How to 'link' object file to executable/compiled binary?

Question

Problem

I wish to inject an object file into an existing binary. As a concrete example, consider a source Hello.c:

#inc         


        

Answer 1

If it were me, I'd look to create Embed.c into a shared object, libembed.so, like so:

gcc -Wall -shared -fPIC -o libembed.so Embed.c

That should created a relocatable shared object from Embed.c. With that, you can force your target binary to load this shared object by setting the environment variable LD_PRELOAD when running it (see more information here):

LD_PRELOAD=/path/to/libembed.so Hello

The "trick" here will be to figure out how to do your instrumentation, especially considering it's a static executable. There, I can't help you, but this is one way to have code present in a process' memory space. You'll probably want to do some sort of initialization in a constructor, which you can do with an attribute (if you're using gcc, at least):

void __attribute__ ((constructor)) my_init()
{
    // put code here!
}