Keith Makan

ELF Files are charged with using their magic to perform two holy tasks in the linux universe. The first being to tell the kernel where to place stuff in memory from the ELF file on disk as well as providing ways to invoke the dynamic loaders functions and maybe even help out with some debugging information. Essentially speaking its telling the kernel where to put it in memory and also the plethora of tools that interpret the file where all the data structures are that hold useful information for making sense of the file. Anyway that's as far as I've figured it out - the actual break down is a little less simple.

I'll demonstrate why this is so here and over the next series of posts in the classic "Learn things by breaking them" style.
ELF Header and Identification fields The first thing that appears in an ELF file is of course the header, which is like most things in file formats just a list of offsets in the file. Its purpose is to indicate essentially what kin…

Introduction to the ELF Format (Part VI) : The Symbol Table and Relocations (Part 1)

This post is part of a series on the ELF format, if you haven't checked out the other parts of the series here they are:

(Part I) : ELF Header https://blog.k3170makan.com/2018/09/introduction-to-elf-format-elf-header.html (Part II) : Program Headers https://blog.k3170makan.com/2018/09/introduction-to-elf-format-part-ii.html (Part III) : Section Header Table https://blog.k3170makan.com/2018/09/introduction-to-elf-file-format-part.html (Part IV) : Section Types and Special Sections https://blog.k3170makan.com/2018/10/introduction-to-elf-format-part-iv.html(Part V) : C Start up https://blog.k3170makan.com/2018/10/introduction-to-elf-format-part-v.htmlthis
In this and the next post I'm going to explore how Elf files manage to pull off the magic of symbol resolution as well as the format, offsets and records in the Elf that represent this information. There are many facets to this mechanism in the format, and before I get into each of them I'd like to provide a gentle intro …

Introduction to the ELF Format Part II : Understanding Program Headers

Welcome back folks! In the previous post I covered pretty much the most trivial parts of the ELF file format. In this post we are actually going to work with one of the most interesting mechanisms in the file - the program headers! I skipped some parts of the ELF header in the previous post and decided to cover them here specifically because they inform on the Program Headers anyway. Lets get started!
Introduction : What are Program Headers?
I mentioned in part 1 that the ELF format performs two tasks. A recipe for how to sublimate dead files into living processes and adds the bells and whistles needed to make the file look pretty to gdb, the dynamic loader and a bunch of other tools. Program Headers (among other functions) are more often for telling the memory loader where to put stuff. It also has some house keeping functions.

We'll get into how these memory loading powers and formats work a little later for now its just important to keep in mind a good idea of what to expect …

Introduction to the ELF Format (Part V) : Understanding C start up .init_array and .fini_array sections

This post is part of a series on the ELF format, if you haven't checked out the other parts of the series here they are:

(Part I) : ELF Header https://blog.k3170makan.com/2018/09/introduction-to-elf-format-elf-header.html (Part II) : Program Headers https://blog.k3170makan.com/2018/09/introduction-to-elf-format-part-ii.html (Part III) : Section Header Table https://blog.k3170makan.com/2018/09/introduction-to-elf-file-format-part.html (Part IV) : Section Types and Special Sections https://blog.k3170makan.com/2018/10/introduction-to-elf-format-part-iv.htmlthis
In this post I'm going to cover how some of the aspects of C start up and mess around with the .init_array and .fini_array sections to show how they work.

C Start Up
So something must happen to get your code in the main function running. This process is called the C start up and it essentially involves running all the initialize code, setting up pointers to some important arrays and then branching over to main.

What the…