segmentation

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asmrox

Joined: 19 Jan 2008
Posts: 160

asmrox 14 Feb 2008, 01:54

cs - segment pointer of .code
ds - segment pointer of .data

Code:

format pe console
section '.code' code readable executable writeable
push dword [ds:0]
push f
call [printf]
add esp,8
ret
section '.data' data readable writeable executable
db 'test',0 ;this is under offset 0
f db '%s',0 ;this is ds:5
section '.idata' import data readable
dd 0,0,0,RVA msvcrt_name,RVA msvcrt_table
dd 5 dup 0
msvcrt_table:
printf dd RVA _pritnf
dd 0
msvcrt_name db 'msvcrt.dll',0
_pritnf db 0,0,'printf',0

Access violation, why?

14 Feb 2008, 01:54

System86

Joined: 15 Aug 2007
Posts: 77

System86 14 Feb 2008, 03:00

Windows does not use segmentation (except for the fs segment register, but that's a different story). It uses flat addressing, and in your code you attempted to access address 0, which your application can't do, so it crashed. Forget about cs/ds/es/ss under Win32, the segment registers always have 0 base, 4 GB limit, so you can ignore them.

14 Feb 2008, 03:00

asmrox

Joined: 19 Jan 2008
Posts: 160

asmrox 14 Feb 2008, 19:13

so why sections was made?

14 Feb 2008, 19:13

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20632
Location: In your JS exploiting you and your system

revolution 14 Feb 2008, 19:20

asmrox wrote:

so why sections was made?

Just 'cause there are effectively no segments doesn't mean it can't use paging. This is how the OS allocates memory to your proggy, by paging it in a section at a time.

14 Feb 2008, 19:20

asmrox

Joined: 19 Jan 2008
Posts: 160

asmrox 14 Feb 2008, 20:31

so even simply hello world will load 4096 bytes memory of?
is that same as heap? can i use rest of memory? Where is the pointer to it?

14 Feb 2008, 20:31

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