ASM Community

Hello,

the solution is probably very simple, but I can't find it... the source code:

SECTION .DATA
		hello:			DB		'Hello world!',10
		helloLen:		EQU		$-hello



SECTION .TEXT
		GLOBAL	_start

		_start:

		CALL	printHW
		XOR	EAX, EAX
		MOV	EBX, string
		MOV	, AL
		CALL	printHW
		CALL	exit
string:
		DB		0xff,0xff,0xff,0xff


		;; write "hello world" to the screen
printHW:
		PUSH	EAX
		PUSH	EBX
		PUSH	ECX
		PUSH	EDX
		MOV		EAX, 4			; system call "write"
		MOV		EBX, 1			; file descriptor stdout
		MOV		ECX, hello		; string to write
		MOV		EDX, helloLen	; num of bytes to write
		INT		80h				; call kernel interrupt
		POP		EDX
		POP		ECX
		POP		EBX
		POP		EAX
		RET



		;; terminate program
exit:
		MOV		EAX, 1		; system call "exit"
		MOV		EBX, 0		; exit code
		INT		80h

compile and execute:

chk@subnetrouter ~/nasm $ nasm -f elf foo.asm
chk@subnetrouter ~/nasm $ ld foo.o -o foo
chk@subnetrouter ~/nasm $ ./foo 
Hello world!
Speicherzugriffsfehler

"Speicherzugriffsfehler" means "segmentation fault", but why?

Using the ald debugger I have found the origin of the error: "MOV , AL". The error is caused by writing the content of AL into the memory, but the pointer (EBX) is initialized correctly at the line above: "MOV EBX, string"

I tried a couple of ideas with LEA instead of MOV, etc...


Anyone any idea?

Thanks for reading
loskornosdelsol

Aaaah, ok. First: Thanks a lot! I didn't know, that such "section flags" exist. Replacing my section definition with yours solved the problem. Ok, it's a small security hole, but I know occurs.

Second: I found this website: http://www.tortall.net/projects/yasm/wiki/ElfObject - it told me (like you), that .DATA would be writeable per default. But, before posting my problem, I tried this too. When I move "string: DB 0xff,0xff,0xff,0xff" into the .DATA section, the problem is still present.

Mhmm, when changing to "SECTION .DATA WRITE ALIGN=16" the problem is really solved, but why is the data section not writable per default. Is this set by the nasm configuration, where can I change the defaults?

I will read the nasm doc....


thanks for help!!!
loskornosdelsol


edit: I can't find information about the section access configuration in the nasm documentation at http://nasm.sourceforge.net/doc/html/nasmdoc0.html

Aaaaah! The section ".data" is writable per default, but ".DATA" is treated as "other", and "other" is not writable :-)