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;01234567890123456789012345678901234567890123456789012345678901234567890
;=======================================================================
;+---------------------------------------------------------------------+
;| |
;| Example using brk() system call for dynamic memory allocations. |
;| |
;| DON'T CONFUSE that brk() used in C Function is different with brk() |
;| systemcall (systemcall 45 for x86 ASM). |
;| |
;| In C program, brk() returns -1 if failed. But in assembly program |
;| that uses brk() systemcall (systemcall 45), the brk() returns |
;| the current break address if failed. |
;| |
;| In C program, when brk() success, it returns 0. But, brk() |
;| systemcall returns the new break address if success. |
;| |
;+---------------------------------------------------------------------+
;| AUTHOR: Nik Mohamad Aizuddin bin Nik Azmi |
;| EMAIL: nickaizuddin93@gmail.com |
;| DATE CREATED: 13/OCT/2014 |
;+---------------------------------------------------------------------+
;| LANGUAGE: x86 Assembly Language |
;| ASSEMBLER: NASM |
;| SYNTAX: Intel |
;| ARCHITECTURE: i386, i586, i686, x86_64, ia32 |
;| KERNEL: Linux 32-bit |
;| FORMAT: ELF32 |
;+---------------------------------------------------------------------+
;| Do whatever you want with this source code :) |
;+---------------------------------------------------------------------+
;=======================================================================
; ---- section read/write data -----------------------------------------
section .data
initial_break:
dd 0x00000000
current_break:
dd 0x00000000
new_break:
dd 0x00000000
; ---- section instruction codes ---------------------------------------
section .text
; etext, edata, end, have nothing to do with brk().
extern etext ;The address of the end of the text segment.
extern edata ;The address of the end of the initialized data segment.
extern end ;The address of the end of the uninitialized (bss segment).
; In gdb, type "x/x etext", "x/x edata", "x/x end" to check the value.
; Use "$ man 3 end" for more info.
global _start
_start:
;+---------------------------------------------------------------------+
;| About brk() system call, please read only at "LINUX NOTES" in |
;| "$ man 2 brk" manpage. |
;| |
;| BECAUSE brk() in C Function is DIFFERENT WITH brk() in systemcall! |
;| |
;| As it says, the brk() system call returns current break address |
;| if failed. If success, brk() returns new break address. |
;| |
;| To get the current break address, pass any value that makes this |
;| system call failed. |
;+---------------------------------------------------------------------+
; get current break address
;-----------------------------------------------------------------------
mov eax, 45 ;system call brk
mov ebx, 0 ;invalid address
int 0x80
mov [current_break], eax
mov [initial_break], eax
; allocate 8 bytes of memory on the heap
;-----------------------------------------------------------------------
mov eax, 45 ;system call brk
mov ebx, [current_break]
add ebx, 8 ;allocate 8 bytes
int 0x80
mov [new_break], eax
mov [current_break], eax
; allocate another 67108864 bytes of memory on the heap
;-----------------------------------------------------------------------
mov eax, 45 ;system call brk
mov ebx, [current_break]
add ebx, 67108864 ;allocate 67108864 bytes
int 0x80
mov [new_break], eax
.b0: ;Break the program here in GDB. Also watch the memory used by
;using "$ top" command. You'll noticed the memory used by this
;program dropped from 65688kb to 148kb.
mov [current_break], eax
; free all allocated memory on the heap
;-----------------------------------------------------------------------
mov eax, 45 ;system call brk
mov ebx, [initial_break] ;reset break address to its initial addr
int 0x80
mov [new_break], eax
.b1: ;Break the program here in GDB, to see the memory drop.
.exit:
mov eax, 0x01 ;system call exit
mov ebx, 0x00 ;return value := 0
int 0x80
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