[Linux x86 NASM] Open, Read & Write syscalls

[Nytro]

[Linux x86 NASM] Open, Read & Write syscalls

section .bss
buffer: resb 2048       ; A 2 KB byte buffer used for read

section .data
buflen: dw 2048         ; Size of our buffer to be used for read

section .text
global _start
_start:
; open(char *path, int flags, mode_t mode);

; Get our command line arguments.
pop ebx         ; argc
pop ebx         ; argv[0] (executable name)
pop ebx         ; argv[1] (desired file name)
mov eax, 0x05   ; syscall number for open
xor ecx, ecx    ; O_RDONLY = 0
xor edx, edx    ; Mode is ignored when O_CREAT isn't specified
int 0x80        ; Call the kernel
test eax, eax   ; Check the output of open()
jns file_read   ; If the sign flag is set (positive) we can begin reading the file

; = If the output is negative, then open failed. So we should exit
exit:
mov eax, 0x01   ; 0x01 = syscall for exit
xor ebx, ebx    ; makes ebx technically set to zero
int 0x80

; = Begin reading the file

file_read:
; read(int fd, void *buf, size_t count);
mov ebx, eax    ; Move our file descriptor into ebx
mov eax, 0x03   ; syscall for read = 3
mov ecx, buffer ; Our 2kb byte buffer
mov edx, buflen ; The size of our buffer
int 0x80
test eax, eax   ; Check for errors / EOF
jz file_out     ; If EOF, then write our buffer out.
js exit         ; If read failed, we exit.
; No error or EOF. Keep reading .

file_out:
; write(int fd, void *buf, size_t count);
mov edx, eax    ; read returns amount of bytes read
mov eax, 0x04   ; syscall write = 4
mov ebx, 0x01   ; STDOUT = 1
mov ecx, buffer ; Move our buffer into the arguments
int 0x80
jmp exit        ; All done 

Sursa: [Linux x86 NASM] Open, Read & Write syscalls - r00tsecurity