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 if (!B->getLHS()->getType()->isPointerType() || 65 !B->getRHS()->getType()->isPointerType()) 66 return; 67 68 const auto *ElemLR = dyn_cast<ElementRegion>(LR); 69 const auto *ElemRR = dyn_cast<ElementRegion>(RR); 70 71 // Allow cases like "(&x + 1) - &x". 72 if (ElemLR && ElemLR->getSuperRegion() == RR) 73 return; 74 // Allow cases like "&x - (&x + 1)". 75 if (ElemRR && ElemRR->getSuperRegion() == LR) 76 return; 77 78 const ValueDecl *DiffDeclL = nullptr; 79 const ValueDecl *DiffDeclR = nullptr; 80 81 if (ElemLR && ElemRR) { 82 const MemRegion *SuperLR = ElemLR->getSuperRegion(); 83 const MemRegion *SuperRR = ElemRR->getSuperRegion(); 84 if (SuperLR == SuperRR) 85 return; 86 // Allow arithmetic on different symbolic regions. 87 if (isa<SymbolicRegion>(SuperLR) || isa<SymbolicRegion>(SuperRR)) 88 return; 89 if (const auto *SuperDLR = dyn_cast<DeclRegion>(SuperLR)) 90 DiffDeclL = SuperDLR->getDecl(); 91 if (const auto *SuperDRR = dyn_cast<DeclRegion>(SuperRR)) 92 DiffDeclR = SuperDRR->getDecl(); 93 } 94 95 if (ExplodedNode *N = C.generateNonFatalErrorNode()) { 96 auto R = 97 std::make_unique<PathSensitiveBugReport>(BT, Msg_MemRegionDifferent, N); 98 R->addRange(B->getSourceRange()); 99 // The declarations may be identical even if the regions are different: 100 // struct { int array[10]; } a, b; 101 // do_something(&a.array[5] - &b.array[5]); 102 // In this case don't emit notes. 103 if (DiffDeclL != DiffDeclR) { 104 auto AddNote = [&R, &C](const ValueDecl *D, StringRef SideStr) { 105 if (D) { 106 std::string Msg = llvm::formatv( 107 "{0} at the {1}-hand side of subtraction", 108 D->getType()->isArrayType() ? "Array" : "Object", SideStr); 109 R->addNote(Msg, {D, C.getSourceManager()}); 110 } 111 }; 112 AddNote(DiffDeclL, "left"); 113 AddNote(DiffDeclR, "right"); 114 } 115 C.emitReport(std::move(R)); 116 } 117 } 118 119 void ento::registerPointerSubChecker(CheckerManager &mgr) { 120 mgr.registerChecker<PointerSubChecker>(); 121 } 122 123 bool ento::shouldRegisterPointerSubChecker(const CheckerManager &mgr) { 124 return true; 125 } 126