[klee-dev] Replacing external printf with an internal version for the Klee tests.

[Willem Pinckaers]

On Oct 8, 2014, at 2:44 AM, Cristian Cadar <c.cadar at imperial.ac.uk> wrote:

> Indeed, I don't think we need to optimize the memory consumption of the test suite, it is not excessive.
> 

It seems I didn’t explain myself very well. 

I would like to be able to run Klee on bit code with a different data layout, in particular a different pointer size. To test if my changes break anything I would like to run the test suite, however the test suite depends on calling external functions. And when external functions are needed/used it means that the data layout has to match with Klee. 

By adding support for an internal printf I was hoping to be able to run the test suite to see if my changes break other parts of Klee, besides external function calls. (Which they do: test/Feature/FunctionPointer.c breaks and can be fixed with minor changes to legalFunctions)

(As a side effect it seems that memory is allocated twice for use with external functions, which could be disabled when external functions are not needed, but this is a minor issue to me at the moment).




I will do more work on testing if Klee can be made to support running bit code with a different data layout and if I am successful revisit this topic. It seems that there is no general benefit to having an internal printf at this point.



> The ability to deal with external calls is one of the most important strengths of dynamic symbolic execution.  However, as Dan pointed out, there is the option of disallowing such calls if needed.  I could understand the  need to special case printf and I'd be happy to consider such a patch, but this should be an option to KLEE, which should not require any changes to the test suite.
> 

Would adding a klee_internal_printf to the intrinsic functions which get aliased to printf when running with external functions disabled be acceptable?



Regards,
    Willem





> Best,
> Cristian
> 
> On 08/10/14 10:34, Daniel Liew wrote:
>> On 7 October 2014 23:47, Willem Pinckaers
>> <willem_list_klee at lekkertech.net> wrote:
>>> Hi List,
>>> 
>>> Currently most tests for Klee depend on an externally provided printf function. Support for external functions has some side effects, among them:
>>> 
>>> - Memory usage is doubled for all non symbolic allocations.
>>> - The bitcode being tested has to have the same data layout as the compiled version of Klee.
>> 
>> For the test suite I really don't think the memory usage matters. They
>> are mostly small programs and testing external function calls is
>> important because it is an important feature of KLEE because real
>> world code inevitably needs to call into a C library or make system
>> calls at some point.
>> 
>>> Without support for external functions Klee memory usage for non symbolic allocations can be reduced by about 50% and Klee could be made to analyze bit code that uses a different data layout.
>> 
>> Outside of the test suite I could see these being potentially useful
>> but if you really need this wouldn't it be simpler if your program had
>> no external function calls? If your program has external calls and you
>> disable external call support in KLEE your program probably isn't
>> going to run.
>> 
>> 
>>>   This currently hits a bug in llvm 3.4 and 3.5 where lli with force-interpreter=true does not properly execute va_start. But (after adopting the tests to link in the new printf) most test cases pass using force-interpreter=false.
>> 
>> Could you report that to the LLVM developers?
>> 
>> Thanks,
>> Dan.
>> 
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> 
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