Simplifying failure Inducing input

Vikas , Purdue

Problem

when testing a program some test case fails how to find the bug ??

problems!!! test input is huge – 800 lines say do a binary search manually ? how much time ?

Problem - example

Mozilla web browser 1999 bugzilla listed more than 370 bug reports faced imminent doom !! opened a program 'Bug a Thorn'

who ever finds bugs will get a prize Finding bug is a SERIOUS PROBLEM

Problem – example

one of the bug Mozilla crashed after 95 user actions It was not able to print a particular html

page of 895 lines simplified to 3 user actions html page simplified to 1 line

Now – 895 lines of input simplified to just 1 line !!

How to solve?

“Delta Debugging algorithm“ take a failing test case and take a passing test case simplify tat and produce a minimal test case the simplified test case still produces the

failure

Example - Continued

<td align=left valign=top><SELECT NAME="op sys" MULTIPLE SIZE=7><OPTION VALUE="All">All<OPTION VALUE="Windows 3.1">Windows 3.1<OPTION VALUE="Windows 95">Windows 95<OPTION VALUE="Windows98">Windows 98<OPTION VALUE="Windows ME">Windows ME<OPTION VALUE="Windows 2000">Windows 2000<OPTION VALUE="WindowsNT">Windows NT<OPTION VALUE="Mac System 7">Mac System 7<OPTION VALUE="Mac System 7.5">Mac System 7.5<OPTION VALUE="MacSystem 7.6.1">Mac System 7.6.1<OPTION VALUE="Mac System 8.0">Mac System 8.0<OPTION VALUE="Mac System 8.5">Mac System8.5<OPTION VALUE="Mac System 8.6">Mac System 8.6<OPTION VALUE="Mac System 9.x">Mac System 9.x<OPTION VALUE="MacOS X">MacOSX<OPTION VALUE="Linux">Linux<OPTION VALUE="BSDI">BSDI<OPTION VALUE="FreeBSD">FreeBSD<OPTION VALUE="NetBSD">NetBSD<OPTIONVALUE="OpenBSD">OpenBSD<OPTION VALUE="AIX">AIX<OPTION VALUE="BeOS">BeOS<OPTION VALUE="HP-UX">HP-UX<OPTIONVALUE="IRIX">IRIX<OPTION VALUE="Neutrino">Neutrino<OPTION VALUE="OpenVMS">OpenVMS<OPTION VALUE="OS/2">OS/2<OPTIONVALUE="OSF/1">OSF/1<OPTION VALUE="Solaris">Solaris<OPTION VALUE="SunOS">SunOS<OPTION VALUE="other">other</SELECT></td><td align=left valign=top><SELECT NAME="priority" MULTIPLE SIZE=7><OPTION VALUE="--">--<OPTION VALUE="P1">P1<OPTION VALUE="P2">P2<OPTION VALUE="P3">P3<OPTION VALUE="P4">P4<OPTIONVALUE="P5">P5</SELECT></td><td align=left valign=top><SELECT NAME="bug severity" MULTIPLE SIZE=7><OPTION VALUE="blocker">blocker<OPTION VALUE="critical">critical<OPTION VALUE="major">major<OPTIONVALUE="normal">normal<OPTION VALUE="minor">minor<OPTION VALUE="trivial">trivial<OPTION VALUE="enhancement">enhancement</SELECT></tr></table>Fig. 1. Printing this HTML page makes Mozilla crash (excerpt)

Conflicting issues

Decomposing a specific bug report into simple test case A bug report should be as specific as possible on the other hand test case should be as

simple as possible Test case simplification does both!

allows for short problem descriptions subsumes all details in bug report

Delta Debugging – how

Define what is a successful test case Feed with a failing test case Ddmin simplifies it by successive testing Stop when a minimal test case is reached Now removing any single input entity

would cause the failure to disappear

Analogous example-flight simulation

problem – flight crashes few seconds after take off , how do we find the bug?? repeat the situation over again and again

under changed circumstances find out what is relevant and what is not

relevant eg , leave the passenger seats – still crashes eg , leave the coffee vending machine – still

crashes ! hence both of them are irrelevant

Example continued - DDMin

1 <SELECT NAME="priority" MULTIPLE SIZE=7> 82 <SELECT NAME="priority" MULTIPLE SIZE=7> 43 <SELECT NAME="priority" MULTIPLE SIZE=7> 44 <SELECT NAME="priority" MULTIPLE SIZE=7> 45 <SELECT NAME="priority" MULTIPLE SIZE=7> 86 <SELECT NAME="priority" MULTIPLE SIZE=7> 87 <SELECT NAME="priority" MULTIPLE SIZE=7> 48 <SELECT NAME="priority" MULTIPLE SIZE=7> 49 <SELECT NAME="priority" MULTIPLE SIZE=7> 410 <SELECT NAME="priority" MULTIPLE SIZE=7> 811 <SELECT NAME="priority" MULTIPLE SIZE=7> 412 <SELECT NAME="priority" MULTIPLE SIZE=7> 413 <SELECT NAME="priority" MULTIPLE SIZE=7> 414 <SELECT NAME="priority" MULTIPLE SIZE=7> 415 <SELECT NAME="priority" MULTIPLE SIZE=7> 416 <SELECT NAME="priority" MULTIPLE SIZE=7> 817 <SELECT NAME="priority" MULTIPLE SIZE=7> 818 <SELECT NAME="priority" MULTIPLE SIZE=7> 819 <SELECT NAME="priority" MULTIPLE SIZE=7> 420 <SELECT NAME="priority" MULTIPLE SIZE=7> 421 <SELECT NAME="priority" MULTIPLE SIZE=7> 422 <SELECT NAME="priority" MULTIPLE SIZE=7> 423 <SELECT NAME="priority" MULTIPLE SIZE=7> 424 <SELECT NAME="priority" MULTIPLE SIZE=7> 425 <SELECT NAME="priority" MULTIPLE SIZE=7> 426 <SELECT NAME="priority" MULTIPLE SIZE=7> 8Fig. 2. Simplifying failure-inducing HTML input

DDMin

Not only minimises the failing input Also maximises the the passing input Not only limited to html input, character

input nor to program input can be applied to all circumstances that

can make a program crash or those which will affect the program execution

input

2 <SELECT NAME="priority" MULTIPLE SIZE=7> x4 <SELECT NAME="priority" MULTIPLE SIZE=7>x

7 <SELECT NAME="priority" MULTIPLE SIZE=7> v6 <SELECT NAME="priority" MULTIPLE SIZE=7> v5 <SELECT NAME="priority" MULTIPLE SIZE=7> v3 <SELECT NAME="priority" MULTIPLE SIZE=7> v1 <SELECT NAME="priority" MULTIPLE SIZE=7> vFig. 3. Isolating a failure-inducing difference

<SELECT> tag is the one which causes the failure !!!

Assumptions – reasons for failure

program code data from storage or input devices the programs environment the specific hardware the operating system “All the above are called circumstances”

Changes that cause failure

We are interested only in the changeble circumstances

These changeable circumstances make up the program input in most of the cases

Definitions

The set of possible circumstances is denoted by “

What is the change ?? ( delta ) - decomposition?

No specific way to get the changes . html example

delta can be a single character can be a single tag can be a single line also

HOW TO DECOMPOSE THEM??

Definitions – composition of changes

Definitions - Test cases and tests

According to POSIX starndart for testing the test can succeed the test can fail the test produce intermediate result

We need a function 'rtest' that takes a program run and gives one of the above output.

Definitions – test case and test

Test cases

is the passing test case

is the failing test case

Minimizing test case

– -A test case which is minimum wrt all possible test cases – requires testing all the 2cx combinations

Minimizing algorithm

Minimizing algorithm

The Algorithm

Complexity of DDMin

Complexity of DDMin is Cx2 + 3Cx Worst case 2 phases

1. When every test has an unresolved input then we go till the maximum granularity of Cx # of tests to be carried out is 2+4+8+.... + 2 Cx ~= 4Cx

2. When testing the last complement fails (∆n)

results in Cx-1 calls Total : 2(Cx-1)+2(Cx-2)+.....+2=~Cx2-Cx

Add up everything :- 4Cx + Cx2 - Cx

Minimizing algorithm

2

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Case Study1 – GCC gets a fatal signal! - run in WYNOT

a program bug.c crashes when compiled with gcc

but the program crashes only with some optimization options given

does not crash with all optimizations enabled!

code is 755 characters – each character a component ( hence , may have a lot of useless C code )

Case study 1

size of Z = 1 Z[1] will segfault

Minimizing the test case

177100

77

Minimizing Gcc options

gcc -o -fforce-addr bug.c

Case study – Mozilla crashing

One of the bug report in Mozilla firefox Following operations cause Mozilla to

crash Start Mozilla Go to bugzilla.mozilla.org Print to file setting the margins to .50 Once its done printing do the exact same

thing on the same file ( /var/tmp/netscape.ps )

This causes browser to crash with a segfault

Mozilla crashes

Mozilla input consists of two items 1 . The sequence of input events

ie the succession of mouse motions pressed keys and clicked buttons used XLAB to capture – 711 actions

2. HTML code of the erroneous www page

Mozilla crashes

out of 711 actions – only 95 were user actions , rest were notifications by X server

out of 95 user actions only 3 are left after 82 test runs . Invoke Print dialog Press mouse button 1 on the print button release mouse button 1

Mozilla crashes

82 runs , 3 user actions

95 user actions

Mozilla crashes – excerpt of input

Mozilla crashes – sample run

Mozilla crashes - run

896 lines !!!

58 runs , 1 line

Mozilla crashes ! - 1 line

<SELECT_NAME=”priority”_MULTIPLE_SIZE=7> - is the culprit

or its just <SELECT> in other words , the bug report is now just

Create an HTML page containing “<SELECT>”

Load the page and print it using Alt+P command

the browser crashes with a segmentation fault

or – printing the <SELECT> crashes !!

Minimizing fuzz

bart Miller and his team examined the robustness of UNIX utilities by sending fuzz input ( a huge number of random characters ) found that 40% of the basic programs

crashed or went into infinite loops ddmin algorithm was tested on fuzz input

sequences for NROFF, TROFF, GLEX, CRTPLOT , UL,UNITS

Minimizing fuzz

Minimizing fuzz

Simplifying failure inducing input

The 3 case studies discussed show that the larger the size of the simplified input , the

higher is the number of tests required because determining 1-minimality of a test

case with n entities req atleast n tests because each individual entity is removed

and tested for flex , the number of tests vary upto 104 for

low precision to 36,000 for high precision .

other approaches ?

Simply stop the process when a certain time limit is reached

Simply stop the process when the input test case is reduced by a certain extent

Better approach is 'Isolation' Find one relevant part of the test

case .removing this particular part makes the failure go away.

Simplifying meant that – the simplified test case had all the relevant parts

Isolating example

7 tests!!

Simplyfying

26 tests

Isolation

Future work

Domain specific methods knowledge about the input structure can very

much enhance the performance for ex – valid program inputs are described

by grammars , would be nice to rely on such grammars

can exclude syntactically invalid inputs