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l : linux-assembly@vger.kernel.org 18 March 2007 • 7:41PM -0400

self modyfing code and GDT
by Henio Paszczak

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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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