Specification-Based Error Localization

Brian DemskyMartin Rinard

Laboratory for Computer ScienceMassachusetts Institute of Technology

Problem

Error Introduced

Execution with

Broken Data

Structure

Crash or Unexpected

Result

• Have to trace symptom back to cause• Corruption may not cause visible

error in test suite

Problem

Error Introduced

Execution with

Broken Data

Structure

Crash or Unexpected

Result

• Solution: discover bugs when • they corrupt data• not when effect becomes visible

• Perform frequent consistency checks• Bug localized between

• first unsuccessful check and• last successful check

Architecture

next

value

value

o1

o1

o1

o1

values

nodes

Concrete Data Structure Abstract Model

Model DefinitionRules

Consistency Constraints

Architecture RationaleWhy use the abstract model?

• Model construction separates objects into sets• Reachability properties• Field values

• Different constraints for objects in different sets• Appropriate division of complexity

• Data structure representation complexity encapsulated in model construction rules

• Consistency property complexity encapsulated in (clean, uniform) model constraint language

Simplified Freeciv Example

tile grid[EDGE][EDGE];structure tile {

int terrain;city *city;

} structure city {

int population;}

PO MM

OO MP

PO MM

PP MP

Terrain Grid

City Structures

O = OceanP = PlainM = Mountain

Sets and Relations in Model

• Sets of objectsset TILE of tilegrid;set CITY of city;

• Relations between objects – values of object fields, referencing relationships between objectsrelation CITYMAP : TILE -> CITY;relation TERRAIN : TILE -> integer;

Model TranslationBits translated to sets and relations in abstract

model using statements of the form:

Quantifiers, Condition Inclusion Constraint

for x in 0..EDGE*EDGE, true grid[x] in TILE

for t in TILE, true t,t.terrain in TERRAIN

for t in TILE, !t.city = NULL t,t.city in CITYMAPfor t in TILE, !t.city=NULL t.city in CITY

Model in Examplegrid[0]

grid[1]

grid[2]

grid[3]

terrain:

1 2 3 4

city: NULL NULLpopulation:

10,000

grid[0]

grid[1]

grid[2]

grid[3]

Tiles

1234

city

Cities

Consistency PropertiesQuantifiers, Body

• Body is first-order property of basic propositions• Inequality constraints on numeric fields • Cardinality constraints on sizes of sets• Referencing relationships for each object• Set and relation inclusion constraints

• Example:for t in TILE, MIN <= t.TERRAIN and t.TERRAIN<=MAXfor c in CITY, size(CITYMAP.c)=1for c in CITY, !(CITYMAP.c).TERRAIN=OCEAN

Consistency ViolationsEvaluate consistency properties, find

violationsfor c in CITY, size(CITYMAP.c)=1

grid[0]

grid[1]

grid[2]

grid[3]

Tiles

1234

city

Cities

Slide about checks

• TODO

Optimized Implementation• Compilation (4.7x speedup)• Fixed point elimination (210x speedup)

• Evaluate model definition rules using simple traversal• Relation construction elimination (500x speedup)

• Evaluate uses of relations directly on data structures• Set construction elimination (3900x speedup)

• Evaluate constraints while traversing data structures• Bottom line

• Interpreted version X times slower than uninstrumented

• Optimized version Y times slower than uninstrumented

Freeciv Case Study

• Multiplayer Client/Server based online game

• Available at www.freeciv.org• Case study looked at the server• Server contains 73,000 lines of code• Added 750 instrumented sites• 20,000 consistency checks performed in

our sample execution

Case Study

• Created three buggy version of Freeciv• Two groups of three developers

• One used conventional tools• One used specification-based

consistency checking• Each participant was asked to spend at

least one hour on each version• Both populations given a pre-

instrumented version of Freeciv

Consistency Properties• Map exists

size(MAP)=1• Grid of tiles exists

size(GRID)=1• Tiles have valid terrain values

for t in TILE, MIN <= t.TERRAIN and t.TERRAIN<=MAX

• Cities are not in the oceanfor c in CITY, !(CITYMAP.c).TERRAIN=OCEAN

• Each city has exactly one reference from the gridfor c in CITY, size(CITYMAP.c)=1

Bugs Introduced

• Actual errors in buggy versions• First error creates invalid terrain values

(violates valid terrain property)• Second causes two tiles to refer to the

same city (violates single reference property)

• Third causes a city to be placed on ocean (violates cities not in ocean property)

Results

• User study shows benefit from approach• With tool

• All developers found and fixed all bugs• Mean of 11 minutes required

• Without tool• Three developers found total of one

bug (out of nine developer/bug combinations)

• Others spent hour debugging (unsuccessfully)

Repair for Deployed Systems

• Consistency specifications for repair• Input: inconsistent data structure• Output: consistent data structure• Technique enables programs to recover

from data structure corruption• And continue to execute successfully

• OOPSLA ’03 paper describes this technique

Full reference here

Related Work

• Specification languages such as UML or Alloy

• Specification-based testing • Korat (Boyapati et. al. ISSTA 2002)• Testera (Marinov and Khurshid)• Eiffel (Meyer)

• Invariant inference and checking• Daikon (Ernst et. al.)• DIDUCE (Hangal and Lam)• Carrot (Pytlik et. Al.)

Conclusion

• Consistency checking to localize data structure corruption bugs

• Good experimental results• With checker, bugs fixed in minutes• Without checker, bugs not fixed in an

hour• Optimizations for good performance• Data structure repair