
Hacking liblzma
---------------

0. Preface

    This document gives some overall information about the internals of
    liblzma, which should make it easier to start reading and modifying
    the code.


1. Programming language

    liblzma was written in C99. If you use GCC, this means that you need
    at least GCC 3.x.x. GCC 2 isn't and won't be supported.

    Some GCC-specific extensions are used *conditionally*. They aren't
    required to build a full-featured library. Don't make the code rely
    on any non-standard compiler extensions or even C99 features that
    aren't portable between almost-C99 compatible compilers (for example
    non-static inlines).

    The public API headers are in C89. This is to avoid frustrating those
    who maintain programs, which are strictly in C89 or C++.

    An assumption about sizeof(size_t) is made. If this assumption is
    wrong, some porting is probably needed:

        sizeof(uint32_t) <= sizeof(size_t) <= sizeof(uint64_t)


2. Internal vs. external API


        Input                         Output
          v     Application             ^
          |     liblzma public API      |
          |     Stream coder            |
          |     Block coder             |
          |     Filter coder            |
          |     ...                     |
          v     Filter coder            ^


        Application
          `-- liblzma public API
                `-- Stream coder
                      |-- Stream info handler
                      |-- Stream Header coder
                      |-- Block Header coder
                      |     `-- Filter Flags coder
                      |-- Metadata coder
                      |     `-- Block coder
                      |           `-- Filter 0
                      |                 `-- Filter 1
                      |                     ...
                      |-- Data Block coder
                      |     `-- Filter 0
                      |           `-- Filter 1
                      |               ...
                      `-- Stream tail coder


x. Designing new filters

    All filters must be designed so that the decoder cannot consume
    arbitrary amount input without producing any decoded output. Failing
    to follow this rule makes liblzma vulnerable to DoS attacks if
    untrusted files are decoded (usually they are untrusted).

    An example should clarify the reason behind this requirement: There
    are two filters in the chain. The decoder of the first filter produces
    huge amount of output (many gigabytes or more) with a few bytes of
    input, which gets passed to the decoder of the second filter. If the
    data passed to the second filter is interpreted as something that
    produces no output (e.g. padding), the filter chain as a whole
    produces no output and consumes no input for a long period of time.

    The above problem was present in the first versions of the Subblock
    filter. A tiny .lzma file could have taken several years to decode
    while it wouldn't produce any output at all. The problem was fixed
    by adding limits for number of consecutive Padding bytes, and requiring
    that some decoded output must be produced between Set Subfilter and
    Unset Subfilter.


x. Implementing new filters

    If the filter supports embedding End of Payload Marker, make sure that
    when your filter detects End of Payload Marker,
      - the usage of End of Payload Marker is actually allowed (i.e. End
        of Input isn't used); and
      - it also checks that there is no more input coming from the next
        filter in the chain.

    The second requirement is slightly tricky. It's possible that the next
    filter hasn't returned LZMA_STREAM_END yet. It may even need a few
    bytes more input before it will do so. You need to give it as much
    input as it needs, and verify that it doesn't produce any output.

    Don't call the next filter in the chain after it has returned
    LZMA_STREAM_END (except in encoder if action == LZMA_SYNC_FLUSH).
    It will result undefined behavior.

    Be pedantic. If the input data isn't exactly valid, reject it.

    At the moment, liblzma isn't modular. You will need to edit several
    files in src/liblzma/common to include support for a new filter. grep
    for LZMA_FILTER_LZMA to locate the files needing changes.