x0reaxeax / smol_helloworld

Shortest Hello World code in C - 0 characters source file!

Very smol C hello world, 0 characters!

Challenge rules:

• No binary modifications
• No external libraries or code
• OS is Linux (3.7 i686)
• GCC compiler (gcc version 4.7.2)
• This (preprocessor macros) is not allowed: $ gcc smoll.c -D"_=int main() {puts(\"hello world\");}"

Approach:

(Ab)Use GCC's __FILE__ preprocessor macro, which holds source filename string.
Example:

/* file - 'test.c' */
  ...
  printf("%s\n", __FILE__);
  ...

Outputs:

test.c

We could either settle for 27 characters with:

int main(){puts(__FILE__);}

...or we could load our source filename with opcodes!
There are 2 things that complicate this:

1. Newline character (0x0a) cuts the filename
2. We can't use nullbyte basically for the exact same reason
   (might be able to overcome this with a pair of double-quotes..)

Workaround: Just avoid movs and leas with 0s!

_start:
    xor eax, eax
    xor ebx, ebx
    xor edx, edx
    
    inc ebx

    add al,  0x4        ; sys_write = 4
    add edx, 0xb        ; strlen = 11 (actually 10, but (int) 10 (0xa) is ASCII newline, which we can't use)
    
    mov ecx, 0x8048285  ; __FILE__ magic address (0x804826d) + offset to "helloworld!"
    int 0x80
    
    xor eax, eax
    inc eax            ; sys_exit = 1
    int 0x80

This with appended "helloworld!" translates to:

unsigned char bytecode[] = {  
        0x31, 0xc0,
        0x31, 0xdb,
        0x31, 0xd2,

        0x43,
        0x04, 0x04,
        0x83, 0xc2, 0x0b,

        0xb9, 0x85, 0x82, 0x04, 0x08,
        0xcd, 0x80,
        
        0x31, 0xc0,
        0x40,
        0xcd, 0x80,

        0x68, 0x65, 0x6c, 0x6c, 0x6f, 0x77, 0x6f, 0x72, 0x6c, 0x64, 0x21
};

Now we have to rename our c source file with these bytes:

/* craft.c */

#include <stdio.h>

int main(void) {
  unsigned char bytecode[] { ... };
  rename("test.c", bytecode);
  return 0;
}

Last thing.. just create an empty source file named test.c
So yea, 0 characters C "Hello World".

Great, now for the final step - compilation!

gcc -nostdlib -z execstack -e 0x804826d -x c {GARBAGE_NAME}

.. or just run make!

$ ./a.out
helloworld!

yaaaaaaaaaaaayyyy