x86 Assembly pushl/popl don't work with "Error: suffix or operands invalid"

Question

I'm a newbie to assembly programming, working through Programming Ground Up on an Ubuntu x86_64 desktop with GNU assembler v2.20.1.

I've been able to assemble/link execute my code, up until I get to using pushl/popl instructions for manipulating the stack. The following code fails to assemble:

 .section .data  # empty

 .section .text
.globl _start
_start:
 pushl $1       # push the value 1 onto the stack
 popl %eax      # pop 1 off the stack and into the %eax register
 int $0x80      # exit the program with exit code '1'

Using "as test.s -o test.o", these errors appear on the terminal and test.o is not created:

test.s: Assembler messages: 
test.s:9: Error: suffix or operands invalid for 'push'
test.s:10:  Error: suffix or operands invalid for 'popl'

I've checked the documentation, and the operands I'm using for pushl and popl are valid. This isn't exactly a debugging question--so what's wrong with my code? Or is it my assembler?

Answer 1

In 64-bit mode you cannot push and pop 32-bit values; you need pushq and popq.

Also, you will not get a proper exit this way. On 32-bit x86, you would need to set %eax to 1 to select the exit() system call, and set %ebx to the exit code you actually wish. On 64-bit x86 (that's what you are using), conventions are different: the system call number for exit() is 60, not 1; the first system call parameter goes in %rdi, not %rbx; the system-call invocation opcode is not int $0x80 but the special, x86-64-only opcode syscall.

Which leads to:

.section .data
.section .text
.globl _start
_start:
    pushq   $60
    popq    %rax
    pushq   $1
    popq    %rdi
    syscall

(each push/pop sequence can be replaced with a simple mov (like mov $60, %eax) of course; I suppose that you are trying to explicitly test push and pop, optimize for code-size, or avoid 0 bytes in the machine code (for an exploit payload))

---

Related:

• What are the calling conventions for UNIX & Linux system calls on i386 and x86-64
• What happens if you use the 32-bit int 0x80 Linux ABI in 64-bit code?