Finding null pointer after environment variables - linux

Question

I'm reading a book(Assembly Language Step by Step, Programming with Linux by Jeff Duntemann) and I'm trying to change this program that show's arguments to instead show the environment variables. I'm trying to only use what was taught thus far(no C) and I've gotten the program to print environment variables but only after I counted how many I had and used an immediate, obviously not satisfying. Here's what I have:
global _start ; Linker needs this to find the entry point!
_start:
nop ; This no-op keeps gdb happy...
mov ebp,esp ; Save the initial stack pointer in EBP
; Copy the command line argument count from the stack and validate it:
cmp dword [ebp],MAXARGS ; See if the arg count exceeds MAXARGS
ja Error ; If so, exit with an error message
; Here we calculate argument lengths and store lengths in table ArgLens:
xor eax,eax ; Searching for 0, so clear AL to 0
xor ebx,ebx ; Stack address offset starts at 0
ScanOne:
mov ecx,0000ffffh ; Limit search to 65535 bytes max
mov edi,dword [ebp+16+ebx*4] ; Put address of string to search in EDI
mov edx,edi ; Copy starting address into EDX
cld ; Set search direction to up-memory
repne scasb ; Search for null (0 char) in string at edi
jnz Error ; REPNE SCASB ended without finding AL
mov byte [edi-1],10 ; Store an EOL where the null used to be
sub edi,edx ; Subtract position of 0 from start address
mov dword [ArgLens+ebx*4],edi ; Put length of arg into table
inc ebx ; Add 1 to argument counter
cmp ebx,44; See if arg counter exceeds argument count
jb ScanOne ; If not, loop back and do another one
; Display all arguments to stdout:
xor esi,esi ; Start (for table addressing reasons) at 0
Showem:
mov ecx,[ebp+16+esi*4] ; Pass offset of the message
mov eax,4 ; Specify sys_write call
mov ebx,1 ; Specify File Descriptor 1: Standard Output
mov edx,[ArgLens+esi*4] ; Pass the length of the message
int 80H ; Make kernel call
inc esi ; Increment the argument counter
cmp esi,44 ; See if we've displayed all the arguments
jb Showem ; If not, loop back and do another
jmp Exit ; We're done! Let's pack it in!
I moved the displacement up past the first null pointer to the first environment variable([ebp+4+ebx*4] > [ebp+16+ebx*4]) in both ScanOne and Showem. When I compare to the number of environment variables I have(44) it will print them just fine without a segfault, comparing to 45 only gives me a segfault.
I've tried using the pointers to compare to zero(in search of null pointer): cmp dword [ebp+16+ebx*4],0h but that just returns a segfault. I'm sure that the null pointer comes after the last environment variable in the stack but it's like it won't do anything up to and beyond that.
Where am I going wrong?

Answer 1

What if your program has 2, 3, or 0 args, would your code still work? Each section is separated by a NULL pointer (4 bytes of 0) You could just get the count of parameters and use that as your array index and skip over the args until you get to the NULL bytes. Now you have your Environment Block:
extern printf, exit
section .data
fmtstr db "%s", 10, 0
fmtint db "%d", 10, 0
global main
section .text
main:
push ebp
mov ebp, esp
mov ebx, [ebp + 4]
.SkipArgs:
mov edi, dword [ebp + 4 * ebx]
inc ebx
test edi, edi
jnz .SkipArgs
.ShowEnvBlock:
mov edi, dword [ebp + 4 * ebx]
test edi, edi
jz .NoMore
push edi
push fmtstr
call printf
add esp, 4 * 2
inc ebx
jmp .ShowEnvBlock
.NoMore:
push 0
call exit
Yes I use printf here, but you just swap that with your system call.

Answer 2

Want to go ahead and apologize, this always happens to me(fix it myself after asking question on stackoverflow). I think when I tried comparing pointer to 0h I typed something wrong. Here's what I did:
inc ebx
cmp dword [ebp+16+ebx*4],0h
jnz ScanOne
and
inc esi
cmp dword [ebp+16+esi*4],0h
jnz Showem
This worked.