Working with ELF files

Question

I need to work with simple ELF files in my C program. I can't use any external libraries, but I can use elf.h.

Let's take hello.o file for source:

int Hello() { return 3; }

How could I access to Hello in ohter C program having only hello.o file?

I should probably load it to memory using mmap or sth like this. At the end I need to work with much complicated ELF files, but I don't know now, how to start.

UPDATE: I need to do this the way I described it, because it is for learning purposes. Whole problem is more complex that what I described.

For this question assume I need to write method:

int HelloFromElfO(const char* helloFile);

which would execute Hello function implemented in helloFile.

I don't want full answer. I don't need any code. I need something to start with.

I have basic knowledge about ELF file structure, but I have no idea how to work in C with binary file without any parser or sth like this.

UPDATE2: OK, apps like readelf are very complicated. So maybe I try this way: lets say again I have hello.o mapped to memory at ptr. How can I get pointer to Hello function?

How can I get any structured data from hello.o? I mean, not pure bytes but something I can work with.

Answer 1

This is way easier than you think. ELF has nothing to do with your problem (and mmap() is even farther...). hello.o is not an ELF file, it's an object file.

You can just link the object file to your executable, then you will be able to access Hello(). Supposing you have compiled your program to an object file as yourCode.o, you link this yourCode.o with hello.o

cc yourCode.o hello.o -o yourExecutable

See here.

EDIT:

If you don't want to link the file but load it dynamically, then

1. mmap() the object file
2. Get Hello() address in memory. You can statically analyze hello.o for this (e.g. using objdump) and get Hello() entry point offset in the file.
3. Add this offset to the address returned by mmap() to get Hello() address.
4. Map Hello() address to a function pointer
5. Call the function.
6. ???
7. Profit