Suppose, we have a macro that writes code into .data and .text sections:
%macro my_macro 0
[section .data]
table dd 1024
[section .text]
dec eax ; some commmands
%endmacro
global _start
section .text:
_start:
jmp .label_jump
my_macro
.label_jump:
mov eax, 1
mov ebx, 0
int 80h
If we have a local label placed after the macro call, it is impossible to jump to it above the macro call:
$ nasm test29.asm
test.asm:11: error: symbol `_start.label_jump' undefined
Is it possible to use the local labels in such circumstances?
Related
How can i access the exception chain (SEH) using masm64?
Using masm32, I get the first exception looking into fs:[0]
But when I checked in Windbg if fs:[0] still pointed at the first exception in x64, I figured that it wasn't.
I'd like to set an exception in x64 the same way I did in x86. Is it feasible (maybe looking at gs register)?
If this is coding related then you use
ml64seh PROC FRAME:ExceptionFilter
this adds your exception handler to .PDATA section RUNTIME_FUNCTION
if this is how to find this exception handler in windbg when the exception has been raised
use !exchain command
or if you want to find it before executing a specific function use .fnent command
a sample for x64 seh and finding it in windbg is as follows
;assemble and link with
;ml64 /Zi ml64seh.asm /link /debug /entry:ml64seh /subsystem:console
.data
safeplace DWORD ?
.code
ExceptionFilter PROC
jmp Handler
ExceptionFilter ENDP
PUBLIC ml64seh
ml64seh PROC FRAME:ExceptionFilter
.ENDPROLOG
mov rax, 0
mov [rax], rax ;access violation
jmp exit
Handler::
lea rax, safeplace
mov [r8+078h], rax ; replacing rax in exception handler so access is possible
mov rax, 0
ret
Exit:
ret
ml64seh ENDP
END
run without stopping in windbg
:\>cdb -g ml64seh.exe
(2aa0.3024): Access violation - code c0000005 (first chance)
ml64seh!ml64seh+0x7:
00007ff7`0e3b1029 488900 mov qword ptr [rax],rax ds:00000000`00000000=????????????????
0:000>
it crashed and broke now locating exception handlers
0:000> .fnent .
Debugger function entry 0000020b`e36c47a8 for:
(00007ff7`0e3b1022) ml64seh!ml64seh+0x7 | (00007ff7`0e3b32b0) ml64seh!$xdatasym
BeginAddress = 00000000`00001022
EndAddress = 00000000`00001042
UnwindInfoAddress = 00000000`000032b0
Unwind info at 00007ff7`0e3b32b0, c bytes
version 1, flags 3, prolog 0, codes 0
handler routine: ml64seh!ILT+0(ExceptionFilter) (00007ff7`0e3b1005), data 0 <<<<<<<<<
0:000> !exchain
3 stack frames, scanning for handlers...
Frame 0x00: ml64seh!ml64seh+0x7 (00007ff7`0e3b1029)
ehandler ml64seh!ILT+0(ExceptionFilter) (00007ff7`0e3b1005) <<<<<<<<<<<<
Frame 0x02: ntdll!RtlUserThreadStart+0x21 (00007ffe`213c26a1)
ehandler ntdll!_C_specific_handler (00007ffe`213fc720)
0:000>
lets see if we go to the handler and return back to re access the faulting place
0:000> bp .
0:000> bp 00007ff7`0e3b1005
0:000> bl
0 e 00007ff7`0e3b1029 0001 (0001) 0:**** ml64seh!ml64seh+0x7
1 e 00007ff7`0e3b1005 0001 (0001) 0:**** ml64seh!ILT+0(ExceptionFilter)
0:000> g
Breakpoint 1 hit
ml64seh!ILT+0(ExceptionFilter):
00007ff7`0e3b1005 e916000000 jmp ml64seh!ExceptionFilter (00007ff7`0e3b1020)
0:000> g
Breakpoint 0 hit
ml64seh!ml64seh+0x7: is accessible now
00007ff7`0e3b1029 488900 mov qword ptr [rax],rax ds:00007ff7`0e3b4000=0000000000000000
0:000>
btw you can use dumpbin or linker to spit out all the unwindinfos in a specific binary using -unwindinfo switch
:\>dumpbin /unwindinfo ml64seh.exe
Microsoft (R) COFF/PE Dumper Version 14.29.30146.0
Copyright (C) Microsoft Corporation. All rights reserved.
Dump of file ml64seh.exe
File Type: EXECUTABLE IMAGE
Function Table (1)
Begin End Info Function Name
00000000 00001022 00001042 000032B0 ml64seh
Unwind version: 1
Unwind flags: EHANDLER UHANDLER
Size of prologue: 0x00
Count of codes: 0
Handler: 00001005 #ILT+0(ExceptionFilter)
MY CODE IS
.model small
.data
arr db 83h, 12h, 0F0h, 0Bh, 89h
cnt db 05h
.code
mov ax, #data
mov ds, ax
mov si, offset arr
mov cl, cnt
sub bl, bl
sub dl, dl
back:
mov al, [si]
and al, 80h
jz skip
inc bl
skip:
inc dl
inc si
dec cl
jnz back
mov ah, 4cH
int 21H
end
THE ERRORS ARE AS FOLLOWS
1.asm:1: error: attempt to define a local label before any non-local labels
1.asm:1: error: parser: instruction expected
1.asm:2: error: attempt to define a local label before any non-local labels
1.asm:9: error: comma, colon, decorator or end of line expected after operand
I've been using the %local directive in NASM to define local variables and thus avoid typing [ebp - 8], [ebp - 24] etc. all the time.
However, I noticed that a local variable defined in one function between the preprocessor context %push and %pop is still available in the rest of the code, which may result in unexpected parsing errors.
Here, I've written a minimal example demonstrating the problem:
struc Rect
.left resd 1
.top resd 1
.width resd 1
.height resd 1
endstruc
%define RECT(x) g_rect + Rect. %+ x
segment .bss
g_rect resd 4
segment .text
; ================================================
function1:
%push
%stacksize flat
%assign %$localsize 0
%local width:dword ; defines local var "width"
push ebp
mov ebp, esp
sub esp, 4
pusha
; ...
popa
leave
ret
%pop
; ================================================
function2:
push ebp
mov ebp, esp
pusha
; ...
mov eax, RECT(height) ; OK
mov ebx, RECT(width) ; Parse error
; ...
popa
leave
ret
The exact error is:
nasm -f elf -d ELF_TYPE -g test.asm
test.asm:42: error: comma, colon, decorator or end of line expected after operand
Obviously, it happens because width is getting substituted with something else, and if I remove the local param definition, the problem goes away.
As you can see, the variable width is still available after the %pop. This doesn't look very local to me! I'd expect NASM to undefine width when the %pop is executed.
Is there a way to use %local but avoid these leaking macros? At the moment they act as a simple %define statement which is confusing.
%include "init.inc"
[org 0x0]
[bits 16]
jmp 0x07C0:start_boot
start_boot:
mov ax, cs
mov ds, ax
mov es, ax
load_setup:
mov ax, SETUP_SEG
mov es, ax
xor bx, bx
mov ah, 2 ; copy data to es:bx from disk.
mov al, 1 ; read a sector.
mov ch, 0 ; cylinder 0
mov cl, 2 ; read data since sector 2.
mov dh, 0 ; Head = 0
mov dl, 0 ; Drive = 0
int 0x13 ; BIOS call.
jc load_setup
lea si, [msg_load_setup]
call print
jmp $
print:
print_beg:
mov ax, 0xB800
mov es, ax
xor di, di
print_msg:
mov al, byte [si]
mov byte [es:di], al
or al, al
jz print_end
inc di
mov byte [es:di], BG_TEXT_COLOR
inc di
inc si
jmp print_msg
print_end:
ret
msg_load_setup db "Loading setup.bin was completed." , 0
times 510-($-$$) db 0
dw 0xAA55
I want to load setup.bin to memory address zero. So, I input 0 value to es register (SETUP_SEG = 0). bx, too. But it didn't work. then, I have a question about this issue. My test is below.
SETUP_SEG's value
0x0000 : fail
0x0010 : success
0x0020 : fail
0x0030 : fail
0x0040 : fail
0x0050 : success
I can't understand why this situation was happened. All test was carried out on VMware. Does anyone have an idea ?
I'm not sure if this is your problem, but your trying to load setup.bin in the Real Mode IVT (Interrupt Vector Table). The IVT contains the location of each interrupt, so I'm assuming that your boatloader is overwriting them when it loads setup.bin into memory! Interrupts can be sneaky and tricky, since they can be called even if you didn't call them. Any interrupt vector you overwrote will likely cause undefined behavior when called, which will cause some problems.
I suggest setting SETUP_SEG to a higher number like 0x2000 or 0x3000, but the lowest you could safely go is 0x07E0. The Osdev Wiki and Wikipedia have some helpful information on conventional memory and memory mapping.
I hope this helps!
I have a problem with printing system date because of cx register size. How can it be solved without any big changes?
title casadatum
zas segment stack
db 256 dup(?)
zas ends
strsize EQU 64
dat segment
print db 'Current System Date is : $'
date db 'dd:mm:rrrr$'
nl db 10,13,'$'
dat ends
code segment
assume cs:code, ss:zas, ds:dat
get_date proc
mov ah,2ah
int 21h
mov al,dl
call convert
mov [bx],ax
mov al,dh
call convert
mov [bx+3],ax
mov al,cx
call convert
mov [bx+6],ax
ret
endp
convert proc
push dx
mov ah,0
mov dl,10
div dl
or ax, 3030h
pop dx
ret
endp
start:
mov ax, seg dat
mov ds,ax
LEA BX, date
CALL GET_date
lea dx,print
mov ah,09h
int 21h
lea dx,date
mov ah,09h
int 21h
koniec:
mov ah, 4ch
int 21h
code ends
end start
Replace this part of your code
mov al,cx
call convert
mov [bx+6],ax
with these instructions
mov al,100
xchg ax,cx
div cl
mov ch,ah
call convert
mov [bx+6],ax
mov al,ch
call convert
mov [bx+8],ax