Lazy Preemption to Enable Path-Based Analysis of Interrupt-Driven Code

1
Lazy Preemption to Enable Path-Based Analysis of
Interrupt-Driven Code
Wei Le, Jing Yang, Mary Lou Soffa, and Kamin
Whitehouse Department of Computer
Science University of Virginia

• SESENA11
• May 22, Waikiki, Honolulu, Hawaii

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WSN Reliability is Important
25 million for a 100,000 node volcano monitoring
network Large money loss if it mistakenly
reports a non-existing eruption Severe
vulnerability and human casualty if it fails to
report a real eruption
However, traditional techniques and tools for
software reliability are handicapped in the WSN
domain !
3
Testing and Debugging?

• Limited by the large range of possible input
  sequences
• Real deployment environments are difficult to
  emulate

• Resource constraints limit the use of runtime
  techniques
• Continuous debugging and reprogramming is hard
  due to remote node deployments

Deployment
Simulation
Real environments
Scale
EmStar
ICE
Realism
Controlled environments
4
A Complementary Static Analysis

• Interrupt-driven code
• Exponential growth of the state space
• Path selection sacrificing coverage
• Path merging sacrificing precision
• Our contribution
• Insight physical world changes much slower than
  software execution
• Solution run interrupt handlers when necessary
  (based on timing) or convenient (end of functions)

5
A Complementary Static Analysis
Task
Interrupt Handler
1, 5, (23), 4 1, 2, 5, 4 1, 3, 5, 4 1, (23), 4,
5 Total 6 paths
1
5
1, (23), 4, 5 Total 2 paths
3
2
4
6
Outline

• Background
• Lazy preemption models
• Path-based fault detector

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Execution Model

• TinyOS
• Tasks, interrupt handlers, and atomic sections
• Task
• In-order execution from the task queue
• Interrupt
• Generated by hardware or environments
• Can preempt the current execution
• Atomic section
• Interrupts are disabled

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Faults Taxonomy
Bug Types Bug Types Number of Bugs
Interrupt Related Bugs Deadlock 4
Interrupt Related Bugs Race Condition 2
Interrupt Related Bugs Atomicity Violation 1
Interrupt Related Bugs Task Queue Overflow 2
Interrupt Related Bugs Stack Overflow 1
Logic violation bugs Logic violation bugs 5
From the most-severe-bug-pool of the TinyOS bug
repository
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Requirements for A Fault Detector

• Both detecting and reporting faults should be
  based on program paths
• The interactions between interrupts and tasks
  should be modeled
• Timing analysis should be performed

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Outline

• Background
• Lazy preemption models
• Path-based fault detector

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Fully Preemptive Model
Task
Interrupt Handler
1, 5, (23), 4 1, 2, 5, 4 1, 3, 5, 4 1, (23), 4,
5 Total 6 paths
1
5
3
2
4
12
Size of Atomic Sections
13
Size of Tasks and Interrupt Handlers
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Non Preemptive Model
Task
Interrupt Handler
1
5
1, (23), 4, 5 Total 2 paths
3
2
4
Only preempt at the end of tasks
15
Restricted Preemptive Model
Task
Interrupt Handler
1
5
1, (23), 4, 5 Total 2 paths
1, (23), 4, 5 1, 3, 5, 4 Total 3 paths
3
2
Preempt when necessary
4
Only preempt at the end of tasks
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Outline

• Background
• Lazy preemption models
• Path-based fault detector

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Framework and Workflow
Static Timing Analysis
Runtime Enforcement
nesC Compiler
WSN App in nesC
C program CFGs
IICFG
Execution based on IICFG
Demand-Driven Analysis
Faults
Fix Bugs
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Static Timing Analysis to Build IICFG

• Input
• Source code
• Arrival frequency for each interrupt
• Required response time for each interrupt
• Output
• Inter-procedural control flow graph (IICFG)
• Preemption points on IICFG

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Demand-Driven, Path-Based Fault Detection
Task
Interrupt Handler
Q5 Len(input)lt32Vul
Q1 Len(b)lt32
b input
strcpy(a, b)
1
5
Q4 4 lt 32 Safe
Q3 Len(b)lt32
b test
2
3

4
Q2 Len(b)lt32
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Runtime Preemption Enforcement

• Record handler
• Invoked whenever an interrupt arrives
• Records the data at the hardware port
• Action handler
• Invoked only at preemption points
• Switches the context
• Executes the original interrupt handler

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Conclusion

• Static analysis in the WSN domain
• Satisfy both coverage and precision
• Two lazy preemption models
• Demand-driven, path-based
• Implementation in progress