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Bold - The Byte Optimized Linker
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
:Author: Amand Tihon
:Contact:
:Version: 0.1.0
:Date: |date|
:Copyright: GNU GPL version 3 + Exception, see copyright file.
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.. contents:: Table of contents
:backlinks: none
Abstract
========
Bold is an ELF linker, currently only targetting x86_64 under Linux. Being
limited in capabilities, it should not be considered as an all-purpose linker.
Rationale
=========
Bold's main purpose is to generate very small executable programs.
While ``ld`` from the GNU binutils can do almost anything anyone would ever
need, some specific goals need an awful lot of tweaking, or can simply not be
achieved. Bold uses several tricks to reduce the size of the final executable
binary.
Getting Bold
============
You can download the tarball from http://www.alrj.org/projects/bold
or get the latest development version with the following git command: ::
git clone http://git.alrj.org/git/bold.git
A gitweb interface is also available at http://git.alrj.org/
Requirements
============
Bold itself is entirely written in Python. There are no additionnal
dependencies.
The runtime library that contains the external symbols resolver is written
in assembler (Intel syntax). An assembler like Nasm or Yasm is needed to
recompile the source code into an object file.
Installation
============
Go into Bold's directory, and run ::
python setup.py build
Then, as root or using sudo, run ::
python setup.py install
Using Bold
==========
Synopsys
--------
bold [options] objfile...
Description
-----------
Bold combines a number of object files, relocate their data and resolves their
symbols references, in order to generate executable binaries.
Bold has only one, very specific purpose: making small executables.
Options
-------
--version
Show program's version and exit.
-h, --help
Show help message and exit.
-e SYMBOL, --entry=SYMBOL
Use SYMBOL as the explicit symbol for beginning execution of your program.
If ``--raw`` is specified, it defaults to ``_start``.
-l LIBNAME, --library=LIBNAME
Link against the shared library specified by LIBNAME. Bold relies on python's
ctypes module to find the libraries. This option may be used any number of
times.
-L DIRECTORY, --library-path=DIRECTORY
This option does nothing, and is present ony for compatibility reasons. It
MAY get implemented in the future, though. This option may be used any number
of times.
-o FILE, --output=FILE
Set the output file name (default value is a.out).
--raw
Don't include the builtin external symbols resolution code. This is
described in details further in this document.
-c, --ccall
Make external symbols directly callable by C, without having to declare the
pointers on functions. This option adds 6 bytes for each externally defined
function. This is described in details further in this document.
-a, --align
Align the wrappers for external symbols on an 8 byte boundary, to take
advantage of the RIP-relative addressing. This is described in details
further in this document.
Notes
-----
The ``LD_PRELOAD`` environment variable may not always work (as expected or
at all).
The ``main()`` function is called without any argument. Its return code is used
as exit code, though.
Internals
=========
External symbols resolution
---------------------------
The "import by hash" method is from parapete, leblane, las, as described on
http://www.pouet.net/topic.php?which=5392
Calling from C
--------------
If you write your code in C and need to call the external symbols, you
basically have two options. The first one is to redefine them (or define new
ones) to call by pointers. For instance, ::
int SDL_Init(int);
would become: ::
int (*SDL_Init)(int);
Repeat it for all functions, or write a tool to automate it (hint: look at
http://research.mercury-labs.org/ibh-i386-0.2.2.tar.gz for help).
There's a second possibility however, and it's the one used by Bold when you
specify the ``--ccall`` option: make the resolved symbol point, not to the
address of the function, but to a JMP instruction to the actual address: ::
global SDL_Init
.text
SDL_Init: jmp [rel _bold__SDL_Init]
SDL_SetVideoMode: jmp [rel _bold__SDL_SetVideoMode]
.bss
_bold__SDL_Init resq ; Filled by the import by hash code
_bold__SDL_SetVideoMode resq
This approach takes 6 bytes (the JMP instruction) for each external function
used.
Aligning
--------
The x86_64 architecture has this nice thing called "RIP-relative addressing".
If all the JMP instructions are in the same order than the pointers to the
functions they refer to, having them aligned with the pointers would result
in identical instructions. This is done with the ``--align`` option.
Adding two null bytes between each JMP enlarges the final executable by
2 x (number of function - 1) bytes, and may seem to go against our goal.
However, the result is a repetition of the *same eight bytes*, something that
can improve compression a lot!
Additional Trick 1: DT_DEBUG
----------------------------
Bold declares a global symbol named ``_dt_debug``, that points to the value of
the ``DT_DEBUG`` entry of the ``DYNAMIC`` table, for easy access. Just in case,
the ``DYNAMIC`` table can also be reached using the global ``_DYNAMIC`` symbol.
Additional Trick 2: Short DYNAMIC table
---------------------------------------
Executables generated by ``ld`` usually have a lot of entries in their
``DYNAMIC`` table. Bold puts only the strict necessary:
- One ``DT_NEEDED`` entry for each shared library to load (obviously).
- A ``DT_SYMTAB`` entry, with null-pointer. Without this one, the interpreter
wouldn't do its job.
- a ``DT_DEBUG`` entry, that will be used for symbol resolution.
And that's it!
Examples
========
The ``examples/`` directory contains a port of the *flow2* intro
(http://www.pouet.net/prod.php?which=30589). Adding the dropper is left as an
exercise for the reader.