tor-browser

writing-matchers.rst (7503B)

---

.. _writing_matchers:

Writing Matchers
================

On this page we will give some information about what a matcher is, and then provide an example of developing a simple match iteratively.

Types of Matchers
-----------------

There are three types of matches: Node, Narrowing, and Traversal.  There isn't always a clear separation or distinction between them, so treat this explanation as illustrative rather than definitive.  Here is the documentation on matchers: `https://clang.llvm.org/docs/LibASTMatchersReference.html <https://clang.llvm.org/docs/LibASTMatchersReference.html>`_

On that page it is not obvious, so we want to note, **cicking on the name of a matcher expands help about that matcher.** Example:

.. image:: documentation-expanded.png

Node Matchers
~~~~~~~~~~~~~

Node matchers can be thought of as 'Nouns'. They specify a **type** of node you want to match, that is, a particular *thing*. A function, a binary operation, a variable, a type.

A full list of `node matchers are listed in the documentation <https://clang.llvm.org/docs/LibASTMatchersReference.html#node-matchers>`_. Some common ones are ``functionDecl()``, ``binaryOperator()``, and ``stmt()``.

Narrowing Matchers
~~~~~~~~~~~~~~~~~~

Narrowing matchers can be thought of as 'Adjectives'. They narrow, or describe, a node, and therefore must be applied to a Node Matcher.  For instance a node matcher may be a ``functionDecl``, and the narrowing matcher applied to it may be ``parameterCountIs``.

The `table in the documentation <https://clang.llvm.org/docs/LibASTMatchersReference.html#narrowing-matchers>`_ lists all the narrowing matchers, which they apply to and how to use them.  Here is how to read the table:

.. image:: narrowing-matcher.png

And some examples:

::

 m functionDecl(parameterCountIs(1))
 m functionDecl(anyOf(isDefinition(), isVariadic()))


As you can see **only one Narrowing Matcher is allowed** and it goes inside the parens of the Node Matcher. In the first example, the matcher is ``parameterCountIs``, in the second it is ``anyOf``.

In the second, we use the singular ``anyOf`` matcher to match any of multiple other Narrowing Matchers: ``isDefinition`` or ``isVariadic``. The other two common combining narrowing matchers are ``allOf()`` and ``unless()``.

If you *need* to specify a narrowing matcher (because it's a required argument to some other matcher), you can use the ``anything()`` narrowing matcher to have a no-op narrowing matcher.

Traversal Matchers
~~~~~~~~~~~~~~~~~~

Traversal Matchers *also* can be thought of as adjectives - at least most of them.  They also describe a specific node, but the difference from a narrowing matcher is that the scope of the description is broader than the individual node.  A narrowing matcher says something about the node in isolation (e.g. the number of arguments it has) while a traversal matcher says something about the node's contents or place in the program.

Again, the `the documentation <https://clang.llvm.org/docs/LibASTMatchersReference.html#traversal-matchers>`_ is the best place to explore and understand these, but here is a simple example for the traversal matcher ``hasArraySize()``:

::

 Given:
   class MyClass { };
   MyClass *p1 = new MyClass[10];


 cxxNewExpr()
   matches the expression 'new MyClass[10]'.

 cxxNewExpr(hasArraySize(integerLiteral(equals(9))))
   does not match anything

 cxxNewExpr(hasArraySize(integerLiteral(equals(10))))
   matches the expression 'new MyClass[10]'.



Example of Iterative Matcher Development
----------------------------------------

When developing matchers, it will be much easier if you do the following:

1. Write out the code you want to match. Write it out in as many different ways as you can. Examples: For some value in the code use a variable, a constant and a function that returns a value. Put the code you want to match inside of a function, inside of a conditional, inside of a function call, and inside of an inline function definition.
2. Write out the code you *don't* want to match, but looks like code you do. Write out benign function calls, benign assignments, etc.
3. Iterate on your matcher and treat it as _code_ you're writing. Indent it, copy it somewhere in case your browser crashes, even stick it in a tiny temporary version-controlled file.

As an example of the above, below is a sample iterative development process of a more complicated matcher.

**Goal**: Match function calls where one of the parameters is an assignment expression with an integer literal, but the function parameter has a default value in the function definition.

::

 int add1(int a, int b) { return a + b; }
 int add2(int c, int d = 8) { return c + d; }

 int main() {
  int x, y, z;

  add1(x, y);     // <- No match, no assignment
  add1(3 + 4, y); // <- No match, no assignment
  add1(z = x, y); // <- No match, assignment, but not an integer literal
  add1(z = 2, y); // <- No match, assignment, integer literal, but function parameter lacks default value
  add2(3, z = 2); // <- Match
 }


Here is the iterative development process:

::

 //-------------------------------------
 // Step 1: Find all the function calls
 m callExpr()
 // Matches all calls, as expected.

 //-------------------------------------
 // Step 2: Start refining based on the arguments to the call
 m callExpr(forEachArgumentWithParam()))
 // Error: forEachArgumentWithParam expects two parameters

 //-------------------------------------
 // Step 3: Figure out the syntax to matching all the calls with this new operator
 m callExpr(
 	forEachArgumentWithParam(
 		anything(),
 		anything()
 	)
 )
 // Matches all calls, as expected

 //-------------------------------------
 // Step 4: Find the calls with a binary operator of any kind
 m callExpr(
   forEachArgumentWithParam(
      binaryOperator(),
      anything()
    )
 )
 // Does not match the first call, but matches the others

 //-------------------------------------
 // Step 5: Limit the binary operator to assignments
 m callExpr(
   forEachArgumentWithParam(
      binaryOperator(isAssignmentOperator()),
      anything()
    )
 )
 // Now matches the final three calls

 //-------------------------------------
 // Step 6: Starting to refine matching the right-hand of the assignment
 m callExpr(
   forEachArgumentWithParam(
      binaryOperator(
      	allOf(
      	  isAssignmentOperator(),
      	  hasRHS()
  	    )),
      anything()
    )
 )
 // Error, hasRHS expects a parameter

 //-------------------------------------
 // Step 7:
 m callExpr(
   forEachArgumentWithParam(
      binaryOperator(
      	allOf(
      	  isAssignmentOperator(),
      	  hasRHS(anything())
  		  )),
      anything()
    )
 )
 // Okay, back to matching the final three calls

 //-------------------------------------
 // Step 8: Refine to just integer literals
 m callExpr(
   forEachArgumentWithParam(
      binaryOperator(
      	allOf(
      	  isAssignmentOperator(),
      	  hasRHS(integerLiteral())
  		  )),
      anything()
    )
 )
 // Now we match the final two calls

 //-------------------------------------
 // Step 9: Apply a restriction to the parameter definition
 m callExpr(
   forEachArgumentWithParam(
      binaryOperator(
      	allOf(
      	  isAssignmentOperator(),
      	  hasRHS(integerLiteral())
  		  )),
      hasDefaultArgument()
    )
 )
 // Now we match the final call