1 //=== PointerSubChecker.cpp - Pointer subtraction checker ------*- C++ -*--===// 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 // This files defines PointerSubChecker, a builtin checker that checks for 10 // pointer subtractions on two pointers pointing to different memory chunks. 11 // This check corresponds to CWE-469. 12 // 13 //===----------------------------------------------------------------------===// 14 15 #include "clang/StaticAnalyzer/Checkers/BuiltinCheckerRegistration.h" 16 #include "clang/StaticAnalyzer/Core/BugReporter/BugType.h" 17 #include "clang/StaticAnalyzer/Core/Checker.h" 18 #include "clang/StaticAnalyzer/Core/CheckerManager.h" 19 #include "clang/StaticAnalyzer/Core/PathSensitive/CheckerContext.h" 20 #include "clang/StaticAnalyzer/Core/PathSensitive/DynamicExtent.h" 21 #include "llvm/ADT/StringRef.h" 22 #include "llvm/Support/FormatVariadic.h" 23 24 using namespace clang; 25 using namespace ento; 26 27 namespace { 28 class PointerSubChecker 29 : public Checker< check::PreStmt<BinaryOperator> > { 30 const BugType BT{this, "Pointer subtraction"}; 31 const llvm::StringLiteral Msg_MemRegionDifferent = 32 "Subtraction of two pointers that do not point into the same array " 33 "is undefined behavior."; 34 35 public: 36 void checkPreStmt(const BinaryOperator *B, CheckerContext &C) const; 37 }; 38 } 39 40 void PointerSubChecker::checkPreStmt(const BinaryOperator *B, 41 CheckerContext &C) const { 42 // When doing pointer subtraction, if the two pointers do not point to the 43 // same array, emit a warning. 44 if (B->getOpcode() != BO_Sub) 45 return; 46 47 SVal LV = C.getSVal(B->getLHS()); 48 SVal RV = C.getSVal(B->getRHS()); 49 50 const MemRegion *LR = LV.getAsRegion(); 51 const MemRegion *RR = RV.getAsRegion(); 52 if (!LR || !RR) 53 return; 54 55 // Allow subtraction of identical pointers. 56 if (LR == RR) 57 return; 58 59 // No warning if one operand is unknown or resides in a region that could be 60 // equal to the other. 61 if (LR->getSymbolicBase() || RR->getSymbolicBase()) 62 return; 63 64 const auto *ElemLR = dyn_cast<ElementRegion>(LR); 65 const auto *ElemRR = dyn_cast<ElementRegion>(RR); 66 67 // Allow cases like "(&x + 1) - &x". 68 if (ElemLR && ElemLR->getSuperRegion() == RR) 69 return; 70 // Allow cases like "&x - (&x + 1)". 71 if (ElemRR && ElemRR->getSuperRegion() == LR) 72 return; 73 74 const ValueDecl *DiffDeclL = nullptr; 75 const ValueDecl *DiffDeclR = nullptr; 76 77 if (ElemLR && ElemRR) { 78 const MemRegion *SuperLR = ElemLR->getSuperRegion(); 79 const MemRegion *SuperRR = ElemRR->getSuperRegion(); 80 if (SuperLR == SuperRR) 81 return; 82 // Allow arithmetic on different symbolic regions. 83 if (isa<SymbolicRegion>(SuperLR) || isa<SymbolicRegion>(SuperRR)) 84 return; 85 if (const auto *SuperDLR = dyn_cast<DeclRegion>(SuperLR)) 86 DiffDeclL = SuperDLR->getDecl(); 87 if (const auto *SuperDRR = dyn_cast<DeclRegion>(SuperRR)) 88 DiffDeclR = SuperDRR->getDecl(); 89 } 90 91 if (ExplodedNode *N = C.generateNonFatalErrorNode()) { 92 auto R = 93 std::make_unique<PathSensitiveBugReport>(BT, Msg_MemRegionDifferent, N); 94 R->addRange(B->getSourceRange()); 95 // The declarations may be identical even if the regions are different: 96 // struct { int array[10]; } a, b; 97 // do_something(&a.array[5] - &b.array[5]); 98 // In this case don't emit notes. 99 if (DiffDeclL != DiffDeclR) { 100 auto AddNote = [&R, &C](const ValueDecl *D, StringRef SideStr) { 101 if (D) { 102 std::string Msg = llvm::formatv( 103 "{0} at the {1}-hand side of subtraction", 104 D->getType()->isArrayType() ? "Array" : "Object", SideStr); 105 R->addNote(Msg, {D, C.getSourceManager()}); 106 } 107 }; 108 AddNote(DiffDeclL, "left"); 109 AddNote(DiffDeclR, "right"); 110 } 111 C.emitReport(std::move(R)); 112 } 113 } 114 115 void ento::registerPointerSubChecker(CheckerManager &mgr) { 116 mgr.registerChecker<PointerSubChecker>(); 117 } 118 119 bool ento::shouldRegisterPointerSubChecker(const CheckerManager &mgr) { 120 return true; 121 } 122