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   if (F.hasFnAttribute(Attribute::NoSanitizeBounds))
146     return false;
147 
148   const DataLayout &DL = F.getParent()->getDataLayout();
149   ObjectSizeOpts EvalOpts;
150   EvalOpts.RoundToAlign = true;
151   ObjectSizeOffsetEvaluator ObjSizeEval(DL, &TLI, F.getContext(), EvalOpts);
152 
153   // check HANDLE_MEMORY_INST in include/llvm/Instruction.def for memory
154   // touching instructions
155   SmallVector<std::pair<Instruction *, Value *>, 4> TrapInfo;
156   for (Instruction &I : instructions(F)) {
157     Value *Or = nullptr;
158     BuilderTy IRB(I.getParent(), BasicBlock::iterator(&I), TargetFolder(DL));
159     if (LoadInst *LI = dyn_cast<LoadInst>(&I)) {
160       if (!LI->isVolatile())
161         Or = getBoundsCheckCond(LI->getPointerOperand(), LI, DL, TLI,
162                                 ObjSizeEval, IRB, SE);
163     } else if (StoreInst *SI = dyn_cast<StoreInst>(&I)) {
164       if (!SI->isVolatile())
165         Or = getBoundsCheckCond(SI->getPointerOperand(), SI->getValueOperand(),
166                                 DL, TLI, ObjSizeEval, IRB, SE);
167     } else if (AtomicCmpXchgInst *AI = dyn_cast<AtomicCmpXchgInst>(&I)) {
168       if (!AI->isVolatile())
169         Or =
170             getBoundsCheckCond(AI->getPointerOperand(), AI->getCompareOperand(),
171                                DL, TLI, ObjSizeEval, IRB, SE);
172     } else if (AtomicRMWInst *AI = dyn_cast<AtomicRMWInst>(&I)) {
173       if (!AI->isVolatile())
174         Or = getBoundsCheckCond(AI->getPointerOperand(), AI->getValOperand(),
175                                 DL, TLI, ObjSizeEval, IRB, SE);
176     }
177     if (Or)
178       TrapInfo.push_back(std::make_pair(&I, Or));
179   }
180 
181   // Create a trapping basic block on demand using a callback. Depending on
182   // flags, this will either create a single block for the entire function or
183   // will create a fresh block every time it is called.
184   BasicBlock *TrapBB = nullptr;
185   auto GetTrapBB = [&TrapBB](BuilderTy &IRB) {
186     if (TrapBB && SingleTrapBB)
187       return TrapBB;
188 
189     Function *Fn = IRB.GetInsertBlock()->getParent();
190     // FIXME: This debug location doesn't make a lot of sense in the
191     // `SingleTrapBB` case.
192     auto DebugLoc = IRB.getCurrentDebugLocation();
193     IRBuilder<>::InsertPointGuard Guard(IRB);
194     TrapBB = BasicBlock::Create(Fn->getContext(), "trap", Fn);
195     IRB.SetInsertPoint(TrapBB);
196 
197     auto *F = Intrinsic::getDeclaration(Fn->getParent(), Intrinsic::trap);
198     CallInst *TrapCall = IRB.CreateCall(F, {});
199     TrapCall->setDoesNotReturn();
200     TrapCall->setDoesNotThrow();
201     TrapCall->setDebugLoc(DebugLoc);
202     IRB.CreateUnreachable();
203 
204     return TrapBB;
205   };
206 
207   // Add the checks.
208   for (const auto &Entry : TrapInfo) {
209     Instruction *Inst = Entry.first;
210     BuilderTy IRB(Inst->getParent(), BasicBlock::iterator(Inst), TargetFolder(DL));
211     insertBoundsCheck(Entry.second, IRB, GetTrapBB);
212   }
213 
214   return !TrapInfo.empty();
215 }
216 
217 PreservedAnalyses BoundsCheckingPass::run(Function &F, FunctionAnalysisManager &AM) {
218   auto &TLI = AM.getResult<TargetLibraryAnalysis>(F);
219   auto &SE = AM.getResult<ScalarEvolutionAnalysis>(F);
220 
221   if (!addBoundsChecking(F, TLI, SE))
222     return PreservedAnalyses::all();
223 
224   return PreservedAnalyses::none();
225 }
226 
227 namespace {
228 struct BoundsCheckingLegacyPass : public FunctionPass {
229   static char ID;
230 
231   BoundsCheckingLegacyPass() : FunctionPass(ID) {
232     initializeBoundsCheckingLegacyPassPass(*PassRegistry::getPassRegistry());
233   }
234 
235   bool runOnFunction(Function &F) override {
236     auto &TLI = getAnalysis<TargetLibraryInfoWrapperPass>().getTLI(F);
237     auto &SE = getAnalysis<ScalarEvolutionWrapperPass>().getSE();
238     return addBoundsChecking(F, TLI, SE);
239   }
240 
241   void getAnalysisUsage(AnalysisUsage &AU) const override {
242     AU.addRequired<TargetLibraryInfoWrapperPass>();
243     AU.addRequired<ScalarEvolutionWrapperPass>();
244   }
245 };
246 } // namespace
247 
248 char BoundsCheckingLegacyPass::ID = 0;
249 INITIALIZE_PASS_BEGIN(BoundsCheckingLegacyPass, "bounds-checking",
250                       "Run-time bounds checking", false, false)
251 INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfoWrapperPass)
252 INITIALIZE_PASS_END(BoundsCheckingLegacyPass, "bounds-checking",
253                     "Run-time bounds checking", false, false)
254 
255 FunctionPass *llvm::createBoundsCheckingLegacyPass() {
256   return new BoundsCheckingLegacyPass();
257 }
258