self modyfing code and GDT

[Henio Paszczak <blurrpp@xxxxxxxxx>]

> Hi.
> 1) I want to write self modyfing code. I'm using 125
> finction (sys_mprotect )
> 
> .section .text
>     
> .global _start
> _start:
> 	nop
> 	movl $125,%eax
> 	movl $smc_address,%ebx
> 	movl $4096,%ecx
> 	movl $7,%edx
> 	int $0x80
> 
> 	orl  %eax, %eax
> 	jnz .error	# if something wrong
> 
> 	movl $0xaabbccdd,smc_address
> 
> 	movl $0x0000000000,%eax
> .equ smc_address, .-4
> 
> 	# eax==0xaabbccdd
> 
>  .error:
> 	movl $1,%eax
> 	movl $0,%ebx
> 	int $0x80
> 
> but it doesn't work ( %eax=0 ). 
> Is there any other way by changing segment
> descriptor
> in GDT ?, and how to do this ( i have no rights to
> write it ).
> 
> 2) How to read 8-bytes of segment descriptor, how to
> get current segment selector and the same question
> about task-state segment ( selector is easy to
> obtain
> by 'str' instruction )
> Thank You in advice for help
> Lukas



 
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