nasm-users

[Nasm-users] can't write argv[0] on stdout

[Holger H. <ha...@gm...>]

Hi all,

i want to write 3 bytes of argv[0] on stdout but it doesn't work. 
Return code -14 (EFAULT Bad address). Am i missing something here? 

System is archlinux 2009.08 x86_64 kernel 2.6.31-rc6 
nasm 2.05.01 Binutils 2.19.1.20090418



stdout	equ	1
write	equ	4
exit	equ	1

section .text
	global	_start
_start:	
	pop	rax		; rax = argc
	pop	rax		; rax = argv[0]

	mov	rdx,	3	; write 3 bytes
	mov	rcx,	rax	; rcx = rax = argv[0]
	mov	rbx,	stdout	
	mov	rax,	write	
	int	0x80
	
	mov	rbx,	0
	mov	rax,	exit
	int	0x80

Re: [Nasm-users] can't write argv[0] on stdout

[Frank K. <fbk...@zy...>]

Holger Hanrath wrote:
> Hi all,
> 
> i want to write 3 bytes of argv[0] on stdout but it doesn't work. 
> Return code -14 (EFAULT Bad address). Am i missing something here? 
> 
> System is archlinux 2009.08 x86_64 kernel 2.6.31-rc6 
> nasm 2.05.01 Binutils 2.19.1.20090418
> 
> 
> 
> stdout	equ	1
> write	equ	4
> exit	equ	1
> 
> section .text
> 	global	_start
> _start:	
> 	pop	rax		; rax = argc
> 	pop	rax		; rax = argv[0]
> 
> 	mov	rdx,	3	; write 3 bytes
> 	mov	rcx,	rax	; rcx = rax = argv[0]
> 	mov	rbx,	stdout	
> 	mov	rax,	write	
> 	int	0x80
> 	
> 	mov	rbx,	0
> 	mov	rax,	exit
> 	int	0x80

64-bit does it a "bit" differently (from the late and sorely missed 
Chuck Crayne:

section .data
     string1 db  "Hello World!",10,0

section .text
     global _start

     _start:
         ; calculate the length of string
         mov     rdi, string1
         mov     rcx, -1
         xor     al,al
         cld
         repnz scasb

         ; place the length of the string in RDX
         mov     rdx, -2
         sub     rdx, rcx

         ; print the string using write() system call
         mov     rsi, string1
         push    0x1
         pop     rax
         mov     rdi,rax
         syscall

         ; exit from the application here
         xor     rdi,rdi
         push    0x3c
         pop     rax
         syscall

That won't do what you want, of course, but will give you an idea which 
registers are used. (the push/pop is just shorter - probably want to use 
"-Ox" to use it) I assume the stack is set up the same for 64-bit as for 
32-bit(?). Let us know how it works - 64-bit examples are in short 
supply, still...

Best,
Frank

Re: [Nasm-users] can't write argv[0] on stdout

[Holger H. <ha...@gm...>]

On Tue, 18 Aug 2009 13:22:01 -0400
Frank Kotler <fbk...@zy...> wrote:

> Holger Hanrath wrote:
> > Hi all,
> > 
> > i want to write 3 bytes of argv[0] on stdout but it doesn't work. 
> > Return code -14 (EFAULT Bad address). Am i missing something here? 
> > 
> > System is archlinux 2009.08 x86_64 kernel 2.6.31-rc6 
> > nasm 2.05.01 Binutils 2.19.1.20090418
> > 
> > 
> > 
> > stdout	equ	1
> > write	equ	4
> > exit	equ	1
> > 
> > section .text
> > 	global	_start
> > _start:	
> > 	pop	rax		; rax = argc
> > 	pop	rax		; rax = argv[0]
> > 
> > 	mov	rdx,	3	; write 3 bytes
> > 	mov	rcx,	rax	; rcx = rax = argv[0]
> > 	mov	rbx,	stdout	
> > 	mov	rax,	write	
> > 	int	0x80
> > 	
> > 	mov	rbx,	0
> > 	mov	rax,	exit
> > 	int	0x80
> 
> 64-bit does it a "bit" differently (from the late and sorely missed 
> Chuck Crayne:
> 
> section .data
>      string1 db  "Hello World!",10,0
> 
> section .text
>      global _start
> 
>      _start:
>          ; calculate the length of string
>          mov     rdi, string1
>          mov     rcx, -1
>          xor     al,al
>          cld
>          repnz scasb
> 
>          ; place the length of the string in RDX
>          mov     rdx, -2
>          sub     rdx, rcx
> 
>          ; print the string using write() system call
>          mov     rsi, string1
>          push    0x1
>          pop     rax
>          mov     rdi,rax
>          syscall
> 
>          ; exit from the application here
>          xor     rdi,rdi
>          push    0x3c
>          pop     rax
>          syscall
> 
> That won't do what you want, of course, but will give you an idea which 
> registers are used. (the push/pop is just shorter - probably want to use 
> "-Ox" to use it) I assume the stack is set up the same for 64-bit as for 
> 32-bit(?). Let us know how it works - 64-bit examples are in short 
> supply, still...
> 
> Best,
> Frank

Your example works fine thanks but i had no problems writing strings 
from the data section to stdout. I know that the rax register holds the 
right address after the second pop because i can copy the bytes i found there
into a string in the data section and then write it to stdout like this: 

;; copy argv[0] into str1 and write to stdout

stdout	equ 	1 
write  	equ 	4
exit   	equ 	1

section .data
str1		db    '#############',10,0
str1len		equ   $ - str1	
	
section .text

_copy:	cmp	BYTE [rax], 0 	    ; check if we reached the end of argv[0]
	je	_print		
	cmp	rcx, rbx	    ; check if argv[0] is not to long
	je	_print

 	mov	dl, [rax]	    ; move one byte into dl
	mov	BYTE [str1+rbx], dl ; move one byte into str1 at pos of rbx
	
	inc	rax		    ; move on in argv[0]
	inc	rbx		    ; move on in str1
	jmp	_copy


	global _start	
_start:
	pop	rax		; load argc
	pop	rax		; load argv[0]
	
	xor	rbx,rbx		; rbx = 0
	mov	rcx,str1len	; load str1len into rcx
	sub	rcx,2		; make sure we always wirte a new line
	xor	dl,dl		; dl = 0
	
	jmp	_copy
	
_print:	mov	rdx, str1len 	; message length
 	mov	rcx, str1	; message to write
	mov	rbx, 1		; file descriptor (stdout)
	mov	rax, 4		; system call number (sys_write)
	int	0x80		; call kernel	

	mov	rbx, 0		; return 0
	mov	rax, exit	; system call number (sys_exit)
	int	0x80		; call kernel
	

that works it's only when try to do the write with the memory address from 
the stack. I think i overlook something here but thanks for trying to 
help me.

Holger

Re: [Nasm-users] can't write argv[0] on stdout - solved

[Holger H. <ha...@gm...>]

On Tue, 18 Aug 2009 21:19:21 +0200
Holger Hanrath <ha...@gm...> wrote:


here is the program what that does what i want in native x86_64:

; write 3 bytes from argv[0] to stdout
; syscall numbers from asm/unistd_64.h
stdout		equ	1
write		equ	1
exit		equ	60
	
section .text
     global _start

     _start:
	pop	rax		; rax = argc 
	pop	rax		; rax = argv[0]
	
         ; place the length of the string in RDX
	mov	rdx,	3	; write 3 bytes
	
	; print the string using write() system call
	mov     rsi, 	rax	; write needs the address of the string in rsi
        mov     rdi,	stdout	
	mov	rax,	write
        syscall

         ; exit from the application here
        xor     rdi,	rdi	; return 0
        mov     rax, 	exit
        syscall

Re: [Nasm-users] can't write argv[0] on stdout - solved

[H. P. A. <hp...@zy...>]

On 08/19/2009 01:48 AM, Holger Hanrath wrote:
> On Tue, 18 Aug 2009 21:19:21 +0200
> Holger Hanrath <ha...@gm...> wrote:
> 
> 
> here is the program what that does what i want in native x86_64:
> 

Yes, but inefficient.

> ; write 3 bytes from argv[0] to stdout
> ; syscall numbers from asm/unistd_64.h
> stdout		equ	1
> write		equ	1
> exit		equ	60
> 	
> 	mov	rax,	write

Small number... you probably want "mov eax,..." instead.

etc.

	-hpa

-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.

Re: [Nasm-users] can't write argv[0] on stdout - solved

[Holger H. <ha...@gm...>]

On Wed, 19 Aug 2009 09:50:17 -0700
"H. Peter Anvin" <hp...@zy...> wrote:

> On 08/19/2009 01:48 AM, Holger Hanrath wrote:
> > On Tue, 18 Aug 2009 21:19:21 +0200
> > Holger Hanrath <ha...@gm...> wrote:
> > 
> > 
> > here is the program what that does what i want in native x86_64:
> > 
> 
> Yes, but inefficient.

true 

> > ; write 3 bytes from argv[0] to stdout
> > ; syscall numbers from asm/unistd_64.h
> > stdout		equ	1
> > write		equ	1
> > exit		equ	60
> > 	
> > 	mov	rax,	write
> 
> Small number... you probably want "mov eax,..." instead.

from the AMD64 Abi:

"3. The number of the syscall has to be passed in register %rax."

Holger

Re: [Nasm-users] can't write argv[0] on stdout - solved

[H. P. A. <hp...@zy...>]

On 08/19/2009 10:56 AM, Holger Hanrath wrote:
> 
> from the AMD64 Abi:
> 
> "3. The number of the syscall has to be passed in register %rax."
> 

Correct.  Loading a value into eax clears the upper 32 bits of rax, however.

	-hpa

-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.

Re: [Nasm-users] can't write argv[0] on stdout - solved

[Frank K. <fbk...@zy...>]

H. Peter Anvin wrote:
> On 08/19/2009 01:48 AM, Holger Hanrath wrote:
>> On Tue, 18 Aug 2009 21:19:21 +0200
>> Holger Hanrath <ha...@gm...> wrote:
>>
>>
>> here is the program what that does what i want in native x86_64:
>>
> 
> Yes, but inefficient.
> 
>> ; write 3 bytes from argv[0] to stdout
>> ; syscall numbers from asm/unistd_64.h
>> stdout		equ	1
>> write		equ	1
>> exit		equ	60
>> 	
>> 	mov	rax,	write
> 
> Small number... you probably want "mov eax,..." instead.
> 
> etc.

Or even:

push write
pop rax

Smaller, if that's the goal, but slower (I think), and it doesn't do 
much for "readability", IMO.

There's a tutorial here:

<http://www.vikaskumar.org/wiki/index.php?title=X86-64_Tutorial>

Might help...

Best,
Frank

Re: [Nasm-users] can't write argv[0] on stdout - solved

[H. P. A. <hp...@zy...>]

On 08/19/2009 10:11 AM, Frank Kotler wrote:
> 
> Or even:
> 
> push write
> pop rax
> 
> Smaller, if that's the goal, but slower (I think), and it doesn't do 
> much for "readability", IMO.
> 

	mov eax,...

... is even smaller (5 bytes.)

	-hpa

Re: [Nasm-users] can't write argv[0] on stdout - solved

[Frank K. <fbk...@zy...>]

H. Peter Anvin wrote:
> On 08/19/2009 10:11 AM, Frank Kotler wrote:
>> Or even:
>>
>> push write
>> pop rax
>>
>> Smaller, if that's the goal, but slower (I think), and it doesn't do 
>> much for "readability", IMO.
>>
> 
> 	mov eax,...
> 
> ... is even smaller (5 bytes.)

I should have specified the "-Ox" switch, or "push byte write".

Best,
Frank

Re: [Nasm-users] can't write argv[0] on stdout - solved

[H. P. A. <hp...@zy...>]

On 08/19/2009 11:43 AM, Frank Kotler wrote:
> H. Peter Anvin wrote:
>> On 08/19/2009 10:11 AM, Frank Kotler wrote:
>>> Or even:
>>>
>>> push write
>>> pop rax
>>>
>>> Smaller, if that's the goal, but slower (I think), and it doesn't do 
>>> much for "readability", IMO.
>>>
>> 	mov eax,...
>>
>> ... is even smaller (5 bytes.)
> 
> I should have specified the "-Ox" switch, or "push byte write".
> 

Ah yes, that does work for values <= 127.  It's really slow, though.

	-hpa

Re: [Nasm-users] can't write argv[0] on stdout

[H. P. A. <hp...@zy...>]

On 08/18/2009 07:34 AM, Holger Hanrath wrote:
> Hi all,
> 
> i want to write 3 bytes of argv[0] on stdout but it doesn't work. 
> Return code -14 (EFAULT Bad address). Am i missing something here? 
> 
> System is archlinux 2009.08 x86_64 kernel 2.6.31-rc6 
> nasm 2.05.01 Binutils 2.19.1.20090418
> 
> 
> 
> stdout	equ	1
> write	equ	4
> exit	equ	1
> 
> section .text
> 	global	_start
> _start:	
> 	pop	rax		; rax = argc
> 	pop	rax		; rax = argv[0]
> 
> 	mov	rdx,	3	; write 3 bytes
> 	mov	rcx,	rax	; rcx = rax = argv[0]
> 	mov	rbx,	stdout	
> 	mov	rax,	write	
> 	int	0x80
> 	
> 	mov	rbx,	0
> 	mov	rax,	exit
> 	int	0x80
> 

int 0x80 is used for 32-bit system calls.  64 bits uses syscall, and
entirely different system call numbers and a different calling convention.

	-hpa

-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.

Re: [Nasm-users] can't write argv[0] on stdout

[Holger H. <ha...@gm...>]

On Tue, 18 Aug 2009 23:33:46 -0700
"H. Peter Anvin" <hp...@zy...> wrote:

> On 08/18/2009 07:34 AM, Holger Hanrath wrote:
> > Hi all,
> > 
> > i want to write 3 bytes of argv[0] on stdout but it doesn't work. 
> > Return code -14 (EFAULT Bad address). Am i missing something here? 
> > 
> > System is archlinux 2009.08 x86_64 kernel 2.6.31-rc6 
> > nasm 2.05.01 Binutils 2.19.1.20090418
> > 
> > 
> > 
> > stdout	equ	1
> > write	equ	4
> > exit	equ	1
> > 
> > section .text
> > 	global	_start
> > _start:	
> > 	pop	rax		; rax = argc
> > 	pop	rax		; rax = argv[0]
> > 
> > 	mov	rdx,	3	; write 3 bytes
> > 	mov	rcx,	rax	; rcx = rax = argv[0]
> > 	mov	rbx,	stdout	
> > 	mov	rax,	write	
> > 	int	0x80
> > 	
> > 	mov	rbx,	0
> > 	mov	rax,	exit
> > 	int	0x80
> > 
> 
> int 0x80 is used for 32-bit system calls.  64 bits uses syscall, and
> entirely different system call numbers and a different calling convention.

i see that now thanks for pointing out

Holger

Re: [Nasm-users] can't write argv[0] on stdout - solved

[Holger H. <ha...@gm...>]

On Wed, 19 Aug 2009 13:11:49 -0400
Frank Kotler <fbk...@zy...> wrote:

> H. Peter Anvin wrote:
> > On 08/19/2009 01:48 AM, Holger Hanrath wrote:
> >> On Tue, 18 Aug 2009 21:19:21 +0200
> >> Holger Hanrath <ha...@gm...> wrote:
> >>
> >>
> >> here is the program what that does what i want in native x86_64:
> >>
> > 
> > Yes, but inefficient.
> > 
> >> ; write 3 bytes from argv[0] to stdout
> >> ; syscall numbers from asm/unistd_64.h
> >> stdout		equ	1
> >> write		equ	1
> >> exit		equ	60
> >> 	
> >> 	mov	rax,	write
> > 
> > Small number... you probably want "mov eax,..." instead.
> > 
> > etc.
> 
> Or even:
> 
> push write
> pop rax
> 
> Smaller, if that's the goal, but slower (I think), and it doesn't do 
> much for "readability", IMO.
> 
> There's a tutorial here:
> 
> <http://www.vikaskumar.org/wiki/index.php?title=X86-64_Tutorial>

good link wish i had it earlier

Holger