Robert Connolly wrote:
> Is this correct?:
It works.
> $ cat nologin.S
> .section .data
> message:
> .ascii "This account is currently not available.\n\0"
> .section .text
> .globl _start
> _start:
> movl $4, %eax
> movl $42, %edx
> movl $message, %ecx
> movl $1, %ebx
> int $0x80
> movl $1, %eax
> movl $1, %ebx
> int $0x80
>
> $ as -o nologin.o nologin.S
> $ ld -o nologin nologin.o
> $ ./nologin ; echo $?
> This account is currently not available.
> 1
A few suggestions:
1.) You don’t need the null byte at the end of message (because the last
argument of write() is how many bytes to be written — it doesn’t
care about null-terminated strings).
2.) Though it might increase the size a smidgen, you might not want to
hardcode the string size into the program (instead, add a symbol
len that is the length of the string. E.g.,
message:
.ascii "This account is currently not available.\n"
len = . - message
And then use $len in place of $42.
3.) You are writing an error message to stdout. Why not change that to
stderr (I do know everyone else also writes to stdout, but why?)?
4.) The “real” nologin (from shadow) will write to syslog. That’s
helpful (at least to me) from a sysadmin standpoint.
P.S.
Just for fun, here’s one for x86-64 assembler:
.section .data
message:
.ascii "This account is currently not available.\n"
len = . - message
.section .text
.globl _start
_start:
movq $len, %rdx
movq $message, %rsi
# movq $2, %rdi
movq $1, %rdi
movq $1, %rax
syscall
movq $1, %rdi
movq $60,%rax
syscall
--
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