sbqs 2013 keynote: cooperative testing and analysis

Tao Xie University of Illinois

at Urbana-Champaign,USA [email protected]

SBQS 2013

IBM's Deep Blue defeated chess champion Garry Kasparov in 1997

IBM Watson defeated top human Jeopardy! players in 2011

Google’s driverless car

Microsoft's instant voice translation tool

IBM Watson as Jeopardy! player

"Completely Automated Public Turing test to tell Computers and Humans Apart"

Movie: Minority Report

CNN News

iPad

Machine is better at task set A Mechanical, tedious, repetitive tasks, … Ex. solving constraints along a long path Human is better at task set B Intelligence, human intent, abstraction, domain

knowledge, … Ex. local reasoning after a loop, recognizing naming

semantics

= A U B 8

Malaysia Airlines Flight 124 @2005 Lisanne Bainbridge, "Ironies of Automation”, Automatica 1983 .

“As the plane passed 39 000 feet, the stall and overspeed warning indicators came on simultaneously—something that’s supposed to be impossible, and a situation the crew is not trained to handle.” IEEE Spectrum 2009


Ironies of Automation “Even highly automated systems, such as electric power networks, need human beings... one can draw the paradoxical conclusion that automated systems still are man-machine systems, for which both technical and human factors are important.”

“As the plane passed 39 000 feet, the stall and overspeed warning indicators came on simultaneously—something that’s supposed to be impossible, and a situation the crew is not trained to handle.” IEEE Spectrum 2009


Ironies of Automation “The increased interest in human factors among engineers reflects the irony that the more advanced a control system is, so the more crucial may be the contribution of the human operator.”

Don’t forget human factors Using your tools as end-to-end solutions Helping your tools Don’t forget cooperations of human and tool;

human and human Human can help your tools too Human and human could work together to help your

tools, e.g., crowdsourcing

11




12

14

“During the past 21 years, over 75 papers and 9 Ph.D. theses have been published on pointer analysis. Given the tones of work on this topic one may wonder, “Haven't we solved this problem yet?'' With input from many researchers in the field, this paper describes issues related to pointer analysis and remaining open problems.”

Michael Hind. Pointer analysis: haven't we solved this problem yet?. In Proc. ACM SIGPLAN-SIGSOFT Workshop on Program Analysis for Software Tools and Engineering (PASTE 2001)

15

Section 4.3 Designing an Analysis for a Client’s Needs

“Barbara Ryder expands on this topic: “… We can all write an unbounded number of papers that compare different pointer analysis approximations in the abstract. However, this does not accomplish the key goal, which is to design and engineer pointer analyses that are useful for solving real software problems for realistic programs.”

17

Zhenmin Li, Shan Lu, Suvda Myagmar, and Yuanyuan Zhou. CP-Miner: a tool for finding copy-paste and related bugs in operating system code. In Proc. OSDI 2004.

MSRA XIAO

Yingnong Dang, Dongmei Zhang, Song Ge, Chengyun Chu, Yingjun Qiu, and Tao Xie. XIAO: Tuning code clones at hands of engineers in practice. In Proc. ACSAC 2012

MSR 2011 Keynote by YY Zhou: Connecting Technology with Real-world Problems – From Copy-paste Detection to Detecting Known Bugs

Human to Determine What are Serious (Known) Bugs

18

Available in Visual Studio 2012 Searching similar snippets for

fixing bug once

Finding refactoring opportunity

Yingnong Dang, Dongmei Zhang, Song Ge, Yingjun Qiu, and Tao Xie. XIAO: Tuning code clones at hands of engineers in practice. In Proc. Annual Computer Security Applications Conference (ACSAC 2012)

XIAO Code Clone Search service integrated into workflow of Microsoft Security Response Center (MSRC)

Microsoft Technet Blog about XIAO: We wanted to be sure to address the vulnerable code wherever it appeared across the Microsoft code base. To that end, we have been working with Microsoft Research to develop a “Cloned Code Detection” system that we can run for every MSRC case to find any instance of the vulnerable code in any shipping product. This system is the one that found several of the copies of CVE-2011-3402 that we are now addressing with MS12-034.

http://blogs.technet.com/b/srd/archive/2012/05/08/ms12-034-duqu-ten-cve-s-and-removing-keyboard-layout-file-attack-surface.aspx

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XIAO enables code clone analysis with High scalability, High compatibility High tunability: what you tune is what you get High explorability:

1. Clone navigation based on source tree hierarchy2. Pivoting of folder level statistics3. Folder level statistics4. Clone function list in selected folder5. Clone function filters6. Sorting by bug or refactoring potential7. Tagging

1 2 3 4 5 6

7

1. Block correspondence2. Block types3. Block navigation4. Copying5. Bug filing6. Tagging

1

2

3

4

1

6

5

How to navigate through the large number of detected clones? How to quickly review a pair of clones?

50 years of automated debugging research N papers only 5 evaluated with actual programmers

“

” Chris Parnin and Alessandro Orso. Are automated debugging techniques actually helping programmers?. In Proc. ISSTA 2011

Academia Tend to leave human out of loop (involving human makes

evaluations difficult to conduct or write) Tend not to spend effort on improving tool usability

▪ tool usability would be valued more in HCI than in SE ▪ too much to include both the approach/tool itself and usability/its evaluation

in a single paper

Real-world Often has human in the loop (familiar IDE integration, social

effect, lack of expertise/willingness to write specs,…) Examples Agitar [ISSTA 2006] vs. Daikon [TSE 2001] Test generation in Pex based on constraint solving

Goal: to identify the future directions in research in formal methods and its transition to industrial practice.

The workshop will bring together researchers and identify primary challenges in the field, both foundational, infrastructural, and in transitioning ideas from research labs to developer tools.

http://goto.ucsd.edu/~rjhala/NSFWorkshop/

“Lack of education amongst practitioners” “Education of students in logic and design for

verification” “Expertise required to create and use a verification

tool. E.g., both Astre for Airbus and SDV for Windows drivers were closely shepherded by verification experts.” “Tools require lots of up-front effort (e.g., to write

specifications)” “User effort required to guide verification tools,

such as assertions or specifications”

“Not integrated with standard development flows (testing)” “Too many false positives and no ranking of errors” “General usability of tools, in terms of false alarms

and error messages. The Coverity CACM paper pointed out that they had developed features that they do not deploy because they baffle users. Many tools choose unsoundness over soundness to avoid false alarms.”

“The necessity of detailed specifications and complex interaction with tools, which is very costly and discouraging for industrial, who lack high-level specialists.” “Feedback to users. It’s difficult to explain to users

why automated verification tools are failing. Counterexamples to properties can be very difficult for users to understand, especially when they are abstract, or based on incomplete environment models or constraints.”

2010 Dagstuhl Seminar 10111

Practical Software Testing: Tool Automation and Human Factors

http://www.dagstuhl.de/programm/kalender/semhp/?semnr=1011




Human Factors



Andy Ko and Brad Myers. Debugging Reinvented: Asking and Answering Why and Why Not Questions about Program Behavior. In Proc. ICSE 2008

Don’t forget human factors Using your tools as end-to-end solutions Helping your tools Don’t forget cooperations of human and tool

intelligence; human and human intelligence Human can help your tools too Human and human could work together to help your


29

Motivation Architecture recovery is challenging (abstraction gap) Human typically has high-level view in mind

Repeat Human: define/update high-level model of interest Tool: extract a source model Human: define/update declarative mapping between

high-level model and source model Tool: compute a software reflexion model Human: interpret the software reflexion model

Until happy Gail C. Murphy, David Notkin. Reengineering with Reflection Models: A Case Study. IEEE Computer 1997

Running Symbolic PathFinder ... … ============================================

========== results no errors detected ============================================

========== statistics elapsed time: 0:00:02 states: new=4, visited=0,

backtracked=4, end=2 search: maxDepth=3,

constraints=0 choice generators: thread=1, data=2 heap: gc=3, new=271, free=22 instructions: 2875 max memory: 81MB loaded code: classes=71, methods=884

…

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Recent advanced technique: Dynamic Symbolic Execution/Concolic Testing Instrument code to explore feasible paths Example tool: Pex from Microsoft Research

(for .NET programs)

P. Godefroid, N. Klarlund, and K. Sen. DART: directed automated random testing. PLDI 2005 K. Sen, D. Marinov, and G. Agha. CUTE: a concolic unit testing engine for C. ESEC/FSE 2005 N. Tillmann and J. de Halleux. Pex - White Box Test Generation for .NET. TAP 2008

L. A. Clarke. A system to generate test data and symbolically execute programs. TSE 1976. J. C. King. Symbolic execution and program testing. CACM 1976.

Code to generate inputs for:

void CoverMe(int[] a) { if (a == null) return; if (a.Length > 0) if (a[0] == 1234567890) throw new Exception("bug"); }

Data



Data

null



Data

null

Execute&Monitor



Data

null

a==null T F

Execute&Monitor



Observed constraints

a==null

Data

null

a==null T F

Execute&Monitor




a==null

Data

null

a==null T F

Choose next path


Constraints to solve

a!=null



a==null

Data

null

a==null T F

Choose next path

Negated condition



a!=null



a==null

Data

null

a==null T F

Solve

Negated condition



a!=null



a==null

Data

null {}

a==null T F

Solve



a!=null



a==null

Data

null {}

a==null T F

Execute&Monitor



a!=null



a==null

Data

null {}

a==null

a.Length>0 T F

T F

Execute&Monitor



a!=null



a==null a!=null && !(a.Length>0)

Data

null {}

a==null

a.Length>0 T F

T F

Execute&Monitor



a!=null




Data

null {}

a==null

a.Length>0 T F

T F

Choose next path



a!=null a!=null && a.Length>0




Data

null {}

a==null

a.Length>0 T F

T F

Choose next path







Data

null {}

a==null

a.Length>0 T F

T F

Solve







Data

null {} {0}

a==null

a.Length>0 T F

T F

Solve







Data

null {} {0}

a==null

a.Length>0 T F

T F

Execute&Monitor







Data

null {} {0}

a==null

a.Length>0

a[0]==123… T

T F

T

F

F

Execute&Monitor






a==null a!=null && !(a.Length>0) a!=null && a.Length>0 && a[0]!=1234567890

Data

null {} {0}

a==null

a.Length>0

a[0]==123… T

T F

T

F

F

Execute&Monitor







Data

null {} {0}

a==null

a.Length>0

a[0]==123… T

T F

T

F

F

Choose next path



a!=null a!=null && a.Length>0 a!=null && a.Length>0 && a[0]==1234567890




Data

null {} {0}

a==null

a.Length>0

a[0]==123… T

T F

T

F

F

Choose next path







Data

null {} {0}

a==null

a.Length>0

a[0]==123… T

T F

T

F

F

Solve







Data

null {} {0} {123…}

a==null

a.Length>0

a[0]==123… T

T F

T

F

F

Solve







Data

null {} {0} {123…}

a==null

a.Length>0

a[0]==123… T

T F

T

F

F

Execute&Monitor






a==null a!=null && !(a.Length>0) a!=null && a.Length>0 && a[0]!=1234567890 a!=null && a.Length>0 && a[0]==1234567890

Data

null {} {0} {123…}

a==null

a.Length>0

a[0]==123… T

T F

T

F

F

Execute&Monitor






a==null a!=null && !(a.Length>0) a!=null && a.Length>0 && a[0]!=1234567890 a!=null && a.Length>0 && a[0]==1234567890

Data

null {} {0} {123…}

a==null

a.Length>0

a[0]==123… T

T F

T

F

F

Done: There is no path left.

Download counts initial 20 months of release Academic: 17,366

Industrial: 13,022 Total: 30,388

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“It has saved me two major bugs (not caught by normal unit tests) that would have taken at least a week to track down and fix normally plus a few smaller issues so I'm a big proponent of Pex.”

Pex detected various bugs (including a serious bug) in a core .NET component (already been extensively tested over 5 years by 40 testers) , used by thousands of developers and millions of end users.

Released since 2008

http://research.microsoft.com/projects/pex/

Method sequences MSeqGen/Seeker [Thummalapenta et al. OOSPLA 11, ESEC/FSE 09],

Covana [Xiao et al. ICSE 2011], OCAT [Jaygarl et al. ISSTA 10], Evacon [Inkumsah et al. ASE 08], Symclat [d'Amorim et al. ASE 06]

Environments e.g., db, file systems, network, …

DBApp Testing [Taneja et al. ESEC/FSE 11], [Pan et al. ASE 11]

CloudApp Testing [Zhang et al. IEEE Soft 12]

Loops Fitnex [Xie et al. DSN 09]

http://people.engr.ncsu.edu/txie/publications.htm

http://people.engr.ncsu.edu/txie/publications.htm

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void test1() { Graph ag = new Graph(); Vertex v1 = new Vertex(0); ag.AddVertex(v1); } 62

00: class Graph { … 03: public void AddVertex (Vertex v) { 04: vertices.Add(v); 05: } 06: public Edge AddEdge (Vertex v1, Vertex v2) { … 15: } 16: }

Class Under Test

void test2() { Graph ag = new Graph(); Vertex v1 = new Vertex(0); ag.AddEdge(v1, v1); }

…

Generated Unit Tests

Manual Test Generation: Tedious, Missing Special/Corner Cases, …

Running Symbolic PathFinder ... … ============================================

========== results no errors detected ============================================

========== statistics elapsed time: 0:00:02 states: new=4, visited=0,

backtracked=4, end=2 search: maxDepth=3,

constraints=0 choice generators: thread=1, data=2 heap: gc=3, new=271, free=22 instructions: 2875 max memory: 81MB loaded code: classes=71, methods=884

…

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object-creation problems (OCP) - 65% external-method call problems (EMCP) – 27%

Total block coverage achieved is 50%, lowest coverage 16%.

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Ex: Dynamic Symbolic Execution (DSE) /Concolic Testing Instrument code to explore feasible paths Challenge: path explosion



38


When desirable receiver or argument

objects are not generated

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A graph example from QuickGraph library

00: class Graph { … 03: public void AddVertex (Vertex v) { 04: vertices.Add(v); // B1 } 06: public Edge AddEdge (Vertex v1, Vertex v2) { 07: if (!vertices.Contains(v1)) 08: throw new VNotFoundException(""); 09: // B2 10: if (!vertices.Contains(v2)) 11: throw new VNotFoundException(""); 12: // B3 14: Edge e = new Edge(v1, v2); 15: edges.Add(e); } } //DFS:DepthFirstSearch 18: class DFSAlgorithm { … 23: public void Compute (Vertex s) { ... 24: if (graph.GetEdges().Size() > 0) { // B4 25: isComputed = true; 26: foreach (Edge e in graph.GetEdges()) { 27: ... // B5 28: } 29: } } } 67

[OOPSLA 11]

68


Includes two classes Graph DFSAlgorithm


[OOPSLA 11]

69



Graph

AddVertex


[OOPSLA 11]

70



Graph

AddVertex AddEdge: requires

both vertices to be in graph


[OOPSLA 11]

71

71

00: class Graph { … 03: public void AddVertex (Vertex v) { 04: vertices.Add(v); // B1 } 06: public Edge AddEdge (Vertex v1, Vertex v2) { 07: if (!vertices.Contains(v1)) 08: throw new VNotFoundException(""); 09: // B2 10: if (!vertices.Contains(v2)) 11: throw new VNotFoundException(""); 12: // B3 14: Edge e = new Edge(v1, v2); 15: edges.Add(e); } } //DFS:DepthFirstSearch 18: class DFSAlgorithm { … 23: public void Compute (Vertex s) { ... 24: if (graph.GetEdges().Size() > 0) { // B4 25: isComputed = true; 26: foreach (Edge e in graph.GetEdges()) { 27: ... // B5 28: } 29: } } }

[OOPSLA 11]

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Test target: Cover true branch (B4) of Line 24

72


[OOPSLA 11]

73


Desired object state: graph should include at least one edge

73


[OOPSLA 11]

74



Target sequence: Graph ag = new Graph(); Vertex v1 = new Vertex(0); Vertex v2 = new Vertex(1); ag.AddVertex(v1); ag.AddVertex(v2); ag.AddEdge(v1, v2); DFSAlgorithm algo = new

DFSAlgorithm(ag); algo.Compute(v1);

74


[OOPSLA 11]



41




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Typically DSE instruments or explores only methods @ project under test; Third-party API external methods (network, I/O, ..):

•too many paths •uninstrumentable


43


Xusheng Xiao, Tao Xie, Nikolai Tillmann, and Jonathan de Halleux. Precise Identification of Problems for Structural Test Generation. In Proc. ICSE 2011

Tackling object-creation problems Seeker [OOSPLA 11] , MSeqGen [ESEC/FSE 09]

Covana [ICSE 11], OCAT [ISSTA 10] Evacon [ASE 08], Symclat [ASE 06]

Still not good enough (at least for now)! ▪ Seeker (52%) > Pex/DSE (41%) > Randoop/random (26%)

Tackling external-method call problems DBApp Testing [ESEC/FSE 11], [ASE 11]

CloudApp Testing [IEEE Soft 12]

Deal with only common environment APIs

@NCSU ASE

84



Target sequence: Graph ag = new Graph(); Vertex v1 = new Vertex(0); Vertex v2 = new Vertex(1); ag.AddVertex(v1); ag.AddVertex(v2); ag.AddEdge(v1, v2); DFSAlgorithm algo = new

DFSAlgorithm(ag); algo.Compute(v1);

84


Tackle object-creation problems with Factory Methods

47

Tackle external-method call problems with Mock Methods or Method Instrumentation

Mocking System.IO.File.ReadAllText

48

Human-Assisted Computing Driver: tool Helper: human Ex. Covana [ICSE 2011]

Human-Centric Computing Driver: human Helper: tool Ex. Pex for Fun [ICSE 2013 SEE]

Interfaces are important. Contents are important too!

49

50

Symptoms

50

Symptoms

external-method call problems (EMCP) all executed external-method calls

50

Symptoms


object-creation problems (OCP) all non-primitive program inputs/fields

50

Symptoms

(Likely) Causes


object-creation problems (OCP) all non-primitive program inputs/fields

Causal analysis: tracing between symptoms and (likely) causes Reduce cost of human consumption ▪ reduction of #(likely) causes ▪ diagnosis of each cause

Solution construction: fixing suspected causes Reduce cost of human contribution ▪ measurement of solution goodness ▪ Inner iteration of human-tool cooperation!

51

52

Symptoms

(Likely) Causes

external-method call problems (EMCP)

object-creation problems (OCP)

Given symptom s foreach (c in LikelyCauses) { Fix(c); if (IsObserved(s)) RelevantCauses.add(c) }

Goal: Precisely identify problems (causes) faced by a tool for causing not to cover a statement (symptom) Insight: Partially-covered conditional has data

dependency on a real problem

53

[ICSE 11]

From xUnit

54

Consider only EMCPs whose arguments have data dependencies on program inputs ▪ Fixing such problem candidates facilitates test-generation tools

From xUnit

Data Dependencies

54

Consider only EMCPs whose arguments have data dependencies on program inputs ▪ Fixing such problem candidates facilitates test-generation tools

From xUnit

55

Partially-covered conditionals have data dependencies on EMCP candidates

Symptom Expression:

55


Symptom Expression: return(File.Exists) == true

55



Element of EMCP Candidate: return(File.Exists)

55



Element of EMCP Candidate: return(File.Exists)

Conditional in Line 1 has data dependency on File.Exists

55


56 From xUnit

Data Dependence

Analysis

Forward Symbolic Execution

Problem Candidates

Problem Candidate

Identification

Runtime Information

Identified Problems

Coverage

Program

Generated Test Inputs

Runtime Events

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[Inputs EMCP]

[EMCP Symptom]

Subjects: xUnit: unit testing framework for .NET ▪ 223 classes and interfaces with 11.4 KLOC

QuickGraph: C# graph library ▪ 165 classes and interfaces with 8.3 KLOC

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Subjects: xUnit: unit testing framework for .NET ▪ 223 classes and interfaces with 11.4 KLOC

QuickGraph: C# graph library ▪ 165 classes and interfaces with 8.3 KLOC

Evaluation setup: Apply Pex to generate tests for program under test Feed the program and generated tests to Covana Compare baseline solution and Covana

58

RQ1: How effective is Covana in identifying the two main types of problems, EMCPs and OCPs? RQ2: How effective is Covana in pruning

irrelevant problem candidates of EMCPs and OCPs?

59

Covana identifies • 43 EMCPs with only 1 false positive and 2 false negatives • 155 OCPs with 20 false positives and 30 false negatives. 60

Covana prunes • 97% (1567 in 1610) EMCP candidates with 1 false positive and 2 false negatives • 66% (296 in 451) OCP candidates with 20 false positives and 30 false negatives

61

Motivation Tools are often not powerful enough Human is good at some aspects that tools are not

What difficulties does the tool face? How to communicate info to the user to get help?

How does the user help the tool based on the info? 62

Iterations to form Feedback Loop

Human-Assisted Computing Driver: tool Helper: human Ex. Covana [ICSE 2011]

Human-Centric Computing Driver: human Helper: tool Ex. Pex for Fun [ICSE 2013 SEE]

Interfaces are important. Contents are important too!

63

1,270,159 clicked 'Ask Pex!'

www.pexforfun.com

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Nikolai Tillmann, Jonathan De Halleux, Tao Xie, Sumit Gulwani and Judith Bishop. Teaching and Learning Programming and Software Engineering via Interactive Gaming. In Proc. ICSE 2013 SEE.

http://research.microsoft.com/en-us/projects/pex4fun/

http://research.microsoft.com/en-us/projects/pex4fun/

Secret Implementation

class Secret { public static int Puzzle(int x) { if (x <= 0) return 1; return x * Puzzle(x-1); } }

Player Implementation

class Player { public static int Puzzle(int x) { return x; } }

class Test { public static void Driver(int x) { if (Secret.Puzzle(x) != Player.Puzzle(x)) throw new Exception(“Mismatch”); } }

behavior Secret Impl == Player Impl

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Coding duels at http://www.pexforfun.com/ Brain exercising/learning while having fun Fun: iterative, adaptive/personalized, w/ win criterion Abstraction/generalization, debugging, problem solving

Brain exercising

http://www.pexforfun.com/

http://pexforfun.com/gradsofteng



Observed Benefits • Automatic Grading • Real-time Feedback (for Both Students and Teachers) • Fun Learning Experiences


“It really got me *excited*. The part that got me most is about spreading interest in teaching CS: I do think that it’s REALLY great for teaching | learning!”

“I used to love the first person shooters and the satisfaction of blowing away a whole team of Noobies playing Rainbow Six, but this is far more fun.”

“I’m afraid I’ll have to constrain myself to spend just an hour or so a day on this really exciting stuff, as I’m really stuffed with work.”

X

70

Internet

class Secret { public static int Puzzle(int x) { if (x <= 0) return 1; return x * Puzzle(x-1); } }

Everyone can contribute Coding duels Duel solutions

Internet Puzzle Games Made from Difficult Constraints or Object-Creation Problems

Supported by MSR SEIF Award Ning Chen and Sunghun Kim. Puzzle-based Automatic Testing: bringing humans into the loop by solving puzzles. In Proc. ASE 2012

http://www.cs.washington.edu/verigames/

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StackMine [Han et al. ICSE 12]

Shi Han, Yingnong Dang, Song Ge, Dongmei Zhang, and Tao Xie. Performance Debugging in the Large via Mining Millions of Stack Traces. In Proc. ICSE 2012

73


Trace Storage Trace collection

Internet


73

Trace analysis



Internet


73

Bug Database

Trace analysis

Bug filing



Internet


73

Problematic Pattern Repository

Bug Database

Trace analysis

Bug filing



Internet


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Pattern Matching

Bug update

Problematic Pattern Repository

Bug Database

Trace analysis

Bug filing



Internet


“We believe that the MSRA tool is highly valuable and much more efficient for mass trace (100+ traces) analysis. For 1000 traces, we believe the tool saves us 4-6 weeks of time to create new signatures, which is quite a significant productivity boost.”

- from Development Manager in Windows

Highly effective new issue discovery on

Windows mini-hang

Continuous impact on future Windows versions


Don’t forget human factors Using your tools as end-to-end solutions Helping your tools Don’t forget cooperations of human and tool

intelligence; human and human intelligence Human can help your tools too Human and human could work together to help your


75

Human-Assisted Computing

Human-Centric Computing Human-Human Cooperation




77

Wonderful current/former students@ASE Collaborators, especially those from Microsoft

Research Redmond/Asia, Peking University Colleagues who gave feedback and inspired me

NSF grants CCF-0845272, CCF-0915400, CNS-0958235, ARO grant W911NF-08-1-0443, an NSA Science of Security, Lablet grant, a NIST grant, a 2011 Microsoft Research SEIF Award

Questions ?

https://sites.google.com/site/asergrp/

Human-Assisted Computing

Human-Centric Computing Human-Human Cooperation

sbqs 2013 keynote: cooperative testing and analysis

Technology

human human

tool human

human factors

cooperations of human

human jeopardy

human intent

human beings

human operator