(rewrite will be better).
+++ /dev/null
-
-Advanced features of liblzma
-----------------------------
-
-0. Introduction
-
- Most developers need only the basic features of liblzma. These
- features allow single-threaded encoding and decoding of .lzma files
- in streamed mode.
-
- In some cases developers want more. The .lzma file format is
- designed to allow multi-threaded encoding and decoding and limited
- random-access reading. These features are possible in non-streamed
- mode and limitedly also in streamed mode.
-
- To take advange of these features, the application needs a custom
- .lzma file format handler. liblzma provides a set of tools to ease
- this task, but it's still quite a bit of work to get a good custom
- .lzma handler done.
-
-
-1. Where to begin
-
- Start by reading the .lzma file format specification. Understanding
- the basics of the .lzma file structure is required to implement a
- custom .lzma file handler and to understand the rest of this document.
-
-
-2. The basic components
-
-2.1. Stream Header and tail
-
- Stream Header begins the .lzma Stream and Stream tail ends it. Stream
- Header is defined in the file format specification, but Stream tail
- isn't (thus I write "tail" with a lower-case letter). Stream tail is
- simply the Stream Flags and the Footer Magic Bytes fields together.
- It was done this way in liblzma, because the Block coders take care
- of the rest of the stuff in the Stream Footer.
-
- For now, the size of Stream Header is fixed to 11 bytes. The header
- <lzma/stream_flags.h> defines LZMA_STREAM_HEADER_SIZE, which you
- should use instead of a hardcoded number. Similarly, Stream tail
- is fixed to 3 bytes, and there is a constant LZMA_STREAM_TAIL_SIZE.
-
- It is possible, that a future version of the .lzma format will have
- variable-sized Stream Header and tail. As of writing, this seems so
- unlikely though, that it was considered simplest to just use a
- constant instead of providing a functions to get and store the sizes
- of the Stream Header and tail.
-
-
-2.x. Stream tail
-
- For now, the size of Stream tail is fixed to 3 bytes. The header
- <lzma/stream_flags.h> defines LZMA_STREAM_TAIL_SIZE, which you
- should use instead of a hardcoded number.
-
-
-3. Keeping track of size information
-
- The lzma_info_* functions found from <lzma/info.h> should ease the
- task of keeping track of sizes of the Blocks and also the Stream
- as a whole. Using these functions is strongly recommended, because
- there are surprisingly many situations where an error can occur,
- and these functions check for possible errors every time some new
- information becomes available.
-
- If you find lzma_info_* functions lacking something that you would
- find useful, please contact the author.
-
-
-3.1. Start offset of the Stream
-
- If you are storing the .lzma Stream inside anothe file format, or
- for some other reason are placing the .lzma Stream to somewhere
- else than to the beginning of the file, you should tell the starting
- offset of the Stream using lzma_info_start_offset_set().
-
- The start offset of the Stream is used for two distinct purporses.
- First, knowing the start offset of the Stream allows
- lzma_info_alignment_get() to correctly calculate the alignment of
- every Block. This information is given to the Block encoder, which
- will calculate the size of Header Padding so that Compressed Data
- is alignment at an optimal offset.
-
- Another use for start offset of the Stream is in random-access
- reading. If you set the start offset of the Stream, lzma_info_locate()
- will be able to calculate the offset relative to the beginning of the
- file containing the Stream (instead of offset relative to the
- beginning of the Stream).
-
-
-3.2. Size of Stream Header
-
- While the size of Stream Header is constant (11 bytes) in the current
- version of the .lzma file format, this may change in future.
-
-
-3.3. Size of Header Metadata Block
-
- This information is needed when doing random-access reading, and
- to verify the value of this field stored in Footer Metadata Block.
-
-
-3.4. Total Size of the Data Blocks
-
-
-3.5. Uncompressed Size of Data Blocks
-
-
-3.6. Index
-
-
-
-
-x. Alignment
-
- There are a few slightly different types of alignment issues when
- working with .lzma files.
-
- The .lzma format doesn't strictly require any kind of alignment.
- However, if the encoder carefully optimizes the alignment in all
- situations, it can improve compression ratio, speed of the encoder
- and decoder, and slightly help if the files get damaged and need
- recovery.
-
- Alignment has the most significant effect compression ratio FIXME
-
-
-x.1. Compression ratio
-
- Some filters take advantage of the alignment of the input data.
- To get the best compression ratio, make sure that you feed these
- filters correctly aligned data.
-
- Some filters (e.g. LZMA) don't necessarily mind too much if the
- input doesn't match the preferred alignment. With these filters
- the penalty in compression ratio depends on the specific type of
- data being compressed.
-
- Other filters (e.g. PowerPC executable filter) won't work at all
- with data that is improperly aligned. While the data can still
- be de-filtered back to its original form, the benefit of the
- filtering (better compression ratio) is completely lost, because
- these filters expect certain patterns at properly aligned offsets.
- The compression ratio may even worse with incorrectly aligned input
- than without the filter.
-
-
-x.1.1. Inter-filter alignment
-
- When there are multiple filters chained, checking the alignment can
- be useful not only with the input of the first filter and output of
- the last filter, but also between the filters.
-
- Inter-filter alignment important especially with the Subblock filter.
-
-
-x.1.2. Further compression with external tools
-
- This is relatively rare situation in practice, but still worth
- understanding.
-
- Let's say that there are several SPARC executables, which are each
- filtered to separate .lzma files using only the SPARC filter. If
- Uncompressed Size is written to the Block Header, the size of Block
- Header may vary between the .lzma files. If no Padding is used in
- the Block Header to correct the alignment, the starting offset of
- the Compressed Data field will be differently aligned in different
- .lzma files.
-
- All these .lzma files are archived into a single .tar archive. Due
- to nature of the .tar format, every file is aligned inside the
- archive to an offset that is a multiple of 512 bytes.
-
- The .tar archive is compressed into a new .lzma file using the LZMA
- filter with options, that prefer input alignment of four bytes. Now
- if the independent .lzma files don't have the same alignment of
- the Compressed Data fields, the LZMA filter will be unable to take
- advantage of the input alignment between the files in the .tar
- archive, which reduces compression ratio.
-
- Thus, even if you have only single Block per file, it can be good for
- compression ratio to align the Compressed Data to optimal offset.
-
-
-x.2. Speed
-
- Most modern computers are faster when multi-byte data is located
- at aligned offsets in RAM. Proper alignment of the Compressed Data
- fields can slightly increase the speed of some filters.
-
-
-x.3. Recovery
-
- Aligning every Block Header to start at an offset with big enough
- alignment may ease or at least speed up recovery of broken files.
-
-
-y. Typical usage cases
-
-y.x. Parsing the Stream backwards
-
- You may need to parse the Stream backwards if you need to get
- information such as the sizes of the Stream, Index, or Extra.
- The basic procedure to do this follows.
-
- Locate the end of the Stream. If the Stream is stored as is in a
- standalone .lzma file, simply seek to the end of the file and start
- reading backwards using appropriate buffer size. The file format
- specification allows arbitrary amount of Footer Padding (zero or more
- NUL bytes), which you skip before trying to decode the Stream tail.
-
- Once you have located the end of the Stream (a non-NULL byte), make
- sure you have at least the last LZMA_STREAM_TAIL_SIZE bytes of the
- Stream in a buffer. If there isn't enough bytes left from the file,
- the file is too small to contain a valid Stream. Decode the Stream
- tail using lzma_stream_tail_decoder(). Store the offset of the first
- byte of the Stream tail; you will need it later.
-
- You may now want to do some internal verifications e.g. if the Check
- type is supported by the liblzma build you are using.
-
- Decode the Backward Size field with lzma_vli_reverse_decode(). The
- field is at maximum of LZMA_VLI_BYTES_MAX bytes long. Check that
- Backward Size is not zero. Store the offset of the first byte of
- the Backward Size; you will need it later.
-
- Now you know the Total Size of the last Block of the Stream. It's the
- value of Backward Size plus the size of the Backward Size field. Note
- that you cannot use lzma_vli_size() to calculate the size since there
- might be padding; you need to use the real observed size of the
- Backward Size field.
-
- At this point, the operation continues differently for Single-Block
- and Multi-Block Streams.
-
-
-y.x.1. Single-Block Stream
-
- There might be Uncompressed Size field present in the Stream Footer.
- You cannot know it for sure unless you have already parsed the Block
- Header earlier. For security reasons, you probably want to try to
- decode the Uncompressed Size field, but you must not indicate any
- error if decoding fails. Later you can give the decoded Uncompressed
- Size to Block decoder if Uncopmressed Size isn't otherwise known;
- this prevents it from producing too much output in case of (possibly
- intentionally) corrupt file.
-
- Calculate the start offset of the Stream:
-
- backward_offset - backward_size - LZMA_STREAM_HEADER_SIZE
-
- backward_offset is the offset of the first byte of the Backward Size
- field. Remember to check for integer overflows, which can occur with
- invalid input files.
-
- Seek to the beginning of the Stream. Decode the Stream Header using
- lzma_stream_header_decoder(). Verify that the decoded Stream Flags
- match the values found from Stream tail. You can use the
- lzma_stream_flags_is_equal() macro for this.
-
- Decode the Block Header. Verify that it isn't a Metadata Block, since
- Single-Block Streams cannot have Metadata. If Uncompressed Size is
- present in the Block Header, the value you tried to decode from the
- Stream Footer must be ignored, since Uncompressed Size wasn't actually
- present there. If Block Header doesn't have Uncompressed Size, and
- decoding the Uncompressed Size field from the Stream Footer failed,
- the file is corrupt.
-
- If you were only looking for the Uncompressed Size of the Stream,
- you now got that information, and you can stop processing the Stream.
-
- To decode the Block, the same instructions apply as described in
- FIXME. However, because you have some extra known information decoded
- from the Stream Footer, you should give this information to the Block
- decoder so that it can verify it while decoding:
- - If Uncompressed Size is not present in the Block Header, set
- lzma_options_block.uncompressed_size to the value you decoded
- from the Stream Footer.
- - Always set lzma_options_block.total_size to backward_size +
- size_of_backward_size (you calculated this sum earlier already).
-
-
-y.x.2. Multi-Block Stream
-
- Calculate the start offset of the Footer Metadata Block:
-
- backward_offset - backward_size
-
- backward_offset is the offset of the first byte of the Backward Size
- field. Remember to check for integer overflows, which can occur with
- broken input files.
-
- Decode the Block Header. Verify that it is a Metadata Block. Set
- lzma_options_block.total_size to backward_size + size_of_backward_size
- (you calculated this sum earlier already). Then decode the Footer
- Metadata Block.
-
- Store the decoded Footer Metadata to lzma_info structure using
- lzma_info_set_metadata(). Set also the offset of the Backward Size
- field using lzma_info_size_set(). Then you can get the start offset
- of the Stream using lzma_info_size_get(). Note that any of these steps
- may fail so don't omit error checking.
-
- Seek to the beginning of the Stream. Decode the Stream Header using
- lzma_stream_header_decoder(). Verify that the decoded Stream Flags
- match the values found from Stream tail. You can use the
- lzma_stream_flags_is_equal() macro for this.
-
- If you were only looking for the Uncompressed Size of the Stream,
- it's possible that you already have it now. If Uncompressed Size (or
- whatever information you were looking for) isn't available yet,
- continue by decoding also the Header Metadata Block. (If some
- information is missing, the Header Metadata Block has to be present.)
-
- Decoding the Data Blocks goes the same way as described in FIXME.
-
-
-y.x.3. Variations
-
- If you know the offset of the beginning of the Stream, you may want
- to parse the Stream Header before parsing the Stream tail.
-
+++ /dev/null
-
-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.
-
+++ /dev/null
-
-Introduction to liblzma
------------------------
-
-Writing applications to work with liblzma
-
- liblzma API is split in several subheaders to improve readability and
- maintainance. The subheaders must not be #included directly. lzma.h
- requires that certain integer types and macros are available when
- the header is #included. On systems that have inttypes.h that conforms
- to C99, the following will work:
-
- #include <sys/types.h>
- #include <inttypes.h>
- #include <lzma.h>
-
- Those who have used zlib should find liblzma's API easy to use.
- To developers who haven't used zlib before, I recommend learning
- zlib first, because zlib has excellent documentation.
-
- While the API is similar to that of zlib, there are some major
- differences, which are summarized below.
-
- For basic stream encoding, zlib has three functions (deflateInit(),
- deflate(), and deflateEnd()). Similarly, there are three functions
- for stream decoding (inflateInit(), inflate(), and inflateEnd()).
- liblzma has only single coding and ending function. Thus, to
- encode one may use, for example, lzma_stream_encoder_single(),
- lzma_code(), and lzma_end(). Simlarly for decoding, one may
- use lzma_auto_decoder(), lzma_code(), and lzma_end().
-
- zlib has deflateReset() and inflateReset() to reset the stream
- structure without reallocating all the memory. In liblzma, all
- coder initialization functions are like zlib's reset functions:
- the first-time initializations are done with the same functions
- as the reinitializations (resetting).
-
- To make all this work, liblzma needs to know when lzma_stream
- doesn't already point to an allocated and initialized coder.
- This is achieved by initializing lzma_stream structure with
- LZMA_STREAM_INIT (static initialization) or LZMA_STREAM_INIT_VAR
- (for exampple when new lzma_stream has been allocated with malloc()).
- This initialization should be done exactly once per lzma_stream
- structure to avoid leaking memory. Calling lzma_end() will leave
- lzma_stream into a state comparable to the state achieved with
- LZMA_STREAM_INIT and LZMA_STREAM_INIT_VAR.
-
- Example probably clarifies a lot. With zlib, compression goes
- roughly like this:
-
- z_stream strm;
- deflateInit(&strm, level);
- deflate(&strm, Z_RUN);
- deflate(&strm, Z_RUN);
- ...
- deflate(&strm, Z_FINISH);
- deflateEnd(&strm) or deflateReset(&strm)
-
- With liblzma, it's slightly different:
-
- lzma_stream strm = LZMA_STREAM_INIT;
- lzma_stream_encoder_single(&strm, &options);
- lzma_code(&strm, LZMA_RUN);
- lzma_code(&strm, LZMA_RUN);
- ...
- lzma_code(&strm, LZMA_FINISH);
- lzma_end(&strm) or reinitialize for new coding work
-
- Reinitialization in the last step can be any function that can
- initialize lzma_stream; it doesn't need to be the same function
- that was used for the previous initialization. If it is the same
- function, liblzma will usually be able to re-use most of the
- existing memory allocations (depends on how much the initialization
- options change). If you reinitialize with different function,
- liblzma will automatically free the memory of the previous coder.
-
-
-File formats
-
- liblzma supports multiple container formats for the compressed data.
- Different initialization functions initialize the lzma_stream to
- process different container formats. See the details from the public
- header files.
-
- The following functions are the most commonly used:
-
- - lzma_stream_encoder_single(): Encodes Single-Block Stream; this
- the recommended format for most purporses.
-
- - lzma_alone_encoder(): Useful if you need to encode into the
- legacy LZMA_Alone format.
-
- - lzma_auto_decoder(): Decoder that automatically detects the
- file format; recommended when you decode compressed files on
- disk, because this way compatibility with the legacy LZMA_Alone
- format is transparent.
-
- - lzma_stream_decoder(): Decoder for Single- and Multi-Block
- Streams; this is good if you want to accept only .lzma Streams.
-
-
-Filters
-
- liblzma supports multiple filters (algorithm implementations). The new
- .lzma format supports filter-chain having up to seven filters. In the
- filter chain, the output of one filter is input of the next filter in
- the chain. The legacy LZMA_Alone format supports only one filter, and
- that must always be LZMA.
-
- General-purporse compression:
-
- LZMA The main algorithm of liblzma (surprise!)
-
- Branch/Call/Jump filters for executables:
-
- x86 This filter is known as BCJ in 7-Zip
- IA64 IA-64 (Itanium)
- PowerPC Big endian PowerPC
- ARM
- ARM-Thumb
- SPARC
-
- Other filters:
-
- Copy Dummy filter that simply copies all the data
- from input to output.
-
- Subblock Multi-purporse filter, that can
- - embed End of Payload Marker if the previous
- filter in the chain doesn't support it; and
- - apply Subfilters, which filter only part
- of the same compressed Block in the Stream.
-
- Branch/Call/Jump filters never change the size of the data. They
- should usually be used as a pre-filter for some compression filter
- like LZMA.
-
-
-Integrity checks
-
- The .lzma Stream format uses CRC32 as the integrity check for
- different file format headers. It is possible to omit CRC32 from
- the Block Headers, but not from Stream Header. This is the reason
- why CRC32 code cannot be disabled when building liblzma (in addition,
- the LZMA encoder uses CRC32 for hashing, so that's another reason).
-
- The integrity check of the actual data is calculated from the
- uncompressed data. This check can be CRC32, CRC64, or SHA256.
- It can also be omitted completely, although that usually is not
- a good thing to do. There are free IDs left, so support for new
- checks algorithms can be added later.
-
-
-API and ABI stability
-
- The API and ABI of liblzma isn't stable yet, although no huge
- changes should happen. One potential place for change is the
- lzma_options_subblock structure.
-
- In the 4.42.0alpha phase, the shared library version number won't
- be updated even if ABI breaks. I don't want to track the ABI changes
- yet. Just rebuild everything when you upgrade liblzma until we get
- to the beta stage.
-
-
-Size of the library
-
- While liblzma isn't huge, it is quite far from the smallest possible
- LZMA implementation: full liblzma binary (with support for all
- filters and other features) is way over 100 KiB, but the plain raw
- LZMA decoder is only 5-10 KiB.
-
- To decrease the size of the library, you can omit parts of the library
- by passing certain options to the `configure' script. Disabling
- everything but the decoders of the require filters will usually give
- you a small enough library, but if you need a decoder for example
- embedded in the operating system kernel, the code from liblzma probably
- isn't suitable as is.
-
- If you need a minimal implementation supporting .lzma Streams, you
- may need to do partial rewrite. liblzma uses stateful API like zlib.
- That increases the size of the library. Using callback API or even
- simpler buffer-to-buffer API would allow smaller implementation.
-
- LZMA SDK contains smaller LZMA decoder written in ANSI-C than
- liblzma, so you may want to take a look at that code. However,
- it doesn't (at least not yet) support the new .lzma Stream format.
-
-
-Documentation
-
- There's no other documentation than the public headers and this
- text yet. Real docs will be written some day, I hope.
-
+++ /dev/null
-
-Using liblzma securely
-----------------------
-
-0. Introduction
-
- This document discusses how to use liblzma securely. There are issues
- that don't apply to zlib or libbzip2, so reading this document is
- strongly recommended even for those who are very familiar with zlib
- or libbzip2.
-
- While making liblzma itself as secure as possible is essential, it's
- out of scope of this document.
-
-
-1. Memory usage
-
- The memory usage of liblzma varies a lot.
-
-
-1.1. Problem sources
-
-1.1.1. Block coder
-
- The memory requirements of Block encoder depend on the used filters
- and their settings. The memory requirements of the Block decoder
- depend on the which filters and with which filter settings the Block
- was encoded. Usually the memory requirements of a decoder are equal
- or less than the requirements of the encoder with the same settings.
-
- While the typical memory requirements to decode a Block is from a few
- hundred kilobytes to tens of megabytes, a maliciously constructed
- files can require a lot more RAM to decode. With the current filters,
- the maximum amount is about 7 GiB. If you use multi-threaded decoding,
- every Block can require this amount of RAM, thus a four-threaded
- decoder could suddenly try to allocate 28 GiB of RAM.
-
- If you don't limit the maximum memory usage in any way, and there are
- no resource limits set on the operating system side, one malicious
- input file can run the system out of memory, or at least make it swap
- badly for a long time. This is exceptionally bad on servers e.g.
- email server doing virus scanning on incoming messages.
-
-
-1.1.2. Metadata decoder
-
- Multi-Block .lzma files contain at least one Metadata Block.
- Externally the Metadata Blocks are similar to Data Blocks, so all
- the issues mentioned about memory usage of Data Blocks applies to
- Metadata Blocks too.
-
- The uncompressed content of Metadata Blocks contain information about
- the Stream as a whole, and optionally some Extra Records. The
- information about the Stream is kept in liblzma's internal data
- structures in RAM. Extra Records can contain arbitrary data. They are
- not interpreted by liblzma, but liblzma will provide them to the
- application in uninterpreted form if the application wishes so.
-
- Usually the Uncompressed Size of a Metadata Block is small. Even on
- extreme cases, it shouldn't be much bigger than a few megabytes. Once
- the Metadata has been parsed into native data structures in liblzma,
- it usually takes a little more memory than in the encoded form. For
- all normal files, this is no problem, since the resulting memory usage
- won't be too much.
-
- The problem is that a maliciously constructed Metadata Block can
- contain huge amount of "information", which liblzma will try to store
- in its internal data structures. This may cause liblzma to allocate
- all the available RAM unless some kind of resource usage limits are
- applied.
-
- Note that the Extra Records in Metadata are always parsed but, but
- memory is allocated for them only if the application has requested
- liblzma to provide the Extra Records to the application.
-
-
-1.2. Solutions
-
- If you need to decode files from untrusted sources (most people do),
- you must limit the memory usage to avoid denial of service (DoS)
- conditions caused by malicious input files.
-
- The first step is to find out how much memory you are allowed consume
- at maximum. This may be a hardcoded constant or derived from the
- available RAM; whatever is appropriate in the application.
-
- The simplest solution is to use setrlimit() if the kernel supports
- RLIMIT_AS, which limits the memory usage of the whole process.
- For more portable and fine-grained limiting, you can use
- memory limiter functions found from <lzma/memlimit.h>.
-
-
-1.2.1. Encoder
-
- lzma_memory_usage() will give you a rough estimate about the memory
- usage of the given filter chain. To dramatically simplify the internal
- implementation, this function doesn't take into account all the small
- helper data structures needed in various places; only the structures
- with significant memory usage are taken into account. Still, the
- accuracy of this function should be well within a mebibyte.
-
- The Subblock filter is a special case. If a Subfilter has been
- specified, it isn't taken into account when lzma_memory_usage()
- calculates the memory usage. You need to calculate the memory usage
- of the Subfilter separately.
-
- Keeping track of Blocks in a Multi-Block Stream takes a few dozen
- bytes of RAM per Block (size of the lzma_index structure plus overhead
- of malloc()). It isn't a good idea to put tens of thousands of Blocks
- into a Stream unless you have a very good reason to do so (compressed
- dictionary could be an example of such situation).
-
- Also keep the number and sizes of Extra Records sane. If you produce
- the list of Extra Records automatically from some untrusted source,
- you should not only validate the content of these Records, but also
- their memory usage.
-
-
-1.2.2. Decoder
-
- A single-threaded decoder should simply use a memory limiter and
- indicate an error if it runs out of memory.
-
- Memory-limiting with multi-threaded decoding is tricky. The simple
- solution is to divide the maximum allowed memory usage with the
- maximum allowed threads, and give each Block decoder their own
- independent lzma_memory_limiter. The drawback is that if one Block
- needs notably more RAM than any other Block, the decoder will run out
- of memory when in reality there would be plenty of free RAM.
-
- An attractive alternative would be using shared lzma_memory_limiter.
- Depending on the application and the expected type of input, this may
- either be the best solution or a source of hard-to-repeat problems.
- Consider the following requirements:
- - You use a maximum of n threads.
- - x(i) is the decoder memory requirements of the Block number i
- in an expected input Stream.
- - The memory limiter is set to higher value than the sum of n
- highest values x(i).
-
- (If you are better at explaining the above conditions, please
- contribute your improved version.)
-
- If the above conditions aren't met, it is possible that the decoding
- will fail unpredictably. That is, on the same machine using the same
- settings, the decoding may sometimes succeed and sometimes fail. This
- is because sometimes threads may run so that the Blocks with highest
- memory usage are tried to be decoded at the same time.
-
- Most .lzma files have all the Blocks encoded with identical settings,
- or at least the memory usage won't vary dramatically. That's why most
- multi-threaded decoders probably want to use the simple "separate
- lzma_memory_limiter for each thread" solution, possibly falling back
- to single-threaded mode in case the per-thread memory limits aren't
- enough in multi-threaded mode.
-
-FIXME: Memory usage of Stream info.
-
-[
-
-]
-
-
-2. Huge uncompressed output
-
-2.1. Data Blocks
-
- Decoding a tiny .lzma file can produce huge amount of uncompressed
- output. There is an example file of 45 bytes, which decodes to 64 PiB
- (that's 2^56 bytes). Uncompressing such a file to disk is likely to
- fill even a bigger disk array. If the data is written to a pipe, it
- may not fill the disk, but would still take very long time to finish.
-
- To avoid denial of service conditions caused by huge amount of
- uncompressed output, applications using liblzma should use some method
- to limit the amount of output produced. The exact method depends on
- the application.
-
- All valid .lzma Streams make it possible to find out the uncompressed
- size of the Stream without actually uncompressing the data. This
- information is available in at least one of the Metadata Blocks.
- Once the uncompressed size is parsed, the decoder can verify that
- it doesn't exceed certain limits (e.g. available disk space).
-
- When the uncompressed size is known, the decoder can actively keep
- track of the amount of output produced so far, and that it doesn't
- exceed the known uncompressed size. If it does exceed, the file is
- known to be corrupt and an error should be indicated without
- continuing to decode the rest of the file.
-
- Unfortunately, finding the uncompressed size beforehand is often
- possible only in non-streamed mode, because the needed information
- could be in the Footer Metdata Block, which (obviously) is at the
- end of the Stream. In purely streamed mode decoding, one may need to
- use some rough arbitrary limits to prevent the problems described in
- the beginning of this section.
-
-
-2.2. Metadata
-
- Metadata is stored in Metadata Blocks, which are very similar to
- Data Blocks. Thus, the uncompressed size can be huge just like with
- Data Blocks. The difference is, that the contents of Metadata Blocks
- aren't given to the application as is, but parsed by liblzma. Still,
- reading through a huge Metadata can take very long time, effectively
- creating a denial of service like piping decoded a Data Block to
- another process would do.
-
- At first it would seem that using a memory limiter would prevent
- this issue as a side effect. But it does so only if the application
- requests liblzma to allocate the Extra Records and provide them to
- the application. If Extra Records aren't requested, they aren't
- allocated either. Still, the Extra Records are being read through
- to validate that the Metadata is in proper format.
-
- The solution is to limit the Uncompressed Size of a Metadata Block
- to some relatively large value. This will make liblzma to give an
- error when the given limit is reached.
-
+++ /dev/null
-
-Introduction to the lzma command line tool
-------------------------------------------
-
-Overview
-
- The lzma command line tool is similar to gzip and bzip2, but for
- compressing and uncompressing .lzma files.
-
-
-Supported file formats
-
- By default, the tool creates files in the new .lzma format. This can
- be overriden with --format=FMT command line option. Use --format=alone
- to create files in the old LZMA_Alone format.
-
- By default, the tool uncompresses both the new .lzma format and
- LZMA_Alone format. This is to make it transparent to switch from
- the old LZMA_Alone format to the new .lzma format. Since both
- formats use the same filename suffix, average user should never
- notice which format was used.
-
-
-Differences to gzip and bzip2
-
- Standard input and output
-
- Both gzip and bzip2 refuse to write compressed data to a terminal and
- read compressed data from a terminal. With gzip (but not with bzip2),
- this can be overriden with the `--force' option. lzma follows the
- behavior of gzip here.
-
- Usage of LZMA_OPT environment variable
-
- gzip and bzip2 read GZIP and BZIP2 environment variables at startup.
- These variables may contain extra command line options.
-
- gzip and bzip2 allow passing not only options, but also end-of-options
- indicator (`--') and filenames via the environment variable. No quoting
- is supported with the filenames.
-
- Here are examples with gzip. bzip2 behaves identically.
-
- bash$ echo asdf > 'foo bar'
- bash$ GZIP='"foo bar"' gzip
- gzip: "foo: No such file or directory
- gzip: bar": No such file or directory
-
- bash$ GZIP=-- gzip --help
- gzip: --help: No such file or directory
-
- lzma silently ignores all non-option arguments given via the
- environment variable LZMA_OPT. Like on the command line, everything
- after `--' is taken as non-options, and thus ignored in LZMA_OPT.
-
- bash$ LZMA_OPT='--help' lzma --version # Displays help
- bash$ LZMA_OPT='-- --help' lzma --version # Displays version
-
-
-Filter chain presets
-
- Like in gzip and bzip2, lzma supports numbered presets from 1 to 9
- where 1 is the fastest and 9 the best compression. 1 and 2 are for
- fast compressing with small memory usage, 3 to 6 for good compression
- ratio with medium memory usage, and 7 to 9 for excellent compression
- ratio with higher memory requirements. The default is 7 if memory
- usage limit allows.
-
- In future, there will probably be an option like --preset=NAME, which
- will contain more special presets for specific file types.
-
- It's also possible that there will be some heuristics to select good
- filters. For example, the tool could detect when a .tar archive is
- being compressed, and enable x86 filter only for those files in the
- .tar archive that are ELF or PE executables for x86.
-
-
-Specifying custom filter chains
-
- Custom filter chains are specified by using long options with the name
- of the filters in correct order. For example, to pass the input data to
- the x86 filter and the output of that to the LZMA filter, the following
- command will do:
-
- lzma --x86 --lzma filename
-
- Some filters accept options, which are specified as a comma-separated
- list of key=value pairs:
-
- lzma --delta=distance=4 --lzma=dict=4Mi,lc=8,lp=2 filename
-
-
-Memory usage control
-
- By default, the command line tool limits memory usage to 1/3 of the
- available physical RAM. If no preset or custom filter chain has been
- given, the default preset will be used. If the memory limit is too
- low for the default preset, the tool will silently switch to lower
- preset.
-
- When a preset or a custom filter chain has been specified and the
- memory limit is too low, an error message is displayed and no files
- are processed.
-
- If the decoder hits the memory usage limit, an error is displayed and
- no more files are processed.
-
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