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The model checking approach

- Create a model of the program in a decidable formalism
- Verify the model algorithmically
- Difficulties
- Model creation is burden on programmer
- The model might be incorrect.
- If verification fails, is the problem in the model or the program?

The axiomatic approach

- Add auxiliary specifications to the program to decompose the verification task into a set of local verification tasks
- Verify each local verification problem
- Difficulties
- Auxiliary spec is burden on programmer
- Auxiliary spec might be incorrect.
- If verification fails, is the problem with the auxiliary specification or the program?

Program

VC

generation

Provability

(theorem proving)

Validity

Specification

Theorem

in a logic

Theorem Proving and Software- Soundness:
- If the theorem is valid then the program meets specification
- If the theorem is provable then it is valid

Meets spec/Found Bug

Overview of the Next Few Lectures

- From programs to theorems
- Verification condition generation
- From theorems to proofs
- Theorem provers
- Decision procedures

Programs ! Theorems = Axiomatic Semantics

- Consists of:
- A language for making assertions about programs
- Rules for establishing when assertions hold
- Typical assertions:
- During the execution, only non-null pointers are dereferenced
- This program terminates with x = 0
- Partial vs. total correctness assertions
- Safety vs. liveness properties
- Usually focus on safety (partial correctness)

Hoare Triples

- Partial correctness: { A } s { B}
- When you start s in any state that satisfies A
- Ifthe execution of s terminates
- It does so in a state that satisfies B
- Total correctness: [ A ] s [ B ]
- When you start s in any state that satisfies A
- The execution of s terminates and
- It does so in a state that satisfies B
- Defined inductively on the structure of statements

Hoare Rules: Assignment

- Example: { A } x := x + 2 {x >= 5 }. What is A?
- General rule:

- Surprising how simple the rule is !
- The key is to compute “backwards” the precondition from the postcondition
- Forward rule is more complicated:

Weakest preconditions

- Dijkstra’s idea: To verify that { A } s { B}
- Let Pre(s, B) = {A’ | { A’} s { B} }
- b) (Pre(s,B), )) is a lattice
- - false Pre(s,B)
- - if Pre(s,B) and Pre(s,B), then Pre(s,B)
- - if Pre(s,B) and Pre(s,B), then Pre(s,B)
- c) WP(s, B) = lub(Pre(s, B))
- d) Compute WP(s, B) and prove A WP(s, B)

Weakest preconditions

- WP(x := E, B) = B[E/x]
- WP(s1; s2, B) = WP(s1, WP(s2, B))
- WP(if E then s1 else s2, B) = E ) WP(s1, B) Æ: E ) WP(s2, B)
- WP(assert E, B) = E B

returns c

requires true

ensures c = a b

bool or(bool a, bool b) {

if (a)

c := true

else

c := b

}

S

WP(S, c = a b) = (a true = a b) (a b = a b)

Conjecture to be proved:

true (a true = a b) (a b = a b)

Weakest preconditions (Contd.)

- What about loops?
- Define a family of weakest preconditions
- WPk(while E do S, B) = weakest precondition from which if the loop terminates in k or fewer iterations, then it terminates in B

WP0 = : E ) B

WPi+1 = WPi (E ) WP(s, WPi))

- WP(while E do S, B) = Æk ¸ 0 WPk = glb {WPk | k ¸ 0}
- Hard to compute
- Can we find something easier yet sufficient ?

condition: VC(s, B)

Not quite weakest preconditions- Recall what we are trying to do:

)

false

true

Pre(s, B)

weak

strong

weakest

precondition: WP(s, B)

A

- We shall construct a verification condition: VC(s, B)
- The loops are annotated with loop invariants !
- VC is guaranteed stronger than WP
- But hopefully still weaker than A: A ) VC(s, B) ) WP(s, B)

Verification condition generation

- Computed in a manner similar to WP
- Except the rule for while:

VC(whileI,T E do s, B) =

I Æ ( I ((E ) VC(s, I)) Æ (:E ) B)) )[T0/T]

- I is the loop invariant (provided by the programmer)
- T is the set of loop targets (can be approximated by scanning the loop body)

I is preserved in

an arbitrary iteration

I holds

on entry

B holds when the

loop terminates

VC(B, t 0 c = a + b - t)

t - 1 0 c + 1 = a + b – (t – 1)

returns c

requires b >= 0

ensures c = a + b

int add(int a, int b) {

int t;

t := b

c := a

invariant

t 0 c = a + b - t

while (t > 0) {

c := c + 1

t := t - 1

}

}

VC(L, c = a + b)

t 0 c = a + b – t

(t 0 c = a + b – t

t > 0 t - 1 0

c + 1 = a + b – (t - 1)

t 0 c = a + b)[c0/c,t0/t]

VC(L, c = a + b)

t 0 c = a + b – t

(t0 0 c0 = a + b – t0

t0 > 0 t0 - 1 0

c0 + 1 = a + b – (t0 - 1)

t0 0 c0 = a + b)

A

L

B

VC(A, c = a + b)

b 0 a = a + b – b

(t0 0 c0 = a + b – t0

t0 > 0 t0 - 1 0

c0 + 1 = a + b – (t0 - 1)

t0 0 c0 = a + b)

Conjecture to be proved:

b 0 VC(A, c = a + b)

Invariants Are Not Easy

- Consider the following code from QuickSort

int partition(int *a, int L0, int H0, int pivot) {

int L = L0, H = H0;

while(L < H) {

while(a[L] < pivot) L ++;

while(a[H] > pivot) H --;

if(L < H) { swap a[L] and a[H] }

L++;

}

return L

}

- Consider verifying only memory safety
- What is the loop invariant for the outer loop ?

Verification conditions (Contd.)

- What about procedure calls?
- Annotate each procedure with a precondition pre and a postcondition post
- VC(f(), B) = pre (post B)

Program verification

- Annotate each procedure with a precondition and a postcondition and each loop with an invariant
- Verify each procedure separately

requires pre

ensures post

f() {

S;

}

Verify that the formula

VC(f) pre VC(S, post)

is valid

Proving verification conditions

- What is the decision procedure for proving validity of VC(f)?
- Depends on the logic in which VC(f) is expressed

VC(f) pre VC(S, post)

Verification condition logic

VC(f) pre VC(S, post)

- Atoms connected by boolean operators
- , , ,
- Atoms depend on the program variables and operations on them
- boolean, integer, memory
- Atoms depend on the language of assertions, i.e., program assertions, loop invariants, preconditions and postconditions
- quantification, reachability predicate

Assume each assertion is a quantifier-free boolean

combination of expressions over program variables.

- VC(f) is a boolean combination of atoms
- Each atom is a relation over terms
- Each term is built using functions and logical constants
- Logical constants are different from program variables
- program variables change over time
- logical constants are fixed
- The logical constants in VC(f) refer to the values of program variables at the beginning of f.

Case I: Boolean programs

- Boolean-valued variables and boolean operations

- Formula := A | |

A Atom := b

b SymBoolConst

returns c

requires true

ensures c = a b

bool or(bool a, bool b) {

if (a)

c := true

else

c := b

}

S

VC(S, c = a b) = (a true = a b) (a b = a b)

Conjecture to be proved:

true (a true = a b) (a b = a b)

Case II: Arithmetic programs

- In addition, integer-valued variables with affine operations

- Formula := A | |

A Atom := b | t = 0 | t < 0 | t 0

t Term := c | x | t + t | t – t | ct

b SymBoolConst

x SymIntConst

c {…,-1,0,1,…}

VC(B, t 0 c = a + b - t)

t - 1 0 c + 1 = a + b – (t – 1)

returns c

requires b >= 0

ensures c = a + b

int add(int a, int b) {

int t;

t := b

c := a

invariant

t 0 c = a + b - t

while (t > 0) {

c := c + 1

t := t - 1

}

}

VC(L, c = a + b)

t 0 c = a + b – t

(t 0 c = a + b – t

t > 0 t - 1 0

c + 1 = a + b – (t - 1)

t 0 c = a + b)[c0/c,t0/t]

VC(L, c = a + b)

t 0 c = a + b – t

(t0 0 c0 = a + b – t0

t0 > 0 t0 - 1 0

c0 + 1 = a + b – (t0 - 1)

t0 0 c0 = a + b)

A

L

B

VC(A, c = a + b)

b 0 a = a + b – b

(t0 0 c0 = a + b – t0

t0 > 0 t0 - 1 0

c0 + 1 = a + b – (t0 - 1)

t0 0 c0 = a + b)

Conjecture to be proved:

b 0 VC(A, c = a + b)

Case III: Memory programs

- In addition, a memory with read and write operations
- an unbounded set of objects
- a finite set of fields in each object
- each field contains a boolean value, an integer value, or a reference to an object
- For each field f, two operations Select and Update
- Select(f,o) is the content of the memory at object o and field f
- Update(f,o,v) is a new memory obtained by updating field f of object o to v

Memory axioms

for all objects o and o’, and memories m:

o = o’ Select(Update(m,o,v),o’) = v

o o’ Select(Update(m,o,v),o’) = Select(m,o’)

Modeling memory operations

Treat each field f as a map variable:

a = b.f a = Select(f,b)

a.f = b f = Update(f,a,b)

{ ? } a.f = 5 { a.f + b.f = 10 }

WP(a.f = 5, a.f + b.f = 10)

WP(f = Update(f,a,5), Select(f,a) + Select(f,b) = 10)

Select(Update(f,a,5),a) + Select(Update(f,a,5),b) = 10

Simplify using memory axiom

Select(Update(f,a,5),a) + Select(Update(f,a,5),b) = 10

iff

5 + Select(Update(f,a,5),b) = 10

iff

Select(Update(f,a,5),b) = 5

iff

a = b 5 = 5

a b Select(f,b) = 5

iff

a b Select(f,b) = 5

A Atom := b | t = 0 | t < 0 | t 0

t Term := c | x | t + t | t – t | ct | Select(m,t)

m MemTerm := f | Update(m,t,t)

b SymBoolConst

x SymIntConst

c {…,-1,0,1,…}

Decision procedures

- Boolean programs
- Propositional satisfiability
- Arithmetic programs
- Propositional satisfiability modulo theory of linear arithmetic
- Memory programs
- Propositional satisfiability modulo theory of linear arithmetic + arrays

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