How to get address of printf function in arm executable

Question

I have assingment to make loader for ARM elf relocatible files. I have managed to parse elf somehow, and managed to make some simple relocations, but i have to do the following and i have trouble. I need to call printf function from inside this ELF file that i am trying to load, and it must be the same printf function as the one from program that is trying to load it. (thats my host program, writen in c++ from ARM arch). How can i get this printf address, and after that how can i relocate symbol in elf file. (I googled and found out that this type of relocation uses 24-bit PC relative addressing) Can anyone help me extract address of printf? Thanx in advance!

Answer 1

If you can understand C++, use the source Luke.

• arm-reloc-property.cc
• arm-reloc-property.h
• arm-reloc.def
• elf32-arm.c

See: Assemblers and Loader, by David Solomon. [some pearls of wisdom, some archaic trivia]

As I understand, the printf is in the same file and you need to patch the address at run-time. In this case, the relocation is a 24-bit PC relative relocation. The answer is you do nothing in this case. As the executable and the implementation are both in the same binary, the 24-bit PC relative is already relocated no matter what the load address is.

If you were relocating a shared library, the process would be different.

Answer 2

Since it is impossible to understand where are you with your progress, best suggestion would be to point some source code. Check Android's linker under bionic git.

Answer 3

I found solution. When I get (unsigned int)printf I get actual 32-bit address of function. Patch code must be added to end of text section in ELF file. Now on place where 24-bit offset is needed, is inserted offset to patch code. Now this patch code is what is hard to make.

edit:

Something like this:

bl printf should be resolved like bl offset_to_patch_code

patch code should be on bottom of .text section. Watch out when calculating offset pipeline made pc point 2 instructions ahead. Patch code should only server to read into pc address of printf. You do not need to save link register.