Detecting and Escaping Infinite Loops with Jolt

1. Detecting and Escaping Infinite Loops with Jolt Michael Carbin with SasaMisailovic, Michael Kling, and Martin Rinard Massachusetts Institute of Technology

2. Exceptions?

3. Our Research Context Researcher Developer User

4. Motivation • You run a program • Edit a document • Search for a keyword • Indent source code files • Program infinite loops • You get no output • You lose your work

5. Our Solution: J lt Automatically detect and escape infinite loops Program produces output User doesn’t lose work Logo courtesy J.D. Rhoades

6. Basic Approach • Compile program with Jolt compiler. (adds instrumentation to mark loop boundaries) • Execute program. • Program stops responding. • Launch Jolt monitor. • Monitor takes snapshots at each loop head. • If two snapshots are same, infinite loop! • Jolt jumps to instruction after loop.

7. Does this work? • Are real infinite loops this simple? • Does Jolt produce a safe execution? • Does Jolt produce better output than Ctrl-C? • Does Jolt match the developer’s fix? • Can this be implemented efficiently?

8. Infinite Loop in Grep conf.in 25th ECOOP grep –E “[0-9]*” --color=always conf.in v2.5 v2.5.1 stdout 25th ECOOP Infinite Loop

9. Infinite Loop in Grep 2.5 1: void prline(regex_t regex, char *beg) { 3: while (true) { 4: int size = 0; 5: int off = match(regex, beg, &size); 6: if (off == -1) { 7: break; 8: } 9: char const *b = beg + off; 10: fwrite(beg, 1, off, stdout); 11: enable_color(); 10: fwrite (b, 1, size, stdout); 11: disable_color(); 11: beg = b + size; 12: } 14: }

10. Applying Jolt to Grep

11. Step 1: Compile Grep Jolt Compiler Adds instructions that mark loop boundaries.

12. Jolt Compiles Grep 1: void prline(regex_t regex, char *beg) { 2: … 3: 4: while (true) { 5: 6: int size = 0; 7: int offset = match(regex, beg, &size); 8: if (offset == -1) { 9: 10: break; 11: } 12: char const *b = beg + offset; 13: fwrite (b, sizeof (char), size, stdout); 14: beg = b + size; 15: } 16: 17: } 18: …

13. Jolt Compiles Grep 1: void prline(regex_t regex, char *beg) { 2: … 3: jolt_loop_entry(ID); 4: while (true) { 5: jolt_loop_body(ID, &jolt_escape_dest); 6: int size = 0; 7: int offset = match(regex, beg, &size); 8: if (offset == -1) { 9: jolt_loop_exit(ID); 10: break; 11: } 12: char const *b = beg + offset; 13: fwrite (b, sizeof (char), size, stdout); 14: beg = b + size; 15: } 16: jolt_escape_dest: 17: … 18: }

14. Step 2: Run Grep • Grep runs as normal. • Control instrumentation overhead ~2.5%.

15. Step 3: Grep stops Responding • Grep enters infinite loop, does not respond!

16. Step 4: Launch Jolt Monitor • Jolt dynamically attaches to grep • Monitors execution • Computes snapshot at each loop head • If two snapshots are the same, infinite loop! Jolt

17. How to Compute a Snapshot Efficiently • Log all registers and memory addresses that each loop iteration accesses. • Record the contents of these registers and addresses at the head of the loop.

18. Comparing Two Snapshots Snapshot 1 • First, check if snapshots have the same accessed • Registers and • Memory addresses • Next check if contents of • Accessed registers and • Accessed memory addresses are the same Snapshot 2

19. Step 5: Jolt Exits the Loop 1: void prline(regex_t regex, char *beg) { 2: jolt_loop_entry(ID); 3: while (true) { 4: jolt_loop_body(ID, &jolt_escape_dest); 5: int size = 0; 6: int offset = match(regex, beg, &size); 7: if (offset == -1) { 8: jolt_loop_exit(ID); 9: break; 10: } 11: char const *b = beg + offset; 12: fwrite (b, sizeof (char), size, stdout); 13: beg = b + size; 14: } 15: jolt_escape_dest: 16: }

20. Output Jolt Ctrl-C

21. Does this work for Grep? • Is it safe? • Yes, no side-effects (except on output files). • Is it better than Ctrl-C? • Yes, continues to match additional lines and files. • Is it equivalent to the correct fix? • Yes, corrected by break if size is 0 (v2.5.1). • 3 years later no, v2.5.3 skips all size 0 matches. 25th ECOOP ‘11 25th ECOOP ‘11 25th ECOOP ‘11

22. Does this work in general?

23. 5 Applications and 8 Infinite Loops • ctags: line numbers of functions in code. • v5.5 : one loop in fortran module. • v5.7b : one loop in python module. • grep(v2.5): matches regexpagainst files (3 loops). • ping (v20100214): icmp utility. • indent (v1.1-svr 4): indents source code. • look (v1.9.1): matches a word against dictionary file.

24. Question #1 Are infinite loops this simple? 7 of 8

25. Question #2 Does escape produce a safe execution? • Methodology • Validated execution with Valgrind and by hand. • Tested over available infinite loop triggering inputs. • Results • Yes, side effects often localized = consistent state. • Or, simple correctness invariants.

26. Question #3 Does Jolt produce a better output than Ctrl-C? • Methodology • Defined output abstraction, and compared outputs. • Results • Yes, errors often isolated to single output unit (e.g., file). • Example • indent: correct indention resumes on next file. • Terminating indent deletes your source code!!! • Development hint: don’t update files in-place.

27. Question #4 Does escape model the developers’ fix? • Methodology • Manually inspected a later version of each application. • Results • ctags: no, output semantically different on some inputs. • grep: jolt matches fix for two of three loops (3 years). • ping, indent, look: yes, in all cases. • Example • ping: developer used continue instead of break.

28. Question #5 Can this be implemented efficiently? *Average over the three loops

29. Observations • Infinite loops can (and often do) frustrate users. • Infinite loops can be (and often are) simple. • Errors are localized in some cases. • Jolt’s results can be (and often are) better than no results at all. • Applying Jolt can (and often does) model the developer’s fix.

30. Did We Just Solve the Halting Problem? No

31. Limitations • Jolt doesn’t detect infinite loops that always change state. • Though still possible to exit loop if user desires. • We did not consider busy-wait loops. • Though a good question is what does it mean when a user thinks a program is waiting too long?

32. Future Work • Off-the-shelf binaries - pure dynamic analysis • Safety guarantees – shepherded execution • Larger classes of loops - symbolic non-termination analysis

33. Related Work • Bounding Loop Length • Detecting and Eliminating Memory Leaks Using Cyclic Memory Allocation (Nguyen and Rinard, ISMM ‘07) • Non-termination Provers • TNT (Gupta, et al.) using Invariant Generation (POPL ‘08) • Looper (Burnim, et al.) using Symbolic Execution (ASE ‘09) • Termination Provers • Terminator (Cook, et al.) (PLDI ‘06)

34. Takeaway Jolt gives users another option to deal with programs that infinite loop. Another hammer in the toolbox to help users deal with otherwise fatal errors.

35. Thank You/Questions? From: "Armando Solar-Lezama" <asolar@csail.mit.edu> To: "Martin Rinard" <rinard@csail.mit.edu> Subject: Thanks I was writing a document in Word this morning, and after about an hour of unsaved work, Word went into an infinite loop that made the application completely frozen. So, having listened to your talks too many times, I got my debugger, paused the program, changed the program counter to a point a few instructions past the end of the loop, and let it keep running from there. Word went back to working as if nothing had ever happened. I was able to finish my document, save it, and close Word without problems. So thanks, Armando.

36. Supplementary Slides

37. Bug Reports grep 2.5 -- 13 Mar2002 2.5.1 -- 29 Oct 2004 2.5.3 -- 02 Aug 2007 indent 1994 ctags 5.5 – 2003 5.7b –2007

38. Instrumentation Overhead

39. Detection Time