1 //===- BoundsChecking.cpp - Instrumentation for run-time bounds checking --===//
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 file implements a pass that instruments the code to perform run-time
11 // bounds checking on loads, stores, and other memory intrinsics.
12 //
13 //===----------------------------------------------------------------------===//
14 
15 #include "llvm/Transforms/Instrumentation.h"
16 #include "llvm/ADT/Statistic.h"
17 #include "llvm/Analysis/MemoryBuiltins.h"
18 #include "llvm/Analysis/TargetFolder.h"
19 #include "llvm/Analysis/TargetLibraryInfo.h"
20 #include "llvm/IR/DataLayout.h"
21 #include "llvm/IR/IRBuilder.h"
22 #include "llvm/IR/InstIterator.h"
23 #include "llvm/IR/Intrinsics.h"
24 #include "llvm/Pass.h"
25 #include "llvm/Support/CommandLine.h"
26 #include "llvm/Support/Debug.h"
27 #include "llvm/Support/raw_ostream.h"
28 using namespace llvm;
29 
30 #define DEBUG_TYPE "bounds-checking"
31 
32 static cl::opt<bool> SingleTrapBB("bounds-checking-single-trap",
33                                   cl::desc("Use one trap block per function"));
34 
35 STATISTIC(ChecksAdded, "Bounds checks added");
36 STATISTIC(ChecksSkipped, "Bounds checks skipped");
37 STATISTIC(ChecksUnable, "Bounds checks unable to add");
38 
39 typedef IRBuilder<true, TargetFolder> BuilderTy;
40 
41 namespace {
42   struct BoundsChecking : public FunctionPass {
43     static char ID;
44 
BoundsChecking__anon79ea1b1a0111::BoundsChecking45     BoundsChecking() : FunctionPass(ID) {
46       initializeBoundsCheckingPass(*PassRegistry::getPassRegistry());
47     }
48 
49     bool runOnFunction(Function &F) override;
50 
getAnalysisUsage__anon79ea1b1a0111::BoundsChecking51     void getAnalysisUsage(AnalysisUsage &AU) const override {
52       AU.addRequired<TargetLibraryInfoWrapperPass>();
53     }
54 
55   private:
56     const TargetLibraryInfo *TLI;
57     ObjectSizeOffsetEvaluator *ObjSizeEval;
58     BuilderTy *Builder;
59     Instruction *Inst;
60     BasicBlock *TrapBB;
61 
62     BasicBlock *getTrapBB();
63     void emitBranchToTrap(Value *Cmp = nullptr);
64     bool instrument(Value *Ptr, Value *Val, const DataLayout &DL);
65  };
66 }
67 
68 char BoundsChecking::ID = 0;
69 INITIALIZE_PASS(BoundsChecking, "bounds-checking", "Run-time bounds checking",
70                 false, false)
71 
72 
73 /// getTrapBB - create a basic block that traps. All overflowing conditions
74 /// branch to this block. There's only one trap block per function.
getTrapBB()75 BasicBlock *BoundsChecking::getTrapBB() {
76   if (TrapBB && SingleTrapBB)
77     return TrapBB;
78 
79   Function *Fn = Inst->getParent()->getParent();
80   IRBuilder<>::InsertPointGuard Guard(*Builder);
81   TrapBB = BasicBlock::Create(Fn->getContext(), "trap", Fn);
82   Builder->SetInsertPoint(TrapBB);
83 
84   llvm::Value *F = Intrinsic::getDeclaration(Fn->getParent(), Intrinsic::trap);
85   CallInst *TrapCall = Builder->CreateCall(F, {});
86   TrapCall->setDoesNotReturn();
87   TrapCall->setDoesNotThrow();
88   TrapCall->setDebugLoc(Inst->getDebugLoc());
89   Builder->CreateUnreachable();
90 
91   return TrapBB;
92 }
93 
94 
95 /// emitBranchToTrap - emit a branch instruction to a trap block.
96 /// If Cmp is non-null, perform a jump only if its value evaluates to true.
emitBranchToTrap(Value * Cmp)97 void BoundsChecking::emitBranchToTrap(Value *Cmp) {
98   // check if the comparison is always false
99   ConstantInt *C = dyn_cast_or_null<ConstantInt>(Cmp);
100   if (C) {
101     ++ChecksSkipped;
102     if (!C->getZExtValue())
103       return;
104     else
105       Cmp = nullptr; // unconditional branch
106   }
107   ++ChecksAdded;
108 
109   BasicBlock::iterator Inst = Builder->GetInsertPoint();
110   BasicBlock *OldBB = Inst->getParent();
111   BasicBlock *Cont = OldBB->splitBasicBlock(Inst);
112   OldBB->getTerminator()->eraseFromParent();
113 
114   if (Cmp)
115     BranchInst::Create(getTrapBB(), Cont, Cmp, OldBB);
116   else
117     BranchInst::Create(getTrapBB(), OldBB);
118 }
119 
120 
121 /// instrument - adds run-time bounds checks to memory accessing instructions.
122 /// Ptr is the pointer that will be read/written, and InstVal is either the
123 /// result from the load or the value being stored. It is used to determine the
124 /// size of memory block that is touched.
125 /// Returns true if any change was made to the IR, false otherwise.
instrument(Value * Ptr,Value * InstVal,const DataLayout & DL)126 bool BoundsChecking::instrument(Value *Ptr, Value *InstVal,
127                                 const DataLayout &DL) {
128   uint64_t NeededSize = DL.getTypeStoreSize(InstVal->getType());
129   DEBUG(dbgs() << "Instrument " << *Ptr << " for " << Twine(NeededSize)
130               << " bytes\n");
131 
132   SizeOffsetEvalType SizeOffset = ObjSizeEval->compute(Ptr);
133 
134   if (!ObjSizeEval->bothKnown(SizeOffset)) {
135     ++ChecksUnable;
136     return false;
137   }
138 
139   Value *Size   = SizeOffset.first;
140   Value *Offset = SizeOffset.second;
141   ConstantInt *SizeCI = dyn_cast<ConstantInt>(Size);
142 
143   Type *IntTy = DL.getIntPtrType(Ptr->getType());
144   Value *NeededSizeVal = ConstantInt::get(IntTy, NeededSize);
145 
146   // three checks are required to ensure safety:
147   // . Offset >= 0  (since the offset is given from the base ptr)
148   // . Size >= Offset  (unsigned)
149   // . Size - Offset >= NeededSize  (unsigned)
150   //
151   // optimization: if Size >= 0 (signed), skip 1st check
152   // FIXME: add NSW/NUW here?  -- we dont care if the subtraction overflows
153   Value *ObjSize = Builder->CreateSub(Size, Offset);
154   Value *Cmp2 = Builder->CreateICmpULT(Size, Offset);
155   Value *Cmp3 = Builder->CreateICmpULT(ObjSize, NeededSizeVal);
156   Value *Or = Builder->CreateOr(Cmp2, Cmp3);
157   if (!SizeCI || SizeCI->getValue().slt(0)) {
158     Value *Cmp1 = Builder->CreateICmpSLT(Offset, ConstantInt::get(IntTy, 0));
159     Or = Builder->CreateOr(Cmp1, Or);
160   }
161   emitBranchToTrap(Or);
162 
163   return true;
164 }
165 
runOnFunction(Function & F)166 bool BoundsChecking::runOnFunction(Function &F) {
167   const DataLayout &DL = F.getParent()->getDataLayout();
168   TLI = &getAnalysis<TargetLibraryInfoWrapperPass>().getTLI();
169 
170   TrapBB = nullptr;
171   BuilderTy TheBuilder(F.getContext(), TargetFolder(DL));
172   Builder = &TheBuilder;
173   ObjectSizeOffsetEvaluator TheObjSizeEval(DL, TLI, F.getContext(),
174                                            /*RoundToAlign=*/true);
175   ObjSizeEval = &TheObjSizeEval;
176 
177   // check HANDLE_MEMORY_INST in include/llvm/Instruction.def for memory
178   // touching instructions
179   std::vector<Instruction*> WorkList;
180   for (inst_iterator i = inst_begin(F), e = inst_end(F); i != e; ++i) {
181     Instruction *I = &*i;
182     if (isa<LoadInst>(I) || isa<StoreInst>(I) || isa<AtomicCmpXchgInst>(I) ||
183         isa<AtomicRMWInst>(I))
184         WorkList.push_back(I);
185   }
186 
187   bool MadeChange = false;
188   for (std::vector<Instruction*>::iterator i = WorkList.begin(),
189        e = WorkList.end(); i != e; ++i) {
190     Inst = *i;
191 
192     Builder->SetInsertPoint(Inst);
193     if (LoadInst *LI = dyn_cast<LoadInst>(Inst)) {
194       MadeChange |= instrument(LI->getPointerOperand(), LI, DL);
195     } else if (StoreInst *SI = dyn_cast<StoreInst>(Inst)) {
196       MadeChange |=
197           instrument(SI->getPointerOperand(), SI->getValueOperand(), DL);
198     } else if (AtomicCmpXchgInst *AI = dyn_cast<AtomicCmpXchgInst>(Inst)) {
199       MadeChange |=
200           instrument(AI->getPointerOperand(), AI->getCompareOperand(), DL);
201     } else if (AtomicRMWInst *AI = dyn_cast<AtomicRMWInst>(Inst)) {
202       MadeChange |=
203           instrument(AI->getPointerOperand(), AI->getValOperand(), DL);
204     } else {
205       llvm_unreachable("unknown Instruction type");
206     }
207   }
208   return MadeChange;
209 }
210 
createBoundsCheckingPass()211 FunctionPass *llvm::createBoundsCheckingPass() {
212   return new BoundsChecking();
213 }
214