A dynamic linker murder mystery

I write a ton of articles about rust. And in those articles, the main focus is about writing Rust code that compiles. Once it compiles, well, we're basically in the clear! Especially if it compiles to a single executable, that's made up entirely of Rust code.

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Thread-local storage

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.

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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.

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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.

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Dynamic linker speed and correctness

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.

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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.

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Loading multiple ELF objects

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.

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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 `elk/samples/hello.asm` global _start section .text _start: mov rdi, 1 ; stdout fd mov rsi, 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 msg: db "hi there", 10
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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.

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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.

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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.
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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.

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Reading files the hard way - Part 1 (node.js, C, rust, strace)

Everybody knows how to use files. You just open up File Explorer, the Finder, or a File Manager, and bam - it's chock-full of files. There's folders and files as far as the eye can see. It's a genuine filapalooza. I have never once heard someone complain there were not enough files on their computer.

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Cross-compilation notes

I'll keep updating this article as I go, just to put stuff in all the same place.

Platforms

Cross-compiling for Linux

I'm pretty sure it's possible to cross-compile for Linux on other OSes, seeing as everything is open-source, but I have never done it - and why would I want to? Linux is the friendliest to build on, so it's better to use it as a build environment.

Cross-compiling for OSX

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