How to set up execve call on nasm right?

Question

The following code shows

relocation truncated to fit: R_386_8 against `.data'

 child:
    mov eax,11
    mov ebx,echo
    mov ecx,argv
    xor edx,edx
    int 0x80

section .data    
echo db '/bin/echo',0

argv0 db 'echo',0
argv1 db 'child excuted',0
argv db argv0, argv1

How I should set up execve call right? I thought to put addresses in stack and use esp but I still need to know how to declare array of strings in nasm.

Answer 1

As @Jester suggested, you need to declare argv properly:

argv dd argv0, argv1, 0

db refers to defining a list of 8-bit bytes. dw would let you define 16-bit shorts. dd is for 32-bit numbers - on 32-bit Linux, this is the proper pointer size. (You would use dq on 64-bit, because that's a 64-bit number, which is the right pointer size in that case.)

execve expects a NULL-terminated list of pointers, so we use dd for each of our pointers, and then NULL (0) to terminate the parameter list.

Also - while execve normally does not return, robust code will take into account the possibility that execve fails, and handle this properly.