<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> <li><p>disassembler_elf_32_arm.h defines the ELF 32-bit arm 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>