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x86 Assembly Questions

Name: Anonymous 2014-04-26 23:52

$ cat bork.asm

section .data
fuck:	dd 0xDEADBEEF

section .text
global _start
_start:
	mov dword [fuck],0xABADC0DA

	mov eax,1
	mov ebx,0
	int 0x80

Why doesn't writing to the .data segment segfault? Shouldn't Linux map it to a read-only page?

$ nasm -f elf bork.asm
$ gold -s -o bork-gold bork.asm
$ ld -s -melf_i386 -o bork-ld bork.o
$ du -bh *
432	bork-gold
132	bork.asm
340	bork-ld
608	bork.o

How does such a minimal binary take up 300+ bytes?

Name: fibsiter.s 2014-05-18 8:37

extern print_number
        
section .text
global asm_main
asm_main:
        enter 0,0
        pusha

        ;; The loop instruction decrements ECX and jumps to the address specified by arg unless decrementing ECX caused its value to become zero.
;;; calculate fib(ecx), must be 2 or more
        mov ecx,1507

        mov eax,0
        mov ebx,1
        dec ecx
        
iter:
        mov edx,ebx
        add ebx,eax
        mov eax,edx
        loop iter

        push ebp
        mov ebp,esp
        push ebx
        call print_number
        add esp,4
        mov esp,ebp
        pop ebp
        
        popa
        mov eax,0 ; return 0
        leave
        ret

Name: Anonymous 2014-05-21 4:05

>>71

enter
pusha
popa
leave

286 QUALITY!

Name: Anonymous 2014-05-21 4:15

Love the "leave" mnemonic - it is 1-char less than the return from C/C++

Newer Posts

x86 Assembly Questions

1 Name: Anonymous 2014-04-26 23:52

71 Name: fibsiter.s 2014-05-18 8:37

72 Name: Anonymous 2014-05-21 4:05

73 Name: Anonymous 2014-05-21 4:15

Name: Anonymous 2014-04-26 23:52

Name: fibsiter.s 2014-05-18 8:37

Name: Anonymous 2014-05-21 4:05

Name: Anonymous 2014-05-21 4:15