1. SLAM internals Sriram K. Rajamani Rigorous Software Engineering Microsoft Research, India

2. C- Types  ::= void | bool | int | ref  Expressionse::=c | x | e 1 op e 2 | &x | *x LExpression l ::= x | *x Declarationd::=  x 1,x 2,…,x n Statementss::=skip | goto L 1,L 2 …L n | L: s | assume(e) | l = e | l = f (e 1,e 2,…,e n ) |return x |s 1 ; s 2 ;… ; s n Procedures p ::=  f (x 1 :  1,x 2 :  2,…,x n :  n ) Programg ::= d 1 d 2 … d n p 1 p 2 … p n

3. C-- Types  ::= void | bool | int Expressionse::=c | x | e 1 op e 2 LExpression l ::= x Declarationd::=  x 1,x 2,…,x n Statementss::=skip | goto L 1,L 2 …L n | L: s | assume(e) | l = e |f (e 1,e 2,…,e n ) |return |s 1 ; s 2 ;… ; s n Procedures p ::= f (x 1 :  1,x 2 :  2,…,x n :  n ) d s Programg ::= d 1 d 2 … d n p 1 p 2 … p n

4. BP Types  ::= void | bool Expressionse::=c | x | e 1 op e 2 LExpression l ::= x Declarationd::=  x 1,x 2,…,x n Statementss::=skip | goto L 1,L 2 …L n | L: s | assume(e) | l = e |f (e 1,e 2,…,e n ) |return |s 1 ; s 2 ;… ; s n Procedures p ::= f (x 1 :  1,x 2 :  2,…,x n :  n ) d s Programg ::= d 1 d 2 … d n p 1 p 2 … p n

5. Syntactic sugar if (e) { S1; } else { S2; } S3; goto L1, L2; L1: assume(e); S1; goto L3; L2: assume(!e); S2; goto L3; L3: S3;

6. Example, in C void cmp (int a, int b) { if (a == b) g = 0; else g = 1; } int g; main(int x, int y){ cmp(x, y); if (!g) { if (x != y) assert(0); }

7. void cmp(int a, int b) { goto L1, L2; L1: assume(a==b); g = 0; return; L2: assume(a!=b); g = 1; return; } int g; main(int x, int y){ cmp(x, y); assume(!g); assume(x != y) assert(0); } Example, in C--

8. c2bp: Predicate Abstraction for C Programs Given P : a C program F = {e 1,...,e n } –each e i a pure boolean expression –each e i represents set of states for which e i is true Produce a boolean program B(P,F) same control-flow structure as P boolean vars {b 1,...,b n } to match {e 1,...,e n } properties true of B(P,F) are true of P

9. Assumptions Given P : a C program F = {e 1,...,e n } –each e i a pure boolean expression –each e i represents set of states for which e i is true Assume: each e i uses either: –only globals (global predicate) –local variables from some procedure (local predicate for that procedure) Mixed predicates: –predicates using both local variables and global variables –complicate “return” processing –covered in Step 2

10. C2bp Algorithm Performs modular abstraction –abstracts each procedure in isolation Within each procedure, abstracts each statement in isolation –no control-flow analysis –no need for loop invariants

11. void cmp (int a, int b) { goto L1, L2 L1: assume(a==b); g = 0; return; L2: assume(a!=b); g = 1; return; } int g; main(int x, int y){ cmp(x, y); assume(!g); assume(x != y) assert(0); } Preds: {x==y} {g==0} {a==b}

12. void cmp (int a, int b) { goto L1, L2 L1: assume(a==b); g = 0; return; L2: assume(a!=b); g = 1; return; } int g; main(int x, int y){ cmp(x, y); assume(!g); assume(x != y) assert(0); } decl {g==0} ; main( {x==y} ) { } void cmp ( {a==b} ) { } Preds: {x==y} {g==0} {a==b}

13. void equal (int a, int b) { goto L1, L2 L1: assume(a==b); g = 0; return; L2: assume(a!=b); g = 1; return; } int g; main(int x, int y){ cmp(x, y); assume(!g); assume(x != y) assert(0); } decl {g==0} ; main( {x==y} ) { cmp( {x==y} ); assume( {g==0} ); assume( !{x==y} ); assert(0); } void cmp ( {a==b} ) { goto L1, L2; L1: assume( {a==b} ); {g==0} = T; return; L2: assume( !{a==b} ); {g==0} = F; return; } Preds: {x==y} {g==0} {a==b}

14. C-- Types  ::= void | bool | int Expressionse::=c | x | e 1 op e 2 LExpression l ::= x Declarationd::=  x 1,x 2,…,x n Statementss::=skip | goto L 1,L 2 …L n | L: s | assume(e) | l = e | f (e 1,e 2,…,e n ) | return | s 1 ; s 2 ;… ; s n Procedures p ::= f (x 1 :  1,x 2 :  2,…,x n :  n ) Programg ::= d 1 d 2 … d n p 1 p 2 … p n

15. Abstracting Assignments Suppose you are given an assignment s if Implies F (WP(s, e i )) is true before s then –e i is true after s if Implies F (WP(s, !e i )) is true before s then –e i is false after s {e i } = Implies F (WP(s, e i )) ?true : Implies F (WP(s, !e i )) ? false : *;

16. Abstracting Expressions via F F = { e 1,...,e n } Implies F (e) –best boolean function over F that implies e ImpliedBy F (e) –best boolean function over F implied by e –ImpliedBy F (e) = !Implies F (!e)

17. Implies F (e) and ImpliedBy F (e) e Implies F (e) ImpliedBy F (e)

18. Computing Implies F (e) F = { e 1,...,e n } minterm m = d 1 &&... && d n –where d i = e i or d i = !e i Implies F (e) –disjunction of all minterms that imply e Naïve approach –generate all 2 n possible minterms –for each minterm m, use decision procedure to check validity of each implication m  e Many optimizations possible

19. do { KeAcquireSpinLock(); nPacketsOld = nPackets; b = true; if(request){ request = request->Next; KeReleaseSpinLock(); nPackets++; b = b ? false : *; } } while (nPackets != nPacketsOld); !b KeReleaseSpinLock(); Example Add new predicate to boolean program (c2bp) b : (nPacketsOld == nPackets) U L L L L U L U U U E

20. Assignment Example Statement in P: Predicates in F: y = y+1; {x==y} Weakest Precondition: WP(y=y+1, x==y) = x==y+1 Implies F ( x==y+1 ) = false Implies F ( x!=y+1 ) = x==y Abstraction of assignment in B: {x==y} = {x==y} ? false : *;

21. Absracting Assumes WP( assume(e), Q) = e  Q assume(e) is abstracted to: assume( ImpliedBy F (e) ) Example: F = {x==2, x<5} assume(x < 2) is abstracted to: assume( {x<5} && !{x==2} )

22. Abstracting Procedures Each predicate in F is annotated as being either global or local to a particular procedure Procedures abstracted in two passes: –a signature is produced for each procedure in isolation –procedure calls are abstracted given the callees’ signatures

23. Abstracting a procedure call Procedure call –a sequence of assignments from actuals to formals –see assignment abstraction Procedure return –NOP for C-- with assumption that all predicates mention either only globals or only locals –with pointers and with mixed predicates: Most complicated part of c2bp Covered in Step 2

24. void cmp (int a, int b) { Goto L1, L2 L1: assume(a==b); g = 0; return; L2: assume(a!=b); g = 1; return; } int g; main(int x, int y){ cmp(x, y); assume(!g); assume(x != y) assert(0); } decl {g==0} ; main( {x==y} ) { cmp( {x==y} ); assume( {g==0} ); assume( !{x==y} ); assert(0); } void cmp ( {a==b} ) { Goto L1, L2 L1: assume( {a==b} ); {g==0} = T; return; L2: assume( !{a==b} ); {g==0} = F; return; } {x==y} {g==0} {a==b}

25. Precision loss during predicate abstraction For program P and F = {e 1,...,e n }, there exist two “ideal” abstractions: –Boolean(P,F) : most precise abstraction –Cartesian(P,F) : less precise abstraction, where each boolean variable is updated independently Theory: –with an “ideal” theorem prover, c2bp can compute Cartesian(P,F) Practice: –c2bp computes a less precise abstraction than Cartesian(P,F) –we use Das/Dill’s technique to incrementally improve precision –with an “ideal” theorem prover, the combination of c2bp + Das/Dill can compute Boolean(P,F)

26. Cartesian abstraction F = { (x!=5), (y 5)} Statement: y = x Abstract statement in the boolean program: {y < 5} = {x != 5} ? * : false {y > 5} = {x != 5} ? * : false Suppose {x != 5} is true before the assignment Then, only one of {y 5} can be true after the assignment This correlation is lost in cartesian abstraction

27. prog. P’ prog. P SLIC rule The SLAM Process boolean program path predicates slic c2bp bebop newton

28. Bebop Model checker for boolean programs Based on CFL reachability Explicit representation of CFG Implicit representation of path edges and summary edges Generation of hierarchical error traces

29. Domains g  Global l  Local f  Frame s  Stack = Frame* State = Global X Local X Stack Transitions: T  (Global X Local) X ( Global X Local) T +  Local X (Local X Frame) T -  (Local X Frame) X Local

30. Transition Relation STEP T (g, l, g’, l’) ________________ (g,l,s)  (g’,l’, s) PUSH T + (l, l’, f) __________________ (g,l,s)  (g’,l’, s.f) POP T - (l, f, l’’) _______________________ (g,l,s.f)  (g’,l’, s) g  Global l  Local f  Frame s  Stack = Frame* State = Global X Local X Stack Transitions: T  (Global X Local) X ( Global X Local) T +  Local X (Local X Frame) T -  (Local X Frame) X Local

31. Naïve model checking STEP T (g, l, g’, l’) ________________ (g,l,s)  (g’,l’, s) PUSH T + (l, l’, f) __________________ (g,l,s)  (g’,l’, s.f) POP T - (l, f, l’’) _______________________ (g,l,s.f)  (g’,l’, s) Start with initial state: (g 0,l 0,  ) Find all the states s such that (g 0,l 0,  )  * s Even if Globals and Locals are finite, the set of reachable states could be infinite since the stack can grow infinite No guarantee for termination Still, assertion checking is decidable Need to use a different algorithm (CFL reachability)

32. CFL reachability CFL-STEP P(g 1, l 1, g 2, l 2 ) T(g 2, l 2, g 3, l 3 ) ________________ P(g 1, l 1, g 3, l 3 ) CFL-PUSH P(g 1, l 1, g 2, l 2 ) T + (l 2, l 3,f, ) __________________ P(g 2, l 3, g 2, l 3 ) CFL-SUM P(g 1, l 1, g 2, l 2 ) T - (l 2, f, l 3, ) __________________ Sum(g 1, l 1, f, g 2, l 3 ) CFL-POP P(g 1, l 1, g 2, l 2 ) T + (l 2, l 3,f, ) Sum( g 2, l 3, f, g 3, l 4 ) _______________________ P(g 1, l 1, g 3, l 4 ) P  (Global X Local) X ( Global X Local) Sum  (Global X Local) X Frame X ( Global X Local) CFL-INIT _______________ P(g 0, l 0, g 0, l 0 ) [Sharir-Pnueli 81] [Reps-Sagiv-Horwitz 95]

33. decl g; void main() decl u,v; [1] u := !v; [2] equal(u,v); [3] if (g) then R: skip; fi [4] return; void equal(a, b) [5] if (a = b) then [6] g := 1; else [7] g := 0; fi [8] return; [1] [2] u!=v g=0 [5] [7] [3] [4] [1] [8] [7] u=v g=0 u!=v g=1 u=v g=1

34. Symbolic CFL reachability  Partition path edges by their “target”  PE(v) = { |  }  What is for boolean programs?  A bit-vector!  What is PE(v)?  A set of bit-vectors  Use a BDD (attached to v) to represent PE(v)

35. decl g; void main() decl u,v; [1] u := !v; [2] equal(u,v); [3] if (g) then R: skip; fi [4] return; void equal(a, b) [5] if (a = b) then [6] g := 1; else [7] g := 0; fi [8] return; [1] [2] u!=v g=0 [5] [7] [3] [4] [1] [8] [7] u=v g=0 u!=v g=1 u=v g=1 g=g’& u=u’& v=v’ g=g’& v!=u’ & v=v’ g=g’ & a=a’ & b=b’ & a!=b g’=0 & v’!=u’ & v=v’ g’=0 & v!=u’ & v=v’ g=g’& a=a’ & b=b’& a!=b g’=0 & a=a’ & b=b’& a!=b

36. Bebop: summary  Explicit representation of CFG  Implicit representation of path edges and summary edges  Generation of hierarchical error traces  Complexity: O(E * 2 O(N) )  E is the size of the CFG  N is the max. number of variables in scope

37. prog. P’ prog. P SLIC rule The SLAM Process boolean program path predicates slic c2bp bebop newton

38. Type 1: False error trace due to approximate predicate abstraction

39. Type 2: False error trace due to insufficient predicates

40. prog. P’ prog. P SLIC rule The SLAM Process (more accurate) boolean program path predicates slic c2bp bebop newton

41. Type 1: False error trace due to approximate predicate abstraction

42. With this technique we can be as precise as boolean abstraction (rather than cartesian)

43. Type 2: False error trace due to insufficient predicates

45. prog. P’ prog. P SLIC rule The SLAM Process (more accurate) boolean program path predicates slic c2bp bebop newton

46. Newton Given an error path p in boolean program B –is p a feasible path of the corresponding C program? Yes: found an error No: find predicates that explain the infeasibility –uses the same interfaces to the theorem provers as c2bp.

47. Newton Execute path symbolically Check conditions for inconsistency using theorem prover (satisfiability) After detecting inconsistency: –minimize inconsistent conditions –traverse dependencies –obtain predicates

48. do { KeAcquireSpinLock(); if(*){ KeReleaseSpinLock(); } } while (*); KeReleaseSpinLock(); Example Model checking boolean program (bebop) U L L L L U L U U U E

49. do { KeAcquireSpinLock(); nPacketsOld = nPackets; if(request){ request = request->Next; KeReleaseSpinLock(); nPackets++; } } while (nPackets != nPacketsOld); KeReleaseSpinLock(); Example Is error path feasible in C program? (newton) U L L L L U L U U U E

50. do { KeAcquireSpinLock(); nPacketsOld = nPackets; b = true; if(request){ request = request->Next; KeReleaseSpinLock(); nPackets++; b = b ? false : *; } } while (nPackets != nPacketsOld); !b KeReleaseSpinLock(); Example Add new predicate to boolean program (c2bp) b : (nPacketsOld == nPackets) U L L L L U L U U U E

51. Symbolic simulation for C-- Domains –variables: names in the program –values: constants + symbols State of the simulator has 3 components: –store: map from variables to values –conditions: predicates over symbols –history: past valuations of the store

52. Symbolic simulation Algorithm Input: path p For each statement s in p do match s with Assign(x,e): let val = Eval(e) in if (Store[x]) is defined then History[x] := History[x]  Store[x] Store[x] := val Assume(e): let val = Eval(e) in Cond := Cond and val let result = CheckConsistency(Cond) in if (result == “inconsistent”) then GenerateInconsistentPredicates() End Say “Path p is feasible”

53. Symbolic Simulation: Caveats Procedure calls –add a stack to the simulator –push and pop stack frames on calls and returns –implement mappings to keep values “in scope” at calls and returns Dependencies –for each condition or store, keep track of which values where used to generate this value –traverse dependency during predicate generation

54. void cmp (int a, int b) { Goto L1, L2 L1: assume(a==b); g = 0; return; L2: assume(a!=b); g = 1; return; } int g; main(int x, int y){ cmp(x, y); assume(!g); assume(x != y) assert(0); }

55. void cmp (int a, int b) { Goto L1, L2 L1: assume(a==b); g = 0; return; L2: assume(a!=b); g = 1; return; } int g; main(int x, int y){ cmp(x, y); assume(!g); assume(x != y) assert(0); } Global: main: (1)x: X (2)y: Y Conditions :

56. void cmp (int a, int b) { Goto L1, L2 L1: assume(a==b); g = 0; return; L2: assume(a!=b); g = 1; return; } int g; main(int x, int y){ cmp(x, y); assume(!g); assume(x != y) assert(0); } Global: main: (1)x: X (2)y: Y cmp : (3)a: A (4)b: B Conditions : Map : X  A Y  B

57. void cmp (int a, int b) { Goto L1, L2 L1: assume(a==b); g = 0; return; L2: assume(a!=b); g = 1; return; } int g; main(int x, int y){ cmp(x, y); assume(!g); assume(x != y) assert(0); } Global: (6) g: 0 main: (1)x: X (2)y: Y cmp : (3)a: A (4)b: B Conditions : (5)(A == B) [3, 4] Map : X  A Y  B

58. void cmp (int a, int b) { Goto L1, L2 L1: assume(a==b); g = 0; return; L2: assume(a!=b); g = 1; return; } int g; main(int x, int y){ cmp(x, y); assume(!g); assume(x != y) assert(0); } Global: (6) g: 0 main: (1)x: X (2)y: Y cmp : (3)a: A (4)b: B Conditions : (5)(A == B) [3, 4] (6)(X == Y) [5] Map : X  A Y  B

59. void cmp (int a, int b) { Goto L1, L2 L1: assume(a==b); g = 0; return; L2: assume(a!=b); g = 1; return; } int g; main(int x, int y){ cmp(x, y); assume(!g); assume(x != y) assert(0); } Global: (6) g: 0 main: (1)x: X (2)y: Y cmp : (3)a: A (4)b: B Conditions : (5)(A == B) [3, 4] (6)(X == Y) [5] (7) (X != Y) [1, 2] Contradictory!

60. void cmp (int a, int b) { Goto L1, L2 L1: assume(a==b); g = 0; return; L2: assume(a!=b); g = 1; return; } int g; main(int x, int y){ cmp(x, y); assume(!g); assume(x != y) assert(0); } Global: (6) g: 0 main: (1)x: X (2)y: Y cmp : (3)a: A (4)b: B Conditions : (5)(A == B) [3, 4] (6)(X == Y) [5] (7) (X != Y) [1, 2] Contradictory!

61. void cmp (int a, int b) { Goto L1, L2 L1: assume(a==b); g = 0; return; L2: assume(a!=b); g = 1; return; } int g; main(int x, int y){ cmp(x, y); assume(!g); assume(x != y) assert(0); } Predicates after simplification: { x == y, a == b }

62. Refinement Using Interpolants Given formulas  -,  + s.t.  -   + is unsatisfiable An interpolant for  -,  + is a formula  s.t. 1.  - implies  2.  has symbols common to  -,  + 3.    + is unsatisfiable  computable from proof of unsat. of  -   + [Henzinger-Jhala-Majumdar-McMillan POPL 04] give a way to generate predicates for refinement from infeasible traces using interpolants

63. Interpolants: Example (1) pc 1 : x = ctr pc 2 : ctr = ctr + 1 pc 3 : y = ctr pc 4 : assume(x = i-1) pc 5 : assume(y  i) TraceTrace Formula Cut + Interpolate at each point Pred. Map: pc i  Interpolant from cut i -- ++ Interpolate x 1 = ctr 0 Predicate Map pc 2 : x= ctr x 1 = ctr 0  ctr 1 = ctr 0 + 1  y 1 = ctr 1  x 1 = i 0 – 1  y 1  i 0

64. pc 1 : x = ctr pc 2 : ctr = ctr + 1 pc 3 : y = ctr pc 4 : assume(x = i-1) pc 5 : assume(y  i) TraceTrace Formula Cut + Interpolate at each point Pred. Map: pc i  Interpolant from cut i -- ++ Interpolate Predicate Map pc 2 : x = ctr pc 3 : x= ctr-1 x 1 = ctr 1 -1 x 1 = ctr 0  ctr 1 = ctr 0 + 1  y 1 = ctr 1  x 1 = i 0 – 1  y 1  i 0 Interpolants: Example (2)

65. -- ++ Interpolate  pc 1 : x = ctr pc 2 : ctr = ctr + 1 pc 3 : y = ctr pc 4 : assume(x = i-1) pc 5 : assume(y  i) TraceTrace Formula y 1 = x 1 + 1 x 1 = ctr 0  ctr 1 = ctr 0 + 1  y 1 = ctr 1  x 1 = i 0 – 1  y 1  i 0 Predicate Map pc 2 : x = ctr pc 3 : x= ctr-1 pc 3 : y= x+1 Cut + Interpolate at each point Pred. Map: pc i  Interpolant from cut i Interpolants: Example (3)

66. pc 1 : x = ctr pc 2 : ctr = ctr + 1 pc 3 : y = ctr pc 4 : assume(x = i-1) pc 5 : assume(y  i) TraceTrace Formula Cut + Interpolate at each point Pred. Map: pc i  Interpolant from cut i -- ++ Interpolate Predicate Map pc 2 : x = ctr pc 3 : x = ctr-1 pc 4 : y = x+1 pc 5 : y= i y 1 = i 0 x 1 = ctr 0  ctr 1 = ctr 0 + 1  y 1 = ctr 1  x 1 = i 0 – 1  y 1  i 0 Interpolants: Example (4)

67. pc 1 : x = ctr pc 2 : ctr = ctr + 1 pc 3 : y = ctr pc 4 : assume(x = i-1) pc 5 : assume(y  i) TraceTrace Formula Predicate Map pc 2 : x = ctr pc 3 : x = ctr-1 pc 4 : y = x+1 pc 5 : y = i Theorem: Predicate map makes trace abstractly infeasible x 1 = ctr 0  ctr 1 = ctr 0 + 1  y 1 = ctr 1  x 1 = i 0 – 1  y 1  i 0 Interpolants: Example (5)

68. Homework Weakest Precondition: WP( *p=3, x==5 ) = ?? What if *p and x alias? Statement in P:Predicates in E: *p = 3 {x==5}

69. Advanced Topics: Pointers

70. prog. P’ prog. P SLIC rule So far.. boolean program path predicates slic c2bp bebop newton

71. C-- Types  ::= void | bool | int Expressionse::=c | x | e 1 op e 2 LExpression l ::= x Declarationd::=  x 1,x 2,…,x n Statementss::=skip | goto L 1,L 2 …L n | L: s | assume(e) | l = e |f (e 1,e 2,…,e n ) |return |s 1 ; s 2 ;… ; s n Procedures p ::= f (x 1 :  1,x 2 :  2,…,x n :  n ) d s Programg ::= d 1 d 2 … d n p 1 p 2 … p n

72. C- Types  ::= void | bool | int | ref  Expressionse::=c | x | e 1 op e 2 | &x | *x LExpression l ::= x | *x Declarationd::=  x 1,x 2,…,x n Statementss::=skip | goto L 1,L 2 …L n | L: s | assume(e) | l = e | l = f (e 1,e 2,…,e n ) |return x |s 1 ; s 2 ;… ; s n Procedures p ::=  f (x 1 :  1,x 2 :  2,…,x n :  n ) d s Programg ::= d 1 d 2 … d n p 1 p 2 … p n

73. Abstracting Procedure Returns Let a be an actual at call-site P(…) –pre(a) = the value of a before transition to P Let f be a formal of a procedure P –pre(f) = the value of f upon entry to P

74. int R (int f) { int r = f+1; f = 0; return r; } WP(x=r, x==2) = r==2 Q() { int x = 1; x = R(x) } pre(f) == x x = r {x==1} {x==2} {f==pre(f)} {r==pre(f)+1} predicatecall/return relationcall/return assign {x==2} = s & {x==1}; } bool R ( {f==pre(f)} ) { {r==pre(f)+1} = {f==pre(f)}; {f==pre(f)} = *; return {r==pre(f)+1}; } WP(f=x, f==pre(f) ) = x==pre(f) f = x x==pre(f) is true at the call to R pre(f)==x and x==1 and r==pre(f)+1 implies r==2 Q() { {x==1},{x==2} = T,F; s = R(T); {x==1} s

75. int R (int f) { int r = f+1; f = 0; return r; } WP(x=r, x==2) = r==2 Q() { int x = 1; x = R(x) } pre(f) == x x = r {x==1} {x==2} {f==pre(f)} {r==pre(f)+1} predicatecall/return relationcall/return assign bool R ( {f==pre(f)} ) { {r==pre(f)+1} = {f==pre(f)}; {f==pre(f)} = *; return {r==pre(f)+1}; } WP(f=x, f==pre(f) ) = x==pre(f) f = x x==pre(f) is true at the call to R pre(f)==x and x==1 and r==pre(f)+1 implies r==2 Q() { {x==1},{x==2} = T,F; s = R(T); {x==1} s {x==1}, {x==2} = *, s & {x==1}; }

76. Pointers and SLAM With pointers, C supports call by reference –Strictly speaking, C supports only call by value –With pointers and the address-of operator, one can simulate call-by-reference Boolean programs support only call-by-value-result –SLAM mimics call-by-reference with call-by-value-result Extra complications: –address operator (&) in C –multiple levels of pointer dereference in C

77. What changes with pointers? C2bp –abstracting assignments –abstracting procedure returns Newton –simulation needs to handle pointer accesses –need to copy local heap across scopes to match Bebop’s semantics –need to refine imprecise alias analysis using predicates Bebop –remains unchanged!

78. Recall : Abstracting Assignments Suppose you are given an assignment s if Implies F (WP(s, e i )) is true before s then –e i is true after s if Implies F (WP(s, !e i )) is true before s then –e i is false after s {e i } = Implies F (WP(s, e i )) ?true : Implies F (WP(s, !e i )) ? false : *;

79. Assignments + Pointers Weakest Precondition: WP( *p=3, x==5 ) = x==5 What if *p and x alias? We use Das’s pointer analysis [PLDI 2000] to prune disjuncts representing infeasible alias scenarios. Correct Weakest Precondition: (p==&x and 3==5) or (p!=&x and x==5) Statement in P:Predicates in E: *p = 3 {x==5}

80. Abstracting Procedure Return Need to account for –lhs of procedure call –mixed predicates –side-effects of procedure Boolean programs support only call-by- value-result –C2bp models all side-effects using return processing

81. Extending Pre-states Suppose formal parameter is a pointer –eg. P(int *f) pre( *f ) –value of *f upon entry to P –can’t change during P * pre( f ) –value of dereference of pre( f ) –can change during P

82. int R (int *a) { *a = *a+1; } Q() { int x = 1; R(&x); } pre(a) = &x {x==1} {x==2} predicatecall/return relationcall/return assign {x==2} = s & {x==1}; } bool R ( {a==pre(a)}, {*pre(a)==pre(*a)} ) { {*pre(a)==pre(*a)+1} = {*pre(a)==pre(*a)} ; return {*pre(a)==pre(*a)+1}; } a = &x Q() { {x==1},{x==2} = T,F; s = R(T,T); {a==pre(a)} {*pre(a)==pre(*a)} {*pre(a)=pre(*a)+1} pre(x)==1 and pre(*a)==pre(x) and *pre(a)==pre(*a)+1 and pre(a)==&x implies x==2 {x==1 } s

83. Newton: what changes with pointers? Simulation needs to handle pointer accesses Need to copy local heap across scopes to match Bebop’s semantics

84. void foo (int *a) { assume(*a > 5); assert(0); } main(int *x){ assume(*x < 5); foo(x); }

85. Contradictory! void foo (int *a) { assume(*a > 5); assert(0); } main(int *x){ assume(*x < 5); foo(x); } main: (1) x: X (2)*X: Y [1] foo : (3) a: A (4)*A: B [3] Conditions : (5)(Y< 5) [1,2] (6)(B < 5) [3,4,5] (7)(B > 5) [3,4] Predicates after simplification: *x 5

86. prog. P’ prog. P SLIC rule Non progress.. boolean program path predicates slic c2bp bebop newton

87. Non-progress due to alias analysis imprecision x = 0; y = 0; p = &y *p = *p + 1; assume(x!=0); p = &x; {x==0} {x==0} = T; skip; {x==0} = *; assume( !{x==0} ); skip; Pts-to(p) = { &x, &y}

88. x = 0; y = 0; p = &y if (p == &x) x = x + 1; else y = y + 1; assume(x!=0); p = &x; x = 0; y = 0; p = &y *p = *p + 1; assume(x!=0); p = &x; {x==0} = T; skip; {x==0} = *; assume(!{x==0} ); skip; Non-progress due to alias analysis imprecision

89. Consider values in abstract trace x = 0; y = 0; p = &y *p = *p + 1; assume(x!=0); p = &x; {x==0} = T; skip; {x==0} = *; assume(!{x==0} ); skip; {x==0} is true {x==0} is false INCONSISTENT

90. Consider values in abstract trace x = 0; y = 0; p = &y *p = *p + 1; assume(x!=0); p = &x; {x==0} = T; skip; {x==0} = *; assume(!{x==0} ); skip; WP(*p = *p +1, !(x==0) ) = ((p == &x) and !(x==-1)) or ((p != &x) and !(x==0)) (p!=&x) gets added as a predicate

91. Generalization of refinement predicates main() { int x; x = 0; while(*) { x++; } assert(x >= 0); } Iterative refinement produces predicates: (x == 0), (x == 1), (x ==2) … Need to generate (x >= 0) automatically! Difficult to come up with general methods to do such generalizations

92. Areas for future work Abstraction-refinement loop with richer models (than just booleans) for solving the progress problem with pointers Combination of over-approximation and under- approximation based methods to avoid refining in irrelevant places in the program