[clang] [clang-tools-extra] [clang-tidy][dataflow] Add `bugprone-null-check-after-dereference` check (PR #84166)

[via cfe-commits]

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+//===-- NullPointerAnalysisModel.cpp ----------------------------*- 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 file defines a generic null-pointer analysis model, used for finding
+// pointer null-checks after the pointer has already been dereferenced.
+//
+// Only a limited set of operations are currently recognized. Notably, pointer
+// arithmetic, null-pointer assignments and _nullable/_nonnull attributes are
+// missing as of yet.
+//
+//===----------------------------------------------------------------------===//
+
+#include "clang/Analysis/FlowSensitive/Models/NullPointerAnalysisModel.h"
+#include "clang/AST/ASTContext.h"
+#include "clang/AST/Decl.h"
+#include "clang/AST/Expr.h"
+#include "clang/AST/Stmt.h"
+#include "clang/ASTMatchers/ASTMatchFinder.h"
+#include "clang/ASTMatchers/ASTMatchers.h"
+#include "clang/Analysis/CFG.h"
+#include "clang/Analysis/FlowSensitive/CFGMatchSwitch.h"
+#include "clang/Analysis/FlowSensitive/DataflowAnalysis.h"
+#include "clang/Analysis/FlowSensitive/DataflowEnvironment.h"
+#include "clang/Analysis/FlowSensitive/DataflowLattice.h"
+#include "clang/Analysis/FlowSensitive/MapLattice.h"
+#include "clang/Analysis/FlowSensitive/NoopLattice.h"
+#include "llvm/ADT/StringRef.h"
+#include "llvm/ADT/Twine.h"
+
+namespace clang::dataflow {
+
+namespace {
+using namespace ast_matchers;
+
+constexpr char kCond[] = "condition";
+constexpr char kVar[] = "var";
+constexpr char kValue[] = "value";
+constexpr char kIsNonnull[] = "is-nonnull";
+constexpr char kIsNull[] = "is-null";
+
+enum class SatisfiabilityResult {
+  // Returned when the value was not initialized yet.
+  Nullptr,
+  // Special value that signals that the boolean value can be anything.
+  // It signals that the underlying formulas are too complex to be calculated
+  // efficiently.
+  Top,
+  // Equivalent to the literal True in the current environment.
+  True,
+  // Equivalent to the literal False in the current environment.
+  False,
+  // Both True and False values could be produced with an appropriate set of
+  // conditions.
+  Unknown
+};
+
+using SR = SatisfiabilityResult;
+
+// FIXME: These AST matchers should also be exported via the
+// NullPointerAnalysisModel class, for tests
+auto ptrToVar(llvm::StringRef VarName = kVar) {
+  return traverse(TK_IgnoreUnlessSpelledInSource,
+                  declRefExpr(hasType(isAnyPointer())).bind(VarName));
+}
+
+auto derefMatcher() {
+  return traverse(
+      TK_IgnoreUnlessSpelledInSource,
+      unaryOperator(hasOperatorName("*"), hasUnaryOperand(ptrToVar())));
+}
+
+auto arrowMatcher() {
+  return traverse(
+      TK_IgnoreUnlessSpelledInSource,
+      memberExpr(allOf(isArrow(), hasObjectExpression(ptrToVar()))));
+}
+
+auto castExprMatcher() {
+  return castExpr(hasCastKind(CK_PointerToBoolean),
+                  hasSourceExpression(ptrToVar()))
+      .bind(kCond);
+}
+
+auto nullptrMatcher() {
+  return castExpr(hasCastKind(CK_NullToPointer)).bind(kVar);
+}
+
+auto addressofMatcher() {
+  return unaryOperator(hasOperatorName("&")).bind(kVar);
+}
+
+auto functionCallMatcher() {
+  return callExpr(hasDeclaration(functionDecl(returns(isAnyPointer()))))
+      .bind(kVar);
+}
+
+auto assignMatcher() {
+  return binaryOperation(isAssignmentOperator(), hasLHS(ptrToVar()),
+                         hasRHS(expr().bind(kValue)));
+}
+
+auto anyPointerMatcher() { return expr(hasType(isAnyPointer())).bind(kVar); }
+
+// Only computes simple comparisons against the literals True and False in the
+// environment. Faster, but produces many Unknown values.
+SatisfiabilityResult shallowComputeSatisfiability(BoolValue *Val,
+                                                  const Environment &Env) {
+  if (!Val)
+    return SR::Nullptr;
+
+  if (isa<TopBoolValue>(Val))
+    return SR::Top;
+
+  if (Val == &Env.getBoolLiteralValue(true))
+    return SR::True;
+
+  if (Val == &Env.getBoolLiteralValue(false))
+    return SR::False;
+
+  return SR::Unknown;
+}
+
+// Computes satisfiability by using the flow condition. Slower, but more
+// precise.
+SatisfiabilityResult computeSatisfiability(BoolValue *Val,
----------------
martinboehme wrote:

Naming nit: This isn't really computing satisfiability; it's computing whether or not the environment proves that a `BoolValue` is necessarily true or false.

I think you could rename this `tryToProve()`, and you could probably do without the `SatisfiabilityResult` type entirely (which isn't really an accurate name either) and instead return a `BoolValue &` (since that's what the caller ends up needing anyway). This would be either a literal true or false (if the environment can prove the value true or false) or `*Val` if the environment can't prove anything about the variable.

https://github.com/llvm/llvm-project/pull/84166