Running a self-relocatable ELF from memory

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.

Everything but ELF

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.

Between libcore and libstd

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!

In the bowels of glibc

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

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.

A no_std Rust binary

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.

More ELF relocations

In our last installment of "Making our own executable packer", we did some code cleanups. We got rid of a bunch of unsafe code, and found a way to represent memory-mapped data structures safely.

In the last article, we managed to load a program (hello-dl) that uses a single dynamic library (libmsg.so) containing a single exported symbol, msg.

Dynamic symbol resolution

Let's pick up where we left off: we had just taught elk to load not only an executable, but also its dependencies, and then their dependencies as well.

Up until now, we've been loading a single ELF file, and there wasn't much structure to how we did it: everyhing just kinda happened in main, in no particular order.

The simplest shared library

In our last article, we managed to load and execute a PIE (position-independent executable) compiled from the following code:

X86 Assembly; in samples/hello-pie.asm

global _start

section .text

_start: mov rdi, 1      ; stdout fd
lea rsi, [rel msg]
mov rdx, 9      ; 8 chars + newline
mov rax, 1      ; write syscall
syscall

xor rdi, rdi    ; return code 0
mov rax, 60     ; exit syscall
syscall

section .data

msg:    db "hi there", 10

ELF relocations

The last article, Position-independent code, was a mess. But who could blame us? We looked at the world, and found it to be a chaotic and seemingly nonsensical place. So, in order to blend in, we had to let go of a little bit of sanity.

Position-independent code

In the last article, we found where code was hiding in our samples/hello executable, by disassembling the whole file and then looking for syscalls.

Running an executable without exec

In part 1, we've looked at three executables:

• sample, an assembly program that prints "hi there" using the write system call.
• entry_point, a C program that prints the address of main using printf
• The /bin/true executable, probably also a C program (because it's part of GNU coreutils), and which just exits with code 0.
What's in a Linux executable?

Executables have been fascinating to me ever since I discovered, as a kid, that they were just files. If you renamed a .exe to something else, you could open it in notepad! And if you renamed something else to a .exe, you'd get a neat error dialog.

Consuming Ethernet frames with the nom crate

Now that we've found the best way to find the "default network interface"... what can we do with that interface?

Reading files the hard way - Part 2 (x86 asm, linux kernel)

Looking at that latest mental model, it's.. a bit suspicious that every program ends up calling the same set of functions. It's almost like something different happens when calling those.

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