[LLVMdev] Lifting ASM to IR

[yekkas]

Indeed, I guess one of the most difficult questions in binary 
translation must be
to discriminate between code and data in an executable. And the most 
difficult
task must be to support self-modifying code.

It seems that though one can use some heuristics to handle both these issues
properly in most cases, to create a robust binary translator one needs 
to have
both a full copy of initial binary and a full-fledged binary 
*interpreter* at
run time. Quite similar to compilation of dynamic languages.


On 03/13/2015 07:58 PM, mats petersson wrote:
> Having spent my formative years (or whatever it's called) many years
> back writing a disassembler that produced "re-assemblable" code (that
> worked most of the time), I'm quite familiar with the fundamentals of
> simply knowing "what is read-only data, what is instructions". I had
> to invent several heuristics to solve for example jump-tables [switch
> statement] that are PC-relative or text-strings embedded in the "code"
> section  - both typically "look kind of like instructions, but aren't
> really" - strings are a little less difficult than jump-tables, but
> both can be very tricky - especially if they are relatively short
> sequences (is "F\n" an instruction or string?)
>
> If you then want to abstract it further, to a higher level language
> (e.g LLVM IR), you have to actually undersstand the semantics of each
> instruction.
>
> Calls are typically not so bad - you can figure out from the system
> calling convention what is part of the call and what isn't (assuming
> the calling convention of the compiler is documented, of course)
>
> --
> Mats
>
> On 13 March 2015 at 16:26, yekkas <yekkas at gmail.com> wrote:
>> On 03/13/2015 05:47 PM, Jonathan Roelofs wrote:
>>> what if void* isn't the same size between the two targets?
>> I believe that the translated program has no other choice but to use the
>> same
>> pointer size and data layout as the original one. Though, memory operations
>> have
>> to be modified to map one memory space into another properly.
>>
>>
>>
>> On 03/13/2015 05:47 PM, Jonathan Roelofs wrote:
>>>
>>>
>>> On 3/12/15 8:14 PM, Daniel Dilts wrote:
>>>> On Thu, Mar 12, 2015 at 6:33 PM, Ahmed Bougacha
>>>> <ahmed.bougacha at gmail.com <mailto:ahmed.bougacha at gmail.com>> wrote:
>>>>
>>>>      > On Thu, Mar 12, 2015 at 05:44:02PM -0700, Daniel Dilts wrote:
>>>>      >> Does there exist a tool that could lift a binary (assembly for
>>>> some
>>>>      >> supported target) to LLVM IR?  If there isn't, does this seem like
>>>>      >> something that would be feasible?
>>>>
>>>>      There's plenty of variations on the idea: Revgen/S2E, Fracture,
>>>> Dagger
>>>>      (my own), libcpu, several closed-source ones used by pentest shops,
>>>>      some that use another representation before going to IR (say
>>>>      llvm-qemu),  and probably others still I forgot about.
>>>>
>>>>      Are you interested in a specific target / use case?
>>>>
>>>>
>>>> I was thinking something along the lines of lifting a binary into IR and
>>>> spitting it out for a different processor.
>>>
>>> This is going to be extremely difficult. Imagine for example how this
>>> function would be compiled:
>>>
>>>    struct Foo {
>>>      void *v;
>>>      int i;
>>>      long l;
>>>    };
>>>
>>>    long bar(Foo *f) {
>>>      return f->l;
>>>    }
>>>
>>> If we pick a particular target, and compile this function for that, then
>>> 'foo' will have some offset into the struct from which it loads 'l'. This is
>>> easy because we know the sizes of the struct's members, and the layout rules
>>> for structs on the target.
>>>
>>> Now turn that around: given an offset into a struct for one target, what's
>>> the offset into the same struct on another target? We're stuck because we
>>> can't reconstruct from this offset what the sizes of v and i are
>>> individually; all we have is their sum (and that doesn't even take alignment
>>> issues into account).  This is because when we're looking at the binary we
>>> don't know, given that offset, that the two elements in front of it are a
>>> void* and an int.
>>>
>>> Now, you might think that: "well, okay we'll just use the offsets from one
>>> target in the other target's binaries". But that isn't going to work either:
>>> what if void* isn't the same size between the two targets? And that's just
>>> the tip of the iceberg.
>>>
>>> TL;DR: binary translation is a very difficult problem.
>>>
>>>
>>> Jon
>>>
>>>> Or maybe a decompiling tool of some kind.
>>>>
>>>>
>>>>
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