[LLVMdev] Re: Hide visible string in variable (Chris Lattner)

[Chris Lattner]

On Mon, 11 Oct 2004, Zhang Qiuyu wrote:
> Following your suggestion, I got some progress. Thanks again. But I am still stuck in some problems.
>
>     Constant *Cstr = GV->getInitializer();
>
> After that, I tried to use
>     a.
>         for(unsigned i=0;i<Cstr->getNumOperands();++i){
>             Cstr->getOperand(i);
>        }
>
>     b. for(User::op_iterator I=Cstr->op_begin(),E=Cstr->op_end(); I!=E;++I){
>
>             std::cerr<<*I;
>     }

These both work, and are equivalent.

>    From either a or b, I could get each element of Global Variable. Supposedly, I will use my arithmetic like XOR etc to encode/hide the string. But I cannot use XOR, I mean I tried (*I)^0x33, it doesn't work. I tried op_xor, but I don't know how to use it. For C level, it is really staightforward. But here, I don't know how to do it. It should be easy way to do it. But I spent several hours on it. For simplifing prolem, I also tried to do the way like
>         a[i]=a[i]+1;
> but I failed. Shy.

The problem is that the operands are 'Constant*'s, not C integers.  Take a
look at the definitions in the include/llvm/Constants.h file.

In particular, you should probably be doing something like this (warning,
untested code):

if (ConstantArray *CA = dyn_cast<ConstantArray>(GV->getInitializer()))
  if (CA->getType()->getElementType()->isInteger()) {
    std::vector<Constant*> NewElements;
    Constant *Magic = ConstantInt::get(CA->getType()->getElementType(), 123);
    for (unsigned i = 0, e = CA->getNumOperands(); i != e; ++i)
      NewElements.push_back(ConstantExpr::getXor(CA->getOperand(i), Magic));
    CA->setInitializer(ConstantArray::get(CA->getType(), NewElements));
  }


Please look through all of Constants.h to familiarize yourself with how
these things work, and in particular, try to figure out what each part of
the above code does, and why it is there.

> > 2. Construct a new ConstantArray with all of the elements of the original
> >    string, but modified according to the function you want (exclusive or
> >    is a reasonable start).
>
>     As you said, how could construct a new ConstantArray? Is it like
>     Constant *pC = new Constant(SBtype); // Constant(const Type *Ty)
>
>     for ConstantArray, replaceUsesOfWithOnConstant(Value *From, Value *To,
>                                            bool DisableChecking = false); this
>     API replaceUsesOfWithOnConstant seems to be able to do what I want, but how could create/construct a new Value with my owner value?

A good way to figure out stuff like this is to look for other
transformations that already exist in LLVM and see how they do it.  In
particular, you use Constant*::get(...) to create constants.  One
invariant that LLVM provides is that all structurally identical constants
have the same address: this means that if you get the same constant twice,
the same Constant* will be returned.  Constant's are also immutable.

I strongly suggest looking for other examples of LLVM transformations if
you can't figure out how to do something.  In particular,
lib/Transforms/Scalar is a rich source of a bunch of intraprocedural
passes and lib/Transforms/IPO is a good source for things that hack on
global variables and functions.

Good luck,

-Chris

> for  3,4,5, those should work well.
>
> I would really appreciated if you can give me a very simple example to show me how to do it.
>
> > 3. Change the initializer of the global variable to the new constant with
> >    setInitializer().
> > 4. Clear the "constant" flag on the string, because the program will be
> >    dynamically hacking on the string: GV->setConstant(false);
> > 5. Insert the for loop that translates the string when main runs.
> >
> > For #5, write the for loop you want, compile it with llvmgcc, then figure
> > out how to generate it at compile time.  Alternatively, you could put the
> > 'decryption' routine in a library and just insert a call to the library.
> >
> > -Chris
> >
>
>
>
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>

-Chris

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