I want to build an interface in a c program which is running on an embedded system. This should accept some bytecode that represents a c function. This code will then be loaded into the memory and executed. This will then be something like remotely inject code into a running app. The only difference here is that i can implement, or change the running code and provide an interface.
The whole thing should be used to inject test code on a target system.
My current problem is that I do not know how to build such a byte code out of an existing c function. Mapping and executing this is no problem if I would knew the start address of the function.
Currently I am working with Ubuntu for testing purposes, this allows me to try some techniques which are not possible in the embedded system (according to missing operating system libs).
I build an shared object and used
dlopen()anddlsym()to run this function. This works fine, the problem is just that i do not have such functions in the embedded system. I read something about loading a shared object into memory and run it, but i could not find examples for that. (see http://www.nologin.org/Downloads/Papers/remote-library-injection.pdf)I also took a simple byte code that just print hello world in stdout. I stored this code in memory using
mmap()and execute it. This also worked fine. Here the problem is that I don't know how to create such a byte code, I just used an hello world example form the internet. (see https://www.daniweb.com/programming/software-development/threads/353077/store-binary-code-in-memory-then-execute-it)I also found something here: https://stackoverflow.com/a/12139145/2479996 which worked very well. But here i need a additional linker script, already for such a simple program.
Further I looked at this post: https://stackoverflow.com/a/9016439/2479996 According to that answer my problem would be solved with the "X11 project". But I did not really find much about that, maybe some of you can provide me a link.
Is there another solution to do that? Did I miss something? Or can someone provide me another solution to this? I hope I did not miss something.
Thanks in advance
I see no easy solution. The closest that I am aware of is GCC's JIT backend (libgccjit). Here is a blog post about it.
As an alternative, you could using a scripting language for that code that needs to be injected. For instance, ChaiScript or Lua. In this question, there is a summary of options. As you are on an embedded device, the overhead might be significant, though.
If using an LLVM based backend instead of GCC is possible, you can have a look at Cling. It is a C++ interpreter based on LLVM and Clang. In my personal experience, it was not always stable, but it is used in production in CERN. I would except that the dynamic compilation features are more advanced in LLVM than in GCC.