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CREST Internal

CREST Internal. Yunho Kim Provable Software Laboratory CS Dept. KAIST. CREST. CREST is a concolic testing tool for C programs Generate test inputs automatically Execute target under test on generated test inputs Explore all possible execution paths of a target systematically

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CREST Internal

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  1. CREST Internal Yunho Kim Provable Software Laboratory CS Dept. KAIST

  2. CREST • CREST is a concolic testing tool for C programs • Generate test inputs automatically • Execute target under test on generated test inputs • Explore all possible execution paths of a target systematically • CREST is a open-source re-implementation of CUTE • mainly written in C++ • CREST’s instrumentation is implemented as a module of CIL(C Intermetiate Language) written in Ocaml

  3. Overview of CREST code C source code CIL EXT cil/src/ext/crestInstrument.ml src/libcrest/crest.cc src/base/symbolic_interpreter.cc src/base/symbolic_execution.cc src/base/symbolic_expression.cc src/base/symbolic_path.cc src/base/symbolic_predicate.cc Instrumented code CREST symbolic execution library Legend Source code GCC External tool src/run_crest/run_crest.cc src/run_crest/concolic_search.cc src/base/yices_solver.cc src/base/symbolic_execution.cc src/base/symbolic_expression.cc src/base/symbolic_path.cc src/base/symbolic_predicate.cc src/base/basic_types.cc CREST constraint yices run_crest next input

  4. Directory Structure • src/ base/ libcrest/ process_cfg/ run_crest/ tools/ • cil/src/ext/crestInstrument.ml • A CIL module for instrumentation • : Base libraries for symbolic execution • : Probe code for collecting symbolic states • : CFG generator for CFG-based search heuristic • : Main function of run_crest and search algorithms • : A tool for printing execution path from szd_execution

  5. CREST Code Metrics

  6. Symbolic Execution Component • Symbolic execution component collects symbolic states during concrete execution and manages symbolic execution paths • Related files

  7. Symbolic Interpreter • Symbolic interpreter performs dynamic symbolic execution during execution of a target program • Symbolic interpreter implements a symbolic machine which has stack-architecture • 4 types of statements • Symbolic variable initialization • Assignments • Applying operators • Branches

  8. Symbolic Machine • Symbolic machine has a symbolic stack, symbolic memory and a symbolic predicate register • Symbolic memory stores symbolic expressions • Symbolic stack element: <symbolic expr, concrete value> • If the top of the stack is a predicate, the predicate is stored in the symbolic predicate register Symbolic predicate register Symbolic stack Symbolic memory

  9. Example Revisited • int a, b, c; • #line 4 /* Initializes symbolic variables a, b, c */ • __CrestInt(& a); __CrestInt(& b); __CrestInt(& c); • … omitted … #line 10 { /* Creates symbolic expression a==b */ __CrestLoad(36, (unsigned long )(& a), (long long )a); __CrestLoad(35, (unsigned long )(& b), (long long )b); __CrestApply2(34, 12, (long long )(a == b)); if (a == b) { • //extern void __CrestBranch(int id , int bid , unsigned char b ) __CrestBranch(37, 11, 1); • /* Creates symbolic expression match = match = 1; */ __CrestLoad(41, (unsigned long )(& match), (long long )match); __CrestLoad(40, (unsigned long )0, (long long )1); __CrestApply2(39, 0, (long long )(match + 1)); __CrestStore(42, (unsigned long )(& match)); match ++; } else { __CrestBranch(38, 12, 0); } } 1 #include <crest.h> 2 main() { 3 inta,b,c, match=0; 4 CREST_int(a); \ CREST_int(b); \ CREST_int(c); 5~9 … omitted… 10 if(a==b) match=match+1; 10~32 … omitted … 33 }

  10. Symbolic Variable Initialization • Creates a symbolic memory element in symbolic memory • A concrete address of a variable is used as a symbolic address • Suppose that we start with the input a = b = c = 0; Symbolic memory • Symbolic variable initialization • int a, b, c; • #line 4 /* Initializes symbolic variables a, b, c */ • __CrestInt(& a); __CrestInt(& b); __CrestInt(& c); Symbolic stack Symbolic predicate register

  11. Symbolic Compare Operator(1/4) • Symbolic compare operator is used for a branch condition and results in a symbolic predicate • The predicate is store in a symbolic predicate register Symbolic memory Symbolic stack • #line 10 • { /* Creates symbolic expression a==b */ • __CrestLoad(36, (unsigned long)(&a), • (long long )a); • __CrestLoad(35, (unsigned long)(&b), • (long long )b); • __CrestApply2(34, 12, (long long )(a == b)); • if (a == b) { Symbolic PC Symbolic predicate register

  12. Symbolic Compare Operator(2/4) • __CrestLoad(int id, unsigned long *ptr, long longval) function loads a symbolic expression which ptr points to and pushes <loaded expr, val> to the stack • If *ptr is a concrete variable, the function pushes <NULL, val> to the stack Symbolic memory Symbolic stack • #line 10 • { /* Creates symbolic expression a==b */ • __CrestLoad(36, (unsigned long)(&a), • (long long )a); • __CrestLoad(35, (unsigned long)(&b), • (long long )b); • __CrestApply2(34, 12, (long long )(a == b)); • if (a == b) { Symbolic PC Symbolic predicate register <a, 0>

  13. Symbolic Compare Operator(3/4) Symbolic memory Symbolic stack • #line 10 • { /* Creates symbolic expression a==b */ • __CrestLoad(36, (unsigned long)(&a), • (long long )a); • __CrestLoad(35, (unsigned long)(&b), • (long long )b); • __CrestApply2(34, 12, (long long )(a == b)); • if (a == b) { <b, 0> Symbolic PC Symbolic predicate register <a, 0>

  14. Symbolic Compare Operator(4/4) • __CrestApply2(int ID, intop_type, long longval) 1. pops two elements from the stack, 2. applies a binary operator corresponding to op_type to the popped elements, 3. pushes a result to the stack if the result is not a predicate • A predicate is stored in the register Symbolic memory Symbolic stack • #line 10 • { /* Creates symbolic expression a==b */ • __CrestLoad(36, (unsigned long)(&a), • (long long )a); • __CrestLoad(35, (unsigned long)(&b), • (long long )b); • __CrestApply2(34, 12, (long long )(a == b)); • if (a == b) {//extern void __CrestBranch(int id , int bid , unsigned char b ) • __CrestBranch(37, 11, 1); Symbolic predicate register Symbolic PC <a==b, 1>

  15. Symbolic Branch(1/2) • Whenever a branch statement is executed, CREST stores which branch is taken by calling __CrestBranch() function. Symbolic stack Symbolic memory • #line 10 • { /* Creates symbolic expression a==b */ • __CrestLoad(36, (unsigned long)(&a), • (long long )a); • __CrestLoad(35, (unsigned long)(&b), • (long long )b); • __CrestApply2(34, 12, (long long )(a == b)); • if (a == b) { • //extern void __CrestBranch(int id , int bid , unsigned char b ) • __CrestBranch(37, 11, 1); Symbolic predicate register <a==b, 1> Symbolic PC

  16. Symbolic Branch(2/2) • Symbolic path is a sequence of <symbolic pred, branch ID> • __CrestBranch(int id, int bid, unsigned char b) function appends a new element <symbolic pred, bid> to the current symbolic path • Symbolic pred comes from the register • If b == 0, negated predicate is appended Symbolic memory Symbolic stack • if (a == b) { • //extern void __CrestBranch(int id , int bid , unsigned char b ) • __CrestBranch(37, 11, 1); • /* Creates symbolic expression match = match = 1; */ • __CrestLoad(41, (unsigned long )(& match), (long long )match); Symbolic PC Symbolic predicate register Symbolic path: <a==b, 11>

  17. Symbolic Arithmetic Operator (1/2) • Symbolic arithmetic operator is similar to symbolic compare operator • Pops operands from the stack, applies operator to the operands, and pushes the result to the stack • if (a == b) { • __CrestBranch(37, 11, 1); • /* Creates symbolic expression match = match = 1; */ • __CrestLoad(41, (unsigned long )(& match), (long long )match); • __CrestLoad(40, (unsigned long )0, • (long long )1); • __CrestApply2(39, 0, (long long )(match + 1)); • __CrestStore(42, (unsigned long )(& match)); • match ++; Symbolic memory Symbolic stack Symbolic PC <NULL, 1> Symbolic predicate register <NULL, 0> Symbolic path: <a==b, 11>

  18. Symbolic Arithmetic Operator (2/2) • If at least one of operands is symbolic, the result is also symbolic • Otherwise, the result is concrete • if (a == b) { • __CrestBranch(37, 11, 1); • /* Creates symbolic expression match = match = 1; */ • __CrestLoad(41, (unsigned long )(& match), (long long )match); • __CrestLoad(40, (unsigned long )0, • (long long )1); • __CrestApply2(39, 0, (long long )(match + 1)); • __CrestStore(42, (unsigned long )(& match)); • match ++; Symbolic memory Symbolic stack Symbolic predicate register <NULL, 2> Symbolic PC Symbolic path: <a==b, 11>

  19. Symbolic Assignment (1/1) • __CrestStore(int id, unsigned long *ptr) function pops one element from the stack and update symbolic memory • If the popped element is concrete, just ignore it • If the element is symbolic • If ptr has an entry in symbolic memory, the corresponding symbolic expression is updated • Otherwise, a new entry is added to symbolic memory Symbolic memory Symbolic stack • __CrestApply2(39, 0, (long long )(match + 1)); • __CrestStore(42, (unsigned long )(& match)); • match ++; Symbolic PC Symbolic predicate register Symbolic path: <a==b, 11>

  20. Conclusion • CREST does not support full ANSI-C semantics • No symbolic pointer dereference • Only linear integer arithmetic • No bit-wise operator • And so on • To support them, we need to improve CREST’s dynamic symbolic interpreter engine • I hope this presentation will be a good starting point

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