Question

I’m learning assembly language programming on FreeBSD. I’m using FreeBSD 9.0 i386 release and nasm assembler.

When I wrote a simple syscall function, I found that I had to push a useless value into the stack to make the code run correctly.

For example:

; File:test.asm
section .text
  global _start
_start:
  xor  eax,eax
  ; Argument of exit()
  push 0x0
  ; Syscall of exit()
  mov  al,1
  int  0x80

I used the following command to assemble and link the above code:

%nasm -f elf test.asm -o test.o
%ld test.o -o test.bin

I used ktrace to inspect the program and found:

%ktrace ./test.bin
%kdump -d -f ./ktrace.out 
2059 ktrace   RET   ktrace 0
2059 ktrace   CALL  execve(-1077940941,-1077941260,-1077941252)
2059 ktrace   NAMI  "./test.bin"
2059 test.bin RET   execve 0
2059 test.bin CALL  exit(1)

So the code didn’t run correctly, cause I provided 0 as the only argument of exit() but the program actually run exit(1).

Then I changed my code.

; File:test.asm
section .text
  global _start
_start:
  xor  eax,eax
  push 0x0
  ; Whatever digits,0x1,0x2...0xFFFFFFFF, ect.
  push 0xFFFFFFFF
  mov  al,1
  int  0x80

Then the code was executed correctly.

At first, I though it was because of something like “stack padding” or “stack alignment”, like Stack allocation, padding, and alignment. So it might respect 16-bit alignment. But I found it not. For example, This following code:

; File:test.asm
section .text
  global _start
_start:
  xor  eax,eax
  push 0x0
  ; Actual argument of exit()
  push 0x3
  push 0xFFFFFFFF
  ; Syscall of exit()
  mov  al,1
  int  0x80

actually executed exit(3). It seemed that it didn’t align bytes. I debug the above code with gdb, when the last line was about to be executed, the stack was something like this:

0xFFFFFFFF  -> esp
0x00000003
0x00000000

So here’s my question: why there’s always a useless argument or is there a method to work around?

Answer 1

It’s a dummy argument to increase performance slightly by preventing a call/ret instruction pair.

See $2.1 in the below link:

http://www.int80h.org/bsdasm/#default-calling-convention