Welcome back to the eighteenth and final part of “Making our own executable packer”.

In the last article, we had a lot of fun. We already had a “packer” executable, minipak, which joined together stage1 (a launcher), and a compressed version of whichever executable we wanted to pack.

What we added, was a whole bunch of abstractions to parse ELF headers using deku, which we used from stage1 to be able to launch the guest executable from memory, instead of writing it to a file and using execve on it.

Welcome back!

In the last article, we did foundational work on minipak, our ELF packer.

It is now able to receive command-line arguments, environment variables, and auxiliary vectors. It can parse those command-line arguments into a set of options. It can make an ELF file smaller using the LZ4 compression algorithm, and pack it together with stage1, our launcher.

And we’re back!

In the last article, we thanked our old code and bade it adieu, for it did not spark joy. And then we made a new, solid foundation, on which we planned to actually make an executable packer.

As part of this endeavor, we’ve made a crate called encore, which only depends on libcore, and provides some of the things libstd would give us, but which we cannot have, because we do not want to rely on a libc.

You’re still here! Fantastic.

I have good news, and bad news. The good news is, we’re actually going to make an executable packer now!

Hurray!

I know right? No lie, we’re actually really going to start working on the final product from this point onwards.

What uhhh what about the previous fourteen parts?

Ah, yes, the previous fourteen parts. Well, we had fun, didn’t we? And we learned a lot about ELF, how it’s basically a database format that different tools look at in different ways, how it’s mapped in memory (more or less), what we really need to set up before starting up another executable, all that good stuff.

Good morning, and welcome back to “how many executables can we run with our custom dynamic loader before things get really out of control”.

In Part 13, we “implemented” thread-local storage. I’m using scare quotes because, well, we spent most of the article blabbering about Addressing Memory Through The Ages, And Other Fun Tidbits.

But that was then, and this is now, which is, uh, nine months later. Not only am I wiser and more productive, I’m also finally done updating all the previous thirteen parts of this series to fix some inconsistencies, upgrade crate versions, and redo all the diagrams as SVG.

Welcome back and thanks for joining us for the reads notes… the thirteenth installment of our series on ELF files, what they are, what they can do, what does the dynamic linker do to them, and how can we do it ourselves.

I’ve been pretty successfully avoiding talking about TLS so far (no, not that one) but I guess we’ve reached a point where it cannot be delayed any further, so.

In Part 11, we spent some time clarifying mechanisms we had previously glossed over: how variables and functions from other ELF objects were accessed at runtime.

We saw that doing so “proper” required the cooperation of the compiler, the assembler, the linker, and the dynamic loader. We also learned that the mechanism for functions was actually quite complicated! And sorta clever!