ASM 120: Files (55 pts)

What You Need

• A Google Cloud Debian 10 Linux Server

Purpose

Writing to a File

section  .text
global   _start

_start:

    mov  eax, 8           ; 8 = sys_creat
    mov  ebx, filename
    mov  ecx, 700o        ; Permissions: 700 octal (rwx------)
    int  0x80

    mov [fd], eax         ; file descriptor

    mov  eax, 4           ; 4 = sys_write
    mov  ebx, [fd]
    mov  ecx, msg
    mov  edx, len
    int  0x80

    mov  eax, 6           ; 6 = sys_close
    mov  ebx, [fd]
    int  0x80

    mov  eax, 1           ; 1 = sys_exit
    int  0x80

section  .data
filename db   "write.txt", 0
msg      db   "Hello World!"
len      equ  $ - msg
fd       db   0, 0, 0, 0

nasm -f elf32 write.asm
ld -m elf_i386 -o write write.o
./write
ls -l w*
cat write.txt

ASM 120.1: File Length (5 pts) In the shell, execute these commands: sudo apt update sudo apt install wget -y wget https://samsclass.info/127/proj/ASM120 Write a program that reads every byte in that file and counts them. The flag is the length of the file in hexadecimal. Useful References Assembly - File Management File Handling - Create

ASM 120.2: Skip Bytes (10 pts) Use the same ASM120 file. Skip the first 230 bytes. Print out the next six bytes to see the flag.

ASM 120.3: Third N (10 pts) Use the same ASM120 file. Find the third capital N. Read five more letters to complete the flag.

ASM 120.4: Fifth (10 pts) Use the same ASM120 file. Print every fifth character to see the flag.

ASM 120.5: Nth (20 pts) Use the same ASM120 file. Gather every Nth character to see the flag. You don't know Nn.

Sources