1 / 19

Model Checking the Garbage Collection Mechanism of SMV

Model Checking the Garbage Collection Mechanism of SMV. Cindy Eisner. IBM Research Laboratory in Haifa. Outline. Motivation Preliminaries Na ï ve translation of C to EDL Radical abstraction A practical application The garbage collection mechanism of SMV Results

oshin
Download Presentation

Model Checking the Garbage Collection Mechanism of SMV

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Model Checking the Garbage Collection Mechanism of SMV Cindy Eisner IBM Research Laboratory in Haifa

  2. Outline • Motivation • Preliminaries • Naïve translation of C to EDL • Radical abstraction • A practical application • The garbage collection mechanism of SMV • Results • False negatives and false positives • Conclusion

  3. Motivation • Model checking hardware is “old hat” • IBM has an industrial strength model checker for hardware • Software is the next frontier • Question: just how far can we go with the existing hardware model checker?

  4. Preliminaries • RuleBase – IBM’s hardware model checker • VHDL/Verilog hardware design • EDL environment • Sugar specification • For software: • C program (translate to EDL) • EDL behavior of inputs • Sugar specification

  5. Preliminaries (EDL) • EDL is a dialect of SMV var a, b, c: boolean; assign next(a) := b & c; define d := a | c;

  6. Preliminaries (Sugar) • Sugar is syntactic sugaring of CTL + regular expressions • One kind of Sugar formula is used in this work: suffix implication Sugar: {true[*],a,b[*],c[+]}(d) CTL: EF(a  EX E[b U E[c U (c & d)]])

  7. Preliminaries (cont.) • Basic idea: • tool c2edl translates C to EDL as shown on next slides • write properties by hand in Sugar • run RuleBase – see how far we get

  8. getmax () { int max, a; 0 a = max = 0; 1 do { 2 if (a > max) 3 max = a; 4 a = input(); 5 } while (a); 6 return(max); } next(a) = if pc=0 then 0 else if pc=4 then {0,1,2,3} else a next(max) = if pc=0 then 0 else if pc=3 then a else max next(pc) = if pc=0 then 1 else if pc=1 then 2 else if pc=2 then if a > max then 3 else 4 … Naïve translation of C to EDL

  9. Naïve translation of C to EDL (cont.) • Problems: • Data ranges need to be severely restricted • Pointers • Function calls, including recursion

  10. f() { … if (a>b) g(); c = d; } g() { if … … return; } next(pc) = if … else if pc=5 then if (a>b) then 6 else 8 else if pc=6 then 9 else if pc=7 then 8 … else if pc=11 then stack[stackp]; next(stackp) := if pc=6 then stackp+1 else if pc=11 then stackp-1 else stackp Naïve translation to EDL (function calls) f() { … 5 if (a>b) { 6 /*pushcall*/ 7 g(); } 8 c = d; } g() { 9 if (…) 10 … 11 return; }

  11. A radical abstraction • Eliminate all variables except for: • program counter • finite stack • Replace variable references with non-deterministic choice: • if (a>b) -> if {0,1}

  12. A practical application • For most programs, the abstraction of the previous slide takes away too much • Is there an application for which this approach is practical? • Yes, at least one – the garbage collection mechanism of SMV

  13. The garbage collection mechanism of SMV • save_bdd() • release_bdd() • mygarbage() • Example: a = save_bdd(and_bdd(b,c)); d = save_bdd(or_bdd(e,f)); mygarbage(); g = or_bdd(a,d);

  14. The garbage collection mechanism of SMV (cont.) • Dangling reference: {true[*], assign_v call_save_bdd_v, (assign_v  call_save_bdd_v)[*], call_mygarbage, assign_v[*], use_v} (false) • All propositions can be expressed as a function of the program counter

  15. The garbage collection mechanism of SMV (cont.) • Memory leak: {true[*], call_save_bdd_v, call_release_bdd_v[*], assign_v} (false)

  16. Results • Run time of a few hours per group of 20 variables checked • No bugs found in SMV version r2.4.4 • Eight real bugs found in HRL version of SMV under development

  17. … b = save_bdd(a); c = b; mygarbage(); d = c; … Many could be eliminated through adherence to coding conventions and/or more precise Sugar specifications False negatives

  18. False positives • Because depth of stack was limited, possibly false positives as well • It is probably possible to show that because of the extent of the non-determinism, some finite depth is sufficient

  19. Conclusion • Our motivation was to see how useful IBM’s existing model checker would be for software • Results were surprising – eight quite sophisticated bugs in real code were found easily • Currently applying hardware model checker to a distributed program with no recursion but without abstracting away the variables

More Related