Added documentation for Courgette internals.

courgette/description.md

+Courgette Internals
+===================
+
+Patch Generation
+----------------
+
+![Patch Generation](generation.png)
+
+- courgette\_tool.cc:GenerateEnsemblePatch kicks off the patch
+  generation by calling ensemble\_create.cc:GenerateEnsemblePatch
+
+- The files are read in by in courgette:SourceStream objects
+
+- ensemble\_create.cc:GenerateEnsemblePatch uses FindGenerators, which
+  uses MakeGenerator to create
+  patch\_generator\_x86\_32.h:PatchGeneratorX86\_32 classes.
+
+- 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:
+
+  - _disassemble_ the old and new binaries into AssemblyProgram
+    objects,
+
+  - _adjust_ the new AssemblyProgram object, and
+
+  - _encode_ the AssemblyProgram object back into raw bytes.
+
+### Disassemble
+
+- The input is a pointer to a buffer containing the raw bytes of the
+  input file.
+
+- 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.
+
+- Done by disassemble:ParseDetectedExecutable, which selects the
+  appropriate Disassembler subclass by looking at the binary file's
+  headers.
+
+  - disassembler\_win32\_x86.h defines the PE/COFF x86 disassembler
+
+  - disassembler\_elf\_32\_x86.h defines the ELF 32-bit x86 disassembler
+
+  - disassembler\_elf\_32\_arm.h defines the ELF 32-bit arm disassembler
+
+- The Disassembler replaces the relocation table with a Courgette
+  instruction that can regenerate the relocation table.
+
+- The Disassembler builds a list of addresses referenced by the
+  machine code, numbering each one.
+
+- The Disassembler replaces and address used in machine instructions
+  with its index number.
+
+- 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.
+
+### Adjust
+
+- 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().
+
+- The goal is the match the indices from the old program to the new
+  program as closely as possible.
+
+- 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.
+
+### Encode
+
+- 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().
+
+- encoded_program.h:EncodedProgram defines the binary format and a
+  WriteTo method that writes to a file.
+
+### bsdiff
+
+- simple_delta.c:GenerateSimpleDelta
+
+Patch Application
+-----------------
+
+![Patch Application](application.png)
+
+- courgette\_tool.cc:ApplyEnsemblePatch kicks off the patch generation
+  by calling ensemble\_apply.cc:ApplyEnsemblePatch
+
+- ensemble\_create.cc:ApplyEnsemblePatch, reads and verifies the
+  patch's header, then calls the overloaded version of
+  ensemble\_create.cc:ApplyEnsemblePatch.
+
+- The patch is read into an ensemble_apply.cc:EnsemblePatchApplication
+  object, which generates a set of patcher_x86_32.h:PatcherX86_32
+  objects for the sections in the patch.
+
+- The original file is disassembled and encoded via a call
+  EnsemblePatchApplication.TransformUp, which in turn call
+  patcher_x86_32.h:PatcherX86_32.Transform.
+
+- 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.
+
+- Finally, EnsemblePatchApplication.TransformDown assembles, i.e.,
+  reverses the encoding and disassembly, on the patched binary data.
+  This is done by calling PatcherX86_32.Reform, which in turn calls
+  the global function encoded_program.cc:Assemble, which calls
+  EncodedProgram.AssembleTo.
+
+
+Glossary
+--------
+
+**Adjust**: Reassign address indices in the new program to match more
+  closely those from the old.
+
+**Assembly program**: The output of _disassembly_.  Contains a list of
+  _Courgette instructions_ and an index of branch target addresses.
+
+**Assemble**: Convert an _assembly program_ back into an object file
+  by evaluating the _Courgette instructions_ and leaving the machine
+  instructions in place.
+
+**Courgette instruction**: Replaces machine instructions in the
+  program.  Courgette instructions replace branches with an index to
+  the target addresses and replace part of the relocation table.
+
+**Disassembler**: Takes a binary file and produces an _assembly
+  program_.
+
+**Encode**: Convert an _assembly program_ into an _encoded program_ by
+  serializing its data structures into byte vectors more appropriate
+  for storage in a file.
+
+**Encoded Program**: The output of encoding.
+
+**Ensemble**: A Courgette-style patch containing sections for the list
+  of branch addresses, the encoded program.  It supports patching
+  multiple object files at once.
+
+**Opcode**: The number corresponding to either a machine or _Courgette
+  instruction_.