I'm trying to print the command line arguments given to my program, using nasm:
GLOBAL main
EXTERN printf
section .rodata
fmt db "Argument: %s", 10, 0
section .text
main:
push ebp ; push ebp0
mov ebp, esp ; [ebp1] == ebp0
push dword[ebp+8] ; push argc
call print_args
mov eax, 0 ; return(0)
mov esp, ebp ; pop
pop ebp ; stack frame
ret
print_args:
push ebp ; pusheo ebp1
mov ebp, esp ; [ebp2] == ebp1
mov edi, dword[ebp+8] ; [ebp+8] == argc
jmp lop
postlop:
mov esp, ebp
pop ebp
ret
lop:
sub edi, 1
cmp edi, 0
jz postlop
mov esi, [ebp] ; [esi] == ebp1
mov ebx, [esi + 12] ; [esi+12] = [ebp1+12] = argv[0]?
push ebx
push fmt
call printf
jmp lop
However, this prints only garbage (I believe this should print argv[0], argc-1 times.).
I'm compiling my code with:
nasm -f elf32 main.asm
gcc -m32 main.o -o main.out
What is wrong?
By the way, using dword[ebp+8] works correctly to pick up argc.
I'm running this on ubuntu. Program does output Argument: ... argc-1 times, but the ... is garbage.
Just like [epb+8]is argc, [esi + 12] is argv, i.e. the address of the array of argument adresses. Thus, in order to find argv[0], you have to dereference once more.
mov esi, [ebp] ; [esi] == ebp1
mov ebx, [esi + 12] ; [esi+12] = [ebp1+12] = argv
push dword [ebx] ; [ebx] = argv[0]
;^^^^^^^^^^^
push fmt
call printf
I worked on this and this is all you need
Assemble as so. On my 32-bit debian 9 VM:
$ nasm -felf32 -g args.s -o args.o
$ gcc -no-pie args.o -o args
segment .data
format db "%s",0x0a,0
segment .text
global main ; let the linker know about main
extern printf ; resolve printf from libc
main:
push ebp ; prepare stack frame for main
mov ebp, esp
sub esp, 8
mov edi, dword[ebp+8] ; get argc into edi
mov esi, dword[ebp+12] ; get first argv string into esi
start_loop:
xor eax, eax
push dword [esi] ; must dereference esi; points to argv
push format
call printf
add esi, 4 ; advance to the next pointer in argv
dec edi ; decrement edi from argc to 0
cmp edi, 0 ; when it hits 0, we're done
jnz start_loop ; end with NULL pointer
end_loop:
xor eax, eax
leave
ret
Related
Hello all.
So I'm learning assembly.And as per my usual learning steps with any new language I pick up I've arrived at networking with assembly.
Which, sadly isn't going that well as I've pretty much failed at step 0, which would be getting a socket through which communication can begin.
The assembly code should be roughly equal to the following C code:
#include <stdio.h>
#include <sys/socket.h>
int main(){
int sock;
sock = socket(AF_INET, SOCK_STREAM, 0);
}
(Let's ignore the fact that it's not closing the socket for now.)
So here's what I did thus far:
Checked the manual. Which would imply that I need to make a socketcall() this is all good and well. The problem starts with that it would need an int that describes what sort of socketcall it should make. The calls manpage isn't helping much with this either as it only describes that:
On a some architectures—for example, x86-64 and ARM—there is no
socketcall() system call; instead socket(2), accept(2), bind(2), and
so on really are implemented as separate system calls.
Yet there are no such calls in the original list of syscalls - and as far as I know the socket(), accept(), bind(), listen(), etc. are calls from libnet and not from the kernel. This got me utterly confused so I've decided to compile the above C code and check up on it with strace. This yielded the following:
socket(PF_INET, SOCK_STREAM, IPPROTO_IP) = 3
While that didn't got me any closer to knowing what socket() is it did explain it's arguments. For witch I don't seem to find the proper documentation (again). I thought that PF_INET, SOCK_STREAM, IPPROTO_IP would be defined in <sys/socket.h> but my grep-ing for them didn't seem to find anything of use. So I decided to just wing it by using gdb in tandem with disass main to find the values. This gave the following output:
Dump of assembler code for function main:
0x00000000004004fd <+0>: push rbp
0x00000000004004fe <+1>: mov rbp,rsp
0x0000000000400501 <+4>: sub rsp,0x10
0x0000000000400505 <+8>: mov edx,0x0
0x000000000040050a <+13>: mov esi,0x1
0x000000000040050f <+18>: mov edi,0x2
0x0000000000400514 <+23>: call 0x400400
0x0000000000400519 <+28>: mov DWORD PTR [rbp-0x4],eax
0x000000000040051c <+31>: leave
0x000000000040051d <+32>: ret
End of assembler dump.
In my experience this would imply that socket() gets it's parameters from EDX (PF_INET), ESI (SOCK_STREAM), and EDI (IPPROTO_IP). Which would be odd for a syscall (as the convention with linux syscalls would be to use EAX/RAX for the call number and other registers for the parameters in increasing order, eg. RBX, RCX, RDX ...). The fact that this is beaing CALL-ed and not INT 0x80'd would also imply that this is not in fact a system call but rather something thats being called from a shared object. Or something.
But then again. Passing arguments in registers is very odd for something that's CALL-ed. Normally as far as I know argument's for called things should be PUSH-ed onto the stack, as the compiler can't know what registers they would try to use.
This behavior becomes even more curious when checking the produced binary with ldd:
linux-vdso.so.1 (0x00007fff4a7fc000)
libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f56b0c61000)
/lib64/ld-linux-x86-64.so.2 (0x00007f56b1037000)
There appears to be no networking library's linked.
And that's the point where I've ran out of ideas.
So I'm asking for the following:
A documentation that describes the x86-64 linux kernel's actual syscalls and their associated numbers. (Preferably as a header file for C.)
The header files that define PF_INET, SOCK_STREAM, IPPROTO_IP as it really bugs me that I wasn't able to find them on my own system.
Maybe a tutorial for networking in assembly on x86-64 linux. (For x86-32 it's easy to find material but for some reason I came up empty with the 64 bits stuff.)
Thanks!
The 64 bit calling convention does use registers to pass arguments, both in user space and to system calls. As you have seen, the user space convention is rdi,rsi, rdx, rcx, r8, r9. For system calls, r10 is used instead of rcx which is clobbered by the syscall instruction. See wikipedia or the ABI documentation for more details.
The definitions of the various constants are hidden in header files, which are nevertheless easily found via a file system search assuming you have the necessary development packages installed. You should look in /usr/include/x86_64-linux-gnu/bits/socket.h and /usr/include/linux/in.h.
As for a system call list, it's trivial to google one, such as this. You can also always look in the kernel source of course.
socket.asm
; Socket
; Compile with: nasm -f elf socket.asm
; Link with (64 bit systems require elf_i386 option): ld -m elf_i386 socket.o -o socket
; Run with: ./socket
%include 'functions.asm'
SECTION .text
global _start
_start:
xor eax, eax ; init eax 0
xor ebx, ebx ; init ebx 0
xor edi, edi ; init edi 0
xor esi, esi ; init esi 0
_socket:
push byte 6 ; push 6 onto the stack (IPPROTO_TCP)
push byte 1 ; push 1 onto the stack (SOCK_STREAM)
push byte 2 ; push 2 onto the stack (PF_INET)
mov ecx, esp ; move address of arguments into ecx
mov ebx, 1 ; invoke subroutine SOCKET (1)
mov eax, 102 ; invoke SYS_SOCKETCALL (kernel opcode 102)
int 80h ; call the kernel
call iprintLF ; call our integer printing function (print the file descriptor in EAX or -1 on error)
_exit:
call quit ; call our quit function
more docs...
this is for x86 system. if you want use for x86_64 system change x86 register to x86_64. for example change 'eax' to 'rax' or 'esp' to 'rsp'. and change syscall value in eax(rax), see https://chromium.googlesource.com/chromiumos/docs/+/master/constants/syscalls.md
[bits 32]
global _start
section .data
msg: db "Socket Failed To Create!",0xa,0
len: equ $-msg
msg1: db "Socket Created",0xa,0
len1: equ $-msg1
msg2: db "Recv Or Send Failed",0xa,0
len2: equ $-msg2
msg3: db "Shutdown Socket Failed",0xa,0
len3: equ $-msg3
DATASIZE: equ 5
SOCK_STREAM: equ 1
AF_INET: equ 2
AF_INET: equ 2
INADDR_ANY: equ 0
MSG_WAITALL: equ 0x100
MSG_DONTWAIT: equ 0x40
SHUT_RDWR: equ 2
SYS_SOCKET: equ 1 ; sys_socket(2)
SYS_BIND: equ 2 ; sys_bind(2)
SYS_CONNECT: equ 3 ; sys_connect(2)
SYS_LISTEN: equ 4 ; sys_listen(2)
SYS_ACCEPT: equ 5 ; sys_accept(2)
SYS_GETSOCKNAME:equ 6 ; sys_getsockname(2)
SYS_GETPEERNAME:equ 7 ; sys_getpeername(2)
SYS_SOCKETPAIR: equ 8 ; sys_socketpair(2)
SYS_SEND: equ 9 ; sys_send(2)
SYS_RECV: equ 10 ; sys_recv(2)
SYS_SENDTO: equ 11 ; sys_sendto(2)
SYS_RECVFROM: equ 12 ; sys_recvfrom(2)
SYS_SHUTDOWN: equ 13 ; sys_shutdown(2)
SYS_SETSOCKOPT: equ 14 ; sys_setsockopt(2)
SYS_GETSOCKOPT: equ 15 ; sys_getsockopt(2)
SYS_SENDMSG: equ 16 ; sys_sendmsg(2)
SYS_RECVMSG: equ 17 ; sys_recvmsg(2)
SYS_ACCEPT4: equ 18 ; sys_accept4(2)
SYS_RECVMMSG: equ 19 ; sys_recvmmsg(2)
SYS_SENDMMSG: equ 20 ; sys_sendmmsg(2)
struc sockaddr_in, -0x30
.sin_family: resb 2 ;2bytes
.sin_port: resb 2 ;2bytes
.sin_addr: resb 4 ;4bytes
.sin_zero: resb 8 ;8bytes
endstruc
struc socket, -0x40
.socketfd resb 4
.connectionfd resb 4
.count resb 4
.data resb DATASIZE
endstruc
section .text
_start:
push ebp
mov ebp, esp
sub esp, 0x400 ;1024byte
xor edx, edx ;or use cdq
;
; int socket(int domain, int type, int protocol);
; domain: The domain argument specifies a communication domain
;
push edx ; Push protocol
push dword SOCK_STREAM ; Push type
push dword AF_INET ; Push domain
mov ecx, esp ; ECX points to args
mov ebx, SYS_SOCKET ;
mov eax, 0x66 ; socketcall()
int 0x80
cmp eax, 0
jl .socket_failed
mov [ebp + socket.socketfd], eax
;
; fill struct sockaddr_in serv_addr;
;
mov word [ebp + sockaddr_in.sin_family], AF_INET
mov word [ebp + sockaddr_in.sin_port], 0x3905
mov dword [ebp + sockaddr_in.sin_addr], INADDR_ANY
push dword [ebp + sockaddr_in.sin_addr]
push word [ebp + sockaddr_in.sin_port]
push word [ebp + sockaddr_in.sin_family]
mov ecx, esp
;
; int bind(int sockfd, const struct sockaddr *addr, socklen_t addrlen);
;
push byte 0x10 ; sizeof(struct sockaddr)
push ecx ; pointer struct sockaddr
push dword [ebp + socket.socketfd]
mov ecx, esp ; ECX points to args
mov ebx, SYS_BIND ;
mov eax, 0x66
int 0x80
cmp eax, 0
jne .socket_failed
;
; int listen(int sockfd, int backlog);
;
push dword 0x10
push dword [ebp + socket.socketfd]
mov ecx, esp
mov ebx, SYS_LISTEN
mov eax, 0x66
int 0x80
cmp eax, 0
jne .socket_failed
;
; int accept(int sockfd, struct sockaddr *addr, socklen_t *addrlen);
;
xor ebx, ebx
push ebx
push ebx
push dword [ebp + socket.socketfd]
mov ecx, esp
mov ebx, SYS_ACCEPT
mov eax, 0x66
int 0x80
cmp eax, -1
je .socket_failed
mov [ebp + socket.connectionfd], eax
mov dword [ebp + socket.count], 0
.again:
lea edi, [ebp + socket.data]
mov ecx, DATASIZE
mov eax, 0
rep stosd
lea eax, [ebp + socket.data]
;
; ssize_t recv(int sockfd, const void *buf, size_t len, int flags);
;
push dword MSG_WAITALL
push dword DATASIZE
push eax
push dword [ebp + socket.connectionfd]
mov ecx, esp
mov ebx, SYS_RECV
mov eax, 0x66
int 0x80
cmp eax, 0
jle .recv_or_send_failed
mov edx, eax
lea ecx, [ebp + socket.data]
call printk
inc dword [ebp + socket.count]
cmp dword [ebp + socket.count], 5
jle .again
.break:
;
; int shutdown(int sockfd, int how);
;
push dword SHUT_RDWR
push dword [ebp + socket.socketfd]
mov ecx, esp
mov ebx, SYS_SHUTDOWN
mov eax, 0x66
int 0x80
cmp eax, 0
jne .shutdown_failed
;
; int close(int fd)
;
mov ebx, [ebp + socket.connectionfd]
mov eax, 0x06
int 0x80
cmp eax, 0
jne .shutdown_failed
jmp .success
.shutdown_failed:
mov edx, len3
mov ecx, msg3
call printk
jmp .end
.recv_or_send_failed:
mov edx, len2
mov ecx, msg2
call printk
jmp .end
.socket_failed:
mov edx, len
mov ecx, msg
call printk
jmp .end
.success:
mov edx, len1
mov ecx, msg1
call printk
jmp .end
.end:
leave
mov ebx,0 ;first syscall argument: exit code
mov eax,1 ;system call number (sys_exit)
int 0x80 ;call kernel
ret
; EDX: message length
; ECX: pointer to message to write
printk:
pusha
mov ebx,1 ;first argument: file handle (stdout)
mov eax,4 ;system call number (sys_write)
int 0x80 ;call kernel
popa
ret
I am trying to execute the following with execve: /bin//nc -lnke /bin/bash -p 4444
When reading the man page for execve, I see the following requirements:
int execve(const char *filename, char *const argv[],
char *const envp[]);
The issue I am running into is pushing arguments to argv; I do not understand how you push an array (in assembly) for this to work properly.
The assembly that I am currently using is below:
global _start
_start:
xor eax, eax
; command
push eax
push 0x636e2f2f
push 0x6e69622f
mov ebx, esp
; args
push eax
push 0x34343434
push 0x20702d20
push 0x68736162
push 0x2f6e6962
push 0x2f20656b
push 0x6e6c2d20
mov ecx, esp
; null, arg 1
push eax
mov edx, esp
; push to stack
push edx
push ecx
push ebx
; command
mov al, 11
int 0x80
It results in the following being passed to execve:
$ strace -f -e execve -s 10000 ./bind
execve("/bin//nc", [0x6e6c2d20, 0x2f20656b, 0x2f6e6962, 0x68736162, 0x20702d20, 0x34343434], 0xffd4c0d4 /* 0 vars */) = -1 EFAULT (Bad address)
How do I pass these arguments properly using assembly?
I am using Linux with nasm
In C, arrays are implicitly converted to pointers to their first elements. So in your case, you need to pass a pointer to an array of pointers to strings. Each array is terminated with a null pointer. Each string is terminated with a NUL byte. The arguments to system calls are passed in ebx, ecx, edx, etc. with the system call number being in eax. Like this:
section .data
; strings
arg0 db "/bin//nc",0
arg1 db "-lnke",0
arg2 db "/bin/bash",0
arg3 db "-p",0
arg4 db "4444",0
; arrays
align 4
argv dd arg0, arg1, arg2, arg3, arg4, 0
envp dd 0
section .text
global _start
_start: mov eax, 11 ; SYS_execve
mov ebx, arg0 ; filanem
mov ecx, argv ; argv
mov edx, envp ; envp
int 0x80 ; syscall
I'm not sure which operating system you are programming for. This example assumes Linux.
If you cannot use the data segment for some reason, proceed like this:
; for easier addressing
mov ebp, esp
; push strings
xor eax, eax
push eax ; - 4
push "4444" ; - 8
push "\0-p\0" ; -12
push "bash" ; -16
push "bin/" ; -20
push "ke\0/" ; -24
push "\0-ln" ; -28
push "//nc" ; -32
push "/bin" ; -36
; push argv, right to left
xor eax, eax
push eax ; NULL
lea ebx, [ebp-8]
push ebx ; "4444\0"
lea ebx, [ebp-11]
push ebx ; "-p\0"
lea ebx, [ebp-21]
push ebx ; "/bin/bash\0"
lea ebx, [ebp-27]
push ebx ; "-lnke\0"
lea ebx, [ebp-36] ; filename
push ebx ; "/bin//nc\0"
mov ecx, esp ; argv
lea edx, [ebp-4] ; envp (NULL)
mov al, 11 ; SYS_execve
int 0x80
In case you can't have null bytes in your data for whatever reason you need to do some cloaking beforehand. For example, you could push each byte xor'ed with 0x80 and xor the data on the stack with 0x80 again afterwards.
I'm trying to write a program that takes two arguments : the path of an executable file and the parameter to launch that executable with.
Example:
$ ./program /bin/ping 127.0.0.1
But the code I wrote does not seem to do anything, can you please tell me what am I doing wrong?
global main
section .text
main:
push ebp
mov ebp, esp
check_argc:
mov eax, [ebp + 8] ; eax <- argc
cmp eax, 1
jg do_execve
jmp done
do_execve:
mov eax,11 ; linux system call number (11) - sys_execve
mov ebx,[ebp+12] ; ebx <- argv[1]
lea ecx,[ebp+12] ; ebx <- &argv[1]
mov edx,0
int 0x80
done:
leave
ret
EDIT:
For compilation I used:
$ nasm -f elf32 program.asm
$ gcc -lc -m32 program.o -o program.exe
The "check_argc" part seems to work, I checked it with puts.
You problem lies here:
mov ebx,[ebp+12] ; ebx <- argv[1]
lea ecx,[ebp+12] ; ebx <- &argv[1]
The C prototype of main is: int main(int argc, char** argv), so what you're doing is actually:
mov ebx,[ebp+12] ; ebx <- argv = &argv[0]
lea ecx,[ebp+12] ; ecx <- &argv
What you want to do is something like the following:
mov ecx, [ebp+12] ; ecx <- &argv[0]
add ecx, 4 ; ecx <- &argv[1]
mov ebx, [ecx] ; ebx <- argv[1]
How do I write a variable to a file using NASM?
For example, if I execute some mathematical operation - how do I write the result of the operation to write a file?
My file results have remained empty.
My code:
%include "io.inc"
section .bss
result db 2
section .data
filename db "Downloads/output.txt", 0
section .text
global CMAIN
CMAIN:
mov eax,5
add eax,17
mov [result],eax
PRINT_DEC 2,[result]
jmp write
write:
mov EAX, 8
mov EBX, filename
mov ECX, 0700
int 0x80
mov EBX, EAX
mov EAX, 4
mov ECX, [result]
int 0x80
mov EAX, 6
int 0x80
mov eax, 1
int 0x80
jmp exit
exit:
xor eax, eax
ret
You have to implement ito (integer to ascii) subsequently len for this manner. This code tested and works properly in Ubuntu.
section .bss
answer resb 64
section .data
filename db "./output.txt", 0
section .text
global main
main:
mov eax,5
add eax,44412
push eax ; Push the new calculated number onto the stack
call itoa
mov EAX, 8
mov EBX, filename
mov ECX, 0x0700
int 0x80
push answer
call len
mov EBX, EAX
mov EAX, 4
mov ECX, answer
movzx EDX, di ; move with extended zero edi. length of the string
int 0x80
mov EAX, 6
int 0x80
mov eax, 1
int 0x80
jmp exit
exit:
xor eax, eax
ret
itoa:
; Recursive function. This is going to convert the integer to the character.
push ebp ; Setup a new stack frame
mov ebp, esp
push eax ; Save the registers
push ebx
push ecx
push edx
mov eax, [ebp + 8] ; eax is going to contain the integer
mov ebx, dword 10 ; This is our "stop" value as well as our value to divide with
mov ecx, answer ; Put a pointer to answer into ecx
push ebx ; Push ebx on the field for our "stop" value
itoa_loop:
cmp eax, ebx ; Compare eax, and ebx
jl itoa_unroll ; Jump if eax is less than ebx (which is 10)
xor edx, edx ; Clear edx
div ebx ; Divide by ebx (10)
push edx ; Push the remainder onto the stack
jmp itoa_loop ; Jump back to the top of the loop
itoa_unroll:
add al, 0x30 ; Add 0x30 to the bottom part of eax to make it an ASCII char
mov [ecx], byte al ; Move the ASCII char into the memory references by ecx
inc ecx ; Increment ecx
pop eax ; Pop the next variable from the stack
cmp eax, ebx ; Compare if eax is ebx
jne itoa_unroll ; If they are not equal, we jump back to the unroll loop
; else we are done, and we execute the next few commands
mov [ecx], byte 0xa ; Add a newline character to the end of the character array
inc ecx ; Increment ecx
mov [ecx], byte 0 ; Add a null byte to ecx, so that when we pass it to our
; len function it will properly give us a length
pop edx ; Restore registers
pop ecx
pop ebx
pop eax
mov esp, ebp
pop ebp
ret
len:
; Returns the length of a string. The string has to be null terminated. Otherwise this function
; will fail miserably.
; Upon return. edi will contain the length of the string.
push ebp ; Save the previous stack pointer. We restore it on return
mov ebp, esp ; We setup a new stack frame
push eax ; Save registers we are going to use. edi returns the length of the string
push ecx
mov ecx, [ebp + 8] ; Move the pointer to eax; we want an offset of one, to jump over the return address
mov edi, 0 ; Set the counter to 0. We are going to increment this each loop
len_loop: ; Just a quick label to jump to
movzx eax, byte [ecx + edi] ; Move the character to eax.
movsx eax, al ; Move al to eax. al is part of eax.
inc di ; Increase di.
cmp eax, 0 ; Compare eax to 0.
jnz len_loop ; If it is not zero, we jump back to len_loop and repeat.
dec di ; Remove one from the count
pop ecx ; Restore registers
pop eax
mov esp, ebp ; Set esp back to what ebp used to be.
pop ebp ; Restore the stack frame
ret ; Return to caller
How come this program is not printing out to the screen, am I missing something on the INT 80 command?
section .bss
section .data
hello: db "Hello World",0xa ;10 is EOL
section .text
global _start
_start:
mov ecx, 0; ; int i = 0;
loop:
mov dl, byte [hello + ecx] ; while(data[i] != EOF) {
cmp dl, 0xa ;
je exit ;
mov ebx, ecx ; store conetents of i (ecx)
; Print single character
mov eax, 4 ; set sys_write syscall
mov ecx, byte [hello + ebx] ; ...
mov edx, 1 ; move one byte at a time
int 0x80 ;
inc ebx ; i++
mov ecx, ebx ; move ebx back to ecx
jmp loop ;
exit:
mov eax, 0x01 ; 0x01 = syscall for exit
int 0x80 ;
ADDITION
My Makefile:
sandbox: sandbox.o
ld -o sandbox sandbox.o
sandbox.o: sandbox.asm
nasm -f elf -g -F stabs sandbox.asm -l sandbox.lst
Modified Code:
section .bss
section .data
hello: db "Hello World",0xa ;10 is EOL
section .text
global _start
_start:
mov ecx, 0; ; int i = 0;
while:
mov dl, byte [hello + ecx] ; while(data[i] != EOF) {
cmp dl, 0xa ;
je exit ;
mov ebx, ecx ; store conetents of i (ecx)
; Print single character
mov eax, 4 ; set sys_write syscall
mov cl, byte [hello + ebx] ; ...
mov edx, 1 ; move one byte at a time
int 0x80 ;
inc ebx ; i++
mov ecx, ebx ; move ebx back to ecx
jmp while ;
exit:
mov eax, 0x01 ; 0x01 = syscall for exit
int 0x80 ;
One of the reasons it's not printing is because ebx is supposed to hold the value 1 to specify stdin, and another is because sys_write takes a pointer (the address of your string) as an argument, not an actual character value.
Anyway, let me show you a simpler way of structuring your program:
section .data
SYS_EXIT equ 1
SYS_WRITE equ 4
STDOUT equ 1
TRAP equ 0x80
NUL equ 0
hello: db "Hello World",0xA,NUL ; 0xA is linefeed, terminate with NUL
section .text
global _start
_start:
nop ; for good old gdb
mov ecx, hello ; ecx is the char* to be passed to sys_write
read:
cmp byte[ecx], NUL ; NUL indicates the end of the string
je exit ; if reached the NUL terminator, exit
; setup the registers for a sys_write call
mov eax, SYS_WRITE ; syscall number for sys_write
mov ebx, STDOUT ; print to stdout
mov edx, 1 ; write 1 char at a time
int TRAP; ; execute the syscall
inc ecx ; increment the pointer to the next char
jmp read ; loop back to read
exit:
mov eax, SYS_EXIT ; load the syscall number for sys_exit
mov ebx, 0 ; return a code of 0
int TRAP ; execute the syscall
It can be simpler to NUL terminate your string as I did, or you could also do $-hello to get it's length at compile time. I also set the registers up for sys_write at each iteration in the loop (as you do), since sys_write doesn't preserve all the registers.
I don't know how you got your code to assemble, but it doesn't assemble over here for a couple of very good reasons.
You cannot use loop as a label name because that name is reserved for the loop instruction.
Your line 20's instruction mov ecx, byte [hello + ebx] doesn't assemble either because the source and destination operands' sizes don't match (byte vs dword). Possible changes:
mov cl, byte [hello + ebx]
mov ecx, dword [hello + ebx]
movzx ecx, byte [hello + ebx]
Was the above not the actual code you had?