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113 lines
4.3 KiB
Text
113 lines
4.3 KiB
Text
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Hacking liblzma
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---------------
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0. Preface
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This document gives some overall information about the internals of
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liblzma, which should make it easier to start reading and modifying
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the code.
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1. Programming language
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liblzma was written in C99. If you use GCC, this means that you need
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at least GCC 3.x.x. GCC 2 isn't and won't be supported.
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Some GCC-specific extensions are used *conditionally*. They aren't
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required to build a full-featured library. Don't make the code rely
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on any non-standard compiler extensions or even C99 features that
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aren't portable between almost-C99 compatible compilers (for example
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non-static inlines).
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The public API headers are in C89. This is to avoid frustrating those
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who maintain programs, which are strictly in C89 or C++.
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An assumption about sizeof(size_t) is made. If this assumption is
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wrong, some porting is probably needed:
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sizeof(uint32_t) <= sizeof(size_t) <= sizeof(uint64_t)
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2. Internal vs. external API
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Input Output
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v Application ^
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| liblzma public API |
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| Stream coder |
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| Block coder |
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| Filter coder |
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| ... |
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v Filter coder ^
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Application
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`-- liblzma public API
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`-- Stream coder
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|-- Stream info handler
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|-- Stream Header coder
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|-- Block Header coder
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| `-- Filter Flags coder
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|-- Metadata coder
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| `-- Block coder
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| `-- Filter 0
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| `-- Filter 1
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| ...
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|-- Data Block coder
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| `-- Filter 0
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| `-- Filter 1
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| ...
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`-- Stream tail coder
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x. Designing new filters
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All filters must be designed so that the decoder cannot consume
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arbitrary amount input without producing any decoded output. Failing
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to follow this rule makes liblzma vulnerable to DoS attacks if
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untrusted files are decoded (usually they are untrusted).
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An example should clarify the reason behind this requirement: There
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are two filters in the chain. The decoder of the first filter produces
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huge amount of output (many gigabytes or more) with a few bytes of
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input, which gets passed to the decoder of the second filter. If the
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data passed to the second filter is interpreted as something that
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produces no output (e.g. padding), the filter chain as a whole
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produces no output and consumes no input for a long period of time.
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The above problem was present in the first versions of the Subblock
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filter. A tiny .lzma file could have taken several years to decode
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while it wouldn't produce any output at all. The problem was fixed
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by adding limits for number of consecutive Padding bytes, and requiring
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that some decoded output must be produced between Set Subfilter and
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Unset Subfilter.
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x. Implementing new filters
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If the filter supports embedding End of Payload Marker, make sure that
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when your filter detects End of Payload Marker,
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- the usage of End of Payload Marker is actually allowed (i.e. End
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of Input isn't used); and
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- it also checks that there is no more input coming from the next
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filter in the chain.
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The second requirement is slightly tricky. It's possible that the next
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filter hasn't returned LZMA_STREAM_END yet. It may even need a few
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bytes more input before it will do so. You need to give it as much
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input as it needs, and verify that it doesn't produce any output.
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Don't call the next filter in the chain after it has returned
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LZMA_STREAM_END (except in encoder if action == LZMA_SYNC_FLUSH).
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It will result undefined behavior.
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Be pedantic. If the input data isn't exactly valid, reject it.
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At the moment, liblzma isn't modular. You will need to edit several
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files in src/liblzma/common to include support for a new filter. grep
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for LZMA_FILTER_LZMA to locate the files needing changes.
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