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Java PathFinder and Model Checking of Programs

Information Sciences & Technology. Java PathFinder and Model Checking of Programs. Guillaume Brat, Dimitra Giannakopoulou, Klaus Havelund, Mike Lowry, Phil Oh, Corina Pasareanu, Charles Pecheur, John Penix, Willem Visser NASA Ames Research Center Automated Software Engineering Group.

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Java PathFinder and Model Checking of Programs

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  1. Information Sciences & Technology Java PathFinder andModel Checking of Programs Guillaume Brat, Dimitra Giannakopoulou, Klaus Havelund, Mike Lowry, Phil Oh, Corina Pasareanu, Charles Pecheur, John Penix, Willem Visser NASA Ames Research Center Automated Software Engineering Group Matt Dwyer, John Hatcliff Kansas State University Department of Computing and Information Sciences Alex Groce, Flavio Lerda Carnegie Mellon University School of Computer Science

  2. Outline • Motivation • Model Checking and Testing • Java PathFinder • Program Model Checking

  3. Software bugs in space do not fly Motivation • Software errors are expensive • Mars Polar Lander • Ariane 501

  4. OK Finite-state model or Error trace Model Checker (F W) Line 5: … Line 12: … Line 15:… Line 21:… Line 25:… Line 27:… … Line 41:… Line 47:… Temporal logic formula Model Checking • Verification and Validation are crucial • Model checking has been shown effective

  5. void add(Object o) { buffer[head] = o; head = (head+1)%size; } Object take() { … tail=(tail+1)%size; return buffer[tail]; } OK Program or Error trace Model Checker (F W) Line 5: … Line 12: … Line 15:… Line 21:… Line 25:… Line 27:… … Line 41:… Line 47:… Temporal logic formula The dream • Model Check Programs

  6. void add(Object o) { buffer[head] = o; head = (head+1)%size; } Object take() { … tail=(tail+1)%size; return buffer[tail]; } Some of the Issues • Semantics Gap • Programming Languages vs. Modeling Languages • Complexity • Not Automated Gap

  7. Outline • Motivation • Model Checking and Testing • Java PathFinder • Program Model Checking

  8. Model Checking and Testing • Software complexity is too high • Some of the presented methods are not sound • This is not model checking anymore • It is “automated” testing

  9. The assumption • Programs have bugs • Knowing that there are doesn’t mean knowing where they are • Testing is not always effective • Requires a lot of knowledge of the system • Model checking can be used to find bugs systematically • If no bug is found we have a non-result

  10. Coverage Metrics • Testing has coverage metrics • They tell you how good your testing is • They can be used to measure confidence • Testing is not very effective for concurrent systems • You don’t just have to guess the inputs but also the timing of the inputs and the scheduling • Model checking can address these issues • We are still missing metrics for concurrent programs

  11. Bug hunting • Bug hunting instead of trying to prove something correct • We can accept unsound methods • We may be able to handle real world examples • If we allow for modeling we are still not checking the correctness of the system itself

  12. Outline • Motivation • Model Checking and Testing • Java PathFinder • Program Model Checking

  13. Model Checker Special JVM Classes Bytecode State Space Model Checking for Java • Explicit State Model Checker • Java Bytecode as Input Language • Assertions, Deadlock Freedom, LTL Properties • Source Level Error Trace • Special JVM • Allows guided execution

  14. Architecture Generic Verification Environment Generic C++ C Search Algorithms (model checking, testing) Java Special JVM Class Loader Storage Subsystem (hash table, bitstate hashing) Expression Evaluator

  15. Outline • Motivation • Model Checking and Testing • Java PathFinder • Program Model Checking

  16. Programs are complex • Enabling Technologies • Slicing • Abstractions • State Compression • Partial Order Reduction • Heuristic Search

  17. indirectly relevant Slice mentioned in property Source program Resulting slice Property-directed Slicing • Slicing criterion automatically generated • Backwards slicing automatically finds dependencies

  18. Abstractions • Remove behaviors but preserve errors • manual or partially automated • Over-approximation • Preserve correctness • Type-based abstractions • Predicate abstraction • Semi-automated

  19. JPF Predicate Abstraction • Annotation used to indicate abstractions • Source-to-source translation • Java PathFinder can find abstract error traces … Abstract.remove(x); Abstract.remove(y); Abstract.addBoolean(“EQ”, x==y); …

  20. Choice-bounded Search • An abstract trace that does not contain any non-deterministic choice correspond to at least one concrete trace • Bias the model checker to look only choice-free traces

  21. Thread Object Object Class Class Stack Frame (Locals, Stack) Fields/Methods Fields/Methods Fields/Methods Fields/Methods Stack Frame (Locals, Stack) Stack Frame (Locals, Stack) Thread Stack Frame (Locals, Stack) Stack Frame (Locals, Stack) Storing the States • States are complex objects • Classes, Instances, Threads, Stack Frames Classes Threads Objects

  22. X11 Y27 Z75 T45 W11 X11 Y27 Z75 T45 W11 State Compression • Instructions modify only part of a state • Different states share common subparts X = X + 1 X0 X1

  23. Class Fields Class Monitors Thread Data Stack Frames Object Fields Object Monitors State Compression State Pools Array Compression is very effective: up to 94%!

  24. X=11 Y=27 X=12 Y=28 Partial Order Reduction • Do not explore “equivalent” traces • Requires analysis before model checking Access to local variable is perfect candidate for partial order reduction. Java does not provide enough information. Assume that every access to a shared object is made in mutual exclusion. Massive use of partial order reduction. Use lockset algorithm to check that mutual exclusion is actually present. Y++ X++ X=11 Y=28 X=12 Y=27 X++ Y++

  25. Heuristic Search • Depth first search leads to very long counter examples • Reactive system often exhibit periodic behavior • It is possible to discover errors at a shorter depth • Heuristic Search • Breadth first like state generation • Priority queue for the states based on some heuristic • The challenge • Find good heuristics: • Based on the property being checked • Based on the program structure • JPF offers an API for user-defined heuristics

  26. DEOS Real time OS from Honeywell 1500 lines of code Subtle concurrency error Testing did not reveal it We (re)discovered the bug! Dependency analysis Type abstraction Choice-free heuristic An example

  27. Conclusion • Model check programs poses some specific issues • Some we can deal with • Some we looked for a way around • Model checking can be used for systematic testing • Can be automated • Can handle concurrent systems • This is still work in progress!

  28. Future directions • Apply the same techniques to C/C++ • Next summer internship proposal • Combine property and heuristic specification • Allow the model checker to direct the search • Combine coverage, model checking and runtime analysis • Develop metrics • Check the system under certain assumptions

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