self modyfing code and GDT

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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