[LLVMdev] Lifting ASM to IR

yekkas yekkas at gmail.com
Fri Mar 13 09:26:51 PDT 2015

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 
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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