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//=== PointerSubChecker.cpp - Pointer subtraction checker ------*- C++ -*--===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
// This files defines PointerSubChecker, a builtin checker that checks for
// pointer subtractions on two pointers pointing to different memory chunks.
// This check corresponds to CWE-469.
//
//===----------------------------------------------------------------------===//
#include "clang/StaticAnalyzer/Checkers/BuiltinCheckerRegistration.h"
#include "clang/StaticAnalyzer/Core/BugReporter/BugType.h"
#include "clang/StaticAnalyzer/Core/Checker.h"
#include "clang/StaticAnalyzer/Core/CheckerManager.h"
#include "clang/StaticAnalyzer/Core/PathSensitive/CheckerContext.h"
#include "clang/StaticAnalyzer/Core/PathSensitive/DynamicExtent.h"
#include "llvm/ADT/StringRef.h"
using namespace clang;
using namespace ento;
namespace {
class PointerSubChecker
: public Checker< check::PreStmt<BinaryOperator> > {
const BugType BT{this, "Pointer subtraction"};
const llvm::StringLiteral Msg_MemRegionDifferent =
"Subtraction of two pointers that do not point into the same array "
"is undefined behavior.";
const llvm::StringLiteral Msg_LargeArrayIndex =
"Using an array index greater than the array size at pointer subtraction "
"is undefined behavior.";
const llvm::StringLiteral Msg_NegativeArrayIndex =
"Using a negative array index at pointer subtraction "
"is undefined behavior.";
const llvm::StringLiteral Msg_BadVarIndex =
"Indexing the address of a variable with other than 1 at this place "
"is undefined behavior.";
bool checkArrayBounds(CheckerContext &C, const Expr *E,
const ElementRegion *ElemReg,
const MemRegion *Reg) const;
public:
void checkPreStmt(const BinaryOperator *B, CheckerContext &C) const;
};
}
bool PointerSubChecker::checkArrayBounds(CheckerContext &C, const Expr *E,
const ElementRegion *ElemReg,
const MemRegion *Reg) const {
if (!ElemReg)
return true;
auto ReportBug = [&](const llvm::StringLiteral &Msg) {
if (ExplodedNode *N = C.generateNonFatalErrorNode()) {
auto R = std::make_unique<PathSensitiveBugReport>(BT, Msg, N);
R->addRange(E->getSourceRange());
C.emitReport(std::move(R));
}
};
ProgramStateRef State = C.getState();
const MemRegion *SuperReg = ElemReg->getSuperRegion();
SValBuilder &SVB = C.getSValBuilder();
if (SuperReg == Reg) {
if (const llvm::APSInt *I = SVB.getKnownValue(State, ElemReg->getIndex());
I && (!I->isOne() && !I->isZero()))
ReportBug(Msg_BadVarIndex);
return false;
}
DefinedOrUnknownSVal ElemCount =
getDynamicElementCount(State, SuperReg, SVB, ElemReg->getElementType());
auto IndexTooLarge = SVB.evalBinOp(C.getState(), BO_GT, ElemReg->getIndex(),
ElemCount, SVB.getConditionType())
.getAs<DefinedOrUnknownSVal>();
if (IndexTooLarge) {
ProgramStateRef S1, S2;
std::tie(S1, S2) = C.getState()->assume(*IndexTooLarge);
if (S1 && !S2) {
ReportBug(Msg_LargeArrayIndex);
return false;
}
}
auto IndexTooSmall = SVB.evalBinOp(State, BO_LT, ElemReg->getIndex(),
SVB.makeZeroVal(SVB.getArrayIndexType()),
SVB.getConditionType())
.getAs<DefinedOrUnknownSVal>();
if (IndexTooSmall) {
ProgramStateRef S1, S2;
std::tie(S1, S2) = State->assume(*IndexTooSmall);
if (S1 && !S2) {
ReportBug(Msg_NegativeArrayIndex);
return false;
}
}
return true;
}
void PointerSubChecker::checkPreStmt(const BinaryOperator *B,
CheckerContext &C) const {
// When doing pointer subtraction, if the two pointers do not point to the
// same array, emit a warning.
if (B->getOpcode() != BO_Sub)
return;
SVal LV = C.getSVal(B->getLHS());
SVal RV = C.getSVal(B->getRHS());
const MemRegion *LR = LV.getAsRegion();
const MemRegion *RR = RV.getAsRegion();
if (!LR || !RR)
return;
// Allow subtraction of identical pointers.
if (LR == RR)
return;
// No warning if one operand is unknown.
if (isa<SymbolicRegion>(LR) || isa<SymbolicRegion>(RR))
return;
const auto *ElemLR = dyn_cast<ElementRegion>(LR);
const auto *ElemRR = dyn_cast<ElementRegion>(RR);
if (!checkArrayBounds(C, B->getLHS(), ElemLR, RR))
return;
if (!checkArrayBounds(C, B->getRHS(), ElemRR, LR))
return;
const ValueDecl *DiffDeclL = nullptr;
const ValueDecl *DiffDeclR = nullptr;
if (ElemLR && ElemRR) {
const MemRegion *SuperLR = ElemLR->getSuperRegion();
const MemRegion *SuperRR = ElemRR->getSuperRegion();
if (SuperLR == SuperRR)
return;
// Allow arithmetic on different symbolic regions.
if (isa<SymbolicRegion>(SuperLR) || isa<SymbolicRegion>(SuperRR))
return;
if (const auto *SuperDLR = dyn_cast<DeclRegion>(SuperLR))
DiffDeclL = SuperDLR->getDecl();
if (const auto *SuperDRR = dyn_cast<DeclRegion>(SuperRR))
DiffDeclR = SuperDRR->getDecl();
}
if (ExplodedNode *N = C.generateNonFatalErrorNode()) {
auto R =
std::make_unique<PathSensitiveBugReport>(BT, Msg_MemRegionDifferent, N);
R->addRange(B->getSourceRange());
// The declarations may be identical even if the regions are different:
// struct { int array[10]; } a, b;
// do_something(&a.array[5] - &b.array[5]);
// In this case don't emit notes.
if (DiffDeclL != DiffDeclR) {
if (DiffDeclL)
R->addNote("Array at the left-hand side of subtraction",
{DiffDeclL, C.getSourceManager()});
if (DiffDeclR)
R->addNote("Array at the right-hand side of subtraction",
{DiffDeclR, C.getSourceManager()});
}
C.emitReport(std::move(R));
}
}
void ento::registerPointerSubChecker(CheckerManager &mgr) {
mgr.registerChecker<PointerSubChecker>();
}
bool ento::shouldRegisterPointerSubChecker(const CheckerManager &mgr) {
return true;
}
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