A Certifying Compiler and Pointer Logic Zhaopeng Li Software Security Lab. Department of Computer Science and Technology, University of Science and Technology of China October 8, 2008 Towards Building Trusted Software

Software Security Lab, USTC2 Outline Motivation Research Goals Our Work A Certifying Compiler PointerC Language Pointer Logic Summary Future Work

Software Security Lab, USTC3 Motivation Software Safety Problems C language Widely used & legacy C codes Not easy to write a safe code with pointers One Solution : Program Verification Program + Annotation + Proof

Software Security Lab, USTC4 Motivation (cont.) Hoare Logic Hoare triple : {P}C{Q} Hard to reason pointer programs Separation Logic Low-level code, or high-level code with restriction Separation Conjunction (P*Q) Example: A Hoare-like Logic for C Language? p l1l1

Software Security Lab, USTC5 Research Goals Verification for C pointer programs Design a C-like language Design a logic Design a certifying compiler Generate codes with proof Minimize Trusted Computing Base

Software Security Lab, USTC6 Outline Motivation Research Goals Our Work A Certifying Compiler PointerC Language Pointer Logic Summary Future Work

Software Security Lab, USTC7 Our Certifying Compiler Source-level Certifying System Pointer Logic VCGen Prover Code Compiler Proof Compiler Certifying Compiler Source Code + Specifications Source Code + Spec. + Proof Assem. Code + Assem. Spec. + Assem. Proof PointerC Language

Software Security Lab, USTC8 Our Certifying Compiler (cont.) Prototype plcc ver1.0 ( ) plcc ver2.0 ( ) Improvements Build-in theorem prover Support limited pointer arithmetic Support more data structures Doubly-linked list

Software Security Lab, USTC9 Supported Programs Singly-linked/doubly-linked list traversal/reversal delete/insert create/clear Binary Tree traversal/rotate delete/insert

Software Security Lab, USTC10 Evaluation

Software Security Lab, USTC11 PointerC Language PointerC: A subset of C language with pointer type Memory management : malloc/free Main Constrains: Pointer Arithmetic is limited No union type No type cast …

Software Security Lab, USTC12 Pointer Logic Motivation PointerC typing rules with side conditions A logic proof system is needed Reason about source programs with complex pointer aliasing Why not separation logic?   p : ptr(struct (…, x: int; …))   p -> x : int ( valid(p) )

Software Security Lab, USTC13 Pointer Logic (cont.) Why not separation logic? p … q = p->next; p->next = p->next->next; free(q); … List_delete.c … q = p->next; t = q->next; p->next = t; free(q); … List_delete_trans.c No Rule for this kind of statement! t q No rule for aliasing inference! NULL struct List{ int data; struct list* next;}

Software Security Lab, USTC14 Basic ideas Precise pointer information collection At each program point Pointer classification Valid pointer set Null pointer set Dangling pointer set Equality between valid pointers

Software Security Lab, USTC15 Specification The information is concise ! Pointer Information

Software Security Lab, USTC16 Specification (cont.) Compare with separation logic Access path is short Low-level address is used in assertion Addresses are used to associate different heaps

Software Security Lab, USTC17 Expressivity Current Application Singly-linked list Doubly-linked list Binary Tree Graph? Equality between pointers is not certain Unable to be expressed in current pointer logic Not well-supported in separation logic either

Software Security Lab, USTC18 Expressivity (cont.) Singly-linked list Flat version Inductive version Singly-linked list from separation logic Flat version p nil p,l 1,l 2, …,l n-1 are distinct!

Software Security Lab, USTC19 Inference Rule Hoare-logic-like rules {P}C{Q} Extend Hoare Logic Calculate pointer information Q using P

Software Security Lab, USTC20 Memory Leak p NULL Pointer Logic Separtion Logic Memory Leak! No rules for this case! Assignment Axiom of Hoare Logic! must using precise assertion to rule out this case!

Software Security Lab, USTC21 Comparison with Separation Logic Common features: Extension of Hoare logic Deal with pointer programs Differences: High-level vs low-level Pointer logic can deal with long access paths Precise information vs information hiding Rule out memory leak via different means

Software Security Lab, USTC22 Outline Motivation Research Goals Our Work A Certifying Compiler PointerC Language Pointer Logic Summary Future Work

Software Security Lab, USTC23 Summary A Certifying Compiler Theorem prover for pointer logic Generate codes with proof A Pointer Logic Verification for PointerC pointer programs Hoare-logic-like rules Compare with separation logic

Software Security Lab, USTC24 Future Work PointerC Language Extension More language features Unlimited pointer arithmetic Pointer Logic Extension Deal more data structures, such as DAG Pointer Logic for Java (static garbage detection etc.) Concurrent programming Realistic Certifying Compiler Verify some codes of mini-OS

Software Security Lab, USTC25 Thanks! Questions?