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GPLAG: Detection of Software Plagiarism by Program Dependence Graph Analysis

Chao Liu, Chen Chen, Jiawei Han, Philip S. Yu

University of Illinois at Urbana-ChampaignIBM T.J. Waston Research Center

Presented by Chao Liu

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Motivations Blossom of open-source projects

SourceForge.net: 125,090 projects as July 2006 Convenience for software plagiarism?

You can always find something online Core-part plagiarism

Ripping off GUIs and irrelevant parts (Illegally) reuse the implementations of core-

algorithms Our goal

Efficient detection of core-part plagiarism

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Challenges Effectiveness

Professional plagiarists Automated plagiarism

Efficiency Only a small part of code is plagiarized, how

to detect it efficiently?

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Outline

Plagiarism Disguises Review of Plagiarism Detection GPLAG: PDG-based Plagiarism Detection Efficiency and Scalability Experiments Conclusions

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Original Program

01 static void02 make_blank (struct line *blank, int count)03 {04 int i;05 unsigned char *buffer;06 struct field *fields;07 blank->nfields = count; 08 blank->buf.size = blank->buf.length = count + 1;09 blank->buf.buffer = (char*) xmalloc (blank->buf.size);10 buffer = (unsigned char *) blank->buf.buffer;11 blank->fields = fields = (struct field *) xmalloc (sizeof (struct field) * count); 12 for (i = 0; i < count; i++){13 ... 14 }15 }

A procedure in a program, called join

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Disguise 1: Format Alteration

01 static void02 make_blank (struct line *blank, int count)03 {04 int i;05 unsigned char *buffer;06 struct field *fields;07 blank->nfields = count; // initialization08 blank->buf.size = blank->buf.length = count + 1;09 blank->buf.buffer = (char*) xmalloc (blank->buf.size);10 buffer = (unsigned char *) blank->buf.buffer;11 blank->fields = fields = (struct field *) xmalloc (sizeof (struct field) * count); 12 for (i = 0; i < count; i++){13 ... 14 }15 }

Insert comments and blanks

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Disguise 2: Identifier Renaming

01 static void02 fill_content (struct line *fill, int num)03 {04 int i;05 unsigned char *buffer;06 struct field *fields;07 fill->nfields = num; // initialization08 fill->buf.size = fill->buf.length = num + 1;09 fill->buf.buffer = (char*) xmalloc (fill->buf.size);10 buffer = (unsigned char *) fill->buf.buffer;11 fill->fields = fields = (struct field *) xmalloc (sizeof (struct field) * num); 12 for (i = 0; i < num; i++){13 ... 14 }15 }

Rename variables consistently

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Disguise 3: Statement Reordering

01 static void02 fill_content (struct line *fill, int num)03 {04 int i;05 unsigned char *buffer;06 struct field *fields;11 fill->fields = fields = (struct field *) xmalloc (sizeof (struct field) * num); 08 fill->buf.size = fill->buf.length = num + 1;09 fill->buf.buffer = (char*) xmalloc (fill->buf.size);10 buffer = (unsigned char *) fill->buf.buffer;07 fill->nfields = num; // initialization 12 for (i = 0; i < num; i++){13 ... 14 }15 }

Reorder non-dependent statements

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Disguise 4: Control Replacement

01 static void02 fill_content (struct line *fill, int num)03 {04 int i;05 unsigned char *buffer;06 struct field *fields;11 fill->fields = fields = (struct field *) xmalloc (sizeof (struct field) * num); 08 fill->buf.size = fill->buf.length = num + 1;09 fill->buf.buffer = (char*) xmalloc (fill->buf.size);10 buffer = (unsigned char *) fill->buf.buffer;07 fill->nfields = num; // initialization 12 i = 0; 13 while (i < num){14 ...15 i++; 16 }

17 }

Use equivalent control structure

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Disguise 5: Code Insertion

01 static void02 fill_content (struct line *fill, int num)03 {04 int i;05 unsigned char *buffer;06 struct field *fields;11 fill->fields = fields = (struct field *) xmalloc (sizeof (struct field) * num); 08 fill->buf.size = fill->buf.length = num + 1;09 fill->buf.buffer = (char*) xmalloc (fill->buf.size);10 buffer = (unsigned char *) fill->buf.buffer;07 fill->nfields = num; // initialization 12 i = 0; 13 while (i < num){14 ... for (int j = 0; j < i; j++);15 i++; 16 }

17 }

Insert immaterial code

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Fully Disguised01 static void02 make_blank (struct line *blank, int count)03 {04 int i;05 unsigned char *buffer;06 struct field *fields;

07 blank->nfields = count;08 blank->buf.size = blank->buf.length = count + 1;09 blank->buf.buffer = (char*) xmalloc (blank->buf.size);10 buffer = (unsigned char *) blank->buf.buffer;11 blank->fields = fields =

(struct field *) xmalloc (sizeof (struct field) * count);

12 for (i = 0; i < count; i++){13 ...14 }15 }

Original C ode

01 static void02 fill_content(int num, struct line* fill)03 {04 (*fill).store.size = fill->store.length = num + 1;05 struct field *tabs;06 (*fill).fields = tabs = (struct field *) xmalloc (sizeof (struct field) * num);07 (*fill).store.buffer = (char*) xmalloc (fill->store.size);08 (*fill).ntabs = num;09 unsigned char *pb;10 pb = (unsigned char *) (*fill).store.buffer;

11 int idx = 0;12 while(idx < num){ // fill in the storage13 ...14 for(int j = 0; j < idx; j++)15 ...16 idx++;17 }18 }

P lagiar ized C ode

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Outline

Plagiarism Disguises Review of Plagiarism Detection GPLAG: PDG-based Plagiarism Detection Efficiency and Scalability Experiments Conclusions

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Review of Plagiarism Detection String-based [Baker et al. 1995]

A program represented as a string Blanks and comments ignored.

AST-based [Baxter et al. 1998, Kontogiannis et al. 1995] A program is represented as an Abstract Syntax Tree (AST) Fragile to statement reordering, control replacement and

code insertion Token-based [Kamiya et al. 2002, Prechelt et al. 2002]

Variables of the same type are mapped to the same token A program is represented as a token string Fingerprint of token strings is used for robustness [Schleimer

et al. 2003] Partially robust to statement reordering, control replacement

and code insertion Representatives: Moss and JPlag

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Outline

Plagiarism Disguises Review of Plagiarism Detection GPLAG: PDG-based Plagiarism Detection Efficiency and Scalability Experiments Conclusions

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Graphic representation of source codeint sum(int array[], int count){ int i, sum; sum = 0; for(i = 0; i < count; i++){ sum = add(sum, array[i]); } return sum;}

int add(int a, int b){ return a + b;}

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Graphic representation of source code

int sum(int array[], int count){ int i, sum; sum = 0; for(i = 0; i < count; i++){ sum = add(sum, array[i]); } return sum;}

int add(int a, int b){ return a + b;}

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Control Dependency

int sum(int array[], int count){ int i, sum; sum = 0; for(i = 0; i < count; i++){ sum = add(sum, array[i]); } return sum;}

int add(int a, int b){ return a + b;}

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Data Dependency

int sum(int array[], int count){ int i, sum; sum = 0; for(i = 0; i < count; i++){ sum = add(sum, array[i]); } return sum;}

int add(int a, int b){ return a + b;}

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Plagiarism Detectible?01 static void02 make_blank (struct line *blank, int count)03 {04 int i;05 unsigned char *buffer;06 struct field *fields;

07 blank->nfields = count;08 blank->buf.size = blank->buf.length = count + 1;09 blank->buf.buffer = (char*) xmalloc (blank->buf.size);10 buffer = (unsigned char *) blank->buf.buffer;11 blank->fields = fields =

(struct field *) xmalloc (sizeof (struct field) * count);

12 for (i = 0; i < count; i++){13 ...14 }15 }

Original Code

01 static void02 fill_content(int num, struct line* fill)03 {04 (*fill).store.size = fill->store.length = num + 1;05 struct field *tabs;06 (*fill).fields = tabs = (struct field *) xmalloc (sizeof (struct field) * num);07 (*fill).store.buffer = (char*) xmalloc (fill->store.size);08 (*fill).ntabs = num;09 unsigned char *pb;10 pb = (unsigned char *) (*fill).store.buffer;

11 int idx = 0;12 while(idx < num){ // fill in the storage13 ...14 for(int j = 0; j < idx; j++)15 ...16 idx++;17 }18 }

P lagiar ized Code

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Corresponding PDGs

3: dec l.,line* blank

8: dec l .,int c ount

12: dec l.,int i

13: ass ign,i = 0

14: inc .,i++

15: c ontro li < c ount

9: as s ig n ,b lan k->b u f.s iz e

= b lan k->...

7: as s ig n ,b lan k->n field s =

co u n t

4: as s ig n ,b lan k->b u f.b u ffer = (ch ai*) xm..

0: as s ig n ,b lan k->field s =

field s = ...

10: as s ig n , b u ffer= (u n s ig n ed ) ...

11: d ec l.,c har* b uffer

5: d ec l.,s tru c t field *

field s

1: as s ig n ,field s =

(s tru c t ...

2: c all-s ite,xmalloc ()

6: c all-s ite,xmalloc ()

3: dec l.,l ine* f i ll

8: de c l.,int num

12: dec l.,int idx

13: as s ign,idx = 0

14: inc .,idx++

15: c ontro lw hile(id x < num)

9: as s ig n ,(*fill).s to re.s iz e

= ...

7: as s ig n ,(*fill).n tab s =

n u m

4: as s ig n ,(*fill).s to re.b u f =

(ch ar*) ...

0: as s ig n ,(*field ).field s =

tab = ...

10: as s ig n , p b =(u n s ig n ed

c har*) (*fill)...

11: dec l.,c har* pb

5: d ec l.,s tru c t field *

tab s

1: as s ig n ,tab s = (s tru c t

...

2: c a ll-s ite,xmalloc ()

6: c a ll-s ite,xmalloc ()

16: dec l.,int j

17: ass ign,j = 0

18: inc .,j++

19: c ontro lj < idx

PDG for the Original Code PDG for the Plagiarized Code

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PDG-based Plagiarism Detection A program is represented as a set of PDGs

Let g be a PDG of Procedure P in the original program Let g’ be a PDG of Procedure P’ in the plagiarism suspect

Subgraph isomorphism implies plagiarism If g is subgraph isomorphic to g’, P’ is likely plagiarized

from P γ-isomorphism: Graph g is γ-isomorphic to g’ if there

exists a subgraph s of g such that s is subgraph isomorphic to g’, and |s|≥ γ |g|.

If g is γ–isomorphic to g’, the PDG pair (g, g’) is regarded as a plagiarized PDG pair, and is then returned to human beings for examination.

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Advantages Robust because it is hard to overhaul PDGs

Dependencies encode program logic Incentive of plagiarism

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Outline

Plagiarism Disguises Review of Plagiarism Detection GPLAG: PDG-based Plagiarism Detection Efficiency and Scalability Experiments Conclusions

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Efficiency and Scalability Search space

If the original program has n procedures and the plagiarism suspect has m procedures n*m subgraph isomorphism testings

Pruning search space Lossless filter Statistical lossy filter

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Lossless filter

Interestingness PDGs smaller than an interesting

size K are excluded from both sides

γ-isomorphism definition A PDG pair (g, g’) is discarded if |

g’| <γ|g|.

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Lossy Filter Observation

If procedure P’ is plagiarized from procedure P, its PDG g’ should look similar to g.

So discard those dissimilar PDG pairs Requirement

This filter must be light-weighted

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Vertex Histogram Represent PDG g by

h(g) = (n1, n2, …, nk), where ni is the frequency of the ith kind of vertices.

Similarly, represent PDG g’ byh(g’) = (m1, m2, …, mk).

Direct similarity measurement? How to define a proper similarity threshold? Is thus defined threshold program-independent?

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Hypothesis Testing-based Approach Basic idea

Estimate a k-dimensional multinomial distribution from h(g)

Test whether h(g’) is likely an observation from

If it is, g’ looks similar to g, and an isomorphism testing is needed.

Otherwise, (g, g’) is discarded

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Technical Details

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Technical Details (cont’d)

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Work-flow of GPLAG PDGs are

generated with Codesurfer

Isomorphism testing is implemented with VFLib.

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Outline

Plagiarism Disguises Review of Plagiarism Detection GPLAG: PDG-based Plagiarism Detection Efficiency and Scalability Experiments Conclusions

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Experiment Design Subject programs

Effectiveness Filter efficiency Core-part plagiarism detection

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Effectiveness

2-hour manual plagiarism, but can be automated? GPLAG detects all plagiarized PDG pairs within 1 second PDG isomorphism also reveals what plagiarism disguises are applied

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Efficiency

Subject programs bc, less and tar. Exact copy as plagiarism.

Lossless and lossy filter Pruning PDG-pairs. Implication to overall time cost.

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Pruning Uninteresting PDG-pairs Lossless only Lossless and

lossy

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Implication to Overall Time Cost

Time-out for subgraph isomorphism testing, time hogs. Lossless filter does not save much time. Lossy filter significantly reduces the time cost. Major time saving comes from the avoidance of time hogs.

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Detection of Core-part Plagiarism

Lower time cost with lossy filter. Lower false positives with lossy filter.

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Outline

Plagiarism Disguises Review of Plagiarism Detection GPLAG: PDG-based Plagiarism Detection Efficiency and Scalability Experiments Conclusions

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Conclusions We developed a new algorithm GPLAG for

software plagiarism detection It is more effective to fight against “professional” plagiarists

We developed a statistical lossy filter, which improves the efficiency of GPLAG

We experimentally verified the effectiveness and efficiency of GPLAG

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Q & A

Thank You!

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systems. In Proc. of 2nd Working Conf. on Reverse Engineering, 1995.[2] I. D. Baxter, A. Yahin, L. Moura, M. Sant’Anna, and L. Bier. Clone detection

using abstract syntax trees. In Proc. of Int. Conf. on Software Maintenance, 1998.

[3] K. Kontogiannis, M. Galler, and R. DeMori. Detecting code similarity using patterns. In Working Notes of 3rd Workshop on AI and Software Engineering, 1995.

[4] T. Kamiya, S. Kusumoto, and K. Inoue. CCFinder: a multilinguistic token-based code clone detection system for large scale source code. IEEE Trans. Softw. Eng., 28(7), 2002.

[5] L. Prechelt, G. Malpohl, and M. Philippsen. Finding plagiarisms among a set of programs with JPlag. J. of Universal Computer Science, 8(11), 2002.

[6] S. Schleimer, D. S. Wilkerson, and A. Aiken. Winnowing: local algorithms for document fingerprinting. SIGMOD, 2003.

[7] V. B. Livshits and T. Zimmermann. Dynamine: Finding common error patterns by mining software revision histories. In Proc. of 13th Int. Symp. on the Foundations of Software Engineering, 2005.

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[9] C. Liu, X. Yan, H. Yu, J. Han, and P. S. Yu. Mining behavior graphs for ”backtrace” of noncrashing bugs. In SDM, 2005.