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Wednesday, July 27, 2016

Tree structures processing and unified AST

The previous article in this series discussed the theory of source code parsing in ANTLR and Roslyn. The article pointed out that a signature-based code analysis in PT Application Inspector is divided into the following stages:
1.        Parsing into a language dependent representation (abstract syntax tree, AST).
2.        Converting AST to a language independent unified format (unified AST, UAST).
3.        A direct comparison with patterns described in the DSL.
The current article focuses on the second stage that includes AST processing using Visitor and Listener strategies, converting AST to a unified format, simplifying an AST, and the algorithm for matching tree structures.

Contents

          AST Traversing
         Visitor and Listener
         Grammar and Visitor in ANTLR
          Types of nodes in a unified AST
          Testing of converters
          Simplifying an UAST
          Conclusion

AST Traversing

As is known, a parser converts the source code into an AST, a parse tree with redundant tokens removed. There are several different ways of processing such a tree. Probably the easiest one implies a recursive in-depth traversal of descendants. However, this approach can only work for rather simple cases, when the number of node types is small and the processing logic is plain to understand. In other cases, we need to split the processing logic into separate methods. To achieve this goal, we use two standard mechanisms (design patterns): Visitor and Listener.

Visitor and Listener

In the Visitor processing of the node descendants requires invoking their traversal methods manually. If the parent has three child nodes and we call methods only for two of them, a part of the subtree will not be processed. In the Listener (Walker) interface processing methods for node descendants are called automatically. The Listener interface contains enterNode() and exitNode() methods, which are invoked when entering and exiting a given node. Using an event mechanism allows implementation of these methods. Unlike methods in the Listener interface, Visitor methods may return objects and may even be typed. For instance, when we declare CSharpSyntaxVisitor, each Visit method returns an AstNode object, a common ancestor of all other nodes in a unified AST.
Thus, using Visitor’s design pattern to convert the tree provides you with dynamic and concise code because there is no need to store information about the visited nodes. The figure below shows that unnecessary HTML and CSS nodes are truncated while converting the PHP language. The order of traversal is indicated by numbers. Listener is usually used to aggregate data (e.g. from CSV files) and to convert one code to another (JSON -> XML). For more information, refer to The Definitive ANTLR 4 Reference.

Differences between Visitor in ANTLR and Roslyn

Visitor and Listener implementations may differ in libraries. The table below provides details about Visitor/Listener classes and methods in Roslyn and ANTLR.

ANTLR
Roslyn
Visitor
AbstractParseTreeVisitor< Result >
CSharpSyntaxVisitor< Result >
Listener
IParseTreeListener
CSharpSyntaxWalker
Default
DefaultResult
DefaultVisit(SyntaxNode node)
Visit
Visit(IParseTree tree)
Visit(SyntaxNode node)
Both Roslyn and ANTLR have the methods returning the default result (if the Visitor method is not overridden for some syntactic structure), and the Visit method that determines which special Visitor method should be called.
ANTLR generates a Visitor for every syntactic grammar rule. There are also special types of methods listed below:
          VisitChild(IRuleNode node); is used to implement the default node traversal.
          VisitTerminal(IRuleNode node); is used when traversing the terminal nodes, i.e. tokens.
          VisitErrorNode(IErrorNode node); is used when traversing the tokens obtained from parsing the code with lexical or syntax errors. For example, if a statement is missing a semicolon at the end, the parser will insert such a token and report it as an error. For more information about parsing errors, see the previous article.
          AggregateResult(AstNode aggregate, AstNode nextResult); a rarely used method intended for aggregating results derived from the traversal of descendants.
          ShouldVisitNextChild(IRuleNode node, AstNode currentResult); a rarely used method intended for determining whether it is necessary to process the next node descendant depending on the result of the currentNode traversal.
Visitor pattern for Roslyn has specific methods for each syntactic structure and a generalized Visit method that will work for all nodes. Unlike ANTLR, it is missing methods to perform traversal of “intermediate” structures. For example, Roslyn does not imply any method for VisitStatement statements, there are only some specific methods like VisitDoStatementVisitExpressionStatement,VisitForStatement, etc. The generalized Visit method can be used as a VisitStatement method. Another difference is that the traversal methods for SyntaxTrivia nodes (i.e. nodes, which can be easily removed without losing code information, like a space, or a comment) are called along with the traversal methods for the main nodes and tokens.
The drawback of using ANTLR visitors is that the names of the generated Visitor methods are directly dependent on the style of grammar rules, so they may fail to fit in with the overall code style. For example, SQL grammars use the so-called Snake case, in which the words are separated with underscore characters. Roslyn methods are written in the style of C# code. Despite the differences, processing techniques for tree structures in Roslyn and ANTLR become more and more unified with each new version (ANTLR version 3 and earlier had no support for Visitor and Listener mechanisms).

Grammar and Visitor in ANTLR

In ANTLR, the
ifStatement
    : If parenthesis statement elseIfStatement* elseStatement?
    | If parenthesis ':' innerStatementList elseIfColonStatement* elseColonStatement? EndIf ';'
    ; rule
will generate a VisitIfStatement(PHPParser.IfStatementContext context) method, wherein the context will have the following fields:
          parenthesis – a single node.
          elseIfStatement* – a node array. If the syntax is missing, then the array length is null.
          elseStatement? – an optional node. If the syntax is missing, then the optional node is null.
          IfEndIf – terminal nodes, start with a capital letter.
          ':'';' – unnamed terminal nodes, are not contained in the context (available only through GetChild ()).
It is worth noting that the fewer rules involved in the grammar, the easier and faster the Visitor can be written. However, the repeating syntax also needs to be brought out to separate rules.

Alternative and element labels in ANTLR

Quite often we have a situation when a rule has other alternatives, and it would be logical to handle these alternatives in the individual methods. Luckily, ANTLR 4 has the alternative labels that begin with an # character and are added after each rule alternative. When generating a parser code, a separate Visitor method is generated for each label, which allows to avoid having a huge amount of code in case if the rule has lots of alternatives. All the alternatives should be marked or none of them. We can use the rule element labels to name the terminal denoting a set of values:
expression
    : op=('+'|'-'|'++'|'--') expression                   #UnaryOperatorExpression
    | expression op=('*'|'/'|'%') expression              #MultiplyExpression
    | expression op=('+'|'-') expression                  #AdditionExpression
    | expression op='&&' expression                       #LogicalAndExpression
    | expression op='?' expression op2=':' expression     #TernaryOperatorExpression
    ;
ANTLR generates VisitExpressionVisitUnaryOperatorExpressionVisitMultiplyExpression, and some other visitors for this rule. Each Visitor will contain an expression array consisting of 1 or 2 elements and an op literal. Labels will keep the code clear and concise:
public override AstNode VisitUnaryOperatorExpression(TestParser.UnaryOperatorExpressionContext context)
{
    var op = new MyUnaryOperator(context.op().GetText());
    var expr = (Expression)VisitExpression(context.expression(0));
    return new MyUnaryExpression(op, expr);
}
public override AstNode VisitMultiplyExpression(TestParser.MultiplyExpressionContext context)
{
    var left = (Expression)VisitExpression(context.expression(0));
    var op = new MyBinaryOpeartor(context.op().GetText());
    var right = (Expression)VisitExpression(context.expression(1));
    return new MyBinaryExpression(left, op, right);
}
public override AstNode VisitTernaryOperatorExpression(TestParser.TernaryOperatorExpressionContextcontext)
{
    var first = (Expression)VisitExpression(context.expression(0));
    var second = (Expression)VisitExpression(context.expression(1));
    var third = (Expression)VisitExpression(context.expression(2));
    return new MyTernaryExpression(first, second, third);
}
...
Without using alternative labels, the processing of Expression is in the same method and the code is as follows:
public override AstNode VisitExpression(TestParser.ExpressionContext context)
{
    Expression expr, expr2, expr3;
    if (context.ChildCount == 2) // Unary
    {
        var op = new MyUnaryOperator(context.GetChild(0).GetText());
        expr = (Expression)VisitExpression(context.expression(0));
        return new MyUnaryExpression(op, expr);
    }
    else if (context.ChildCount == 3) // Binary
    {
        expr = (Expression)VisitExpression(context.expression(0));
        var binaryOp = new MyBinaryOpeartor(context.GetChild(0).GetText());
        expr2 = (Expression)VisitExpression(context.expression(1));
        return new MyBinaryExpression(expr, binaryOp, expr2);
        ...
    }
    else // Ternary
    {
        var first = (Expression)VisitExpression(context.expression(0));
        var second = (Expression)VisitExpression(context.expression(1));
        var third = (Expression)VisitExpression(context.expression(2));
        return new MyTernaryExpression(first, second, third);
    }
}
Alternative labels exist not only in ANTLR, but also in other tools for describing grammars. For example, unlike with ANTLR, an assignment operator label in Nitra is located to the left of the alternative:
syntax Expression
    {
      | IntegerLiteral
      | BooleanLiteral
      | NullLiteral            = "null";
      | Parenthesized          = "(" Expression ")";
      | Cast1                  = "(" !Expression AnyType ")" Expression;
      | ThisAccess             = "this";
      | BaseAccessMember       = "base" "." QualifiedName;
      | RegularStringLiteral;

Types of nodes in a unified AST

The development of the structure for a unified AST was guided by the structure of the NRefactory AST. We find this structure quite simple, at the same time, fidelity (a tree converted to code character by character) is not required. Every node is inherited from the AstNode and has its own type (NodeType), which is used at the stage of matching with patterns and deserialization from JSON. The structure of nodes looked like this:
In addition to the type, each node has a property that stores the location in the code (TextSpan), which is used to display it in the source code when comparing with the pattern. A nonterminal node keeps a list of child nodes, and terminal - a numeric, string or other primitive value.
In order to compare AST nodes of different languages we created a table, where each line represents the syntax of certain nodes and each column is their implementation in C#, Java, and PHP languages. The table looked as follows:



The explanation to the terms given in this table:
          Expression; the expression has a return value.
          Statement; the statement (instruction), has no return value.
          Literal; a terminal node.
          Most Common Ast (MCA) This node is built if all three languages contain a node of this or similar type (e.g. IfStatement, AssignmentExpression).
          Most Detail Ast (MDA) This node is built if at least one language contains a node of this type (e.g. FixedStatenemt fixed (a) { } for C#). These nodes are more relevant to SQL like languages due to the fact they are classified as declarative, and the difference between T-SQL and C# is much more significant than between PHP and C#.
In addition to the nodes seen in the figure (and pattern nodes described in the next section) there are also artificial nodes required to build the Most Common Ast node with as little loss in syntax as possible. The examples of such nodes are:
          MultichildExpression is inherited from the Expression, but it contains a collection of other Expression type nodes;
          WrapperExpression is inherited from the Expression, but it contains an arbitrary type node;
          WrapperStatement is inherited from the Statement, but contains an arbitrary type node.
Expression and Statement are basic constructs in imperative programming languages. First ones have a return value, while the second are used to execute operations. Therefore in this module we have focused mostly on them. These constructs are the basic building blocks of both the CFG implementation and other source code representations required for taint analysis. The detection of vulnerabilities in the source code requires no knowledge about the syntactic sugar, generics or other things specific to a particular language. Thus we can rewrite the syntactic sugar to basic constructs and remove some specific details.
Artificial nodes representing user templates are called pattern nodes. For example, a range of numbers and regular expressions are used as literal patterns.

Testing of converters

Testing of the entire code (instead of its parts) is a task of high priority for the code analyzer. To accomplish this, we decided to override Visitor methods for all the node types. Thus if vizitor is not used, it generates an exception new ShouldNotBeVisitedException(context). This approach simplifies the development, because the IntelliSense knows which methods were overridden, and which were not. Therefore, it helps to identify which Visitor methods have already been implemented.
We also have some suggestions on improving the code coverage analysis. Each node of the unified AST keeps the location of the corresponding source code. At the same time all the terminals are associated with lexemes, i.e. certain sequences of characters. Since all the lexemes should be processed, the coverage ratio can be expressed in the following form where uterms — terminals of the unified AST, and terms — terminals of the typical AST in Roslyn or ANTLR:
This metric represents code coverage using a single coefficient that should tend to unity. The evaluation through this coefficient is approximate, however it could be used to refactor and improve the Visitor's code. We can use graphical representation of the covered terminals to obtain a more reliable analysis.

Simplifying an UAST

After converting an AST to UAST the latter should be simplified. The simplest and most effective optimization method is a constant folding. For example, there are some code vulnerabilities related to setting the excessively long lifetime of the cookie: cookie.setMaxAge(2147483647); the argument in brackets can be both written as a single number, e.g. 86400, and some arithmetic expression, 60 * 60 * 24. Another example is related to string concatenation when searching SQL-injection and other vulnerabilities.
To achieve this goal Visitor for the UAST was implemented. Since simplification of an AST reduces the number of nodes in the tree, the Visitor is typed: it accepts and returns the same type. The reflection feature in .NET allows the implementation of such a Visitor with small code size. Since each node contains other nodes or terminal primitive values, then using reflection enables to extract all possible members of the particular node and to process them, calling other visitors in a recursive scenario.

Algorithm for matching AST and patterns

The algorithm is trying to match the pattern introduced as a tree structure with a tree fragment rooted at the current node. First, the node type is compared and then the following operations are performed depending on its type:
          Recursive comparison of descendants.
          Comparison of simple literal types (identifier, strings, and numbers).
          Comparison of extended literal types (regular expressions, ranges). Comments are included in this type.
          Comparison of complex extended types (expressions, the Statement sequence).
This approach is based on simple principles to achieve high performance with a relatively small amount of code for their implementation. The latter is achieved due to the fact that the CompareTo method to compare nodes is implemented for the base class, terminals, and a small number of other nodes. It is not yet required to implement more sophisticated finite-state machine algorithms improving the performance. However, it is difficult (or even impossible) to use this algorithm for more advanced analysis, e.g., the one sensitive to semantics and covering links between different AST nodes.

Conclusion

In this article we went over Visitor and Listener patterns used to process trees. We also talked about the structure for a unified AST. Next time we will tell you about:
          Methods for storing code patterns (Hardcoded, Json, DSL).
          Developing and using the DSL to describe patterns.
          Examples of some actual patterns and principles of searching them in open source projects.


Author: Ivan Kochurkin, Positive Technologies

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