Thread: Made a patch in C, now how to bind it into an existing DLL ?

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First of - Hello and thank you for reading this,

I have one DLL which I do not have the source code but need to add some functionalities into it.

I made up another DLL implementing all these needed functionalities in C - using Visual Studio.

Now I need to insert the generated code from this new DLL into the target DLL (it has to be done at the file level {not at runtime}).

I am probably creating a new PE section on the target DLL and put there all the code/data/rdata from the dll I made up. The problem is that I need somehow to fix the IAT and the relocs relative to this new inserted code on the target DLL.

My question is:

What is the best way to do it?

It would be nice if Visual Studio came up with an option to build using only (mostly) relative addressing - This would save me a lot when dealing with the relocs. I guess I could encapsulate all my vars and constants into a struct, hopefully MSVC would then only need to relocate the address of this "container" struct and use relative addressing to access its members. But don't know if this is a good idea.

I could even go further and get rid of the IAT by making a function pointer which would dynamically load the needed function module (kind of the Delay Load Module). And again, put this function pointer inside the "container" struct I said before.

The last option I have is to make it all by hand, manually editing the binary in hex... which I really didn`t want to do, because it would take some good time to do it for every single IAT entry and reloc entry. I have already written a PE file encryptor some time ago so I know most of the inner workings and know it can be done, just want to know your thoughts and maybe a tool already exists to help me out?

Any suggestions is highly appreciated!

Thanks again for your time for reading this!