nullability/pointer_nullability.cc

// Part of the Crubit project, under the Apache License v2.0 with LLVM
// Exceptions. See /LICENSE for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception

#include "nullability/pointer_nullability.h"

#include <cassert>
#include <optional>

#include "nullability/type_nullability.h"
#include "clang/AST/Type.h"
#include "clang/Analysis/FlowSensitive/DataflowAnalysisContext.h"
#include "clang/Analysis/FlowSensitive/DataflowEnvironment.h"
#include "clang/Analysis/FlowSensitive/Formula.h"
#include "clang/Analysis/FlowSensitive/Value.h"
#include "clang/Basic/LLVM.h"
#include "clang/Basic/Specifiers.h"
#include "llvm/ADT/StringRef.h"

namespace clang::tidy::nullability {

using dataflow::AtomicBoolValue;
using dataflow::BoolValue;
using dataflow::DataflowAnalysisContext;
using dataflow::Environment;
using dataflow::Formula;
using dataflow::PointerValue;
using dataflow::RecordStorageLocation;
using dataflow::TopBoolValue;
using dataflow::Value;

/// The nullness information of a pointer is represented by two properties
/// which indicate if its source was nullable, and if its value is null.
constexpr llvm::StringLiteral kFromNullable = "from_nullable";
constexpr llvm::StringLiteral kNull = "is_null";

NullabilityKind getNullabilityKind(QualType Type, ASTContext &Ctx) {
  return Type->getNullability().value_or(NullabilityKind::Unspecified);
}

PointerValue *getPointerValueFromExpr(const Expr *PointerExpr,
                                      const Environment &Env) {
  return cast_or_null<PointerValue>(Env.getValue(*PointerExpr));
}

absl::Nullable<PointerValue *> getPointerValueFromSmartPointer(
    absl::Nullable<RecordStorageLocation *> SmartPointerLoc,
    const Environment &Env) {
  if (SmartPointerLoc == nullptr) return nullptr;
  return cast_or_null<dataflow::PointerValue>(
      Env.getValue(SmartPointerLoc->getSyntheticField(PtrField)));
}

bool hasPointerNullState(const dataflow::PointerValue &PointerVal) {
  return PointerVal.getProperty(kFromNullable) != nullptr &&
         PointerVal.getProperty(kNull) != nullptr;
}

PointerNullState getPointerNullState(const PointerValue &PointerVal) {
  Value *FromNullableProp = PointerVal.getProperty(kFromNullable);
  Value *NullProp = PointerVal.getProperty(kNull);

  return {
      isa<TopBoolValue>(FromNullableProp)
          ? nullptr
          : &cast<BoolValue>(FromNullableProp)->formula(),
      isa<TopBoolValue>(NullProp) ? nullptr
                                  : &cast<BoolValue>(NullProp)->formula(),
  };
}

static bool tryCreatePointerNullState(PointerValue &PointerVal,
                                      dataflow::Arena &A,
                                      const Formula *FromNullable = nullptr,
                                      const Formula *IsNull = nullptr) {
  if (hasPointerNullState(PointerVal)) return false;
  if (!FromNullable) FromNullable = &A.makeAtomRef(A.makeAtom());
  if (!IsNull) IsNull = &A.makeAtomRef(A.makeAtom());
  PointerVal.setProperty(kFromNullable, A.makeBoolValue(*FromNullable));
  PointerVal.setProperty(kNull, A.makeBoolValue(*IsNull));
  return true;
}

void initPointerNullState(PointerValue &PointerVal,
                          DataflowAnalysisContext &Ctx,
                          std::optional<PointerTypeNullability> Source) {
  auto &A = Ctx.arena();
  if (tryCreatePointerNullState(PointerVal, A,
                                Source ? &Source->isNullable(A) : nullptr)) {
    // The symbolic/nonnull check is not needed for correctness, but it avoids
    // adding meaningless (false => !null) invariant clauses.
    // TODO: remove this once such clauses are recognized and dropped.
    if (Source &&
        (Source->isSymbolic() || Source == NullabilityKind::NonNull)) {
      if (const Formula *IsNull = getPointerNullState(PointerVal).IsNull)
        Ctx.addInvariant(
            A.makeImplies(Source->isNonnull(A), A.makeNot(*IsNull)));
    }
  }
}

void forgetFromNullable(dataflow::PointerValue &PointerVal,
                        DataflowAnalysisContext &Ctx) {
  PointerVal.setProperty(kFromNullable, Ctx.arena().makeTopValue());
}

void forgetIsNull(dataflow::PointerValue &PointerVal,
                  DataflowAnalysisContext &Ctx) {
  PointerVal.setProperty(kNull, Ctx.arena().makeTopValue());
}

void initNullPointer(PointerValue &PointerVal, DataflowAnalysisContext &Ctx) {
  tryCreatePointerNullState(PointerVal, Ctx.arena(),
                            /*FromNullable=*/&Ctx.arena().makeLiteral(true),
                            /*IsNull=*/&Ctx.arena().makeLiteral(true));
}

bool isNullable(const PointerValue &PointerVal, const Environment &Env,
                const dataflow::Formula *AdditionalConstraints) {
  auto &A = Env.getDataflowAnalysisContext().arena();
  auto [FromNullable, Null] = getPointerNullState(PointerVal);

  // A value is nullable if two things can be simultaneously true:
  // - We got it from a nullable source
  //   (values from unknown sources may be null, but are not nullable)
  // - The value is actually null
  //   (if a value from a nullable source was checked, it's not nullable)
  const Formula *ForseeablyNull = &A.makeLiteral(true);
  if (FromNullable) ForseeablyNull = &A.makeAnd(*ForseeablyNull, *FromNullable);
  if (Null) ForseeablyNull = &A.makeAnd(*ForseeablyNull, *Null);
  if (AdditionalConstraints)
    ForseeablyNull = &A.makeAnd(*ForseeablyNull, *AdditionalConstraints);

  return Env.allows(*ForseeablyNull);
}

NullabilityKind getNullability(const dataflow::PointerValue &PointerVal,
                               const dataflow::Environment &Env,
                               const dataflow::Formula *AdditionalConstraints) {
  auto &A = Env.getDataflowAnalysisContext().arena();
  if (auto *Null = getPointerNullState(PointerVal).IsNull) {
    if (AdditionalConstraints) Null = &A.makeAnd(*AdditionalConstraints, *Null);
    if (Env.proves(A.makeNot(*Null))) return NullabilityKind::NonNull;
  }
  return isNullable(PointerVal, Env, AdditionalConstraints)
             ? NullabilityKind::Nullable
             : NullabilityKind::Unspecified;
}

NullabilityKind getNullability(const Expr *E, const dataflow::Environment &Env,
                               const dataflow::Formula *AdditionalConstraints) {
  dataflow::PointerValue *P = nullptr;
  if (isSupportedRawPointerType(E->getType()))
    P = getPointerValueFromExpr(E, Env);
  else if (isSupportedSmartPointerType(E->getType()))
    P = getPointerValueFromSmartPointer(
        cast_or_null<dataflow::RecordStorageLocation>(
            Env.getStorageLocation(*E)),
        Env);
  if (P != nullptr) return getNullability(*P, Env, AdditionalConstraints);
  return clang::NullabilityKind::Unspecified;
}

}  // namespace clang::tidy::nullability