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NASM Forum => Using NASM => Topic started by: Kyi Soe Thin on June 10, 2017, 03:57:23 AM

Title: Segmentation fault error using nasm
Post by: Kyi Soe Thin on June 10, 2017, 03:57:23 AM

Hi
I started learning assembly language and I tested jmp instruction with a small program.
Here are my code

Code: [Select]

section .data
        pointer dd one,two,three
        dis1 dd "One"
        dis2 dd "Two"
        dis3 dd "Three"
        lendis1 equ $-dis1
        lendis2 equ $-dis2
        lendis3 equ $-dis3

section .bss
var resd 1

section .text
global main
main:
mov eax,3
mov ebx,0
mov ecx,var
mov edx,1
int 80h
mov edx,var
mov eax,[pointer + edx*4]
jmp [eax]
one:
mov eax,4
mov ebx,1
mov ecx,dis1
mov edx,lendis1
int 80h
jmp exit
two:
mov eax,4
                mov ebx,1
                mov ecx,dis2
                mov edx,lendis2
                int 80h
jmp exit

three:
mov eax,4
                mov ebx,1
                mov ecx,dis3
                mov edx,lendis3
                int 80h
jmp exit

exit:
mov eax,1
int 80hWhen I ran this as following

Code: [Select]

root@kalihost:~# nasm -f elf64 jump.asm
root@kalihost:~# gcc -o jump jump.o
root@kalihost:~# ./jump
1
Segmentation fault
root@kalihost:~#
root@kalihost:~#
Please find me the error.

Title: Re: Segmentation fault error using nasm
Post by: Frank Kotler on June 10, 2017, 05:29:38 AM

I don't know anything about 64-bit code, but I know pointers are 64 bits!

What you've got there looks like pretty good 32-bit code. Mixing 64- and 32-bit code may not be a good idea. You might want to assemble it as 32-bit code, and tell gcc "-m32".

In any case, you're calculating the lengths wrong. Put the equs immediately after what you want the lengths of.

You need to convert stdin from characters to a number. You're using the address of "var" in your effective address.

My advice, if you're just beginning, would be to start with something simpler and take smaller steps.

Best,
Frank

Title: Re: Segmentation fault error using nasm
Post by: Kyi Soe Thin on June 10, 2017, 03:12:23 PM

I want to ask one more question.
Can jmp instruction be used with variable ?
For example,

Code: [Select]

section .data
        pointer dw "one","two","three"
        dis1 dd "one"
        lendis1 equ $-dis1
dis2 dd "Two"
        lendis2 equ $-dis2
dis3 dd "Three"
        lendis3 equ $-dis3

section .bss
var resd 1

section .text
global main
main:
mov eax,3
mov ebx,0
mov ecx,var
mov edx,1
int 80h
mov eax,pointer
jmp eax
one:
mov eax,4
mov ebx,1
mov ecx,dis1
mov edx,lendis1
int 80h
jmp exit
two:

mov ecx,dis2
mov eax,4
                mov ebx,1
                mov edx,lendis2
                int 80h
jmp exit

three:
mov eax,4
                mov ebx,1
                mov ecx,dis3
                mov edx,lendis3
                int 80h
jmp exit

exit:
mov eax,1
int 80hPlease help me.

Title: Re: Segmentation fault error using nasm
Post by: Frank Kotler on June 11, 2017, 04:13:43 AM

Quote

I want to ask one more question.
Can jmp instruction be used with variable ?

Yes.

... not with random garbage in the variable, though.

Since this is mostly 32-bit code (64-bit uses "syscall" not "int 80h"), I tried it as 32 bit.

Code: [Select]

; nasm -f elf32 myprog.asm
; ld -m elf_i386 -o myprog myprog.o

section .data
        pointer dd one,two,three

        dis1 db "one"
        lendis1 equ $-dis1
dis2 db "Two"
        lendis2 equ $-dis2
dis3 db "Three"
        lendis3 equ $-dis3

section .bss
var resd 1

section .text
global _start
_start:
mov eax,3
mov ebx,0
mov ecx,var
mov edx,1
int 80h

mov edi, [var]
sub edi, '0' ; convert character to number
dec edi ; pointer + 0 is "one"

mov eax,[pointer + edi * 4]
jmp eax
; or...
jmp [pointer + edi * 4]


one:
mov eax,4
mov ebx,1
mov ecx,dis1
mov edx,lendis1
int 80h
jmp exit
two:

mov ecx,dis2
mov eax,4
                mov ebx,1
                mov edx,lendis2
                int 80h
jmp exit

three:
mov eax,4
                mov ebx,1
                mov ecx,dis3
                mov edx,lendis3
                int 80h
jmp exit

exit:
mov eax,1
int 80h

Works for me. It may not be what you have in mind...

Best,
Frank

Title: Re: Segmentation fault error using nasm
Post by: Kyi Soe Thin on June 12, 2017, 01:49:19 PM

Yesss, I get it.
It really works for me !!!
I really really thank you, Frank Kotler.