Runtime Verification Ali Akkaya Boğaziçi University

Motivation The Remote Agent Experiment During the May 1999 RAX mission, the satellite deadlocked in space, causing the ground crew to put the spacecraft on standby. Ariane 5 Flight 501 Airane 5 Flight 501 was destroyed 40 seconds after take off. The US$1 billion prototype rocket self-destructed due to a bug in the on-board guidance software

Motivation Air-Traffic Control System in LA Airport The controllers lost contact with the planes when the main voice communications system shut down unexpectedly. To make matters worse, a backup system that was supposed to take over in such an event crashed within a minute after it was turned on. The outage disrupted about 800 flights ac ross the country.

 Introduction  Runtime Verification Tools  Java PathExplorer (JPaX)  Java MultiPathExplorer (JMPaX)  Conclusion  Further Study Outline

 Runtime Verification is the study of monitoring and analyzing system executions to detect/recover faults.  Two important aspects of program verification are  Testing  Use of Formal Methods Runtime Verification

Testing Formal Methods Ideal Runtime Verification Scalibility Coverage

Runtime Verification Architecture Reaction Instrumentation Specification Code Monitoring Execution

while (true) { lock(r1); processShared(); unlock(r2); } while (true) { lock(r1); logLock(p,r1); processShared(); release(r2); logRelease(p,r1); } Instrumentation Execution Traces: lock(p1,r1) release(p1,r1) lock(p2,r1) release(p2,r1)

 Dispatching of trace events to a set of specification rules.  Specification Language  Boolean Logic provides formulation of statements for a specific time.  Not sufficient to express time based changes in states Monitoring

 If A happens now, B must happen   (A → ◊ B) Future Time Temporal Logic AB p ∧ q = p and q p ∨ q = p or q p → q = p implies q ¬p = not p  p = always p ◊ p = eventually p p U q = p until q

 If A happens now, B must have happened   (A → ♦ B) Past Time Temporal Logic BA p ∧ q = p and q p ∨ q = p or q p → q = p implies q ¬p = not p ■ p = sofar p ♦ p = previously p p S q = p since q

 Offline  Monitor does not run in parallel but runs after program  Online  Outline: Runs in parallel with program as an external entity.  Inline: Runs in parallel with program as embedded in the code. Monitoring

 Action to be taken in case faults are detected  Error mesage  Exception  Seperate code execution  Integrated code execution Reaction

 Java PathExplorer (JPaX)  Java MultiPathExplorer (JMPaX)  Temporal ROVER (Commercial)  Cadence, Synopsys, Mentor (Commercial HW Tools)  Java MaC  Partial Order Trace Analyzer (POTA)  …. Runtime Verification Tools

Java PathExplorer (JPaX)  Monitors Java programs by analyzing (exploring) particular execution traces.  The observer performs two kinds of verification  Logic based monitoring  Future Time Temporal Logic  Past Time Temporal Logic  Error pattern analysis  Deadlocks  Data Races

JPaX Architecture

Data Race Analysis class Value { private int x = 1 ; public synchronized void add(Value v) { x = x + v.get() } ; public int get() { return x ; } } class Task extends Thread { Value v1 ; Value v2 ; public Task(Value v1, Value v2) {this.v1=v1;this.v2=v2;this.start()} public void run(){v1.add(v2)} ; } class Main { public static void main(String [] args) { Value d1 = new Value() ; Value d2 = new Value() ; new Task(d1, d2) ; new Task(d2, d1) ; }

Data Race Analysis  Task 1 start() d1.lock.acquire() d1. add(d2) d1.x = d1.x + d2.get() R 1 = d2.get() = 1 d1.x = 1 + R 1 = 2  Task 2 start() d2.lock.acquire() d2. add(d1) d2.x = d2.x + d1.get() R2 = d1.get() = 1 d2.x = 1 + R 2 = 2

Data Race Analysis  Task 1 Task 2 start() d1.lock.acquire() Thread-map[Task1] = {d1.lock} d1. add(d2) start() d2.lock.acquire() Thread-map[Task 2 ] = {d2.lock} d2. add(d1) d1.x = d1.x + d2.get() Variable-map[d1] = {d1.lock} R 1 = d2.get() = 1 Variable-map[d2] = {d 1.lock} d1.x = 1 + R 1 = 2 d2.x = d2.x + d1.get() Variable-map[d1] = {} R2 = d1.get() = 2 Variable-map[d1] = {} d2.x = 1 + R 2 = 3

Deadlock Analysis class Value { private int x = 1 ; public synchronized void add(Value v) { x = x + v.get() } ; public synchronized int get() { return x ; } } class Task extends Thread { Value v1 ; Value v2 ; public Task(Value v1, Value v2) {this.v1=v1;this.v2=v2;this.start()} public void run(){v1.add(v2)} ; } class Main { public static void main(String [] args) { Value d1 = new Value() ; Value d2 = new Value() ; new Task(d1, d2) ; new Task(d2, d1) ; }

Deadlock Analysis  Task 1 start() d1.lock.acquire() d1. add(d2) d1.x = d1.x + d2.get() d2.lock.acquire()  Task 2 start() d2.lock.acquire() d2. add(d1) d2.x = d2.x + d1.get() d1.lock.acquire() Deadlock occurred!!

Deadlock Analysis  Task 1 Task 2 start() d1.lock.acquire() Thread-map[Task1] = {d1.lock} d1. add(d2) d1.x = d1.x + d2.get() d2.lock.acquire() Thread-map[Task1] = {d1.lock, d2.lock} d1.lock → d2.lock R 1 = d2.get() = 1 d1.x = 1 + R 1 = 2 start() d2.lock.acquire() Thread-map[Task2] = {d2.lock} d2. add(d1) d2.x = d2.x + d1.get() d1.lock.acquire() Thread-map[Task 2 ] = {d 2.lock, d 1.lock} d2.lock → d1.lock Cycle!! R2 = d1.get() = 2 d2.x = 1 + R 2 = 3

Possible Implementation class Value { private int x = 1 ; public void add(Value v) { x = x + v.get() } ; public int get() { return x ; } } class Task extends Thread { Value v1 ; Value v2 ; public Task(Value v1, Value v2) {this.v1=v1;this.v2=v2;this.start()} public void run(){ synchronized (lock) { v1.add(v2)} ; } class Main { public static Object lock = new Object(); public static void main(String [] args) { Value d1 = new Value() ; Value d2 = new Value() ; new Task(d1, d2) ; new Task(d2, d1) ; }

Possible Implementation  Task 1 start() lock.acquire() d1. add(d2) d1.x = d1.x + d2.get() R 1 = d2.get() = 1 d1.x = 1 + R 1 = 2 lock.release ()  Task 2  start() lock.acquire() d2. add(d1) d2.x = d2.x + d1.get() R2 = d1.get() d2.x = 1 + R2 lock.release()

Java MultiPathExplorer (JMPaX)  Monitors multithreaded Java programs.  The observer performs Logic based monitoring based on Past Time Temporal Logic  Have the ability to predict safety violation errors in multithreaded programs by observing successful executions.

JMPaX Architecture

Vector Clocks  Vector Clocks is an algorithm for generating a partial ordering of events in a distributed system and detecting causality violations. A A:0 B B:0 C C:0 C:1 B:1 C:1 B:2 C:1 A:1 B:2 C:1 A:2 B:2 C:1 B:3 C:1 A:3 B:4 C:1 B:3 C:2 B:3 C:3 A:3 B:3 C:3

Example  Suppose that one wants to monitor some safety property of the multithreaded program below. The program involves relevant variables x, y and z: Initially: x = −1; y = 0; z = 0; thread T1{... x++;... y = x + 1;... } thread T2{... z = x + 1;... x++;... }

Example

Multithreaded Safety Analysis  Checking safety against single run  Suppose we want to monitor “if (x > 0), then (x = 0) has been true in the past, and since then (y > z) was always false.” (x > 0) → [(x = 0), y >z)s (−1, 0, 0), (0, 0, 0), (0, 0, 1), (0, 1, 1), (1, 1, 1) -> satisfied (−1, 0, 0), (0, 0, 0), (0, 1, 0), (0, 1, 1), (1, 1, 1) -> not satisfied

Multithreaded Safety Analysis  Checking safety against all runs  The major hurdle in monitoring all possible runs is that the number of possible runs can be exponential in the length of the computation  The problem is avoided by traversing the computation lattice level by level.

JPaX vs JMPaX  JPaX uses total ordering of events  JMPaX uses partial ordering of events  In JPaX it is possible to reveal errors in multithreaded programs that are hard to detect by observing successful executions.  JMPaX extends JPaX

Conclusion  Runtime verification combines testing and formal methods to provide scalable solutions with bigger coverage.  Several academic and commercial tools available to be used for runtime verification.

Further Study  Other runtime verification tools.  Use of tools on small scale real-life problems.

References  “ Runtime Safety Analysis of Multithreaded Programs ”, Koushik Sen, Grigore Rosu, and Gul Agha.  “ Monitoring Java Programs with Java PathExplore ”, K. Havelund and G. Rosu,    

Thank you Questions ?