1

Welcome to CS 119

Reliable Software: Testing and Monitoring

First half of lectures will cover testing• Taught by me (agroce@gmail.com)• http://www.cs.cmu.edu/~agroce• http://www.cs.cmu.edu/~agroce/CS119

Last half of class will focus on monitoring• Taught by Klaus Havelund

“In runtime verification a software component,an observer, monitors the execution of a programand checks its conformity with a requirementspecification.” - Klaus

2

Today

Some general background

• Topics of the class

• Testing project

Basic definitions

Black box testing (FSM) algorithms• Why is testing difficult, in theory and

practice?

3

Before we start

What do I know about testing, anyway?• I’ve written programs and tested them

• So have most of you, I would bet• Split my time at JPL between model checking &

testing research• E.g., testing the file systems that will be used in

the Mars Science Laboratory – JPL’s next big Mars mission

4

they turn the file system off during EDL (Entry, Descent, and Landing), which helps me sleep at night

5

Black box (Finite State Machine) testing

Design for testability

Coverage measures

Random testing

Constraint-based testing

Debugging and test case minimization

Using model checkers for testing

Coverage revisited (“small model property”)

Topics in Testing We’ll Cover

6

Read All About It

No textbook for this class, only papers

Books I like that have something important to say about testing (though none of these are about testing):

• The Practice of Programming, Kernighan and Pike

• Programming Pearls, Bentley

• Why Programs Fail: A Guide to Systematic Debugging, Zeller

• Code Complete, McConnell

• The Mythical Man-Month, Brooks

7

Read All About It

Book about testing:

• Introduction to Software Testing, Ammons and Offutt• I like it myself• Recommended by colleagues who’ve taught classes on testing

(and are first-rate testing researchers)• Book is thorough and cleverly organized, provokes some real

thought about how to test programs

• I might just follow this book if doing a whole class on testing• As it is, we’ll take more of a “hit the highlights” approach –• More concentrated on automated techniques: random

testing, constraint-based testing, and model checking• Won’t stop me from using some of their slides for areas they

cover well

8

Testing Project

Start with source code for YAFFS (Yet Another Flash File System): an open source flash file system• Plus a set of (buggy) variations of the

YAFFS code• A (minimal) automated testing framework

Project:• Write a better automated tester

• Must be efficient: spend no more than 45 minutes to test a version of YAFFS

• Write a test report on the YAFFS versions

9

Testing Project

You will also turn in two new buggy variations of YAFFS• Make sure your tester finds the bug• Produce a test case• Make sure the test case succeeds for original YAFFS!• You get to debug programs in lots of classes and in real

life – in this class you get to bug a program intentionally, for once in your life

I will apply all of your testers to these bugs plus another (top secret) set of mutations of YAFFS that I generate

Let me know any time you think you find a bug in the original YAFFS!

10

Testing Project

Grading criteria• Design/implementation of the tester• Effectiveness of the tester• Quality of the test report

• Can I figure out how you tested YAFFS?• Can I figure out what wasn’t tested?• Can I figure out how reliable you think

YAFFS is, and how buggy the various versions are?

• How “interesting” and hard-to-find your new bugs are

11

Testing Project

Expectations• You can program in C• You can use makefiles / build system

Hopes• Maybe we’ll find some previously unknown

bugs in YAFFS – that would be cool• I hope you’ll help me make sure MSL

figures out if there was life on Mars

Get started with YAFFS at http://www.yaffs.net

“he who learns to play the harp learns to play by playing it”- Aristotle, Metaphysics, Book IX

12

Basic Definitions: Testing

What is software testing?• Running a program• In order to find faults

• a.k.a. defects• a.k.a. errors • a.k.a. flaws• a.k.a. faults• a.k.a. BUGS

13

Bugs

“It has been just so in all of my inventions. The first step is an intuition, and comes with a burst, then difficulties arise—this thing gives out and [it is] then that 'Bugs'—as such little faults and difficulties are called—show themselves and months of intense watching, study and labor are requisite. . .” – Thomas Edison

“an analyzing process must equally have been performed in order to furnish the Analytical Engine with the necessary operative data; and that herein may also lie a possible source of error. Granted that the actual mechanism is unerring in its processes, the cards may give it wrong orders. ” – Ada, Countess Lovelace (notes on Babbage’s Analytical Engine)

Hopper’s“bug” (mothstuck in arelay on anearly machine)

14

Testing

What isn’t software testing?• Purely static analysis: examining a program’s

source code or binary in order to find bugs, but not executing the program

• Good stuff, and very important, but it’s not testing

• Fuzzy borderline: if we only symbolically execute the program

• For this class, we’ll stick to testing where the program actually runs (but maybe in a virtual machine)

15

Why Testing?

Ideally: we prove codecorrect, using formalmathematical techniques (with a computer, not chalk)

• Extremely difficult: for some trivial programs (100 lines) and many small (5K lines) programs

• Simply not practical to prove correctness in most cases – often not even for safety or mission critical code

16

Why Testing?

Nearly ideally: use symbolic or abstract model checking to prove the system correct• Automatically extracts a mathematical abstraction from

a system• Proves properties over all possible executions

• In practice, can work well for very simple properties (“this program never crashes in this particular way”), but can’t handle complex properties (“this is a working file system”)

• Doesn’t work well for programs with complex data structures (like a file system)

17

As a last resort…

… we can actually run the program, to see if it works

This is software testing• Always necessary, even when you can prove

correctness – because the proof is seldom directly tied to the actual code that runs

“Beware of bugs in the above code; I have only proved it correct, not tried it” – Knuth

18

Why Does Testing Matter?

NIST report, “The Economic Impacts of Inadequate Infrastructure for Software Testing” (2002)

• Inadequate software testing costs the US alone between $22 and $59 billion annually

• Better approaches could cut this amount in half

Major failures: Ariane 5 explosion, Mars Polar Lander, Intel’s Pentium FDIV bug

Insufficient testing of safety-critical software can cost lives: THERAC-25 radiation machine: 3 dead

We want our programs to be reliable• Testing is how, in most cases, we find out if

they are

Mars PolarLander crashsite?

THERAC-25 design

Ariane 5:exception-handlingbug : forced selfdestruct on maidenflight (64-bit to 16-bitconversion: about370 million $ lost)

19

Testing and Monitoring

In this first half of the class, we’ll look at which executions of a program to run• I’ll call this problem “the” testing problem

Second problem: how do we know if an execution reveals a bug?• Key question when monitoring deployed

programs to handle faults or send in bug reports from the field

• I’ll (mostly) take this for granted: we have a reference model or assertions to check

20

Example: File System Testing

File system is a library, called by other components of the flight software

Accepts a fixed set of operations that manipulate files:

Operation Result

mkdir (“/eng”, …) SUCCESS

mkdir (“/data”, …) SUCCESS

creat (“/data/image01”, …) SUCCESS

creat (“/eng/fsw/code”, …) ENOENT

mkdir (“/data/telemetry”, …) SUCCESS

unlink (“/data/image01”) SUCCESS

/

/eng /data

image01 /telemetry

File system

21

Example: File System Testing

Easy to detect many errors: we have access to many working file systems, and can just compare results

Choose operation F

Perform F on Tested FSPerform F on Reference

(if applicable)

Compare return values

Compare error codes

Compare file systems

Check invariants

(inject a fault?)

(in this unusual case, the problem Klaus will discuss is not much of a problem)

22

Example: File System Testing

How hard would it be to just try “all” the possibilities?

Consider only core 7 operations (mkdir, rmdir, creat, open, close, read, write)• Most of these take either a file name or a

numeric argument, or both• Even for a “reasonable” (but not provably safe)

limitation of the parameters, there are 26610

executions of length 10 to try• Not a realistic possibility (unless we have 1012

years to test)

23

The Testing Problem

This is the topic of the first half of the class: what “questions” do we pose to the software, i.e., • How do we select a small set of executions out

of a very large set of executions?

• Fundamental problem of software testing research and practice

• An open (and essentially unsolvable, in the general case) problem

24

The Testing Problem / Terms

This is not a class in the management or even the basic practices of testing• Hard, important problem• But not the focus of this class

This class is going to focus on state-of-the-art automated approaches• Using tools• To catch the bugs that you don’t catch with basic

practices

I will briefly cover some basic terms of testing and testing management today, then we’ll mostly dive into “How To Test It” at a more technical level

25

Terms: Verification and Validation

These two terms appear a lot, often in vague or sloppy ways, in the literature• Verification is checking that a program

matches a specification• Validation is making sure it meets the

original requirements – satisfies customers, operates ok onboard the spacecraft, etc.

Verification: “you built it right”

Validation: “you built the right thing”

(our focus, forthe most part)

26

Terms: Unit, Integration, System Testing

Stages of testing• Unit testing is the first phase, done by

developers of modules• Integration testing combines unit tested

modules and tests how they interact• System testing tests a whole program to

make sure it meets requirements

• “Design testing” is testing prototypes or very abstract models before implementation – seldom mentioned, but when possible it can save your bacon

• Exhaustive model checking may be possible at this stage

27

Terms: Functional Testing

Functional testing is a related term• Tests a program from a “user’s” perspective – does it

do what it should?• Opposed to unit testing, which often proceeds from

the perspective of other parts of the program• Module spec/interface, not user interaction• Sort of a fuzzy line – consider a file system – how different is

the use by a program and use of UNIX commands at a prompt by a user?

• Building inspector does “unit testing”; you, walking through the house to see if its livable, perform “functional testing”

• Kick the tires vs. take it for a spin?

28

Terms: Regression Testing

Regression testing• Changes can break code, reintroduce old bugs

• Things that used to work may stop working (e.g., because of another “fix”) – software regresses

• Usually a set of cases that have failed (& then succeeded) in the past

• Finding small regressions is an ongoing research area – analyze dependencies

“. . . as a consequence of the introduction of new bugs, program maintenance requires far more system testing. . . . Theoretically, after each fix one must run the entire batch of test cases previously run against the system, to ensure that it has not been damaged in an obscure way. In practice, such regression testing must indeed approximate this theoretical idea, and it is very costly." - Brooks, The Mythical Man-Month

29

Terms: The Oracle Problem (Klaus)

The oracle problem• How to know if a test fails• If the oracle says every execution is good, why

bother running the program?• Some obvious, easily automated approaches:

• The program probably shouldn’t crash• Assertions shouldn’t be violated

• Automatable, but more difficult to apply:• Differential testing (McKeeman, etc.) – when you

have another program, likely correct, that does the same thing, just compare outputs over same inputs

• Last resort, not automatable:• Hand inspection of executions

(oracle: a magical source of truth, often cryptic, given by the gods)

30

Terms: Test (Case) vs. Test Suite

Test (case): one execution of the program, that may expose a bug

Test suite: a set of executions of a program, grouped together• A test suite is made of test cases

Tester: a program that generates tests

Line gets blurry when testing functions, not programs – especially with persistent state

31

Terms: Black Box Testing

Black box testing• Treats a program or system as a • That is, testing that does not look at source

code or internal structure of the system• Send a program a stream of inputs, observe the

outputs, decide if the system passed or failed the test

• Abstracts away the internals – a useful perspective for integration and system testing

• Sometimes you don’t have access to source code, and can make little use of object code

• True black box? Access only over a network

32

Terms: White Box Testing

White box testing• Opens up the box!

• (also known as glass box, clear box, or structural testing)

• Use source code (or other structure beyond the input/output spec.) to design test cases

• Brings us to the idea of coverage

33

Terms: Coverage

Coverage measures or metrics• Abstraction of “what a test suite tests” in a

structural sense• Best explained by giving examples• Common measures:

• Statement coverage• A.k.a line coverage or basic block coverage• Which statements execute in a test suite

• Decision coverage• Which boolean expressions in control structures

evaluated to both true and false during suite execution• Path coverage

• Which paths through a program’s control flow graph are taken in the test suite

34

Terms: Coverage Measures

In general, used to measure the quality of a test suite• Even in cases where the suite was designed for

some other purpose (such as testing lots of different use scenarios)

• Not always a very good measure of suite quality, but “better than nothing”

• We “open the box” in white box testing partly in order to look at (and design tests to achieve) coverage

We’ll cover coverage in much more detail

35

Terms: Mutation Testing

A mutation of a program is a version of the program with one or more random changes

Mutation testing is another way to measure the quality of a test suite• Amman and Offutt call it syntax-based coverage

Idea: generate a large number of mutants• Run the test suite on these

• If few mutants are detected, the test suite may not be very good

• Difficulties• Cost of testing many versions of a program• How to generate mutants (operators)

• In principle, can subsume many otherforms of coverage

36

Black Box (Finite State Machine) Testing

37

Black box(FSM) testing

Let’s step back from software testing

Let’s look at a simpler model• Finite state machines

• Software is a finite state machine• What? Software is a Turing machine, right?

Lego “Turing machine”

Only with an infinite tape.

That is, only if your software has accessto infinite memory.

38

Black box (FSM) testing

With static memory allocation or with limited dynamic allocation nothing is infinite• Even if you add in disk or network storage• We don’t have infinite electrons, much

less memory

So software systems are finite state machines, in reality

Don’t you feel better now?• No more late nights worrying about the halting

problem!

there are only ~1079 of these little guys, y’know?

39

Black box (FSM) testing

Theoretical issues aside, why do we care about testing finite state machines?• Abstraction: designs can often be best

understood as finite-state machines• String processing/searching• Protocols – communication, cache coherence, etc.• Control component of any discrete system

• Automatic abstraction:• Tools that take systems and produce (coarse) finite

state abstractions

40

Black box (FSM) testing

Useful for modeling aspects of many designs

FD = open (“/foo”) close(FD)

read(FD, buf, nbytes)

write(FD, buf, nbytes)

41

Very Simple FSM Model

FSM is a tuple, <S, , T, I>• S is a set of states is the input alphabet• T is the transition relation

• T: S x x S• I S is the initial state

Further assume:• Machine is deterministic

• T is a (partial) function S x S

• Given an input from , machine either• Outputs 0 (if no transition)• Or outputs 1 and takes the transition to s’

a

c

a

ba

d

a 1b 1c 0

42

Conformance Testing

How do we test finite state machines?

Let’s say we have• Known FSM A

• Know all states and transitions• Unknown FSM B (same alphabet)

• Can only perform experiments• How do we tell if A = B?

Known as the conformance testing or equivalence testing problem• As stated, we cannot solve the problem• Why?

a

c

a

ba

d

43

Combination Lock Machine

How many states does B have?• If we don’t know, we can never be sure it is the

same machine as A

• B is a combination lock: looks like A unless we input exact sequence “b u g” – in which case it deadlocks

Machine A

a-z

Machine B

a, c-z

a-t, v-z

b

a-f, h-z

u g

44

Combination Lock Machine

Even if we know upper limit n on B’s size, for alphabet of size ||• It takes n|| tests to check equivalence to this

particular A• This pathological case imposes some limits on

conformance testing in general

Machine A

a-z

Machine B

a, c-z

a-t, v-z

b

a-f, h-z

u g

45

Conformance Testing (VC Algorithm)

Algorithm due to Vasilevskii and Chow for conformance testing• Assumptions

• A is minimized, has m states• B has no more than n states• A, B both have a reliable reset

• We can start from initial state at will

• Worst-case complexity: O(n2 m ||n-m+1)

• I’ll cover this quickly and informally, skipping over the sub-algorithms

46

Conformance Testing (VC Algorithm)

First, we find a path to each state of A

Typically, we compute a spanning tree• For example, by a depth first search (DFS)

Call this set P

a

c

a

b

d

a

a b

d

Read the paths off of the tree:

<no input>

a

a a

a a d

a b

a

47

Conformance Testing (VC Algorithm)

Next, compute a characterizing (or distinguishing) set for A

Set W of input sequences such that s, s’ S• s s’• Exists w W

• Output for w from s not equal to output from s’ • i.e., we can use W to tell what state we’re in

48

Conformance Testing (VC Algorithm)

Next, compute a characterizing (or distinguishing) set for A

For example, W for A might be:• {aa, b}

• aa: 11, 10, 01, 00, 10• Distinguishes all but these two states• Which are distinguished by b (1 vs. 0)

Can we find another (better?) set?

a

c

a

b

a

d

49

Conformance Testing (VC Algorithm)

Now we can compute Z:• W U W U2 W U• …m-n W

To test B for conformance with A• Run the tests produced by taking cross-product

of P and Z on both A and B

50

Conformance Testing (VC Algorithm)

P: {<>, a, aa, aad, ab}

W: {aa, b}

Let’s say we know B has no more than 6 states

The complete testing sequence (with reset before each test on each machine) is:• {aa, b, a aa, a b, aa aa, aa b, aad aa, aad b, ab

aa, ab b, a aa, b aa, c aa, d aa, a b, b b, c b, d b, a a aa, a b aa, a c aa, a d aa, a a b, a b b, a c b, a d b, aa a aa, aa b aa, aa c aa, aa d aa, aa a b, aa b b, aa c b, aa d b, aad a aa, aad b aa, aad c aa, aad d aa, aad a b, aad b b, aad c b, aad d b, ab a aa, ab b aa, ab c aa, ab d aa, ab a b, ab b b, ab c b, ab d b}

a

c

a

b

a

d

51

Conformance Testing (VC Algorithm)

As this small example shows, exhaustive tests can be very expensive• In general, we cannot computationally afford to

perform complete testing• We will always face the risk of missing errors• Even when we reduce our problem to the

simplest model• The complexity of testing full equivalence to a

reference model is simply too high• Exhaustion is exhausting

52

From FSM Testing to the Big Picture

Testing (almost always) is an attempt to• Cover some measure of a structure

• Nodes of a graph (e.g., VC’s spanning tree)• Inputs that give different outputs (e.g., VC’s

distinguishing set)• All possible inputs (e.g., m-n)

• Logical expression evaluations• Predicates over program variables• Pairs of where a variable is defined and where it is

used (data flow)• Usually, we can’t even guarantee that coverage

directly correlates to more bugs found

53

Basic Definitions: Testing

What is software testing?• Running a program• In order to find faults

• a.k.a. defects• a.k.a. errors • a.k.a. flaws• a.k.a. faults• a.k.a. BUGS

• But also, in order to• Increase our confidence that the program has high quality and

low risk• Because we can never be sure we caught all bugs

• How does a set of executions increase confidence?• Sometimes, by algorithmic argument (VC)• Sometimes by less formal arguments (coverage in general)

54

FSM Testing: Further Reading

Yannakakis and Lee, “Principles and Methods of Testing Finite State Machines: A Survey”

Chow, “Testing Software Design Modeled by Finite-State Machines”

Links on the class website

55

“Assignment 0”

Download the YAFFS tarball from the class website

cd direct

make

look at yaffscfg2k.c

run ./directtest2k

write a very simple test (e.g., open a file, write something, and read it back) and add it to the main

get it to compile and run, and email the test case (and results) to me at agroce@gmail.com