openbsd

I am wondering whether it would be possible to bootstrap fasm or fasmg on a 64bit OpenBSD. I know this has been discussed in other threads, but the infos are sparse and outdated. Most discussions pre-date the x86_64 port for Linux.

I thought my best bet is to take the x86_64 Linux version and adapt the syscalls part.

Before trying this myself I wanted to see if I can produce at least a working 64bit executable with fasm/fasmg so I installed a 32bit OpenBSD virtual machine using the native vmd, grabbed the libC version of fasm and linked it with clang -nopie. For fasmg I had to start from Linux, make a fasmg.o and then link it under 32bit OBSD.
So now I have 32 bits fasm/fasmg on OBSD.

I cannot make however a working 64bit executable. My best try is:

Code:

format ELF64

section '.note.openbsd.ident' align 2
        align   2
        dd      8,4,1
        db      "OpenBSD",0
        dd      0
        align   2

section '.text' executable align 16
        public _start
        _start:
        mov rax, 1
        mov rdi, 7 ; exit code 7
        syscall

I compiled it with fasm 32bit and linked it on the 64bit OS with ld.bfd
If I run it I get Abort trap. If I compile it with clang -nopie (with main instead of _start) I get:
./t64[1]: syntax error: `)' unexpected

On a 64bit machine the following is working with NASM

Code:

section .note.openbsd.ident note
        align   2
        dd      8,4,1
        db      "OpenBSD",0
        dd      0
        align   2
section .text

        global  _start
        _start:

        mov     rax, 1
        mov     rdi, 7         ; exit code 7
        syscall

One difference is that if I compare the elf headers, the .note.openbsd.ident section is not marked as note in fasm, however I don't think it's enough to solve the problem, if I omit "note" in the nasm example I get the section marked as PROGBITS like with fasm, but the program still runs.

I tried to compile the example above with fasmg aswell, including the selfhost.inc file for linux (I am not using any of the 32on64 parts) but I still get the same abort trap.

Can someone give me hints on how to proceed? Do I need implement an elf64 format specific for OpenBSD or is there much more that I am missing? I am still trying to assess how complicated this is

Thanks