[POC] Generating DWARF debugging information

["Carlos Maniero" <carlos@maniero.me>]

Hi everyone.

Since it is not that simple to find information about about how to
generate debugging information in a way that the program can be
debuggable on gdb, I'll dump what I found here.

First of all I took the minimal example we have currently in our code
base and manually converted it to assembly.

Minimal example:

  fn main(): u32 {
    return 0
  }

Correspondent assembly:

  .text
  .globl  _start

  _start:
    call main
    mov %eax, %edi
    mov $60, %eax
    syscall

  main:
    nop   ; (this nop is mocking a function prelude)
    mov $32, %eax
    ret

After a few reverse engineer with GCC and C I could be able to make it
work with the assembly bellow:

  .text
  .globl  _start

  _start:
    call main
    mov %eax, %edi
    mov $60, %eax
    syscall

  .file 1 "main_exit.0"
  main:
    .loc 1 1 1
    nop
    .loc 1 2 1
    mov $32, %eax
    ret

compiling:

  as main_exit.s -gdwarf-2; ld a.out -o a

And that is it. 

It was really hard to found any documentation of the gnu assembler (gas)
directives. So I'm going to document it here:

https://software-dl.ti.com/codegen/docs/tiarmclang/rel3_2_0_LTS/gnu_syntax_arm_asm_language/gnu_arm_directives/debugging_directives.html#gar-debug-directives

That's it!