How can I traverse every character of GlobalVariable name in LLVM? - llvm

Question

I have written a demo of getting every character of GlobalVariable name and doing XOR operation. Here is my part of my demo code(Function entry block has been created privously, gvar is the GlobalVariable and len is the GlobalVariable length):
BasicBlock *preHeaderBB=builder.GetInsertBlock();
ConstantInt* string_length = ConstantInt::get(mod->getContext(), APInt(32, len));
BasicBlock* for_body = BasicBlock::Create(mod->getContext(), "for-body", fdecode);
builder.CreateBr(for_body);
builder.SetInsertPoint(for_body);
PHINode *variable = builder.CreatePHI(Type::getInt32Ty(mod->getContext()), 2, "i");
Value *startValue = builder.getInt32(0);
Value *endValue = builder.getInt32(len);
variable->addIncoming(startValue, preHeaderBB);
LoadInst *Load=builder.CreateLoad(gvar);
Load->setAlignment(8);
Value* indexList[2] = {ConstantInt::get(startValue->getType(), 0), startValue};
ConstantInt* const_key = ConstantInt::get(mod->getContext(), APInt(8, key));
Value *GEP=builder.CreateGEP(gvar,ArrayRef<Value*>(indexList, 2),"arrayIdx");
LoadInst *loadElement=builder.CreateLoad(GEP);
loadElement->setAlignment(1);
Value *Xor = builder.CreateXor(loadElement,const_key,"xor");
StoreInst *Store = builder.CreateStore(Xor, loadElement);
Value *stepValue = builder.getInt32(1);
Value *nextValue = builder.CreateAdd(startValue, stepValue, "next-value");
Value *endCondition = builder.CreateICmpULT(startValue, endValue, "end-condition");
endCondition = builder.CreateICmpNE(endCondition, builder.getInt32(0), "loop-condion");
BasicBlock *loopEndBB = builder.GetInsertBlock();
builder.CreateCondBr(endCondition, loopEndBB, entry);
variable->addIncoming(nextValue, loopEndBB);
builder.SetInsertPoint(entry);
I used the LoadInst to get every character of GloablVariable name, but it came with error. I wander where can I improve this code. Thank you.

Answer 1

In LLVM's SSA form IR, the GV's name is merely an "alias" for the variable, and possibly appears in the SymbolTable, depending on the LinkageType. There is nothing as "Loading the variable's name", since the name is the variable itself. The GEP Instruction you are creating is actually an index into whatever value being represented by the GV, not the name. So that's something fundamentally wrong here. You will need to writing some code to parse the binary format and runtime and retrieve the name from the SymbolTable, providing the GV's LinkageType is not PrivateLinkage or InternalLinkage and the binary is not stripped