Model-checking Concurrent Java Software Model-checking Concurrent Java Software Using the Bandera Tool SetUsing the Bandera Tool Set

Matthew Dwyer

John Hatcliff

Radu Iosif

Yu Chen

Georg Jung

Todd Wallentine

Faculty Students and Post-docs

Robby

Roby Joehanes

Venkatesh Ranganath

Oksana Tkachuk

(http://www.cis.ksu.edu/santos/bandera)

FundingUS National Science Foundation (NSF)US National Aeronautics and Space Agency (NASA)US Department of Defense Advanced Research Projects Agency (DARPA) US Army Research Office (ARO)

Rockwell-Collins ATCHoneywell Technology Center and NASA Langley Sun Microsystems

SAnToS Laboratory, Kansas State University, USA

Goals of the ProjectGoals of the Project

II. Integration with commonly used design notations, methods, and processes… UML artifacts, JML

e.g., checking, specification

… automatic generation of synchronization code with dedicated checking

I. Provide platform for construction of and experimentation with technologies for model-checking concurrent Java software

… property specification languagese.g., temp logic, state machines

… model-reduction techniquese.g., abstraction, slicing, compiler-based optimizations

… model-checking enginese.g., explicit-state, symbolic

III. Evaluation using safety-critical military and civilian applications as well as non-critical popular open-source software

… integration with developmentand certification of safety-criticalsystems.

Model CheckingModel Checking

OK

orFinite-state model

Temporal logic formula

Model Checker

Error trace

Line 5: …Line 12: …Line 15:…Line 21:…Line 25:…Line 27:… …Line 41:…Line 47:…

What makes model-checking What makes model-checking software difficult?software difficult?

Model construction

OK

Error trace

orFinite-state model

Temporal logic formula

Model Checker

State explosion

Problems using existing checkers:

Property specification Output interpretation

Line 5: …Line 12: …Line 15:…Line 21:…

Model Construction ProblemModel Construction Problem

Semantic gap:

Model Description

Model CheckerProgram

void add(Object o) { buffer[head] = o; head = (head+1)%size;}

Object take() { … tail=(tail+1)%size; return buffer[tail];}

Gap

Programming Languages

Model Description Languages

methods, inheritance, dynamic creation, exceptions, etc.

automata

What makes model-checking What makes model-checking software difficult?software difficult?

Model construction

OK

Error trace

orFinite-state model

Temporal logic formula

Model Checker

State explosion

Problems using existing checkers:

Property specification Output interpretation

Line 5: …Line 12: …Line 15:…Line 21:…

Property Specification ProblemProperty Specification Problem

Difficult to formalize a requirement in temporal logic

“Between the window open and the window close, button X can be pushed at most twice.”

[]((open /\ <>close) -> ((!pushX /\ !close) U (close \/ ((pushX /\ !close) U (close \/ ((!pushX /\ !close) U (close \/ ((pushX /\ !close) U (close \/ (!pushX U close))))))))))

…is rendered in LTL as...

Property Specification ProblemProperty Specification Problem

We want to write source level specifications...

(((_collect(heap_b) == 1)\ && (BoundedBuffer_col.instance[_index(heap _b)].head == BoundedBuffer_col.instance[_index(heap _b)].tail) )\|| ((_collect(heap _b) == 3)\ && (BoundedBuffer_col_0.instance[_index(heap _b)].head == BoundedBuffer_col_0.instance[_index(heap _b)].tail) )\|| ((_collect(heap _b) == 0) && TRAP))

Heap.b.head == Heap.b.tail

We are forced to write model level specifications...

Forced to state property in terms of model rather than source:

Requirement:

If a buffer instance becomes full,it will eventually become non-full.

Consider multiple instances of a bounded buffer class...

In general, a heap object has no program-level name that persists throughout the lifetime of the object.

b1 b2 b3 Variables

Heap object

Property Specification ProblemProperty Specification Problem

Complications due to the dynamic nature of OO software…

What makes model-checking What makes model-checking software difficult?software difficult?

Model construction

OK

Error trace

orFinite-state model

Temporal logic formula

Model Checker

State explosion

Problems using existing checkers:

Property specification Output interpretation

Line 5: …Line 12: …Line 15:…Line 21:…

State Explosion ProblemState Explosion Problem

Moore’s law and algorithm advances can help– Holzmann: 7 days (1980) ==> 7 seconds (2000)

Explosive state growth in software limits scalability

Bit x1,…,xN 2^N states

Cost is exponential in the number of components

What makes model-checking What makes model-checking software difficult?software difficult?

Model construction

OK

Error trace

orFinite-state model

Temporal logic formula

Model Checker

State explosion

Problems using existing checkers:

Property specification Output interpretation

Line 5: …Line 12: …Line 15:…Line 21:…

Output Interpretation ProblemOutput Interpretation Problem

Raw error trace may be 1000’s of steps long

Model DescriptionProgram

void add(Object o) { buffer[head] = o; head = (head+1)%size;}

Object take() { … tail=(tail+1)%size; return buffer[tail];}

Gap

Error trace

Line 5: …Line 12: …Line 15:…Line 21:…Line 25:…Line 27:… …Line 41:…Line 47:…

Must map line listing onto model description Mapping to source is made difficult by

– Semantic gap & clever encodings of complex features– multiple optimizations and transformations

Bandera:Bandera:An open tool set for model-checking Java source codeAn open tool set for model-checking Java source code

Checker Inputs

CheckerOutputs

Optimization Control

Transformation &Abstraction Tools

ModelCheckers

Java Source

void add(Object o) { buffer[head] = o; head = (head+1)%size;}

Object take() { … tail=(tail+1)%size; return buffer[tail];}

Bandera Temporal Specification

Graphical User Interface

Error Trace Mapping

Bandera

Addressing theAddressing the Model Construction ProblemModel Construction Problem

Numerous analyses, optimizations,two intermediate languages, multiple back-ends

Slicing, abstract interpretation, specialization Variety of usage modes: simple...highly tuned

Model extraction: compiling to model checker inputs:

Java Source

void add(Object o) { buffer[head] = o; head = (head+1)%size;}

Object take() { … tail=(tail+1)%size; return buffer[tail];}

Model DescriptionModel Compiler

Static Analyses

Abstract Interpretation

Slicing Optimizations

Addressing theAddressing the Property Specification ProblemProperty Specification Problem

An extensible language based on field-tested temporal property specification patterns

[]((open /\ <>close) -> ((!pushX /\ !close) U (close \/ ((pushX /\ !close) U (close \/ ((!pushX /\ !close) U (close \/ ((pushX /\ !close) U (close \/ (!pushX U close))))))))))

Using the pattern system: 2-bounded existence

Between {open} and {close} {pushX} exists atMost {2} times;

Addressing theAddressing the State Explosion ProblemState Explosion Problem

Aggressive customization via slicing, abstract interpretation, program specialization

Java Source

void add(Object o) { buffer[head] = o; head = (head+1)%size;}

…

Model DescriptionsModel Compiler

Property

Generate models customized wrt property!

Result: multiple models --- even as many as one per property

Addressing theAddressing the Output Interpretation ProblemOutput Interpretation Problem

Run error traces forwards and backwards Program state queried Heap structures navigated Locks, wait sets, blocked sets displayed

Like a debugger: error traces mapped back to source

Java Source

void add(Object o) { buffer[head] = o; head = (head+1)%size;}

Object take() { … tail=(tail+1)%size; return buffer[tail];}

Model Compiler

ModelChecker

Intermediate Representations

Error traceLine 5: …Line 12: …Line 15:…Line 21:…

ModelDescription

+ simulator

Bandera ArchitectureBandera Architecture

BIRC BIR

Simulator

AbstractionEngine

Slicer

Analyses

Translators

SPIN

dSPIN

SMV

JPF

Property Tool

JavaJimple

Parser

Error Trace Display

Bounded Buffer Bounded Buffer class BoundedBuffer { Object [] buffer; int head; /* next available slot */ int tail; /* last available slot */ int bound; /* max # of elements */

public BoundedBuffer(int b) {…}

public synchronized boolean isEmpty() {…}

public synchronized void add(Object o) {…}

public synchronized Object take () {…}}

Initialization

head tail

Add,Add

head tail

Add,Take,Take

headtail

Property SpecificationProperty Specification

class BoundedBuffer { Object [] buffer; int head, tail, bound;

public synchronized void add(Object o) {…}

public synchronized Object take () {…}}

Requirement:

If a buffer becomes full,it will eventually becomenon-full.

Bandera Specification:

FullToNonFull: {Full(b)} leads to {!Full(b)} globally;

/** * @observable * EXP Full: (head == tail); */

forall[b:BoundedBuffer].

Property SpecificationProperty Specification

Requirement:

Empty buffers must added to before being taken from

Bandera Specification:

NoTakeWhileEmpty: {take.Return(b)} is absent after {Empty(b)} until {add.Call(b)};

forall[b:BoundedBuffer].

/** * @observable * EXP Empty: * head == ((tail+1) % bound);*/class BoundedBuffer { int head, tail, bound;

public synchronized void add(Object o) {…}

public synchronized Object take () {…}}

/** * @observable INVOKE Call; */

/** * @observable RETURN Return; */

Property-directed SlicingProperty-directed Slicing

slicing criterion generated automatically from observables mentioned in the property

backwards slicing automatically finds all components that might influence the observables.

Source program Resulting slice

Slice

mentionedin property

indirectlyrelevant

Property-directed SlicingProperty-directed Slicing/** * @observable EXP Full: (head == tail) */

class BoundedBuffer { Object [] buffer_; int bound; int head, tail; public synchronized void add(Object o) { while ( tail == head ) try { wait(); } catch ( InterruptedException ex) {}

buffer_[head] = o; head = (head+1) % bound; notifyAll(); }...}

Included inslicingcritirion

Slicing Criterion

All statementsthat assign tohead, tail.

indirectlyrelevant

removed byslicing

Abstraction EngineAbstraction Engine

int x = 0;if (x == 0) x = x + 1;

Data domains

(n<0) : neg(n==0): zero(n>0) : pos

Signs

neg poszero

int

Code

Signs x = zero;if (x == zero) x = pos;

Collapses data domains via abstract interpretation:

Abstraction Component Abstraction Component FunctionalityFunctionality

VariableConcrete Type

Abstract Type

Inferred Type

AbstractionLibrary

BanderaAbstractionSpecificationLanguage

BASLCompiler

PVS

JimpleJimple AbstractionEngine

AbstractedJimple

xydonecount

ob

intintbool

ObjectBuffer

int….

SignsSignsSigns

intAbsBool

….PointBuffer

Abstraction SpecificationAbstraction Specificationabstraction Signs abstracts intbegin TOKENS = { NEG, ZERO, POS };

abstract(n) begin n < 0 -> {NEG}; n == 0 -> {ZERO}; n > 0 -> {POS}; end

operator + add begin (NEG , NEG) -> {NEG} ; (NEG , ZERO) -> {NEG} ; (ZERO, NEG) -> {NEG} ; (ZERO, ZERO) -> {ZERO} ; (ZERO, POS) -> {POS} ; (POS , ZERO) -> {POS} ; (POS , POS) -> {POS} ; (_,_)-> {NEG, ZERO, POS}; /* case (POS,NEG), (NEG,POS) */ end

public class Signs { public static final int NEG = 0; // mask 1 public static final int ZERO = 1; // mask 2 public static final int POS = 2; // mask 4 public static int abstract(int n) { if (n < 0) return NEG; if (n == 0) return ZERO; if (n > 0) return POS; }

public static int add(int arg1, int arg2) { if (arg1==NEG && arg2==NEG) return NEG; if (arg1==NEG && arg2==ZERO) return NEG; if (arg1==ZERO && arg2==NEG) return NEG; if (arg1==ZERO && arg2==ZERO) return ZERO; if (arg1==ZERO && arg2==POS) return POS; if (arg1==POS && arg2==ZERO) return POS; if (arg1==POS && arg2==POS) return POS; return Bandera.choose(7); /* case (POS,NEG), (NEG,POS) */ }

Compiled

Specification Creation ToolsSpecification Creation Toolsabstraction Signs abstracts intbegin TOKENS = { NEG, ZERO, POS };

abstract(n) begin n < 0 -> {NEG}; n == 0 -> {ZERO}; n > 0 -> {POS}; end

operator + add begin (NEG , NEG) -> {NEG} ; (NEG , ZERO) -> {NEG} ; (ZERO, NEG) -> {NEG} ; (ZERO, ZERO) -> {ZERO} ; (ZERO, POS) -> {POS} ; (POS , ZERO) -> {POS} ; (POS , POS) -> {POS} ; (_,_)-> {NEG, ZERO, POS}; end

AutomaticGeneration

Forall n1,n2: neg?(n1) and neg?(n2) implies not pos?(n1+n2)

Forall n1,n2: neg?(n1) and neg?(n2) implies not zero?(n1+n2)

Forall n1,n2: neg?(n1) and neg?(n2) implies not neg?(n1+n2)

Proof obligations submitted to PVS...

Example: Start safe, then refine: +(NEG,NEG)={NEG,ZERO,POS}

Bounded Buffer BIRBounded Buffer BIR

process BoundedB() BoundedBuffer_rec = record { bound : range -1..4; head : range -1..4; tail : range -1..4; BIRLock : lock wait reentrant; }; BoundedBuffer_col : collection [3] of BoundedBuffer_rec; BoundedBuffer_col_0 : collection [3] of BoundedBuffer_rec;

BoundedBuffer_ref = ref { BoundedBuffer_col, BoundedBuffer_col_0 };

State Declarations

static identification of threads

object state as record

qualified lock representation

Reference type indicates mini-heaps that can be pointed to.

Easily express results of “points-to” analysis

bounded integer values

“mini-heaps” – one per allocator site

Bounded Buffer BIRBounded Buffer BIR

loc s34: live { b2, b1, T_0, T_6, T_8 } when true do invisible { T_8 := (T_6 % T_8); } goto s35; … loc s36: live { b2, b1, T_0 } when true do { notifyAll(T_0.BIRLock); } goto s37; … loc s37: live { b2, b1, T_0 } when true do { unlock(T_0.BIRLock); } goto s38;

Guarded Transitions

control point labellive variable information

used to optimize back-end code

annotation denotinginvisible transition which canbe merged with followingtransition

built-in operations on lock representations

Case Study: DEOS Kernel (with NASA Ames)Case Study: DEOS Kernel (with NASA Ames)

A real-time operating system for integrated modular avionics systems

Some interesting history…Non-trivial concurrent Java program: 1443 lines of code, 20 classes, 6 threads

With a known bug

Honeywell Dynamic Enforcement Operating System (DEOS)

Application processes are guaranteed to be scheduled for their budgeted time during a scheduling unit

Requirement:

DEOS ArchitectureDEOS Architecture

Requirement Monitor

Environment

System Clock & Timer

User Process 1

User Process 2

...

DEOS Kernel

...if(...) assert(false);...

class Thread

class StartofPeriodEvent

class ListofThreads

class Scheduler

Verification of DEOSVerification of DEOS

We used Bandera and Java PathFinder (JPF)

Verification of the system exhausted 4 Gigabytes

of memory without completing

– no information about satisfaction of requirement

To verify property or produce a counter-example

– to reduce the state space to a tractable size

– some form of abstraction is needed

Variable SelectionVariable Selection

Requirement Monitor

Environment

System Clock & Timer

User Process 1

User Process 2

...

Control dependencies:

29 conditionals

16 methods

32 variables

DEOS Kernel

int itsPeriodId = 0; ...public int currentPeriod() { return itsPeriodId; }public void pulseEvent(...) {... if(countDown == 0) { itsPeriodId=itsPeriodId + 1; ... }

class StartofPeriodEvent

int itsLastExecution; ...public void startChargingCPUTime(){ int cp=itsEvent.currentPeriod(); if(cp == itsLastExecution) { ... }

class Thread

...if(...) assert(false);...

Variable SelectionVariable Selection

Requirement Monitor

Environment

System Clock & Timer

User Process 1

User Process 2

...

Control dependencies:

29 conditionals

16 methods

32 variables

DEOS Kernel

int itsPeriodId = 0; ...public int currentPeriod() { return itsPeriodId; }public void pulseEvent(...) {... if(countDown == 0) { itsPeriodId=itsPeriodId + 1; ... }

class StartofPeriodEvent

int itsLastExecution; ...public void startChargingCPUTime(){ int cp=itsEvent.currentPeriod(); if(cp == itsLastExecution) { ... }

class Thread

...if(...) assert(false);...

Unbounded!

Variable SelectionVariable Selection

Requirement Monitor

Environment

System Clock & Timer

User Process 1

User Process 2

...

DEOS Kernel

int itsPeriodId = 0; ...public int currentPeriod() { return itsPeriodId; }public void pulseEvent(...) {... if(countDown == 0) { itsPeriodId=itsPeriodId + 1; ... }

class StartofPeriodEvent

int itsLastExecution; ...public void startChargingCPUTime(){ int cp=itsEvent.currentPeriod(); if(cp == itsLastExecution) { ... }

class Thread

...if(...) assert(false);...

Data dependencies

Attaching Abstract TypesAttaching Abstract Types

Requirement Monitor

Environment

System Clock & Timer

User Process 1

User Process 2

...

DEOS Kernel

int itsPeriodId = 0; ...public int currentPeriod() { return itsPeriodId; }public void pulseEvent(...) {... if(countDown == 0) { itsPeriodId=itsPeriodId + 1; ... }

class StartofPeriodEvent

int itsLastExecution; ...public void startChargingCPUTime(){ int cp=itsEvent.currentPeriod(); if(cp == itsLastExecution) { ... }

class Thread

...if(...) assert(false);...

SIGNS

SIGNS

SIGNS

Code TransformationCode Transformation

Requirement Monitor

Environment

System Clock & Timer

User Process 1

User Process 2

...

DEOS Kernel

Signs itsPeriodId = ZERO; ...public Signs currentPeriod() { return itsPeriodId; }public void pulseEvent(...) {... if(countDown == 0) { itsPeriodId=Signs.add(itsPeriodId ,POS);... }

class StartofPeriodEvent

Signs itsLastExecution; ...public void startChargingCPUTime(){ Signs cp=itsEvent.currentPeriod(); if(Signs.eq(cp,itsLastExecution)){ ... }

class Thread

...if(...) assert(false);...

Verification of Abstracted DEOS Verification of Abstracted DEOS

JPF completed the check– produced a 464 step counter-example

Does the counter-example correspond to a feasible execution?– difficult to determine– because of abstraction, we may get spurious errors

We re-ran JPF to perform a customized search– found a guaranteed feasible 318 step counter-example

After fixing the bug– the requirement was verified

SummarySummary A collection of tools for reasoning about concurrent systems. Has been applied to find bugs in safety-critical systems.

– Can it be used effectively by others – people familiar with quality assurance issues?

Still a research prototype, but the implementation is maturing. Is it possible to integrate tools like this into the develop process for

certified systems?– As a simple debugging utitlity?

– As an officially sanctioned (certified) tool?

– Can it complement existing tools used in certification – especially when concurrency is considered?

The JPF back-end has been modified to produce coverage information.

– Can this complement existing coverage tools?

We would be happy to have challenge problems/case studies.

Primary ChallengesPrimary Challenges

Will there be so much technical knowledge and effort required that it will only be used by research teams or quality assurance in highly safety-critical applications?

Can it be integrated well enough with standard development processes so it can be used not only with designs but also mesh well with rigorous testing?

Can software engineers be trained in the concepts of abstraction and temporal logic, e.g., in a one semester course, so with little further training they can use tools effectively, in concurrent software development?

Can effective “canned” solutions be provided for particular application domains – thus enabling developers with less training?

Unclear how far model-checking can be pushed down the hierarchy of software developers