Assembly Code keep showing segment fault

Question

Why this assembly code assemble and link fine but show segment fault in runtime. Commented after the instruction to give a idea what I wanted to do.

  jmp short init

action:
  pop esi
  xor eax, eax
  mov byte [esi+24], al ;null terminating the string.
  mov dword [esi+25],24 ;length of the string

  mov al,4 ;syscall write
  mov ebx,1 ;standard out
  lea ecx,[esi]   ;<<---------- Unsure about this. probably load the address of the string to ecx  
  mov edx,[esi+25] ;<<-- load edx with string length
  int 80h


init:
  call action
  db "what a pity! not working#LLLL"

I am using NASM to assemble and ld to link. This program will run on a 64-bit machine but I want it to be 32-bit compatible.

Your string appears to be in section .text (which Nasm will default to, if you don't say) which is read-only memory. Attempting to alter it will segfault. If it ran, it would loop forever. I don't see a sys_exit in there... — Frank Kotler
– Frank Kotler, Commented Nov 3, 2013 at 6:15
Well the orginal program have a sys_exit. How to make it writiable? — Shakil Ahamed
– Shakil Ahamed, Commented Nov 3, 2013 at 12:56
Looks recursive to me - drops through to :init and calls action again... — Martin James
– Martin James, Commented Nov 4, 2013 at 18:14

Devolus · Accepted Answer · 2013-11-03 07:28:04Z

1

You want to have the address of the string in ecx. So why do you pop esi ? Use pop ecx and you already have it in the correct register. You don't need it anyway for some other purpose.

Apart from that, you are writing to the code segment, which is not writable by default.

edited Nov 3, 2013 at 7:28

answered Nov 3, 2013 at 7:18

Devolus

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2 Comments

Shakil Ahamed Over a year ago

Thanks for your recommendation to pop in ecx. But is here a problem with pop esi. Also how to make the code writable

Frank Kotler Over a year ago

section .text write will make your .o file writable. Dump it. See, writable! However, ld changes it back to readonly. :( section .kode exec write seems to work. I think there's a way to make ld do this with .text, but I don't know it. Better yet, don't!

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