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CS5103 Software
Engineering
Lecture 17Debugging
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Today’s class
Delta Debugging Motivation
Algorithm
In practice
Statistical Debugging Tarantula
Dynamic Slicing
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Debugging
Something we do when testing find a bug
Basic Process Reproduce the bug
Locate the fault
Fix
Bug localization: Basic idea Suspicious Score (s) = failing tests cover (s) / all tests
cover (s)
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Debugging
Sometimes the inputs is too complex… Quite common in real world (compiler, office,
browser, database, OS, …)
Locate the relevant inputs
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Consider Mozilla Firefox
Taking html pages as inputs A large number of bugs are related to
loading certain html pages Corner cases in html syntax
Incompatibility between browsers
Corner cases in Javascripts, css, …
Error handling for incorrect html, Javascript, css, …
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How do we go from this<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<OPTIONVALUE="Windows 98">Windows 98<OPTION VALUE="Windows ME">Windows ME<OPTION VALUE="Windows 2000">Windows2000<OPTION VALUE="Windows NT">Windows NT<OPTION VALUE="Mac System 7">Mac System 7<OPTION VALUE="Mac System7.5">Mac System 7.5<OPTION VALUE="Mac System 7.6.1">Mac System 7.6.1<OPTION VALUE="Mac System 8.0">Mac System8.0<OPTION VALUE="Mac System 8.5">Mac System 8.5<OPTION VALUE="Mac System 8.6">Mac System 8.6<OPTION VALUE="MacSystem 9.x">Mac System 9.x<OPTION VALUE="MacOS X">MacOS X<OPTION VALUE="Linux">Linux<OPTIONVALUE="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">HPUX<OPTION VALUE="IRIX">IRIX<OPTION VALUE="Neutrino">Neutrino<OPTION VALUE="OpenVMS">OpenVMS<OPTIONVALUE="OS/2">OS/2<OPTION VALUE="OSF/1">OSF/1<OPTION VALUE="Solaris">Solaris<OPTIONVALUE="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<OPTIONVALUE="P4">P4<OPTION VALUE="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<OPTIONVALUE="enhancement">enhancement<
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To this…
<SELECT NAME="priority" MULTIPLE SIZE=7>
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Motivation
Turning bug reports with real web pages to minimized test cases
The minimized test case should still be able to reveal the bug
Benefit of simplification Easy to communicate
Remove duplicates
Easy debugging Involve less potentially buggy code Shorter execution time
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Delta Debugging
The problem definition A program exhibit an error for an input
The input is a set of elements
E.g., a sequence of API calls, a text file, a serialized object, …
Problem: Find a smaller subset of the input that still cause the
failure
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A generic algorithm
How do people handle this problem?
Binary search Cut the input to halves
Try to reproduce the bug
Iterate
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Delta Debugging Version 1
The set of elements in the bug-revealing input is I
Assumptions Each subset of I is a valid input:
Each Subset of I -> success / fail
A single input element E causes the failure
E will cause the failure in any cases (combined with any other elements) (Monotonic)
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Solution is simple
Go with the binary search process
Throw away half of the input elements, if the rest input elements still cause the failure
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Solution is simple
Go with the binary search process
Throw away half of the input elements, if the rest input elements still cause the failure
A single element: we are done!
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Example
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Delta Debugging Version 1
This is just binary search: easy to automate
The assumptions do not always hold
Let’s look at the assumptions:
(I1 U I2) =
-> I1 = and I2 =
or I1 = and I2 =
It is interesting to see if this is not the case
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Case I: multiple failing branches
What happened if I1 = and I2 = ?
A subset of I1 fails and also a subset of I2 fails
We can simply continue to search I1 and I2 And we find two fail-causing elements
They may be due to the same bug or not
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Case II: Interference
What happened if I1 = and I2 = ?
This means that a subset of I1 and a subset of I2
cause the failure when they combined
This is called interference
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Handling Interference
The cute trick Consider I1 = and I2 =
But I1 U I2 =
An element D1 in I1 and an element D2 in I2 cause the
failure
We do binary search in I2 with I1
Split I2 to P1 and P2, try I1 U P1 and I1 U P2
Continue until you find D2, so that I1 U D2 cause the
failure
Then we do binary search in I1 with D2 until find D1
Return D1 U D2
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Example I: Handle interference
Consider 8 input elements, of which 3 and 7 cause the failure when they applied together
Configuration Result1 2 3 4
5 6 7 81 2 3 4 5 61 2 3 4 7 8
1 2 3 4 7
1 2 7 3 4 7 3 7
Interference!
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Example II: Handle multiple interference
Consider 8 input elements, of which 3, 5 and 7 cause the failure when they applied together
Configuration Result1 2 3 4
5 6 7 81 2 3 4 5 61 2 3 4 7 8
1 2 3 4 5 6 7
1 2 3 4 5 7 1 2 5 7 3 4 5 7
Interference!
Second Interference! What to do?
3 5 7
Go on with I1 U P1!
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Delta Debugging Version 2
The set of elements in the bug-revealing input is I
New Assumptions Each subset of I is a valid input
A subset of input elements E causes the failure
E will cause the failure in any cases (combined with any other elements)
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Delta Debugging Version 2
Algorithm Split I to I1 and I2
Case I: I1 = and I2 =
Try I1
Case I: I1 = and I2 =
Try I2
Case I: I1 = and I2 =
try both I1 and I2
Case II: I1 = and I2 =
Handle interference for I1 and I2
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Real example: GNU Compiler
This input program (bug.c)
causes Gcc 2.59.2 to crash
when all optimitization are
enabled
Minimize it to debug gcc
Consider each character
as an element
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Real example: GNU Compiler
Our delta debugging process Create the appropriate subset of bug.c
Feed it to gcc
Continue according to whether Gcc crashes77
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GCC compiler example
The minimized code:
The test case is 1-minimal No single character can be removed
Even every space is removed
The function name has been changed from mult to a signle t
Gcc is executed for 700+ times
Input reduce to 10% of the initial input
t(double z[],int n){int i,j;for(;;){i=i+j+1;z[i]=z[i]*(z[0]+0);}return z[n];}
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Another example: GDB
GDB is the debugger from GNU
It updates from 4.16 to 4.17
The version 4.17 no longer compatible with DDD (a GUI for GNU software development tools)
178, 000 lines of code change from 4.16
How to know which code change(s) cause the failure
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Results
After a lot of work (by machine) 178KLOC change grouped to 8700 groups (commits)
Use delta debugging
Work it out in 470 tests
It took 48 hours
Doing this by hand would be a nightmare!
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Importance of input elements
It is important to have good input element definition So that subset of input elements are valid for input
The size of input is small
Consider the examples GCC example: we use characters as elements, which
is simple but not so good, if the bug happens after parser, the bug is not monotonic due to syntax errors
GDB example: we group LOC to groups to reduce input size to 5% of the original size. 2 days are acceptable, what about 40 days?
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Limitations of Delta debugging
Rely on the assumptions Monotonicity does not always hold
Rely on good input elements, always providing valid inputs will enhance efficiency
Require automatic test oracles Good for regression testing No good for development-time testing
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Statistical Debugging
Delta Debugging Narrow down the input to be considered
Statistical Debugging Narrow down the code to be considered
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Statistical Debugging
Basic Idea Consider a number of test cases, some of
which pass and some of which fail
If a statement is covered mostly by failed test cases, it is highly likely to be the buggy part of the code
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Tarantula A classical tool for statistical debugging
Use the following formulas Color = red + pass/(fail + pass) * (green ) Brightness = max (pass, fail)
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Tarantula: Illustration
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Context based statistical debugging Not just consider a statement
Runtime Control Flow Graph
Also consider connections Outcomes of branches Connections on a runtime-CFG
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Runtime Control Flow Graph1: void replaceFirst (sx, sy) {2: for (int i=0;i<len;i++) {3: if (arr[i]==sx){4: arr[i] = sz;5: //should break;6: }7: if (arr[i]==sy)){8: arr[i] = sz;9: //should break;10: }11: }12:}
pass passFail
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Limitations Questions:
If a statement is covered only by passed test cases, can it be the root cause of the bug found?
If a statement is covered only by failed test cases, it must be the root cause of the bug found?
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Example
void f(int a, int b){ if (a > 0){ //error: should be >= do something; } if (b < 0){ do something }}
Test Cases:3, 22, 1, 0, -12, 0
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Dynamic Slicing Another way to narrow down code to be
considered in debugging
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Data Dependencies
Data dependencies are the dependency from the usage of a variable to the definition of the variable
Example:s1: x = 3;s2: if(y > 5){s3: y = y + x; //data depend on x in s1s4: }
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Control Dependencies
Control dependencies are the dependency from the branch basic blocks to the predicate
Example:
s1: x = 3;s2: if(y > 5){s3: y = y + x; //control depend on y in s2s4: }
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Dynamic Slicing Describe dependencies among code
elements
If a variable has incorrect value, the bug should be in its backward dynamic slice
Like runtime control flow graph A map from static slicing to the executed
code
Algorithm
A dependence edge is introduced from a load to a store if during execution, at least once, the value stored by the store is indeed read by the load (mark dependence edge)
No static analysis is needed.
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51
71
81
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Algorithm II Example
1: b=0
2: a=2
3: 1 <=i <=N
4: if ((i++)%2= =1)
5: a=a+1 6: b=a*2
7: z=a+b
8: print(z)
T F
T
F
For input N=1, the trace is:
Efficiency: Summary
For an execution of 130M instructions: space requirement: reduced from 1.5GB to 94MB (I further reduced the size by a facto
r of 5 by designing a generic compression technique [MICRO’05]). time requirement: reduced from >10 Mins to <30 seconds.
http://jslice.sourceforge.net/
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Summary of debugging
Debugging is a follow-up step of testing
Bug localization, and bug fixing are tasks highly depend on human intelligence
Tools can help us to narrow the scope to consider Bug localization
Reduce the code to be considered
Delta debugging Reduce the inputs to be considered