Program to print all evironment variables

Question

I'm learning x86 assembly on Linux and this is the first program I wrote, so I'm looking for all kinds of ways to improve it.

; Build using these commands:
;   nasm -f elf printenv.asm
;   ld -o printenv printenv.o -m elf_i386 -s

section .data
section .bss

section .text

global _start

_start:
  pop ecx                 ; load argument count
  lea esp,[esp+ecx*4+4]   ; move past every argument and null ptr

  PopEnv:
  pop esi                 ; get next env variable
  cmp esi,0               ; null means end
  je Exit                 ; exit if null

  ; calculate length
  mov ecx,0ffffffffh      ; max length
  xor eax,eax             ; search for null byte
  mov edi,esi             ; where to look for
  repne scasb             ; search
  dec edi                 ; edi points one past the null
  mov byte [edi],0ah      ; replace null with EOL
  sub edi,esi             ; length
  mov edx,edi             ; set arg for sys_write
  inc edx                 ; include EOL

  ; print env to console
  mov eax,4               ; sys_write
  mov ebx,1               ; stdout
  mov ecx,esi             ; location
  int 80h                 ; call
  cmp eax,edx             ; check return value
  jb ExitFailure          ; error

  jmp PopEnv              ; get another env

  Exit:
  mov eax,1               ; sys_exit
  mov ebx,0               ; EXIT_SUCCESS
  int 80h                 ; make the call

  ExitFailure:
  mov eax,1               ; sys_exit
  mov ebx,-1              ; EXIT_FAILURE
  int 80h                 ; make the call

Answer 1

There's always a question of what 'improve' might mean. I don't see anything here that is 'wrong,' so mostly I'm commenting on 'style.' That said, if I were doing a code review, these are some of the things I'd mention:

• You have lots of very clear comments. That's a good thing, especially in assembler. I can't count the number of times I've read code and thought "What the heck was this guy THINKING?" This can be especially important when the code is wrong: He thinks it's doing x, but the code actually does y, so I am probably safe fixing it.
• You don't have a comment that describes what the program does. While your SE post has a (misspelled) description, the code does not.
• While it is not an issue for such a short program, you might want to start thinking in terms of 'routines.' Having a chunk of code that counts the length of a string (or that prints a string) that you can copy into your next program can be useful.
• While it works, I wouldn't get into the habit of using repne. It's not the best performing way to do this kind of loop (see https://stackoverflow.com/a/34141835/2189500). BTW, this is one of the reasons to use a high level language (like C). Compilers already know all this stuff and (try to) use the best possible construct.
• I expect that with a bit of fiddling (by using an offset on the mov) you can remove the dec edi/inc edx.
• I might consider changing the jb ExitFailure. jb can only jump over a certain amount of code, so your existing construct will work even if PopEnv was far away. But since your code is small, you could do:

a

jnb PopEnv              ; get another env

ExitFailure:
mov eax,1               ; sys_exit
mov ebx,-1              ; EXIT_FAILURE
int 80h                 ; make the call

Exit:
mov eax,1               ; sys_exit
mov ebx,0               ; EXIT_SUCCESS
int 80h                 ; make the call

In summary, my comments are mostly just piddly 'style' stuff.