Question

I’m trying to define some subroutines that have calls to printf in them.
A very trivial example is as follows:

extern printf
LINUX        equ     80H
EXIT         equ     60

section .data
    intfmt: db "%ld", 10, 0

segment .text
    global  main

main:
    call os_return      ; return to operating system

os_return:
    mov  rax, EXIT      ; Linux system call 60 i.e. exit ()
    mov  rdi, 0     ; Error code 0 i.e. no errors
    int  LINUX      ; Interrupt Linux kernel

test:
    push rdi
    push rsi
    mov rsi, 10
    mov rdi, intfmt
    xor rax, rax
    call printf
    pop rdi
    pop rsi
    ret

Here test just has a call to printf that outputs the number 10 to the screen. I would not expect this to get called as I have no call to it.

However when compiling and running:

nasm -f elf64 test.asm
gcc -m64 -o test test.o

I get the output:

10
10

I’m totally baffled and wondered if someone could explain why this is happening?

Answer 1

int 80H invokes the 32-bit system call interface, which a) uses the 32-bit system call numbers and b) is intended for use by 32-bit code, not 64-bit code. Your code is actually performing a umask system call with random parameters.

For a 64-bit system call, use the syscall instruction instead:

...
os_return:
    mov  rax, EXIT      ; Linux system call 60 i.e. exit ()
    mov  rdi, 0     ; Error code 0 i.e. no errors
    syscall         ; Interrupt Linux kernel
...