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/Instruction.h"
23 #include "llvm/IR/Instructions.h"
24 #include "llvm/IR/Intrinsics.h"
25 #include "llvm/IR/Value.h"
26 #include "llvm/InitializePasses.h"
27 #include "llvm/Pass.h"
28 #include "llvm/Support/Casting.h"
29 #include "llvm/Support/CommandLine.h"
30 #include "llvm/Support/Debug.h"
31 #include "llvm/Support/raw_ostream.h"
32 #include <cstdint>
33 #include <utility>
34 
35 using namespace llvm;
36 
37 #define DEBUG_TYPE "bounds-checking"
38 
39 static cl::opt<bool> SingleTrapBB("bounds-checking-single-trap",
40                                   cl::desc("Use one trap block per function"));
41 
42 STATISTIC(ChecksAdded, "Bounds checks added");
43 STATISTIC(ChecksSkipped, "Bounds checks skipped");
44 STATISTIC(ChecksUnable, "Bounds checks unable to add");
45 
46 using BuilderTy = IRBuilder<TargetFolder>;
47 
48 /// Gets the conditions under which memory accessing instructions will overflow.
49 ///
50 /// \p Ptr is the pointer that will be read/written, and \p InstVal is either
51 /// the result from the load or the value being stored. It is used to determine
52 /// the size of memory block that is touched.
53 ///
54 /// Returns the condition under which the access will overflow.
55 static Value *getBoundsCheckCond(Value *Ptr, Value *InstVal,
56                                  const DataLayout &DL, TargetLibraryInfo &TLI,
57                                  ObjectSizeOffsetEvaluator &ObjSizeEval,
58                                  BuilderTy &IRB, ScalarEvolution &SE) {
59   uint64_t NeededSize = DL.getTypeStoreSize(InstVal->getType());
60   LLVM_DEBUG(dbgs() << "Instrument " << *Ptr << " for " << Twine(NeededSize)
61                     << " bytes\n");
62 
63   SizeOffsetEvalType SizeOffset = ObjSizeEval.compute(Ptr);
64 
65   if (!ObjSizeEval.bothKnown(SizeOffset)) {
66     ++ChecksUnable;
67     return nullptr;
68   }
69 
70   Value *Size   = SizeOffset.first;
71   Value *Offset = SizeOffset.second;
72   ConstantInt *SizeCI = dyn_cast<ConstantInt>(Size);
73 
74   Type *IntTy = DL.getIntPtrType(Ptr->getType());
75   Value *NeededSizeVal = ConstantInt::get(IntTy, NeededSize);
76 
77   auto SizeRange = SE.getUnsignedRange(SE.getSCEV(Size));
78   auto OffsetRange = SE.getUnsignedRange(SE.getSCEV(Offset));
79   auto NeededSizeRange = SE.getUnsignedRange(SE.getSCEV(NeededSizeVal));
80 
81   // three checks are required to ensure safety:
82   // . Offset >= 0  (since the offset is given from the base ptr)
83   // . Size >= Offset  (unsigned)
84   // . Size - Offset >= NeededSize  (unsigned)
85   //
86   // optimization: if Size >= 0 (signed), skip 1st check
87   // FIXME: add NSW/NUW here?  -- we dont care if the subtraction overflows
88   Value *ObjSize = IRB.CreateSub(Size, Offset);
89   Value *Cmp2 = SizeRange.getUnsignedMin().uge(OffsetRange.getUnsignedMax())
90                     ? ConstantInt::getFalse(Ptr->getContext())
91                     : IRB.CreateICmpULT(Size, Offset);
92   Value *Cmp3 = SizeRange.sub(OffsetRange)
93                         .getUnsignedMin()
94                         .uge(NeededSizeRange.getUnsignedMax())
95                     ? ConstantInt::getFalse(Ptr->getContext())
96                     : IRB.CreateICmpULT(ObjSize, NeededSizeVal);
97   Value *Or = IRB.CreateOr(Cmp2, Cmp3);
98   if ((!SizeCI || SizeCI->getValue().slt(0)) &&
99       !SizeRange.getSignedMin().isNonNegative()) {
100     Value *Cmp1 = IRB.CreateICmpSLT(Offset, ConstantInt::get(IntTy, 0));
101     Or = IRB.CreateOr(Cmp1, Or);
102   }
103 
104   return Or;
105 }
106 
107 /// Adds run-time bounds checks to memory accessing instructions.
108 ///
109 /// \p Or is the condition that should guard the trap.
110 ///
111 /// \p GetTrapBB is a callable that returns the trap BB to use on failure.
112 template <typename GetTrapBBT>
113 static void insertBoundsCheck(Value *Or, BuilderTy &IRB, GetTrapBBT GetTrapBB) {
114   // check if the comparison is always false
115   ConstantInt *C = dyn_cast_or_null<ConstantInt>(Or);
116   if (C) {
117     ++ChecksSkipped;
118     // If non-zero, nothing to do.
119     if (!C->getZExtValue())
120       return;
121   }
122   ++ChecksAdded;
123 
124   BasicBlock::iterator SplitI = IRB.GetInsertPoint();
125   BasicBlock *OldBB = SplitI->getParent();
126   BasicBlock *Cont = OldBB->splitBasicBlock(SplitI);
127   OldBB->getTerminator()->eraseFromParent();
128 
129   if (C) {
130     // If we have a constant zero, unconditionally branch.
131     // FIXME: We should really handle this differently to bypass the splitting
132     // the block.
133     BranchInst::Create(GetTrapBB(IRB), OldBB);
134     return;
135   }
136 
137   // Create the conditional branch.
138   BranchInst::Create(GetTrapBB(IRB), Cont, Or, OldBB);
139 }
140 
141 static bool addBoundsChecking(Function &F, TargetLibraryInfo &TLI,
142                               ScalarEvolution &SE) {
143   if (F.hasFnAttribute(Attribute::NoSanitizeBounds))
144     return false;
145 
146   const DataLayout &DL = F.getParent()->getDataLayout();
147   ObjectSizeOpts EvalOpts;
148   EvalOpts.RoundToAlign = true;
149   ObjectSizeOffsetEvaluator ObjSizeEval(DL, &TLI, F.getContext(), EvalOpts);
150 
151   // check HANDLE_MEMORY_INST in include/llvm/Instruction.def for memory
152   // touching instructions
153   SmallVector<std::pair<Instruction *, Value *>, 4> TrapInfo;
154   for (Instruction &I : instructions(F)) {
155     Value *Or = nullptr;
156     BuilderTy IRB(I.getParent(), BasicBlock::iterator(&I), TargetFolder(DL));
157     if (LoadInst *LI = dyn_cast<LoadInst>(&I)) {
158       if (!LI->isVolatile())
159         Or = getBoundsCheckCond(LI->getPointerOperand(), LI, DL, TLI,
160                                 ObjSizeEval, IRB, SE);
161     } else if (StoreInst *SI = dyn_cast<StoreInst>(&I)) {
162       if (!SI->isVolatile())
163         Or = getBoundsCheckCond(SI->getPointerOperand(), SI->getValueOperand(),
164                                 DL, TLI, ObjSizeEval, IRB, SE);
165     } else if (AtomicCmpXchgInst *AI = dyn_cast<AtomicCmpXchgInst>(&I)) {
166       if (!AI->isVolatile())
167         Or =
168             getBoundsCheckCond(AI->getPointerOperand(), AI->getCompareOperand(),
169                                DL, TLI, ObjSizeEval, IRB, SE);
170     } else if (AtomicRMWInst *AI = dyn_cast<AtomicRMWInst>(&I)) {
171       if (!AI->isVolatile())
172         Or = getBoundsCheckCond(AI->getPointerOperand(), AI->getValOperand(),
173                                 DL, TLI, ObjSizeEval, IRB, SE);
174     }
175     if (Or)
176       TrapInfo.push_back(std::make_pair(&I, Or));
177   }
178 
179   // Create a trapping basic block on demand using a callback. Depending on
180   // flags, this will either create a single block for the entire function or
181   // will create a fresh block every time it is called.
182   BasicBlock *TrapBB = nullptr;
183   auto GetTrapBB = [&TrapBB](BuilderTy &IRB) {
184     if (TrapBB && SingleTrapBB)
185       return TrapBB;
186 
187     Function *Fn = IRB.GetInsertBlock()->getParent();
188     // FIXME: This debug location doesn't make a lot of sense in the
189     // `SingleTrapBB` case.
190     auto DebugLoc = IRB.getCurrentDebugLocation();
191     IRBuilder<>::InsertPointGuard Guard(IRB);
192     TrapBB = BasicBlock::Create(Fn->getContext(), "trap", Fn);
193     IRB.SetInsertPoint(TrapBB);
194 
195     auto *F = Intrinsic::getDeclaration(Fn->getParent(), Intrinsic::trap);
196     CallInst *TrapCall = IRB.CreateCall(F, {});
197     TrapCall->setDoesNotReturn();
198     TrapCall->setDoesNotThrow();
199     TrapCall->setDebugLoc(DebugLoc);
200     IRB.CreateUnreachable();
201 
202     return TrapBB;
203   };
204 
205   // Add the checks.
206   for (const auto &Entry : TrapInfo) {
207     Instruction *Inst = Entry.first;
208     BuilderTy IRB(Inst->getParent(), BasicBlock::iterator(Inst), TargetFolder(DL));
209     insertBoundsCheck(Entry.second, IRB, GetTrapBB);
210   }
211 
212   return !TrapInfo.empty();
213 }
214 
215 PreservedAnalyses BoundsCheckingPass::run(Function &F, FunctionAnalysisManager &AM) {
216   auto &TLI = AM.getResult<TargetLibraryAnalysis>(F);
217   auto &SE = AM.getResult<ScalarEvolutionAnalysis>(F);
218 
219   if (!addBoundsChecking(F, TLI, SE))
220     return PreservedAnalyses::all();
221 
222   return PreservedAnalyses::none();
223 }
224 
225 namespace {
226 struct BoundsCheckingLegacyPass : public FunctionPass {
227   static char ID;
228 
229   BoundsCheckingLegacyPass() : FunctionPass(ID) {
230     initializeBoundsCheckingLegacyPassPass(*PassRegistry::getPassRegistry());
231   }
232 
233   bool runOnFunction(Function &F) override {
234     auto &TLI = getAnalysis<TargetLibraryInfoWrapperPass>().getTLI(F);
235     auto &SE = getAnalysis<ScalarEvolutionWrapperPass>().getSE();
236     return addBoundsChecking(F, TLI, SE);
237   }
238 
239   void getAnalysisUsage(AnalysisUsage &AU) const override {
240     AU.addRequired<TargetLibraryInfoWrapperPass>();
241     AU.addRequired<ScalarEvolutionWrapperPass>();
242   }
243 };
244 } // namespace
245 
246 char BoundsCheckingLegacyPass::ID = 0;
247 INITIALIZE_PASS_BEGIN(BoundsCheckingLegacyPass, "bounds-checking",
248                       "Run-time bounds checking", false, false)
249 INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfoWrapperPass)
250 INITIALIZE_PASS_END(BoundsCheckingLegacyPass, "bounds-checking",
251                     "Run-time bounds checking", false, false)
252 
253 FunctionPass *llvm::createBoundsCheckingLegacyPass() {
254   return new BoundsCheckingLegacyPass();
255 }
256