[fuzzer] Improve memory constraints (#141)

Author: tonkosi

tools/fuzzer/expr_gen.cc

@@ -561,6 +561,12 @@ std::optional<Expr> ExprGenerator::gen_cast_expr_impl(
     return {};
   }
 
+  const auto& memory_constraints = constraints.memory_constraints();
+  if (!cfg_.valid_pointer_cast_enabled && memory_constraints.must_be_valid() &&
+      memory_constraints.required_freedom_index() > 0) {
+    return {};
+  }
+
   auto maybe_type = gen_type(weights, constraints.type_constraints());
   if (!maybe_type.has_value()) {
     return {};
@@ -723,9 +729,15 @@ std::optional<Expr> ExprGenerator::gen_member_of_ptr_expr_impl(
 
   Field field = rng_->pick_field(fields);
   TypeConstraints new_type_constraints(field.containing_type());
-  ExprConstraints new_constraints = ExprConstraints(
-      new_type_constraints.make_pointer_constraints(),
-      memory_constraints.from_member_of(constraints.must_be_lvalue(), 1));
+  // If the field is a reference or a virtually inherited field, assume a read
+  // from memory.
+  MemoryConstraints new_memory_constraints =
+      field.is_reference_or_virtual()
+          ? MemoryConstraints(true, 1)
+          : memory_constraints.from_member_of(constraints.must_be_lvalue(), 1);
+  ExprConstraints new_constraints =
+      ExprConstraints(new_type_constraints.make_pointer_constraints(),
+                      std::move(new_memory_constraints));
   auto maybe_expr = gen_with_weights(weights, std::move(new_constraints));
   if (!maybe_expr.has_value()) {
     return {};

tools/fuzzer/expr_gen.h

@@ -147,6 +147,11 @@ struct GenConfig {
   // Probability that sizeof will take a type as a argument.
   float sizeof_gen_type_prob = 0.3f;
 
+  // A cast such as `((AnotherStruct*)struct_ptr)->field` isn't allowed. Setting
+  // this flag to false avoids casting between different pointer types if read
+  // from memory is expected.
+  bool valid_pointer_cast_enabled = false;
+
   BinOpMask bin_op_mask = BinOpMask::all_set();
   UnOpMask un_op_mask = UnOpMask::all_set();
 

tools/fuzzer/symbol_table.cc

@@ -243,17 +243,21 @@ class ClassAnalyzer {
     std::string name;
     Type type;
     uint32_t id;
+    bool is_reference;
     bool is_virtual;  // Is it a virtually inherited field?
 
-    FieldInfo(std::string name, Type type, uint32_t id, bool is_virtual = false)
+    FieldInfo(std::string name, Type type, uint32_t id, bool is_reference,
+              bool is_virtual = false)
         : name(std::move(name)),
           type(std::move(type)),
           id(id),
+          is_reference(is_reference),
           is_virtual(is_virtual) {}
 
     friend bool operator==(const FieldInfo& lhs, const FieldInfo& rhs) {
-      return lhs.id == rhs.id && lhs.is_virtual == rhs.is_virtual &&
-             lhs.name == rhs.name && lhs.type == rhs.type;
+      return lhs.id == rhs.id && lhs.is_reference == rhs.is_reference &&
+             lhs.is_virtual == rhs.is_virtual && lhs.name == rhs.name &&
+             lhs.type == rhs.type;
     }
 
     // Needed for std::unordered_set.
@@ -310,7 +314,7 @@ class ClassAnalyzer {
       auto maybe_type = convert_type(field.GetType(), ignore_qualified_types_);
       if (maybe_type.has_value()) {
         fields.emplace(fix_name(field.GetName()), std::move(maybe_type.value()),
-                       next_field_id());
+                       next_field_id(), field.GetType().IsReferenceType());
       }
     }
 
@@ -398,7 +402,9 @@ ClassAnalyzer load_tagged_types(SymbolTable& symtab, lldb::SBFrame& frame,
       const auto& info = classes.get_class_info(type);
       for (const auto& field : info.fields()) {
         if (info.is_unique_field_name(field.name)) {
-          symtab.add_field(tagged_type, field.name, field.type);
+          bool reference_or_virtual = field.is_reference || field.is_virtual;
+          symtab.add_field(tagged_type, field.name, field.type,
+                           reference_or_virtual);
         }
       }
     }

tools/fuzzer/symbol_table.h

@@ -42,15 +42,20 @@ struct VariableFreedomPair {
 
 class Field {
  public:
-  Field(TaggedType containing_type, std::string name)
-      : containing_type_(std::move(containing_type)), name_(std::move(name)) {}
+  Field(TaggedType containing_type, std::string name,
+        bool is_reference_or_virtual = false)
+      : containing_type_(std::move(containing_type)),
+        name_(std::move(name)),
+        is_reference_or_virtual_(is_reference_or_virtual) {}
 
   const TaggedType& containing_type() const { return containing_type_; }
   const std::string& name() const { return name_; }
+  bool is_reference_or_virtual() const { return is_reference_or_virtual_; }
 
  private:
   TaggedType containing_type_;
   std::string name_;
+  bool is_reference_or_virtual_;
 };
 
 class Function {
@@ -100,9 +105,9 @@ class SymbolTable {
   }
 
   void add_field(TaggedType containing_type, std::string field_name,
-                 Type field_type) {
-    fields_by_type_[std::move(field_type)].emplace_back(containing_type,
-                                                        std::move(field_name));
+                 Type field_type, bool reference_or_virtual) {
+    fields_by_type_[std::move(field_type)].emplace_back(
+        containing_type, std::move(field_name), reference_or_virtual);
 
     tagged_types_.insert(std::move(containing_type));
   }