Assembled program gives error: /a.out: cannot execute binary file: Exec format error

Question

I'm new to coding 64-bit x86, but I am running Ubuntu 14.04 (trusty), and I have a very simple 64-bit piece of code that I assemble using as. The output I get has strange permissions and file type.

When I run:

as file.s

I get a file a.out with 770 permissions.

I get this error when I execute it:

bash: ./a.out: cannot execute binary file: Exec format error

When I run:

file ./a.out 

I get:

./a.out: ELF 64-bit LSB relocatable, x86-64, version 1 (SYSV), not stripped

The assembly code I am using is:

.section    .data
.LC0:
    .string "/bin/sh"

.LC1:
    .string "/bin/sh"

.LC3:
    .quad .LC1, 0

    .text
    .globl  _start
_start:
.LFB0:

    pushq   %rbp
    movq    %rsp, %rbp

    movq    $59,%rax     # System Call to execve
    movq    $.LC0, %rdi  # Pass program to execute
    movq    $.LC3, %rsi  # Pass command line arguments
    syscall

    movl    $0, %eax
    popq    %rbp
    ret

Answer 1

You want to assemble the program with GNU Assembler (AS) first and then use the linker to generate the final executable. To do that you should be able to use something like:

as file.s -o file.o
ld file.o -o file

The first command tells the assembler to output an ELF64 object called file.o using the -o option. The second command links file.o to a 64-bit ELF executable called file using the -o option.

The typical default behaviour on a 64-bit system is for the assembler and linker to generate 64-bit objects and executables.

You can then run it with:

./file