Smallest x86 ELF Hello World | Hacker News Reader

Smallest x86 ELF Hello World

timelessname.com

89 pointsshawndumas36 comments

The classic resource for this (does the same sort of tricks, taken to an extreme: code overlaid with tables, tables overlaid with other tables, etc.):

http://www.muppetlabs.com/~breadbox/software/tiny/

(62-byte hello world among other things)

how about 45 bytes...

http://www.muppetlabs.com/~breadbox/software/tiny/teensy.htm...

Indeed, except that one doesn't print "Hello world". :)

Oh, right, you got me, I totally didn't remember that, only the fact, that it was really small and I dug the fact that he put it inside the header.

How about 31 bytes?

http://www.returninfinity.com/baremetal-helloworld.html

Why use ELF or Linux? Flat binaries (and a custom OS) to the rescue! :)

Why limit yourself to an OS? Design your own instruction set, with a single-bit 'hw' instruction!

As with all games (even Nomic) a game is only a game if there are rules that limit what one can do.

The esoteric language HQ9+ does this.

Many years ago I used to have a 58-byte version, but around the time that x86-64 was being developed, Linux started examining (and thus validating) a couple more fields in the ELF header. It's really a shame, because the corner-cutting in the 62-byte version isn't nearly as impressive (in my own opinion).

Other than as an interesting intellectual exercise is there any reason to do this on a modern computer with Virtual memory?

Surely, once you get below page-size (e.g. 4K) you are just wasting memory that will get loaded anyway?

Practice and understanding for when you've got to work on something that comes with 4k of RAM in total, if you're lucky.

I edited out the words "apart from on an embedded device with a tiny memory" as I had hoped that the mention of the modern computer and virtual memory would suffice.

But this was a genuine question rather than a put-down. I was hoping to have my assumption debunked.

EDIT: I have no idea why you're being down-voted seems unfair

Well, on one of the Dart threads, bashing its hello world size, this[1] came up:

Compile "Hello world" into an executable, and see how many kilobytes it takes on most architectures, despite the object file being a few bytes.

This thread pretty much responds to that. It is no 17000 lines, nor 17000K, it is just 142 bytes (other comments suggest it is 62).

[1] http://news.ycombinator.com/item?id=3098660

It makes more sense for VM byte-code as the bytes not only have to be loaded (probably by the page, again) but the bytes have to be run and possibly jitted, too.

Doing it in C with mixed assembly

  [surki@linux-vrse tt]$ cat | gcc -nostdlib -x c - -o helloworld
  #define SYS_exit  1
  #define SYS_write 4
  #define stdout    1
  
  int strlen(const char *str)
  {
    long len = 0;
    while (str && *str++)
    {
        len++;
    }
  
    return len;
  }
  
  
  void print(const char *str) 
  {
      int len = strlen(str);
  
      long ret;
  
      /* Can't touch ebx directly, PIC uses it */
      __asm__ __volatile__ ("pushl %%ebx\n"
                            "movl  %%esi, %%ebx\n"
                            "int   $0x80\n;"
                            "popl  %%ebx"
                            
                            "a" (SYS_write),
                              "S" ((long) stdout),
                              "c" ((long) str),
                              "d" ((long) len));
      return;
  }
  
  void _start()
  {
    main();
  
    __asm__ __volatile__ (
         "xorl %%ebx, %%ebx\n"
         "int $0x80\n"
         
         "a" (SYS_exit));
  }
  
  int main()
  {
    print("Hello World\n");
    return 0;
  }

  [surki@linux-vrse tt]$ strip -R .comment -R .comment.SUSE.OPTs -R .note.gnu.build-id helloworld
  [surki@linux-vrse tt]$ ll helloworld 
  -rwxr-xr-x 1 suresh users 540 2010-07-21 13:19 helloworld
  
  [surki@linux-vrse tt]$ ./helloworld 
  Hello World

can get a few bits off by using some well known shellcode techniques. I think xor ebx,ebx is smaller than mov ebx,0, for instance.

You are right in that

    31 c0             xor %eax, %eax

is shorter than

    b8 00 00 00 00    mov $0, %eax

but that's not the main reason it's often used in shellcode. The xor reg, reg is preferred because it means the resulting shellcode will not contain a null byte. This is desirable since the null-byte is often used as a string terminator, in environment variables for example.

Another reason to use xor: in the old days, ALU reg-reg commands were generally faster than reg-mem moves.

The 8086 assembly tutorial book I used then recommended to use "sub ax, ax" instead because it was less surprising.

same for win32 PEs: http://www.phreedom.org/solar/code/tinype/

I wonder if this could be improved with genetic algorithms or similar. The code is pretty small, but the seach space is still pretty huge.

Did you post in the wrong thread?

Nope. Why couldn't you use a generic optimizer to optimize a piece of code?

Because it doesn't make any sense? What, are you planning just randomly try every permutation of 142 bytes?

You can't have a genetic algorithm run on something where almost every attempt will simply produce invalid code. Genetic algorithms work when you can try various strategies to maximize a fitness function. But here the fitness is yes or no, does it work or not. There is nothing to maximize - and you can't try to maximize the "size fitness" because pretty much all the attempts will produce invalid code, so there is nothing to mix together for various generations.

I disagree completely. The fitness function is clear, and that fact that many children will produce invalid code is not that much of a problem, really. You just eliminate them.

Google "automatic programming" if you think it's never been done, your mind is about to be expanded.

You should really learn to think for a while before you dismiss ideas as "nonsensical".

Check this out:

https://secure.wikimedia.org/wikipedia/en/wiki/Genetic_progr...

a cool project would be to make a webapp that implements the following contest: create an ELF binary to do X with the smallest number of bytes.

...why not just use the Stack Exchange "Code Golf" website?

(http://codegolf.stackexchange.com/)

Is this the next "how fast is Fibonacci" fiasco?

Very impressive.

$ echo "Hello World" > hw.php && php hw.php Hello World $ ls -l hw.php -rw-r--r-- 1 gareth staff 12 Oct 11 11:23 hw.php

By this stupid metric, PHP is the greatest programming language ever made.

12 byte Hello World must mean that it's also the ideal solution for every other programming problem, right?

Your implementation is not x86 ELF. Regardless, I don't recall the author's insistence that finding the smallest executable was desirable. "Stupid metrics" make for great educational exercises.

First, PHP is the greatest programming language ever made. (imagine DiCaprio screaming "I'm the king of the world!" on the Titanic)

Second, there is no point in using it for that when you have bash ... because bash has echo built-in and bash is a programming language:

$ echo "Hello World!"

1 line bitches! I can feel my unix beard growing ...

Unless you set +H in bash, including a '!' insisde double quotes is a bad idea.

touché

I believe you entirely missed the point. This was more of an "Look at how you can abuse the loader and play with ELF headers" and in general show an interesting 'hacky' article, not "this is smaller, thus more efficient" argument.

I figured this was posted as a related article to the Dash "Hello World" issue where people are pointing at 17,000 lines of Hello World code, because it includes the runtime when compiled to Javascript.

There was another comment about how this will be the next "Fibonacci debacle", related to how Ted Dzubia trolled the node.js crowd by pointing out it's weakness for CPU-intensive tasks, then had a ton of people wasting time on writing faster Fibonacci algorithms in node just to prove him wrong.

My point was that comparing languages by the size of their Hello World programs is a useless exercise.

I think you need to include the size of the php executable...

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