Append Code To Text Segment With Global Variables?

Question

I have a powerpc computer that takes ELF files for its code. I have made a tool that allows me to extract and inject segments successfully, but I am missing a small piece of information regarding the modifications I would like to make. How can I call the original code, and how can I store the variables that need to be global?

I can change where the exported functions point, which is all I need to enable the changes that I want. I am not completely sure how I should implement the stub for the functions in ASM to call some C code without messing with the stack or registers unless I explicitly want that to happen. My thought is to make a few small functions that branch out to a second stub that would handle all the state management to prepare for the C code. I was thinking I can use this "dual stage stub" in order to have the first part of the originalText+function*4 and do some easy math for that, and only need to change the exported functions whenever I add/remove a hooked function.

Edit: I figured I should add some clarification to this. I mainly want to have the program connect to a remote device and log some stuff. So I require an active socket connection to be maintained using method calls. I am also using a proprietary OS that has a slightly modified ELF format, but the only extra is the zLib compression.

Edit 2: Ok, I understand the calling better. I was overcomplicating this some, just need the one branch. I also found an address to stick a malloc'd pointer for my data. Now all I need to figure out is how to get GCC to do a relitave jump when calling an address with a PIE. Since I am tacking my code on the end I know I am X away, but I do not know the real address because my exacutable can move in memory.

Answer 1

Ok, after more research I came up with an answer to this problem.

You only need the one branch handler, since I could branch into the C code just fine because they share the same calling method.

I ran out of space in the text segment before memory was getting mapped over my code, so I had to do some more invasive things via a different method.

Now to make GCC play nice with the relative function calls I told it my code was located at 0x00000000+sizeof(.text) so it would use relative branches to the elf and absolute to the system stuff that does not move.