Question

I’m trying to create a Shared library (*.so) in ASM and I’m not sure that i do it correct…

My code is:

    .section .data
    .globl var1
var1:
    .quad     0x012345

    .section .text
    .globl func1
func1:
    xor %rax, %rax
  # mov var1, %rcx       # this is commented
    ret

To compile it i run

gcc ker.s -g -fPIC -m64 -o ker.o
gcc ker.o -shared -fPIC -m64 -o libker.so

I can access variable var1 and call func1 with dlopen() and dlsym() from a program in C.

The problem is in variable var1. When i try to access it from func1, i.e. uncomment that line, the compiler generates an error:

/usr/bin/ld: ker.o: relocation R_X86_64_32S against `var1' can not be used when making a shared object; recompile with -fPIC
ker.o: could not read symbols: Bad value
collect2: ld returned 1 exit status

I don’t understand. I’ve already compiled with -fPIC, so what’s wrong?

Answer 1

Ok, i think i found something…

First solution from drhirsch gives almost the same error but the relocation type is changed. And type is always ended with 32. Why is it? Why 64 bit program uses 32-bit relocation?

I found this from googling: http://www.technovelty.org/code/c/relocation-truncated.html

It says:

For code optimisation purposes, the default immediate size to the mov
instructions is a 32-bit value

So that’s the case. I use 64-bit program but relocation is 32-bit and all i need is to force it to be 64 bit with movabs instruction.

This code is assembling and working (access to var1 from internal function func1 and from external C program via dlsym()):

    .section .data 
    .globl var1 
var1: 
    .quad     0x012345

    .section .text 
    .globl func1 
func1: 
    movabs var1, %rax       # if one is symbol, other must be %rax
    inc %rax
    movabs %rax, var1
    ret

But i’m in doubt about Global Offset Table. Must i use it, or this “direct” access is absolutely correct?