[klee-dev] Question about how Klee performs symbolic execution

[Shehbaz Jaffer]

Hi Rick,


Although I am new to Klee and as such dont know the internals of Klee, I'd like to answer the questions to the best of my ability.

186 cases being generated in case 2 is because there are a lot of redundant testcases.


If you inspect each of your testcases using ktest-tool as below:


ktest-tool klee-last/test000001.ktest
ktest file : 'klee-last/test000001.ktest'
args       : ['sample_str.o']
num objects: 2
object    0: name: b'str'
object    0: size: 10
object    0: data: b'\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00'
object    1: name: b'offset'
object    1: size: 4
object    1: data: b'\n\x00\x00\x00'


you will find the values given to "str" and "offset" values for each testcase. To remove redundancy, please try the following command:


klee --only-output-states-covering-new program_name.bc


this will create 4 testcases, for different values of output, even when you dont put klee_assume() intrinsic function.

Again, you can check the four testcase values by using ktest-tool as shown above.


Hope this helps.


Shehbaz


________________________________
From: klee-dev-bounces at imperial.ac.uk <klee-dev-bounces at imperial.ac.uk> on behalf of Rutledge, Richard L <rrutledge at gatech.edu>
Sent: Wednesday, November 9, 2016 8:10:04 PM
To: klee-dev at imperial.ac.uk
Subject: [klee-dev] Question about how Klee performs symbolic execution

I hope someone can clarify up a little klee-fundlement on my part regarding how Klee performs symbolic execution. Consider the following example program:

--------------------------------------------------------
#include <klee/klee.h>

#define MAXSTR 10

int test(char *str, int offset) {

  if (str[offset] == '\n') {
    return 1;
  }
  return 0;
}

int main(int argc, char *argv[]) {

  char str[MAXSTR];
  int offset;

  klee_make_symbolic(str, MAXSTR, "str");
  offset = klee_int("offset");
  klee_assume(offset >= 0);
  klee_assume(offset < MAXSTR);

  int result = test(str, offset);

  return result;
}
--------------------------------------------------------

There are two paths through this program. As expected, Klee finds exactly two paths and generates two test cases.

However, if I delete to two klee_assume() statements, then Klee finds 186 paths/test cases.

What leads to the path explosion? Without the assumes, the program admits undefined behavior, but does not introduce any new paths. It still only has two and execution should fork into two states at the test() if statement. What condition forks the other 184 states?

Cheers and Thanks!
Rick Rutledge




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