pass a local variable to a function llvm

Question

I am using llvm 4.0. I need to hook a function in which the argument is one of the local variable in IR e.g. I want to pass %10 value to a function. I know how to hook a function. Following is my code:

hookFunc2 = M.getOrInsertFunction("funcName", Type::getVoidTy(M.getContext()),Type::getInt8PtrTy(M.getContext()),NULL);    
hook1= cast<Function>(hookFunc2);
IRBuilder <> builder1(a); //where a is the basic block pointer
BasicBlock::iterator BC = a->begin();
builder1.SetInsertPoint(&(*BC));
Value* args = builder1.CreateGlobalStringPtr(%10); //this is wrong.what should be here instead of %10? I want %10 value.
builder1.CreateCall(hook1, args)

You need to obtain a reference for that variable somehow. Can you show the code where %10 is defined. — arrowd
%10 = getelementptr inbounds [10000 x i8], [10000 x i8]* %line, i32 0, i32 0 %11 = load %struct._IO_FILE*, %struct._IO_FILE** %fp, align 8 %12 = call i8* @fgets(i8* %10, i32 10000, %struct._IO_FILE* %11) — Aatira Anum

Aatira Anum Aatira Anum · Accepted Answer · 2018-05-16T15:41:22

Thanks for reply. I got it. The code is %10 = getelementptr inbounds [10000 x i8], [10000 x i8]* %line, i32 0, i32 0 %11 = load %struct._IO_FILE*, %struct._IO_FILE** %fp, align 8 %12 = call i8* @fgets(i8* %10, i32 10000, %struct._IO_FILE* %11)

I can get %10 by getting fgets 1 argument using

   CallInst abc = cast<CallInst>(&(*i)); //where i is the instruction pointer where fgets is called.
   Value* args = abc->getOperand(0);
   builder1.CreateCall(hook1, args)

pass a local variable to a function llvm

1 Answers