State of ruby-macho, Weeks of 6/13 and 6/20

Jun 26, 2016 Tags: gsoc, programming, ruby, ruby-macho

As of the last two weeks:

PPC parsing is here, and live by default in Homebrew!

…on test-bot, anyways.

ruby-macho/24 was merged, prompting the release of version 0.2.3. This was followed up by 0.2.4, currently the latest version (and primarily just an expansion of the test suite and known CPU types/sub-types). This version was then merged into Homebrew via brew/378, along with some tweaks to the glue code. With this, Homebrew is now successfully using ruby-macho by default on test-bot instances (i.e., the CI)!

The API is getting cleaner!

Previously, ruby-macho’s public interface was a bit of a mixed bag in terms of style and usability. Many methods (like MachOFile#cputype and MachOFile0#cpusubtype) returned (long and unwieldy) string literals as representations of the integer constants, while others (like MachOFile#[] and Section#flag?) expected symbols as parameters. This is finally (slowly) improving, with MachOFile#filetype returning a new friendly symbol for each Mach-O type instead of the old "MH_*" stringified constant names. These changes can be seen in progress in ruby-macho/29 and merged as ruby-macho/aeea94b.

Load commands are getting DRYer!

One of the ugliest features of ruby-macho were the load command definitions. Because each load command type has different fields following a group of common ones, their constructors would end up looking something like this:

  # @api private
  def initialize(raw_data, endianness, offset, cmd, cmdsize, ilocalsym,
      nlocalsym, iextdefsym, nextdefsym, iundefsym, nundefsym, tocoff,
      ntoc, modtaboff, nmodtab, extrefsymoff, nextrefsyms, indirectsymoff,
      nindirectsyms, extreloff, nextrel, locreloff, nlocrel)
    super(raw_data, endianness, offset, cmd, cmdsize)
    @ilocalsym = ilocalsym
    @nlocalsym = nlocalsym
    @iextdefsym = iextdefsym
    @nextdefsym = nextdefsym
    @iundefsym = iundefsym
    @nundefsym = nundefsym
    @tocoff = tocoff
    @ntoc = ntoc
    @modtaboff = modtaboff
    @nmodtab = nmodtab
    @extrefsymoff = extrefsymoff
    @nextrefsyms = nextrefsyms
    @indirectsymoff = indirectsymoff
    @nindirectsyms = nindirectsyms
    @extreloff = extreloff
    @nextrel = nextrel
    @locreloff = locreloff
    @nlocrel = nlocrel
  end

These fields are auto-populated by LoadCommand.new_from_bin, but even still are an eyesore and repeat much of the same (general) information over and over. Since fields like raw_data and offset are not constituents of the load command itself but rather metadata about the command (namely, its source blob and the offset at which it begins in that blob), the decision was made to group them - along with an endianness symbol - into a single MachOView class. Instances of this class then represent an abstract “view” into the Mach-O data associated with some ruby-macho class or structure. As such, constructors like the above trade in three arguments for one:

  # @api private
  def initialize(view, cmd, cmdsize, ilocalsym,
      nlocalsym, iextdefsym, nextdefsym, iundefsym, nundefsym, tocoff,
      ntoc, modtaboff, nmodtab, extrefsymoff, nextrefsyms, indirectsymoff,
      nindirectsyms, extreloff, nextrel, locreloff, nlocrel)
    super(view, cmd, cmdsize)
    # ...
  end

It’s not a huge reduction, but it’s certainly a step in the right direction! These changes can be seen in progress in ruby-macho/30 and merged as ruby-macho/a62c23a.

Support for complex inspection is on the horizon!

The introduction of MachOView paves the way for more robust Mach-O inspection, as dedicated inspection classes declared within individual load commands can now rely on a uniform view into the binary data that their command references. Currently, ruby-macho/32 is the only example of this new interface’s advantages, although richer support for data referenced by sections and dyld commands will be coming shortly. This also has the added benefit of reducing the current pattern of returning control flow to the current MachOFile or FatFile object for calculations not strictly restricted to the data in a given load command.

Thanks for reading!

- William

ENOSUCHBLOG

Programming, philosophy, pedaling.

State of ruby-macho, Weeks of 6/13 and 6/20

Jun 26, 2016 Tags: gsoc, programming, ruby, ruby-macho