Compiling GAS code doesn't detect -fPIC option

Question

I am trying to compile some GAS code for a project using the GCC gnu compiler. Here is how I am compiling it:

gcc -c boot.s -o boot.o -fPIC

After I compile my kernel.c file with the -fPIC argument, I try to link it with this command:

gcc -N -T linker.ld -o Slack\ Berry.bin -ffreestanding -nostdlib kernel.o boot.o -lgcc

It comes up with:

/usr/bin/ld: boot.o: relocation R_X86_64_32 against '.multiboot' can not be used when making a PIE object; recompile with -fPIC

This leads me to think that it is not compiling my GAS code with -fPIC. How can I fix this?

Answer 1

First of all you probly need -fPIE rather than -fPIC. -fPIE allows compiler to generate more efficient code but can only be used for code that's part of main executable (not shared library).

Now both -fPIC and -fPIE are compiler-only flags and are not passed to assembler. You'll need to explicitly use PIC-specific mnemonics in your assembly code instead of position-dependent calls and branches e.g instead of

movq $bar, %rdx

use

movq bar@GOTPCREL(%rip), %rdx

(normally to get the syntax I need I just run gcc -fPIE -S -o- on matching C snippet).