Segmentation Fault happening before any code is run Assembly

Question

I am trying to write a shell code program that will call execve and spawn a shell. I am working in a 32 bit virtual machine that was offered for this class . The code is as follows:

section .text

global _start

_start:
;clear out registers
xor eax, eax
xor ebx, ebx
xor ecx, ecx
xor edx, edx
;exacve("/bin/sh",Null,NULL)
;ascii for /bin/sh;
;2f 62 9 6e 2f 73 68 3b
push 0x3b68732f
push 0x6e69622f
mov ebx, esp
mov al, 11
int 0x80
;exit(int status)
movv al, 1
xor ebx, ebx
int 0x80

I compile with nasm -f elf -g shell.asm and link with ld -o shell shell.o When I try to run it, I get a segmentation fault. I tried using gdb to see where I made the mistake, but, it segfaults even if a set a break point at _start+0. It says that there was a segfault at the address after the last instruction for the code. ie if The last line has an address of 0x804807c then the segmentation fault happens at 0x804807e before any of the code has a chance to run.

Could any one point me in the right direction so I can figure out how to fix this?

Answer 1

One mistake in your code is, that there is no 0x3b in ascii code of the string:

;exacve("/bin/sh",Null,NULL)
;ascii for /bin/sh;
;2f 62 9 6e 2f 73 68 3b
push 0x3b68732f
push 0x6e69622f

This following code shall fix this problem (assuming you do work with a little-endian machine):

;exacve("/bin/sh",Null,NULL)
;ascii for /bin/sh;
;2f 62 99 6e 2f 73 68 00
push 0x0068732f
push 0x6e69622f