Truth, Computing and Fail

This is meant for NASM running on a linux machine. First things first: everything in UNIX is a file. A device is a file, a network socket is a file and everything else you can imagine is a file. Each file has a file descriptor attached to it. Some standard file descriptors are 0 for STDIN (standard input), 1 for STDOUT (standard output) and 2 for STDERR (standard error output). Since everything is a file, you can read() and write() on them provided you have the file descriptor.

read(), write() and open() are some of the very important syscalls in linux that would allow you to manipulate any file (that is, provided you have the permissions). There are some 150+ of them if you are really interested to find out. For syscalls, you need to pass the necessary parameters in the ebx, ecx, edx registers and so on, depending upon the number of parameters. You can also call libc functions by declaring them as extern printf for example and pushing the necessary parameters on the stack, the last parameter to be pushed first. To understand what kind of parameters need to be passed for the syscalls, read the developer man pages. Like man 2 write for write(). Use this for any other syscall like read() or open() and also keep the list of syscalls mentioned earlier in this post handy.

Regarding passing arguments to the program, they are stored in the stack as

argc
argv[0]
argv[1]
argv[2]

and so on…
argv[0] is the program name itself, so we can pop and discard it safely.

The following is a small program in NASM that takes a filename as an argument and reads 8kB from the file and displays it on the screen.

section .data
buf times 8 db 0
bufsize db 8192

section .text
global _start

_start:
pop ebx
pop ebx
pop ebx
mov eax,5
mov ecx,2	;the read mode
int 80h

test eax,eax
jns file_read

mov ebx,eax
mov eax,1
int 80h
ret

file_read:
mov ebx,eax	;move the file descriptor
mov eax,3	;3 for read()
mov ecx,buf	;the buffer
mov edx,bufsize	;the buffer size
int 80h

mov edx,eax	;move the number of bytes read from the previous syscall to edx
mov eax,4	;4 for write()
mov ebx,1	;1, the STDOUT file descriptor
int 80h

mov eax,1
mov ebx,0
int 80h		;exit with error code 0
ret

To assemble: nasm -f elf filename.asm
To link: ld -s -o executable-name filename.o

To know more about the read and write modes, the file descriptors and some increased masochistic behavior, start with reading the /usr/include/unistd.h and /usr/include/fcntl.h files. Poke around that directory for some more knowledge.