[klee-dev] Working with fixed memory locations.

[Marco Vanotti]

After letting it run for a few hours I've observed that klee spawns a
subprocess that keeps growing on memory until it reaches ~100GiB and then
it stops and restarts again.
Nothing is being printed indicating an error, but I'm not sure if the
behavior is normal. This is with KLEE from the docker container.

I've tried building KLEE from source, both with STP and Z3 support, and
running my program makes it crash with a segfault :(

Here is the backtrace for the crash with the STP solver:
https://pastebin.com/raw/xpf9D9VD

Best Regards,
Marco

On Thu, Jun 16, 2022 at 3:48 PM Marco Vanotti <mvanotti at dc.uba.ar> wrote:

> Hi Martin, Manuel,
>
> Thanks for your answer :) !
>
> On Thu, Jun 16, 2022 at 1:19 PM Nowack, Martin <m.nowack at imperial.ac.uk>
> wrote:
>
>> Hi Marco,
>>
>> Maybe the following helps you:
>>
>> https://github.com/klee/klee/blob/292600cf54d5fd73278f67a4f98c2f955cbdaa10/test/Feature/DefineFixedObject.c
>>
>
> This seems to be what I am looking for, thanks!. I tried using it for
> small variables and it works. However, for a big object (0x256000 bytes) it
> shows the following warning:
>
> *KLEE: WARNING ONCE*: flushing 2449408 bytes on read, may be slow and/or
> crash: MO195[2449408] allocated at main():  call void
> @klee_define_fixed_object(i8* inttoptr (i64 8404992 to i8*), i64 2449408),
> !dbg !171
>
> KLEE is still running, so maybe it just means it is slow.
>
> I went with the approach of having my blob as a global variable, and then
> `memcpy` it into the address after calling define_fixed_object.
>
> Best,
>> Martin
>>
>> On 16. Jun 2022, at 20:43, Carrasco, Manuel G <m.carrasco at imperial.ac.uk>
>> wrote:
>>
>> Hi Marco!
>>
>> I have a program that when compiled, adds a program header that loads a
>> data blob into a fixed memory location.
>>
>> I'm sorry to ask, but could you explain a bit more how this works? At
>> first glance, I'd say that if any of this happens on a stage later than
>> LLVM-IR, it may be hard to mimic in KLEE.
>>
>> I have a bunch of files that I add as .incbin into a section, and then my
> linker scripts put them in a fixed address when it links the program
> altogether. I think there is no way this would work with LLVM IR.
>
>>
>> As far as I understand, when KLEE executes a LLVM-IR load instruction
>> <https://github.com/klee/klee/blob/master/lib/Core/Executor.cpp#L2722>,
>> it will try to find
>> <https://github.com/klee/klee/blob/master/lib/Core/Executor.cpp#L4191> the
>> MemoryObjects (more than one if it is a symbolic pointer) that contain the
>> address. Conceptually, you want KLEE to somehow have a MemoryObject at the
>> hardcoded address.
>>
>> One way to go could be modelling this as a LLVM-IR GlobalVariable at your
>> fixed address with the content of your blob.  If this makes sense, you may
>> want to check this function
>> <https://github.com/klee/klee/blob/master/lib/Core/Executor.cpp#L648> and
>> addExternalObject perhaps as well.
>>
>> Thanks! This looks interesting, but I am a bit puzzled about how to go
> with this. Should I recompile KLEE to add support for my use case? I
> checked on the MemoryManager class and it seems like it just allocates
> stuff at whatever place is available.
>
>>
>> I apologise if you already know this!
>>
>>
> I did not know any of that :) This is the second time I am using KLEE, and
> the first one was a big failure :P
>
> Thanks!
> Marco
>
>
>
>>
>> Best regards,
>> Manuel.
>>
>> ------------------------------
>> *From:* klee-dev-bounces at imperial.ac.uk <klee-dev-bounces at imperial.ac.uk>
>> on behalf of Marco Vanotti <mvanotti at dc.uba.ar>
>> *Sent:* 16 June 2022 18:55
>> *To:* klee-dev <klee-dev at imperial.ac.uk>
>> *Subject:* [klee-dev] Working with fixed memory locations.
>>
>> Hi klee-dev!
>>
>> I am new to KLEE, and have a question about using it with one of my
>> programs.
>>
>> I have a program that when compiled, adds a program header that loads a
>> data blob into a fixed memory location.
>>
>> This means that my program has this fixed memory location hardcoded all
>> around the place (also this blob has references to itself).
>>
>> I would like to load my program in KLEE to get a better understanding of
>> how it works. The problem I am facing is that I have no idea how to make
>> KLEE understand that I need this blob mapped in that address.
>>
>> This are the things I've tried:
>>
>> * Using wllvm/gclang to get the full program linked together, following
>> my link script, then extracting the bc and running that with KLEE. This
>> didn't work. KLEE complains that the pointers are invalid.
>>
>> * Manually embedding the blob into my program as an array, then calling
>> `mmap` with `MAP_FIXED` to map the area that I want and copying over the
>> blob.
>>
>> The issue here is that MAP_FIXED returns EPERM because probably the
>> address range I am trying to map is already mapped.
>>
>>
>> * Setting the KLEE deterministic allocations to encompass the range that
>> I care about, then doing a big `malloc` and making sure that my range is
>> inside that malloc chunk.
>>
>> For this last one, I am using flags like:
>> --allocate-determ --allocate-determ-start-address=8404992
>> --allocate-determ-size=3145728
>>
>> One of the things that I see is that KLEE fails to mmap big chunks (in
>> the order of 100MiB). But even if I decrease the size, I still get failures
>> when I try to assert things like:
>>
>> uintptr_t malloc_addr = (uintptr_t) malloc(malloc_size);
>> klee_assert(BASE_ADDR >= malloc_addr);
>> klee_assert(BASE_ADDR < malloc_addr + malloc_size);
>>
>> ------
>>
>> Something that might be relevant is that in reality I need two of these
>> blobs loaded into different regions of memory, but so far I can't even get
>> to load one. And they are not too far apart from each other, so if, for
>> example, the malloc approach works, I could just increase the size and make
>> the two allocations.
>>
>> One thing that might complicate things, is that these addresses might
>> collide with where KLEE tries to load the program. I don't know how to deal
>> with that either.
>>
>> Any advice on how to tune KLEE for this use case?
>>
>> Best Regards,
>> Marco
>> _______________________________________________
>> klee-dev mailing list
>> klee-dev at imperial.ac.uk
>> https://mailman.ic.ac.uk/mailman/listinfo/klee-dev
>>
>>
>>
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