//=== 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 "llvm/ADT/StringRef.h"

using namespace clang;
using namespace ento;

namespace {
class PointerSubChecker
  : public Checker< check::PreStmt<BinaryOperator> > {
  const BugType BT{this, "Pointer subtraction"};

public:
  void checkPreStmt(const BinaryOperator *B, CheckerContext &C) const;
};
}

void PointerSubChecker::checkPreStmt(const BinaryOperator *B,
                                     CheckerContext &C) const {
  // When doing pointer subtraction, if the two pointers do not point to the
  // same memory chunk, 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;

  const MemRegion *BaseLR = LR->getBaseRegion();
  const MemRegion *BaseRR = RR->getBaseRegion();

  if (BaseLR == BaseRR)
    return;

  // Allow arithmetic on different symbolic regions.
  if (isa<SymbolicRegion>(BaseLR) || isa<SymbolicRegion>(BaseRR))
    return;

  if (ExplodedNode *N = C.generateNonFatalErrorNode()) {
    constexpr llvm::StringLiteral Msg =
        "Subtraction of two pointers that do not point to the same memory "
        "chunk may cause incorrect result.";
    auto R = std::make_unique<PathSensitiveBugReport>(BT, Msg, N);
    R->addRange(B->getSourceRange());
    C.emitReport(std::move(R));
  }
}

void ento::registerPointerSubChecker(CheckerManager &mgr) {
  mgr.registerChecker<PointerSubChecker>();
}

bool ento::shouldRegisterPointerSubChecker(const CheckerManager &mgr) {
  return true;
}