[klee-dev] how to dampen ktest exuberance with symbolic index into symbolic array

[Dan Liew]

Hi Richard,

On 10 January 2017 at 00:19, Richard Rutledge <rrutledge at gatech.edu> wrote:
> Consider the following program:
>
> //-----------------------------------------------------------------
> #include <stdlib.h>
> #include <klee/klee.h>
>
> #define BUFFER_SIZE 16
>
> int main(int argc, char *argv[]) {
>
>   char *buffer = malloc(BUFFER_SIZE);
>   int index;
>
>   klee_make_symbolic(buffer, BUFFER_SIZE, "buffer");
>   klee_make_symbolic(&index, sizeof(index), "index");
>
>   if (buffer[index] == '\0') {
>     return 1;
>   }
>   return 0;
> }
>
> //-----------------------------------------------------------------
>
> Given this program, klee generates approx 180 test cases (the actual number
> seems to vary). I understand from prior correspondence here that klee forks
> a path for each object which can be referenced. The forking occurs in
> Executor::executeMemoryOperation().
>
> My objective is to only generate inbounds values of buffer and index that
> induce each of the two paths through the program. My attempts to modify
> AddressSpace::resolve to only return the single desired memory object have
> so far been disappointing (i.e. abject failure).
>
> Any suggestions as to how I should proceed?

For this particular program you could add

```
klee_asume(index >= 0);
klee_assume(index < BUFFER_SIZE);
```

before doing any indexing.

If you want KLEE to always be in bounds you'll have to modify KLEE. It
sounds like you're looking roughly in the right area but this is a
part of the code I've not looked at in detail.

Looking at a glance what you need to change is
`Executor::executeMemoryOperation()`. You can see that it tries to
resolve to check if the pointer can resolve to a single memory object
(`AddressSpace::resolveOne()` then a few additional checks). If it
possible for the pointer to be out of bounds then an unbounded search
using `AddressSpace::resolve()` occurs. You can limit the bound by not
passing `0` as `maxResolutions` (arguably that should be a command
line option that defaults to 0). You could also just delete this code
and fork once and on the forked copy add the constraint that the
pointer doesn't resolve to the previously found object and then
terminate it as an error state for being out of bounds.

To be honest I find the code very hard to follow because the
ImmutableTree that stores the memory object is basically not documents
so I have no idea why the search in `AddressSpace::resolveOne()` is
done the way it is).

HTH,
Dan.