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ASM-Ext: An Extension to ASM

Introduction

We are calling this project ASM-Ext for it is an extension to ASM and is based on ASM. ASM-Ext allows high level data and control flow analysis. This tool proves to be useful in research project when one wishes to quickly write a prototype static analysis to test his/her algorithms.

For those who are not familiar with ASM, ASM is a general, low-level transformation and analysis framework for Java bytecode. Java bytecodes are used not only for Java but a number of other programming langauges, notably Scala, also compiles to Java bytecodes. Since ASM provides its users with only primitive facilities, it can be used to analyze or transform any Java bytecode file regardless of the source language according to which the file is produced. We refer the readers to the official website of ASM (asm.objectweb.org) so as to get more information about this useful analysis/transformation framework.

As we stated above, ASM features primitive facilities that enable its users analyze or transform general JVM bytecode files. These primitive tools are, though fast, hard to use for research purposes where we need to quickly implement and test different static analysis algorithms. There are many "high-level" analysis frameworks out there but ASM-Ext is the first analysis framework written on top of ASM which will be our main tool during my PhD studies when I want to implement some algorithm. When compared to Soot framework, ASM-Ext runs faster, but this is not a virtue as Soot does a rather complicated transformation before presenting the object program to the programmer, and there are a handful of rival analyses that also run fast. So even though, to the best of our knowledge it features the most accurate control-flow construction algortihm, we are not going to call this programming work a research contribution; this is just a warm up exercise of mine before joining Software Evolution and Testing (SET) lab at UT Dallas.

As said above, ASM-Ext is a high-level analysis framework, by this we mean uisng ASM-Ext, in order to do control flow analysis and construct call-graph for an object progrm, all you need to do is to specify the name of its main class, and the path on which the class files reside. Similarly, in order to do a data-flow analysis all you need to do is to specify an instanceof Monotone Framework. Thanks to the facilities provided by Java 8, such a specification will be super easy. Pleade note that the functionality of ASM-Ext is highly extensible, e.g. one can write his/her desired fixed-point algorithm.

Organization

In this report, first we are going to review packages of the ASM-Ext, then through a number of examples, we will show how does call-graph analysis and control-flow graph construction of ASM-Ext works. Next we will do a classic data-flow analysis using ASM-Ext. Finally, we will describe the difficulties that one may face if they want to implement a control flow graph construction program for an object-oriented programming language like Java.

Packages

In this section, we are going to review the purpose of each package making up ASM-Ext. The package asmext, as the name suggests, is the package that contains the main class of the program. In this package there are classes to model Java classes, class members such as fields and methods. The package also contains classes for modeling different kinds of data types available in Java. The main class of ASM-Ext, receives the name of the main class of the object program through its first command line argument. Other parameters such as class path are provided through properties. The main class loads all classes that are directly or indirectly referenced by the main class of the object program, construsts class heirarchy of the program, and constructs its call graph. It is worth mentioning that the call-graph construction algorithm is based on simple CHA, an indeed constructs a gross approximation of the real call-graph of the object program. While ASM-Ext loads application and library classes, it does a very lightweighted abstract interpretation on the method bodies to infer the type of parameters fed to certian kind of bytecode instructions. This type information proves very useful in constructing accurate control-flow graphs.

Inside asmext, the package asmext.instruction contains abstractions for Java bytecode instructions. We have grouped Java bytecode instructions based on their functionality, e.g. all load instructions are modeled by the interface Load. From our previous experience of using Soot framework, we know that using such abstractions makes an analysis framework safe (in that programming under such a framework becomes less error prone) and more user-friendly. Among the instructions the class NeutralInst is used in control-flow graph construction to create "dummy" head nodes. Such dummy nodes do not correspond to any bytecode instruction inside the body of a method, rather they are added to control-flow graphs to make single-headed, easy-to-deal-with control-flow graphs. This package contains over 80 classes, and the root of all them is the interface Inst, which specifies the most general interface that is applicable in the case of any Java bytecode instruction, thereby enabling us treat them in a uniform manner.

When one needs to convert Java bytecodes to their corresponding abstract representation (e.g. during construction of control-flow graphs), instantiating the correct class based on the opcode of a bytecode instruction is a tedious and error prone task. The class InstFactory inside the package asmext.instruction.factory abstracts this process by providing a "builder" that by recieving an specification of the instruction to be created, instantiates the Inst class that correctly models the specified bytecode instruction.

The package controlflow contains three sub-packages controlflow.callgraph, controlflow.cfg, and controlflow.analysis. We describe these packages in the following paragraphs.

The interface CallGraph in the package controlflow.callgraph defines the general interface of any call-graph. As it is expected, any call-graph must, at least, have two method to query the methods that each method may call, and the methods that may be called at each call site. These functionality is provided by the two overloads of mayCall method.

Inside the package controlflow.cfg, the class CFG models control-flow graphs whose nodes are BasicBlocks and whose edges represent flow of control from source basic block to destination basic block. The class has methods to query successors or predecessors of each basic block. It also has methods to retrieve heads (which is always a single basic block in our case) and tails of the control-flow graph. By definition head is any node that does not have any predecessor and tail is any node that lacks successor. The class also offers a method dot to print out the graph in the form that can be fed to GraphViz program so as to produce a graphical representation of the graph. Any basic block is either an InstSeq (which represents sequence of instructions) or a TryCatchBlock (which represents a specification of a try-catch-block inside a method body). Although one may desire to insert specifications of try-catch blocks inside control-flow graphs they contruct, our control-flow graph construction algorithm does not include TryCatchBlocks inside the graphs it builds.

The package controlflow.analysis contains two important classes CHA and CFGBuilder that are responsible for control flow analysis. The class CHA offers a method doCFA that builds a call graph of the object program based on class hierarchy information that is gathered during class loading. The class CFGBuilder receives a method, and construct a control-flow graph for the method. The control-flow graph has not only normal flows of control, but also contains so-called exceptional control flows. As we shall describe in the next sections, building a control-flow graph in the presence of exception can be tricky. The algorithm implemented in CFGBuilder is very accurate in that, in comparison with the control-flow graphs produced by Soot, it contains less spurious edges. Removing such false edges, that are necessary in order to have a sound analysis, deteriorates the precision of any analysis based on the graphs.

The package dataflow, as the name suggests, contains classes necessary for data-flow analysis. The class MonotoneFramework represents instances of Monotone Framework. Each instance of MonotoneFramework is a sextuple (cfg, dep, initializer, lte, join, transferFunction) where

  • cfg is a reference to the control-flow graph of the method to be analyzed.
  • dep is a two place function that given the cfg and a node should return its successors or predecessors (depending on direction of the analysis which might be "forward" or "backward").
  • initializer is a function that given an extremal node (head or tail depending on direction of the analysis) should return the initial value for the analysis.
  • lte is a two-place predicate that should represent less-than-or-equal relation on the lattice based on which we are conducting our analysis.
  • join is a binary function that (depending on the nature of the analysis that might be "may" or "must") instructs how to join the data coming from different branchs of the cfg.
  • transferFunction represents the transfer function, it does the (abstract) interpretation by transforming input data to output data based on the kind of instruction being met.

The class MonotoneFramework is parameterized with the type of lattice on which the analysis is based. The contains the method doAnalysis that given a fixed-point computation strategy finds the maximal fixed-point solution (aka MFP). Please note that MFP is a misnomer and is present in the static analysis literature due to historical reasons; actually a solution to an analysis might be minimal fixed-point. The fixed-point algorithm shipped with ASM-Ext is general enough to handle both cases.

Fixed-point computation strategy is modeled by the abstract class FixedPointAlgorithm which is parameterized with the type of lattice on which the analysis is based. Any fixed-point computation algorithm in order to be compatible with our framework, needs to override the abstract method perform. Given an instance of MonotoneFramework, the method should return an MFP solution. ASM-Ext is shipped with the class Worklist which contains a general implementation of Worklist algorithm, a popular fixed-point algorithm with decent efficiency.

The interface SemiLattice specifies a general interface of a semi-lattice (aka join semi-lattice; a partially ordered set with pair of join and meet operations that yield least upper bound and greatest lower bound, resp., for each pair of elements). The interface FiniteSet is used to represent finite semi-lattices. The class FiniteHashSet is an implementation of FiniteSet based on Java's HashSet, and BitVectorSet is an implementation of the interface based on bit vectors.

Finally, the package util contains a number of useful classes on which our implementation depends. The class BiDiGraph_Int represent a directed graph. The implementation of the class is both fast (as it is purely based on integer manipulation) and space-efficient (as it chooses to use bytes or ints depending on the size of graph). The class BitVector, as the name suggests, implements a bit vector. Bit vectors are useful in many analyzes, and it is used (for some other purpose) in several places in our implementation of ASM-Ext. The parameterized class ImmutablePair represents pairs. Lastly, the class Utilities offers two methods for printing collections.

Using ASM-Ext

In this section, we are going to show how a programmer uses ASM-Ext in order to conduct data and control flow analyses. In what follows first, we show how to access call-graph of the object program and next we shall show how to use ASM-Ext to do a Reaching Definition Analysis.

ASM-Ext construct a call-graph once you specify the name of the main class of the object program, and as soon as it loads all application classes and all relevant library classes. In order to access the call-graph constructed for the object program, all you need to do is to get the public field cg of the ASMExt singleton class. Given an object program, the following example code snippet prints the names of te set of all methods that are directly called by a method of that program, provided that the Class is the name of a class in of the program and the class declares a at least one method named method.

Method theMethod = ASMExt.v().getClassByName("Class")
				.getDeclaredMethodByName("method")
				.get(0);
				
List<Method> callees = ASMExt.v().cg.mayCall(theMethod);
		
callees.stream().forEach(m -> System.out.println(m.name));

Now we are going to explain how to use ASM-Ext to doing Reaching Definition Analysis (RDA) on a method of the object program. We can specify RDA as an instance of Monotone Framework, first component of which is the control-flow graph of the object program piece. To compute control-flow of the method theMethod (retrieved above) we write:

CFG cfg = (new CFGBuider(theMethod)).build();

Suppose that the object program and the method theMethod is defined as follows:

class Class {
	//... something
	public static void method() {
		int x = 5;
		int y;
		int z;
		
		y = x;
		z = 1;
		while(y > 1) {
			z = z * y;
			y = y - 1;
		}
		y = 0;
		System.out.println(z);
	}
	//... something else
}

The program simply computes factorial of the local variable x, and prints out the result that is left in the variable z. For ease of presentation (and write less code!), we shall represent the local variable x with integer 0, y with 1, and z with 2. We are going to use pair (v, b) to represent variable v may be last assigned in a basic block whose number is b. We shall use (v, -1) to model the fact that we do not know where the local variable v is last assigned. The lattice on which we are going to conduct our RDA is the powersets of the finite set VARS x BASIC_BLOCKS, where VARS = {0, 1, 2} is the set of variables and BASIC_BLOCK is some finite subset of the set of natural numbers.

The rest of code for RDA can be written as follows. We leave as an exercise to the dedicated reader to explain the code.

List<Integer> VARS = Arrays.asList(0, 1, 2);
		
FiniteHashSet<ImmutablePair<Integer, Integer>> iota = new FiniteHashSet<>();
		
for(int var : VARS) {
	iota.add(new ImmutablePair<>(var, -1));
}
		
BiFunction<CFG, Integer, FiniteHashSet<ImmutablePair<Integer, Integer>>> initializer 
	= (_cfg, _node) -> _cfg.isHead(_node) ? iota : new FiniteHashSet<>();
@SuppressWarnings("unchecked")
BiFunction<FiniteHashSet<ImmutablePair<Integer, Integer>>, FiniteHashSet<ImmutablePair<Integer, Integer>>, FiniteHashSet<ImmutablePair<Integer, Integer>>> join 
	= (_s1, _s2) -> (FiniteHashSet<ImmutablePair<Integer, Integer>>) _s1.join(_s2);
ThreeVarFunction<CFG, Integer, FiniteHashSet<ImmutablePair<Integer, Integer>>, FiniteHashSet<ImmutablePair<Integer, Integer>>> transferFunction
	= (_cfg, _node, _input) -> {
		InstSeq iSeq = (InstSeq) _cfg.getNodeDescriptor(_node);
		List<Integer> assigned = new ArrayList<>();
		
		for(Inst inst : iSeq.instructions) {
			if(inst instanceof StoreIntLocal) {
				assigned.add(((StoreIntLocal) inst).varIndex); 
			}
		}
		_input = new FiniteHashSet<ImmutablePair<Integer, Integer>>();
		_input.addAll(_input.stream().filter(ip -> assigned.contains(ip.first())).collect(Collectors.toList()));
		for(int assignedVar : assigned)
			_input.add(new ImmutablePair<>(assignedVar, _node));
		return _input;
	};
		
MonotoneFramework<FiniteHashSet<ImmutablePair<Integer, Integer>>> rda 
	= new MonotoneFramework<>(cfg,
			CFG::succs,
			initializer, 
			FiniteHashSet<ImmutablePair<Integer, Integer>>::lte, 
			join,
			transferFunction);
			
rda.doAnalysis(new Worklist<>(rda));

Challenges

Constructing control-flow graphs in the absence of exceptions is all sunshine and flowers, but once we consider the exceptions that each instruction may throw, and also consider exception handling mechanism, we face difficulties that we are going to point out some of them.

In the latest version of Soot, the tool constructs control-flow graph (suppose that we are talking about Soot's ExceptionalUnitGraph, though the same problem occurs in ExceptionalBlockGraph) for a peice of code like the one below, it connects every instruction in the try block to the corresponding handler.

try {
	I1; ... In;
} catch (SOME_SUBTYPE_OF_Throwable e) {
	J1; ... Jk;
}

At the first sight, this is necessary for constructing a sound control-flow graph (the one that does not miss any flow edges), but with a careful examination we realize that not all instructions in the try block throw exceptions that the catch block may catch. ASM-Ext takes into account the set of all possible exceptions that an intruction may throw and connects only those intructions to the exception handler that may throw an exception whose type is compatible with SOME_SUBTYPE_OF_Throwable which is stated in the catch block. In particular, we do a light-weight abstract interpretation to infer the type Throwable object that each ATHROW instruction may through. It is worth noting that such processes do not deteriorate the performance of ASM-Ext.

The next complication arises if we consider the possibility of failure of the first intruction (I1 in the above code) in executing successfully. In such a situation, we must add an edge from I1 each predecessor to the handler, thereby modeling the state of machine in case that I1 fails to execute successfully. When constructing ExceptionalUnitGraphs, since Soot is commited to be faithful to the underlying bytecode, it cannot add nodes to the control-flow graphs that does not correspond to any bytecode instruction. Therefore, in the case of a grotesque like this (which is not so uncommon in JDK library),

static {
	try {
		I1; ... In;
	} catch (SOME_SUBTYPE_OF_Throwable e) {
		J1; ... Jk;
	}
}

Soot constructs two heads for the control-flow graph of the method. According to John Jorgensen, this is intended to take into account the occasions in which I1 fails to execute successfully (please note that even though he admits that this is a poor design decision, he had no choice; as I said, Soot is intended to be faithful to the underlying bytecode representation and the tool could not create a dummy head node so as to construct uniform, easy-to-deal-with control-flow graphs). In such occasions, ASM-Ext adds a dummy head to the control-flow graph (using the neutral instruction NeutralInst) and creates a single-headed control-flow graph.

Future Work

In later versions of ASM-Ext, once we build a points-to analysis for it, we shall refine the call-graph by consulting points-to information which can in turn refine points-to analysis by providing it with a more accurate call graph.

While dynamic loading and reflection is an important feature of Java, current version of ASM-Ext does not support dynamic loading or Java reflection. We need to add support for the features in the future versions. Since ASM-Ext is a fast analysis framework, we hope that we can conduct remarkably precise analyses so as to improve precision of the existing reflection analyses without incurring significant performance penalty.

Current version of ASM-Ext does not support JAR files, but since JAR file are simple ZIP files, we can extend ASM-Ext easily to handle JAR files.

Cloning & Building

Unfortunately, ASM-Ext lacks any kind of build script, so we need to explain how to build it. First you need a Java 8 compiler. Since I was using Eclipse Oxygen 4.7.0, I recommend to add the clonned project to that version of Eclipse. In Run Configuration window, you need to pass the name of main class of the object program as Program Argument, enable assertions by passing -ea to VM, and pass the following properties to ASM-Ext. You need to specify class path, i.e. where class files of the object program are stored; you can do that through VM argument -Dasmext.cp=[class-path], in which you should replace [class-path] with the actual class path. You can also ask ASM-Ext to not to print out messages about every single task it does, by passing VM the argument -Dasmext.verbose=false.

Please note that current version of ASM-Ext does not support JAR files, so in order to analyse Java programs stored in JAR files, you need to first unzip them.