A Casual Introduction to Abstract Interpretation Examples of Abstractions
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CMACS Workshop on Systems Biology and Formals Methods (SBFM'12) Abstractions of Dora Maar by Picasso A casual introduction to Abstract Interpretation NYU, 29–30 March 2012 Patrick Cousot cs.nyu.edu/~pcousot di.ens.fr/~cousot CMACS Workshop on Systems Biology and Formals Methods (SBFM'12), NYU, 29–30 March 2012 1 © P. Cousot CMACS Workshop on Systems Biology and Formals Methods (SBFM'12), NYU, 29–30 March 2012 3 © P. Cousot Pixelation of a photo by Jay Maisel Examples of Abstractions www.petapixel.com/2011/06/23/how-much-pixelation-is-needed-before-a-photo-becomes-transformed/ P. Cousot & R. Cousot. A gentle introduction to formal verification of computer systems by abstract interpretation. In Logics and Languages for Reliability and Security, J. Esparza, O. Grumberg, & Image credit: Photograph by Jay Maisel M. Broy (Eds), NATO Science Series III: Computer and Systems Sciences, © IOS Press, 2010, Pages 1—29. CMACS Workshop on Systems Biology and Formals Methods (SBFM'12), NYU, 29–30 March 2012 2 © P. Cousot CMACS Workshop on Systems Biology and Formals Methods (SBFM'12), NYU, 29–30 March 2012 4 © P. Cousot II.P. Combination of abstract domains Abstract interpretation-based tools usually use several different abstract domains, since the design of a complex one is best decomposed into a combination of simpler abstract domains. Here are a few abstract 2 An old idea... domain examples used inNumerical the Astree´ static abstractions analyzer: in Astrée y y y 20 000 years old picture in a spanish cave: x x x Collecting semantics:1, 5 Intervals:20 Simple congruences:24 partial traces x [a, b] x a[b] ⌃ ⌅ y y y t x x The concrete is not always well-known! Octagons:25 Ellipses:26 Exponentials:27 x y a x2 + by2 axy d abt y(t) abt ± ± ⇥ − ⇥ − ⇥ ⇥ CMACS Workshop on Systems Biology and Formals Methods (SBFM'12), NYU, 29–30 March 2012 5 © P. Cousot NFM 2012 — 4th NASA Formal Methods Symposium — Norfolk, VA, April 3–5, 2012 7 © P Cousot Such abstract domains (and more) are described in more details in Sects. III.H–III.I. The following classic abstract domains, however, are not used in Astree´ because they are either too imprecise, not scalable, di⌅cult to implement correctly (for instance, soundness may be an issue in the event of floating-point rounding), or out of scope (determining program properties which are usually of no interest Abstractions of a man / crowd to prove the specification): y y y ... x x x Height Individual heights A slightly more detailled Polyhedra:9 Signs:7 Linear congruences:28 too costly too imprecise out of scope Fingerprint ... example Because abstract domains do not use a uniform machine representation of the information they manip- ulate, combining them is not completely trivial. The conjunction of abstract program properties has to be performed, ideally, by a reduced product 7 for Galois connection abstractions. In absence of a Galois connec- Eye color min, max tion or for performance reasons, the conjunction is performed using an easily computable but not optimal over-approximation of this combination of abstract domains. Assume that we have designed several abstract domains and compute lfp⊆ F D ,...,lfp⊆ F D 1 ⌃ 1 n ⌃ n in these abstract domains D1,...,Dn, relative to a collecting semantics t I. The combination of these DNA , C analyses is sound as t I γ1(lfp⊆ F1) γn(lfp⊆ Fn). However, only combining the analysis results is CMACS Workshop on Systems Biology and Formals Methods (SBFM'12), NYU, 29–30 March 2012 6 © P. Cousot CMACS Workshop Con Systems Biology⇧ and Formals Methods (SBFM'12),∩ ··· NYU,∩ 29–30 March 2012 8 © P. Cousot not very precise, as it does not permit analyses to improve each other duringJ theK computation. Consider, for instance, that intervalJ andK parity analyses find respectively that x [0, 100] and x is odd at some iteration. Combining the results would enable the interval analysis to continue⌃ with the interval x [1, 99] and, e.g., avoid a useless widening. This is not possible with analyses carried out independently. ⌃ Combining the analyses by a reduced product, the proof becomes “let F ( x1,...,xn ) ⇥( F1(x1),..., ⌦ Fn(xn ) and r1,...,rn =lfp⊆ F in t I γ1(r1) γn(rn)” where ⇥ performs the reduction between abstract⌦ domains. For example⌦ ⇥( [0,C100], odd⇧ )= ∩[1···, 99]∩, odd . ⌦ ⌦ J K 10 of 38 American Institute of Aeronautics and Astronautics Set of functions abstraction A less precise abstraction fi(t) f(t) i=4 i=3 i=2 i=1 i=0 t t How to approximate { f1, f 2, f 3, f 4 } ? CMACS Workshop on Systems Biology and Formals Methods (SBFM'12), NYU, 29–30 March 2012 9 © P. Cousot CMACS Workshop on Systems Biology and Formals Methods (SBFM'12), NYU, 29–30 March 2012 11 © P. Cousot Set of functions abstraction Concrete questions answeredon the fi in the abstract f(t) f(t) l h ∃ i, t ∈ [l,h] : fi(t) > M ? M m t t ∃ i, t ∈ [l, h]: fi(t) < m ? No Min/max questions on the fi CMACS Workshop on Systems Biology and Formals Methods (SBFM'12), NYU, 29–30 March 2012 10 © P. Cousot CMACS Workshop on Systems Biology and Formals Methods (SBFM'12), NYU, 29–30 March 2012 12 © P. Cousot Concrete questions answered in the abstract A more precise/refined abstraction f(t) l h f(t) i, t ∈ [l,h]: fi(t) > M ? I don’t know M ∃ m t t ∃ i, t ∈ [l,h] : fi(t) < m ? No Min/max questions on the fi CMACS Workshop on Systems Biology and Formals Methods (SBFM'12), NYU, 29–30 March 2012 13 © P. Cousot CMACS Workshop on Systems Biology and Formals Methods (SBFM'12), NYU, 29–30 March 2012 15 © P. Cousot Soundness of the abstraction An even more precise/refined abstraction • No concrete case is ever forgotten: f(t) f(t) t t CMACS Workshop on Systems Biology and Formals Methods (SBFM'12), NYU, 29–30 March 2012 14 © P. Cousot CMACS Workshop on Systems Biology and Formals Methods (SBFM'12), NYU, 29–30 March 2012 16 © P. Cousot Passing to the limit The hierarchy of abstractions f(t) t Sound and complete abstraction for min/max questions on the fi CMACS Workshop on Systems Biology and Formals Methods (SBFM'12), NYU, 29–30 March 2012 17 © P. Cousot CMACS Workshop on Systems Biology and Formals Methods (SBFM'12), NYU, 29–30 March 2012 19 © P. Cousot A non-comparable abstraction f(t) Elements of Abstract Interpretation Theory Explained with ... Flowers Patrick Cousot & Radhia Cousot. Static Determination of Dynamic Properties of Programs. In B. Robinet, editor, Proceedings of the second international symposium on Programming, Paris, France, pages 106—130, April 13-15 1976, Dunod, Paris. t Patrick Cousot, Radhia Cousot: Abstract Interpretation: A Unified Lattice Model for Static Analysis of Programs by Construction or Approximation of Fixpoints. POPL 1977: 238-252 Patrick Cousot, Radhia Cousot: Systematic Design of Program Analysis Frameworks. POPL 1979: 269-282 Patrick Cousot. Méthodes itératives de construction et d'approximation de points fixes d'opérateurs monotones sur un treillis, analyse sémantique des programmes. Thèse És Sciences Sound and incomplete abstraction for min/max questions on Mathématiques, Université Joseph Fourier, Grenoble, France, 21 March 1978 Patrick Cousot. Semantic foundations of program analysis. In S.S. Muchnick & N.D. Jones, editors, Program Flow Analysis: Theory and Applications, Ch. 10, pages 303—342, Prentice- the fi Hall, Inc., Englewood Cliffs, New Jersey, U.S.A., 1981. CMACS Workshop on Systems Biology and Formals Methods (SBFM'12), NYU, 29–30 March 2012 1811 © P. Cousot CMACS Workshop on Systems Biology and Formals Methods (SBFM'12), NYU, 29–30 March 2012 20 © P. Cousot A mini graphical language • Objects o ∈ O Elements of Abstract • Operations on objects On O, n ≥ 0 Interpretation Theory • Logical operations on objects On Booleans, n ≥ 0 Explained with ... Flowers Patrick Cousot & Radhia Cousot. Static Determination of Dynamic Properties of Programs. In B. Robinet, editor, Proceedings of the second international symposium on Programming, Paris, France, pages 106—130, April 13-15 1976, Dunod, Paris. Patrick Cousot, Radhia Cousot: Abstract Interpretation: A Unified Lattice Model for Static Analysis of Programs by Construction or Approximation of Fixpoints. POPL 1977: 238-252 Patrick Cousot, Radhia Cousot: Systematic Design of Program Analysis Frameworks. POPL 1979: 269-282 Patrick Cousot. Méthodes itératives de construction et d'approximation de points fixes d'opérateurs monotones sur un treillis, analyse sémantique des programmes. Thèse És Sciences Mathématiques, Université Joseph Fourier, Grenoble, France, 21 March 1978 Patrick Cousot. Semantic foundations of program analysis. In S.S. Muchnick & N.D. Jones, editors, Program Flow Analysis: Theory and Applications, Ch. 10, pages 303—342, Prentice- Hall, Inc., Englewood Cliffs, New Jersey, U.S.A., 1981. CMACS Workshop on Systems Biology and Formals Methods (SBFM'12), NYU, 29–30 March 2012 21 © P. Cousot CMACS Workshop on Systems Biology and Formals Methods (SBFM'12), NYU, 29–30 March 2012 23 © P. Cousot Objects • An object o ∈ O is defined by • An origin (a reference point ) • A set of (infinitely small) black pixels (on a white The concrete world background) • Example I of object: CMACS Workshop on Systems Biology and Formals Methods (SBFM'12), NYU, 29–30 March 2012 22 © P. Cousot CMACS Workshop on Systems Biology and Formals Methods (SBFM'12), NYU, 29–30 March 2012 24 © P. Cousot An example II of object: a flower Constant objects • A petal is an example of constant object petal = CMACS Workshop on Systems Biology and Formals Methods (SBFM'12), NYU, 29–30 March 2012 25 © P.