                        The UPX Hacker's Guide
                        ======================


Foreword
--------

  The precompiled UPX versions are linked against the NRV compression
  library instead of the UCL library. Using the same compression algorithms,
  NRV achieves a better compression ratio. NRV is not publicly
  available, though, and probably never will be.

  While you may be disappointed that you don't have access to the
  latest state-of-the-art compression technology this is actually
  a safe guard for all of us. The UPX source code release makes
  it very easy for any evil-minded person to do all sort of bad
  things. By not providing the very best compression ratio it is much
  more difficult to create fake or otherwise disguised UPX versions (or
  similar trojans), as any end user will notice when the compression
  has gotten worse with a new "version" or "product".

  Finally please be aware that you now have your hands on the source
  code of the most sophisticated executable packer ever.
  Let's join our forces to make it even better :-)

  Share and enjoy,
  Markus & Laszlo


Short overview
--------------

  The UPX source code consists of two mainly independent parts:

  1) The src/stub directory contains the decompression stubs that
     will get added to each compressed executable.
     The stubs are mainly written in assembler and get "compiled"
     into ordinary C header files.

  2) The src directory contains the actual packer sources. The stubs
     are #included by the individual executable format handlers.


Tools needed to build/modify the UPX sources
--------------------------------------------

  - A C++ compiler that supports C++ 17: clang-5, gcc-8 or msvc-2019 16.11
    (older or other compilers may work but are unsupported, use at your own risk)

  - GNU make

  - CMake 3.13 or better; see https://cmake.org/


To compile the packer sources
-----------------------------

  - just run "make" in the top-level source directory


If you want to modify the stub sources you'll also need
-------------------------------------------------------

  - Podman/Docker: see misc/rebuild-stubs-with-podman - this
    is the preferred way of rebuilding the stubs

  - otherwise you will need:
    - a Linux host system with Perl, Python2 and some older compat-libs
    - upx-stubtools - a number of cross-assemblers and cross-compilers.
      Precompiled binaries for amd64-linux hosts are available from
      https://github.com/upx/upx-stubtools/releases


Misc. notes
-----------

  As the docs say: UPX is a portable, extendable and endian neutral
  program, so if you want to add some new stuff, try not to break these
  nice properties.

  - Use the types LE16, LE32, BE16 and BE32 for fields in file headers.
  - Use [sg]et_[bl]e(16|32) for getting/setting values in the data
    stream.
  - Use gcc extensions and other compiler specific stuff only through
    macros.


Some conventions:
-----------------

  - follow our coding style
  - indent level = 4
  - expand all tabulators

  - Use throwSomeException() functions instead of throw SomeException():
    this makes the code shorter if used often.

# vim:set ts=4 sw=4 et:
