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revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20449
Location: In your JS exploiting you and your system

revolution 05 Apr 2008, 16:09

White-spirit: You have a stack imbalance.

For all your asm routines that are called from C, don't use "ret num", use only "ret".

For all your asm routines that call into C code use "add esp,number_of_parameters*4".

If you still can't see it, use a debugger to follow through and watch the stack to see what happens.

05 Apr 2008, 16:09

White-spirit

Joined: 26 Mar 2008
Posts: 27

White-spirit 06 Apr 2008, 14:48

Thanks revolution .

I've added a prolog to my call_test function and it works ,at least, it doesn't reboot but the ( . ) is not printed on the screen :s

Here's the code :

Code:

format ms coff

public call_test

call_test:
push ebp
mov ebp, esp
sub esp, 4
push '.'
call putchar
add esp,4
leave
ret

06 Apr 2008, 14:48

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20449
Location: In your JS exploiting you and your system

revolution 06 Apr 2008, 14:57

Why do you have "sub esp, 4", that is not needed

06 Apr 2008, 14:57

White-spirit

Joined: 26 Mar 2008
Posts: 27

White-spirit 06 Apr 2008, 15:34

Without the "sub esp, 4" the kernel reboot, and I've found it here : http://www.milw0rm.com/papers/52 .

06 Apr 2008, 15:34

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20449
Location: In your JS exploiting you and your system

revolution 06 Apr 2008, 16:09

White-spirit wrote:

Without the "sub esp, 4" the kernel reboot, and I've found it here : http://www.milw0rm.com/papers/52 .

I think you must have a major problem with your kernel. The "sub esp,4" is really not needed unless you are using that stack space for a local variable. But in your function you don't use it for local storage. And indeed the "leave" will restore the stack properly so whether you have the "sub esp,4" or not will make no difference at all to either your function or to the kernel. You could even put "sub esp,1000" and it wouldn't matter, it is just wasted stack space.

06 Apr 2008, 16:09

White-spirit

Joined: 26 Mar 2008
Posts: 27

White-spirit 06 Apr 2008, 16:14

Can I post here my boot sector ( in asm ) and my kernel's ( C ) source code and my GCC and binutils version ?

Thanks

06 Apr 2008, 16:14

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20449
Location: In your JS exploiting you and your system

revolution 06 Apr 2008, 16:19

White-spirit wrote:

Can I post here my boot sector ( in asm ) and my kernel's ( C ) source code and my GCC and binutils version ?

Well I expect the asm code will be acceptable, but this is not really the place for a kernel in C or binutils and things.

Have a search around here for other boot sectors people have posted previously. You might get some insight from reading how other approached the problem.

06 Apr 2008, 16:19

White-spirit

Joined: 26 Mar 2008
Posts: 27

White-spirit 06 Apr 2008, 16:34

Okay thanks, here's my boot sector's code :

Code:

use16
org 0x7C00

;;;;;;;;; Loading the Kernel from floppy to 0x1000 ;;;;;;;;;
xor ax, ax                                ; ax <== 0
mov es, ax                             ; es <== ax <== 0
mov bx, 0x1000       ; destination adress
mov ah, 0x2                             ; function 02h ( reading sectors )
mov al, 0xE                               ; 14 sectors
xor ch, ch                              ;  ch <== 0 ( cylinder 0 )
mov cl, 0x2                            ; sector 2
xor dx,dx                         ; ( dh <== 0 , dl <== 0 )
int 0x13
;;;;;;;;; Memory infos ;;;;;;;;;
; coming soon ;

;;;;;;;;; Entering Protected Mode ;;;;;;;;;
cli                                            ; disables interrupts
xor ax,ax                              ; ax <== 0
mov ds,ax                              ; ds <== 0
lgdt [gdt_desc]
mov eax,cr0                         ; eax <== cr0
xor eax,eax
inc eax                                      ; eax <== 1
mov cr0,eax                           ; cr0 <== 1

;;;;;;;;; Jumping to Kernel ;;;;;;;;;
jmp dword 0x8:jmp_kernel
use32                                    ; using 32 bits instructions
jmp_kernel:
mov ax,0x10                          ; ax <== 10h
mov ds,ax                            ; ds <== 10h
mov ss,ax                            ; ss <== 10h
mov es,ax                            ; es <== 10h
mov fs,ax                            ; fs <== 10h
mov gs,ax                            ; eg <== 10h
mov esp, 0x90000             ; sets stack at 090000h
finit                                        ; initializing FPU registers
jmp dword 0x8:0x1000
jmp $

;;;;;;;;; Initializing GDT ;;;;;;;;;

gdt:

gdt_null:

gdt_desc:
     dw gdt_end - gdt - 0x1
      dd gdt
      dw 0x0

gdt_code:
 dw 0xFFFF
   dw 0x0
      db 0x0
      db 0x9A
     db 0xCF
     db 0x0
      
gdt_data:
   dw 0xFFFF
   dw 0x0
      db 0x0
      db 0x92
     db 0xCF
     db 0x0
      
gdt_end:
    
times 510-($-$$) db 0x90    ; Fills the file with Nop's
dw 0xAA55                                       ; MBR signature

06 Apr 2008, 16:34

DJ Mauretto

Joined: 14 Mar 2007
Posts: 464
Location: Rome,Italy

DJ Mauretto 06 Apr 2008, 19:23

Hello

Code:

;;;;;;;;; Initializing GDT ;;;;;;;;; 
align 16
gdt: 

gdt_null: 
        dw 0x0
        dw 0x0
        dw 0x0 
        dw 0x0

gdt_code: 
        dw 0xFFFF 
        dw 0x0 
        db 0x0 
        db 0x9A 
        db 0xCF 
        db 0x0 
         
gdt_data: 
        dw 0xFFFF 
        dw 0x0 
        db 0x0 
        db 0x92 
        db 0xCF 
        db 0x0 
gdt_end: 
gdt_desc: 
        dw gdt_end - gdt - 0x1 
        dd gdt

Code:

;;;;;;;;; Entering Protected Mode ;;;;;;;;; 
cli                                             ; disables interrupts 
xor ax,ax                               ; ax <== 0 
mov ds,ax                               ; ds <== 0 
lgdt [gdt_desc] 
mov eax,cr0                             ; eax <== cr0 
or eax,1                                 ; eax <== 1 
mov cr0,eax                             ; cr0 <== 1

06 Apr 2008, 19:23

White-spirit

Joined: 26 Mar 2008
Posts: 27

White-spirit 07 Apr 2008, 07:44

Thanks but it's still the same :s

07 Apr 2008, 07:44

edfed

Joined: 20 Feb 2006
Posts: 4353
Location: Now

edfed 07 Apr 2008, 08:34

Code:

mov ax, 100h                              ; ax <== 100h 
mov es, ax                              ; es <== ax <== 100h 
xor bx,bx        ; sometimes, it bugs to put a non zero offset in BX.
mov ah, 0x2                             ; function 02h ( reading sectors ) 
mov al, 0xE                             ; 14 sectors 
xor ch, ch                              ;  ch <== 0 ( cylinder 0 ) 
mov cl, 0x2                             ; sector 2 
xor dx,dx                               ; ( dh <== 0 , dl <== 0 ) 
int 0x13

are you sure the kernel is orged at 1000h ???

Mods: note you can move this thread in OS construction.

07 Apr 2008, 08:34

White-spirit

Joined: 26 Mar 2008
Posts: 27

White-spirit 07 Apr 2008, 10:44

Otherwise the kernel would not have shown messages on the screen before restarting, right?

PS : I thought it was juste a problem with linking the MS COFF object using ld so I asked here .

07 Apr 2008, 10:44

White-spirit

Joined: 26 Mar 2008
Posts: 27

White-spirit 07 Apr 2008, 12:46

Yeah, i used "format elf" instead of MS COFF and it works =')

07 Apr 2008, 12:46

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