Why am I getting segmentation fault?

[alzamabar]

Hi all,

I'm an Assembly newby and am studying Jeff Duntemann's book on programming Assembly on Linux. As much as I love Assembly and I have to learn it if I want to do anything significant in IT since it's part of a journey, I am finding difficult to get started because there are so many variables.

My environment is Ubuntu (Linux Debian) 9.10 64-bit and I am using NASM as compiler. Following Duntemann's suggestion, I created a sort of "sandbox" program where I can simply run few instructions and see the effect that these have on registers.

This is the simple program I'm writing:

;  Executable name : 
;  Version         : 1.0
;  Created date    : 
;  Last update     : 
;  Author          : Marco Tedone
;  Description     : A template to create sandbox programs
;
;  Build using these commands:
;    nasm -f elf64 -g -F stabs <your-asm-name>.asm
;    ld -o <your-asm-name> <your-asm-name>.o
;

SECTION .data			; Section containing initialised data
  
SECTION .text			; Section containing code

global _start			; Linker needs this to find the entry point!
	
_start:  			; Write your instructions between the two noops
nop
mov eax,0
inc eax
mov ebx,eax
nop

SECTION .bss			; Section containing uninitialised variables

[code]

The program compiles fine but when I run it I get segmentation fault. Why?

Also here there is another program (which actually does something useful) which runs fine: 

[code]
;  Executable name : EATSYSCALL
;  Version         : 1.0
;  Created date    : 1/7/2009
;  Last update     : 2/18/2009
;  Author          : Jeff Duntemann
;  Description     : A simple program in assembly for Linux, using NASM 2.05,
;    demonstrating the use of Linux INT 80H syscalls to display text.
;
;  Build using these commands:
;    nasm -f elf -g -F stabs eatsyscall.asm
;    ld -o eatsyscall eatsyscall.o
;

SECTION .data			; Section containing initialised data
	
	EatMsg: db "Eat at Marco's  !",10
	EatLen: equ $-EatMsg	
	
SECTION .bss			; Section containing uninitialized data	

SECTION .text			; Section containing code

global 	_start			; Linker needs this to find the entry point!
	
_start:
	nop			; This no-op keeps gdb happy...
	mov eax,4		; Specify sys_write call
	mov ebx,1		; Specify File Descriptor 1: Standard Output
	mov ecx,EatMsg		; Pass offset of the message
	mov edx,EatLen		; Pass the length of the message
	int 80H			; Make kernel call

	MOV eax,1		; Code for Exit Syscall
	mov ebx,0		; Return a code of zero	
	int 80H			; Make kernel call
[code]

 

[Prattaratt]

The only difference I can see is in the code that runs, you have a space between global and _start, where you do not in the code that doesn't run.

 

[monkhandle]

Um, you need to terminate program properly by calling exit system call.
Use same 3 lines at end as in your second example.
What are you trying to achieve actually?

 

[alzamabar]

Hi guys, thanks for your answer. Actually the author explained me that the "sandbox" code above was incomplete, since it missed the three lines to return control to the OS. What I am trying to achieve is to have a kind of "blueprint" program which I can use as a template to create other, simple programs (e.g. if I want to see the value in some registers, etc). I solved the issue now.

Thanks.

M.