<h1>Courgette Internals</h1>

<h2>Patch Generation</h2>

<p><img src="generation.png" alt="Patch Generation" title="" /></p>

<ul>
<li><p>courgette_tool.cc:GenerateEnsemblePatch kicks off the patch
generation by calling ensemble_create.cc:GenerateEnsemblePatch</p></li>
<li><p>The files are read in by in courgette:SourceStream objects</p></li>
<li><p>ensemble_create.cc:GenerateEnsemblePatch uses FindGenerators, which
uses MakeGenerator to create
patch_generator_x86_32.h:PatchGeneratorX86_32 classes.</p></li>
<li><p>PatchGeneratorX86_32's Transform method transforms the input file
using Courgette's core techniques that make the bsdiff delta
smaller.  The steps it takes are the following:</p>

<ul>
<li><p><em>disassemble</em> the old and new binaries into AssemblyProgram
objects,</p></li>
<li><p><em>adjust</em> the new AssemblyProgram object, and</p></li>
<li><p><em>encode</em> the AssemblyProgram object back into raw bytes.</p></li>
</ul></li>
</ul>

<h3>Disassemble</h3>

<ul>
<li><p>The input is a pointer to a buffer containing the raw bytes of the
input file.</p></li>
<li><p>Disassembly converts certain machine instructions that reference
addresses to Courgette instructions.  It is not actually
disassembly, but this is the term the code-base uses.  Specifically,
it detects instructions that use absolute addresses given by the
binary file's relocation table, and relative addresses used in
relative branches.</p></li>
<li><p>Done by disassemble:ParseDetectedExecutable, which selects the
appropriate Disassembler subclass by looking at the binary file's
headers.</p>

<ul>
<li><p>disassembler_win32_x86.h defines the PE/COFF x86 disassembler</p></li>
<li><p>disassembler_elf_32_x86.h defines the ELF 32-bit x86 disassembler</p></li>
</ul></li>
<li><p>The Disassembler replaces the relocation table with a Courgette
instruction that can regenerate the relocation table.</p></li>
<li><p>The Disassembler builds a list of addresses referenced by the
machine code, numbering each one.</p></li>
<li><p>The Disassembler replaces and address used in machine instructions
with its index number.</p></li>
<li><p>The output is an assembly_program.h:AssemblyProgram class, which
contains a list of instructions, machine or Courgette, and a mapping
of indices to actual addresses.</p></li>
</ul>

<h3>Adjust</h3>

<ul>
<li><p>This step takes the AssemblyProgram for the old file and reassigns
the indices that map to actual addresses.  It is performed by
adjustment_method.cc:Adjust().</p></li>
<li><p>The goal is the match the indices from the old program to the new
program as closely as possible.</p></li>
<li><p>When matched correctly, machine instructions that jump to the
function in both the new and old binary will look the same to
bsdiff, even the function is located in a different part of the
binary.</p></li>
</ul>

<h3>Encode</h3>

<ul>
<li><p>This step takes an AssemblyProgram object and encodes both the
instructions and the mapping of indices to addresses as byte
vectors.  This format can be written to a file directly, and is also
more appropriate for bsdiffing.  It is done by
AssemblyProgram.Encode().</p></li>
<li><p>encoded_program.h:EncodedProgram defines the binary format and a
WriteTo method that writes to a file.</p></li>
</ul>

<h3>bsdiff</h3>

<ul>
<li>simple_delta.c:GenerateSimpleDelta</li>
</ul>

<h2>Patch Application</h2>

<p><img src="application.png" alt="Patch Application" title="" /></p>

<ul>
<li><p>courgette_tool.cc:ApplyEnsemblePatch kicks off the patch generation
by calling ensemble_apply.cc:ApplyEnsemblePatch</p></li>
<li><p>ensemble_create.cc:ApplyEnsemblePatch, reads and verifies the
patch's header, then calls the overloaded version of
ensemble_create.cc:ApplyEnsemblePatch.</p></li>
<li><p>The patch is read into an ensemble<em>apply.cc:EnsemblePatchApplication
object, which generates a set of patcher</em>x86<em>32.h:PatcherX86</em>32
objects for the sections in the patch.</p></li>
<li><p>The original file is disassembled and encoded via a call
EnsemblePatchApplication.TransformUp, which in turn call
patcher<em>x86</em>32.h:PatcherX86_32.Transform.</p></li>
<li><p>The transformed file is then bspatched via
EnsemblePatchApplication.SubpatchTransformedElements, which calls
EnsemblePatchApplication.SubpatchStreamSets, which calls
simple_delta.cc:ApplySimpleDelta, Courgette's built-in
implementation of bspatch.</p></li>
<li><p>Finally, EnsemblePatchApplication.TransformDown assembles, i.e.,
reverses the encoding and disassembly, on the patched binary data.
This is done by calling PatcherX86<em>32.Reform, which in turn calls
the global function encoded</em>program.cc:Assemble, which calls
EncodedProgram.AssembleTo.</p></li>
</ul>

<h2>Glossary</h2>

<p><strong>Adjust</strong>: Reassign address indices in the new program to match more
  closely those from the old.</p>

<p><strong>Assembly program</strong>: The output of <em>disassembly</em>.  Contains a list of
  <em>Courgette instructions</em> and an index of branch target addresses.</p>

<p><strong>Assemble</strong>: Convert an <em>assembly program</em> back into an object file
  by evaluating the <em>Courgette instructions</em> and leaving the machine
  instructions in place.</p>

<p><strong>Courgette instruction</strong>: Replaces machine instructions in the
  program.  Courgette instructions replace branches with an index to
  the target addresses and replace part of the relocation table.</p>

<p><strong>Disassembler</strong>: Takes a binary file and produces an <em>assembly
  program</em>.</p>

<p><strong>Encode</strong>: Convert an <em>assembly program</em> into an <em>encoded program</em> by
  serializing its data structures into byte vectors more appropriate
  for storage in a file.</p>

<p><strong>Encoded Program</strong>: The output of encoding.</p>

<p><strong>Ensemble</strong>: A Courgette-style patch containing sections for the list
  of branch addresses, the encoded program.  It supports patching
  multiple object files at once.</p>

<p><strong>Opcode</strong>: The number corresponding to either a machine or <em>Courgette
  instruction</em>.</p>