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/Support/Casting.h"
27#include "llvm/Support/CommandLine.h"
28#include "llvm/Support/Debug.h"
29#include "llvm/Support/raw_ostream.h"
30#include <cstdint>
31#include <utility>
32
33using namespace llvm;
34
35#define DEBUG_TYPE "bounds-checking"
36
37static cl::opt<bool> SingleTrapBB("bounds-checking-single-trap",
38 cl::desc("Use one trap block per function"));
39
40static cl::opt<bool> DebugTrapBB("bounds-checking-unique-traps",
41 cl::desc("Always use one trap per check"));
42
43STATISTIC(ChecksAdded, "Bounds checks added");
44STATISTIC(ChecksSkipped, "Bounds checks skipped");
45STATISTIC(ChecksUnable, "Bounds checks unable to add");
46
47using BuilderTy = IRBuilder<TargetFolder>;
48
49/// Gets the conditions under which memory accessing instructions will overflow.
50///
51/// \p Ptr is the pointer that will be read/written, and \p InstVal is either
52/// the result from the load or the value being stored. It is used to determine
53/// the size of memory block that is touched.
54///
55/// Returns the condition under which the access will overflow.
56static Value *getBoundsCheckCond(Value *Ptr, Value *InstVal,
57 const DataLayout &DL, TargetLibraryInfo &TLI,
58 ObjectSizeOffsetEvaluator &ObjSizeEval,
59 BuilderTy &IRB, ScalarEvolution &SE) {
60 TypeSize NeededSize = DL.getTypeStoreSize(Ty: InstVal->getType());
61 LLVM_DEBUG(dbgs() << "Instrument " << *Ptr << " for " << Twine(NeededSize)
62 << " bytes\n");
63
64 SizeOffsetValue SizeOffset = ObjSizeEval.compute(V: Ptr);
65
66 if (!SizeOffset.bothKnown()) {
67 ++ChecksUnable;
68 return nullptr;
69 }
70
71 Value *Size = SizeOffset.Size;
72 Value *Offset = SizeOffset.Offset;
73 ConstantInt *SizeCI = dyn_cast<ConstantInt>(Val: Size);
74
75 Type *IndexTy = DL.getIndexType(PtrTy: Ptr->getType());
76 Value *NeededSizeVal = IRB.CreateTypeSize(DstType: IndexTy, Size: NeededSize);
77
78 auto SizeRange = SE.getUnsignedRange(S: SE.getSCEV(V: Size));
79 auto OffsetRange = SE.getUnsignedRange(S: SE.getSCEV(V: Offset));
80 auto NeededSizeRange = SE.getUnsignedRange(S: SE.getSCEV(V: NeededSizeVal));
81
82 // three checks are required to ensure safety:
83 // . Offset >= 0 (since the offset is given from the base ptr)
84 // . Size >= Offset (unsigned)
85 // . Size - Offset >= NeededSize (unsigned)
86 //
87 // optimization: if Size >= 0 (signed), skip 1st check
88 // FIXME: add NSW/NUW here? -- we dont care if the subtraction overflows
89 Value *ObjSize = IRB.CreateSub(LHS: Size, RHS: Offset);
90 Value *Cmp2 = SizeRange.getUnsignedMin().uge(RHS: OffsetRange.getUnsignedMax())
91 ? ConstantInt::getFalse(Context&: Ptr->getContext())
92 : IRB.CreateICmpULT(LHS: Size, RHS: Offset);
93 Value *Cmp3 = SizeRange.sub(Other: OffsetRange)
94 .getUnsignedMin()
95 .uge(RHS: NeededSizeRange.getUnsignedMax())
96 ? ConstantInt::getFalse(Context&: Ptr->getContext())
97 : IRB.CreateICmpULT(LHS: ObjSize, RHS: NeededSizeVal);
98 Value *Or = IRB.CreateOr(LHS: Cmp2, RHS: Cmp3);
99 if ((!SizeCI || SizeCI->getValue().slt(RHS: 0)) &&
100 !SizeRange.getSignedMin().isNonNegative()) {
101 Value *Cmp1 = IRB.CreateICmpSLT(LHS: Offset, RHS: ConstantInt::get(Ty: IndexTy, V: 0));
102 Or = IRB.CreateOr(LHS: Cmp1, RHS: Or);
103 }
104
105 return Or;
106}
107
108/// Adds run-time bounds checks to memory accessing instructions.
109///
110/// \p Or is the condition that should guard the trap.
111///
112/// \p GetTrapBB is a callable that returns the trap BB to use on failure.
113template <typename GetTrapBBT>
114static void insertBoundsCheck(Value *Or, BuilderTy &IRB, GetTrapBBT GetTrapBB) {
115 // check if the comparison is always false
116 ConstantInt *C = dyn_cast_or_null<ConstantInt>(Val: Or);
117 if (C) {
118 ++ChecksSkipped;
119 // If non-zero, nothing to do.
120 if (!C->getZExtValue())
121 return;
122 }
123 ++ChecksAdded;
124
125 BasicBlock::iterator SplitI = IRB.GetInsertPoint();
126 BasicBlock *OldBB = SplitI->getParent();
127 BasicBlock *Cont = OldBB->splitBasicBlock(I: SplitI);
128 OldBB->getTerminator()->eraseFromParent();
129
130 if (C) {
131 // If we have a constant zero, unconditionally branch.
132 // FIXME: We should really handle this differently to bypass the splitting
133 // the block.
134 BranchInst::Create(GetTrapBB(IRB), OldBB);
135 return;
136 }
137
138 // Create the conditional branch.
139 BranchInst::Create(GetTrapBB(IRB), Cont, Or, OldBB);
140}
141
142static bool addBoundsChecking(Function &F, TargetLibraryInfo &TLI,
143 ScalarEvolution &SE) {
144 if (F.hasFnAttribute(Kind: Attribute::NoSanitizeBounds))
145 return false;
146
147 const DataLayout &DL = F.getDataLayout();
148 ObjectSizeOpts EvalOpts;
149 EvalOpts.RoundToAlign = true;
150 EvalOpts.EvalMode = ObjectSizeOpts::Mode::ExactUnderlyingSizeAndOffset;
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>(Val: &I)) {
160 if (!LI->isVolatile())
161 Or = getBoundsCheckCond(Ptr: LI->getPointerOperand(), InstVal: LI, DL, TLI,
162 ObjSizeEval, IRB, SE);
163 } else if (StoreInst *SI = dyn_cast<StoreInst>(Val: &I)) {
164 if (!SI->isVolatile())
165 Or = getBoundsCheckCond(Ptr: SI->getPointerOperand(), InstVal: SI->getValueOperand(),
166 DL, TLI, ObjSizeEval, IRB, SE);
167 } else if (AtomicCmpXchgInst *AI = dyn_cast<AtomicCmpXchgInst>(Val: &I)) {
168 if (!AI->isVolatile())
169 Or =
170 getBoundsCheckCond(Ptr: AI->getPointerOperand(), InstVal: AI->getCompareOperand(),
171 DL, TLI, ObjSizeEval, IRB, SE);
172 } else if (AtomicRMWInst *AI = dyn_cast<AtomicRMWInst>(Val: &I)) {
173 if (!AI->isVolatile())
174 Or = getBoundsCheckCond(Ptr: AI->getPointerOperand(), InstVal: AI->getValOperand(),
175 DL, TLI, ObjSizeEval, IRB, SE);
176 }
177 if (Or)
178 TrapInfo.push_back(Elt: std::make_pair(x: &I, y&: 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 Function *Fn = IRB.GetInsertBlock()->getParent();
187 auto DebugLoc = IRB.getCurrentDebugLocation();
188 IRBuilder<>::InsertPointGuard Guard(IRB);
189
190 if (TrapBB && SingleTrapBB && !DebugTrapBB)
191 return TrapBB;
192
193 TrapBB = BasicBlock::Create(Context&: Fn->getContext(), Name: "trap", Parent: Fn);
194 IRB.SetInsertPoint(TrapBB);
195
196 Intrinsic::ID IntrID = DebugTrapBB ? Intrinsic::ubsantrap : Intrinsic::trap;
197 auto *F = Intrinsic::getDeclaration(M: Fn->getParent(), id: IntrID);
198
199 CallInst *TrapCall;
200 if (DebugTrapBB) {
201 TrapCall =
202 IRB.CreateCall(Callee: F, Args: ConstantInt::get(Ty: IRB.getInt8Ty(), V: Fn->size()));
203 } else {
204 TrapCall = IRB.CreateCall(Callee: F, Args: {});
205 }
206
207 TrapCall->setDoesNotReturn();
208 TrapCall->setDoesNotThrow();
209 TrapCall->setDebugLoc(DebugLoc);
210 IRB.CreateUnreachable();
211
212 return TrapBB;
213 };
214
215 // Add the checks.
216 for (const auto &Entry : TrapInfo) {
217 Instruction *Inst = Entry.first;
218 BuilderTy IRB(Inst->getParent(), BasicBlock::iterator(Inst), TargetFolder(DL));
219 insertBoundsCheck(Or: Entry.second, IRB, GetTrapBB);
220 }
221
222 return !TrapInfo.empty();
223}
224
225PreservedAnalyses BoundsCheckingPass::run(Function &F, FunctionAnalysisManager &AM) {
226 auto &TLI = AM.getResult<TargetLibraryAnalysis>(IR&: F);
227 auto &SE = AM.getResult<ScalarEvolutionAnalysis>(IR&: F);
228
229 if (!addBoundsChecking(F, TLI, SE))
230 return PreservedAnalyses::all();
231
232 return PreservedAnalyses::none();
233}
234