRBuilder<>::InsertPointGuard Guard(IRB);
 191     TrapBB = BasicBlock::Create(Fn->getContext(), "trap", Fn);
 192     IRB.SetInsertPoint(TrapBB);
 193
 194     auto *F = Intrinsic::getDeclaration(Fn->getParent(), Intrinsic::trap);
 195     CallInst *TrapCall = IRB.CreateCall(F, {});
 196     TrapCall->setDoesNotReturn();
 197     TrapCall->setDoesNotThrow();
 198     TrapCall->setDebugLoc(DebugLoc);
 199     IRB.CreateUnreachable();
 200
 201     return TrapBB;
 202   };
 203
 204   // Add the checks.
 205   for (const auto &Entry : TrapInfo) {
 206     Instruction *Inst = Entry.first;
 207     BuilderTy IRB(Inst->getParent(), BasicBlock::iterator(Inst), TargetFolder(DL));
 208     insertBoundsCheck(Entry.second, IRB, GetTrapBB);
 209   }
 210
 211   return !TrapInfo.empty();
 212 }
 213
 214 PreservedAnalyses BoundsCheckingPass::run(Function &F, FunctionAnalysisManager &AM) {
 215   auto &TLI = AM.getResult<TargetLibraryAnalysis>(F);
 216   auto &SE = AM.getResult<ScalarEvolutionAnalysis>(F);
 217
 218   if (!addBoundsChecking(F, TLI, SE))
 219     return PreservedAnalyses::all();
 220
 221   return PreservedAnalyses::none();
 222 }
 223
 224 namespace {
 225 struct BoundsCheckingLegacyPass : public FunctionPass {
 226   static char ID;
 227
 228   BoundsCheckingLegacyPass() : FunctionPass(ID) {
 229     initializeBoundsCheckingLegacyPassPass(*PassRegistry::getPassRegistry());
 230   }
 231
 232   bool runOnFunction(Function &F) override {
 233     auto &TLI = getAnalysis<TargetLibraryInfoWrapperPass>().getTLI(F);
 234     auto &SE = getAnalysis<ScalarEvolutionWrapperPass>().getSE();
 235     return addBoundsChecking(F, TLI, SE);
 236   }
 237
 238   void getAnalysisUsage(AnalysisUsage &AU) const override {
 239     AU.addRequired<TargetLibraryInfoWrapperPass>();
 240     AU.addRequired<ScalarEvolutionWrapperPass>();
 241   }
 242 };
 243 } // namespace
 244
 245 char BoundsCheckingLegacyPass::ID = 0;
 246 INITIALIZE_PASS_BEGIN(BoundsCheckingLegacyPass, "bounds-checking",
 247                       "Run-time bounds checking", false, false)
 248 INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfoWrapperPass)
 249 INITIALIZE_PASS_END(BoundsCheckingLegacyPass, "bounds-checking",
 250                     "Run-time bounds checking", false, false)
 251
 252 FunctionPass *llvm::createBoundsCheckingLegacyPass() {
 253   return new BoundsCheckingLegacyPass();
 254 }