Creating Teensy ELF Executables for Linux (Or, "Size Is Everything") (1999) | Hacker News Reader

Creating Teensy ELF Executables for Linux (Or, "Size Is Everything") (1999)

muppetlabs.com

87 pointsBluestein22 comments

Part 2: https://www.muppetlabs.com/~breadbox/software/tiny/revisit.h...

Discussed here a couple months ago: https://news.ycombinator.com/item?id=48654411

Part 3: https://www.muppetlabs.com/~breadbox/software/tiny/somewhat....

This is actually a lot of fun. Never did it on Linux, but Windows PE EXE files make similar tricks possible. Windows 95 in particular was very permissive. Using a (very primitive, by today's standards) hex editor I tried making the ".text" section begin halfway through the header and bang! ..it worked, a revelation of sorts. I imagine today's loaders may be a bit more strict because of security but I guess you can always make your own loader :)

There are no relevant security concerns, but they may require page alignment for efficient mapping.

I wonder how much of this could be done by a clever linker? There’s a lot of skill in there but how would an llm do, for the rest of us?

A linker could do this, but "clever" isn't the adjective I'd use when something randomly breaks due to it not working to standard.

First experience with proper ELFs was using PS2 Linux ELFs to run homebrew and games and get past Sony’s restrictions

This is the wrong end of the issue. ELF for executables and dynamic libs is obsolete on modern hardware (same for microsoft PE+).

I am now using for most of my projects a exe/dynamic lib format of my own. This file format is so miminal a simple RFC will suffice. BUT, I will need to build dynamic libs which are system interfaces (for instance userland drivers like mesa) with this file format: problem, c++ and its runtime (and probably other similar languages with their runtimes)... as usual...

Well, I am still removing stuff from my file format, because I think this is not yet "minimal and enough" on modern hardware architecture (I think I have still too much complexity in the tracker of loaded dynamic libs).

> ELF for executables and dynamic libs is obsolete on modern hardware (same for microsoft PE+)

This is... extremely not true?

Then you are of extremely bad faith?

come on...

> I am now using for most of my projects a exe/dynamic lib format of my own. This file format is so miminal a simple RFC will suffice.

That's great! Have you published it? If not, please consider doing so.

Well, everything is open source, I have a partial and very limited (but currently enough for my needs) linker for x86_64 and RISC-V 64bits (but written in plain and simple C which compiles with tinycc and cproc/qbe!).

I even have an ELF capsule for executables (for x86_64, written in assembly).

So it is out there, but not "tidy" (aka a wip messy tarball), as I said, I am not sorta sure about how to handle the in-process loader, which is beyond simple, but I still believe I can make it even simpler (yeah, not more gigantic external "interpreter"/"loader).

Once I am satisfied, I'll ship that to a nicely tidied GIT repo (but with support of noscript/basic HTML browsers, unlike ... you know which ones...).

Oh, ELF file manipulation. I thought this would be about getting Linux to run on a Teensy.

https://forum.pjrc.com/index.php?threads/linux-on-teensy-4-x...

The program described in this article is as simple as possible. What about something more complex, like basic "hello world"? It requires at least one more syscall and several bytes more data.

Take a look at the collections by the same author:

https://www.muppetlabs.com/~breadbox/software/tiny/home.html

There are some more complete programs there.

My approach to crafting "small" ELFs (in the hundreds-of-bytes range, written in relatively normal C) is like so: https://github.com/DavidBuchanan314/kurl/blob/main/golfed/el...

i386 are perhaps starting to die out. Has anything like this been done for amd64? Can we play such tricks with arm and aarch64?

84 byte aarch64 ELF: https://tmpout.sh/2/14.html

57 byte x86-64 ELF https://tmpout.sh/5/3.html

4 byte x86 .mut ELF https://tmpout.sh/4/13.html

Fantastic, well done you lot.

Lot of useful info here! Good work!

if you want to do this yourself, definitely read the actual Linux ELF loader linux/fs/binfmt_elf.c instead of making guesses based on the specification. Because it's open source, you can do this.

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