xz4rust

Memory safe pure Rust no-std & no alloc XZ decoder

Usage

With standard library

use std::fs::File;
use std::io::Read;
use xz4rust::XzReader;

fn main() -> std::io::Result<()> {
    //This file contains Hello\nWorld!
    let file = File::open("../test_files/good-1-block_header-1.xz")?;
    let mut reader = XzReader::new(file);
    
    let mut result = Vec::new();
    reader.read_to_end(&mut result)?;
    
    println!("{}", String::from_utf8_lossy(&result));
    Ok(())
}

no_std + no allocator

Note: XzDecoder itself uses about 32k memory on the stack. This example needs about 100k stack. See below for an alternative that uses much less stack.

use xz4rust::{XzDecoder, XzNextBlockResult};

/// I am aware that the print! macro is not available in no_std, but other than that everything
/// here should work in no_std environments.
fn main() {
  //This file contains Hello\nWorld!
  let compressed_data = include_bytes!("../test_files/good-1-block_header-1.xz");
  // The size of the dictionary depends on the input file, this one has a 64kib dictionary.
  // If you use xz-utils you can set the dict size when encoding.
  // The largest preset in xz-utils (-9) uses a 65mb dictionary.
  // Using a dictionary that is too small will cause an err when decoding.
  // Note: You may have to move this off the stack to the heap or static memory if this becomes too large for your stack.
  let mut dictionary_buffer = [0u8; 65536];
  let mut decompressed_data_buffer = [0u8; 16];

  let mut decoder = XzDecoder::with_fixed_size_dict(&mut dictionary_buffer);
  let mut input_position = 0usize;
  loop {
    match decoder.decode(
      &compressed_data[input_position..],
      &mut decompressed_data_buffer,
    ) {
      Ok(XzNextBlockResult::NeedMoreData(input_consumed, output_produced)) => {
        input_position += input_consumed;
        if output_produced > 0 {
          // Note: We know this input file contains only ascii characters
          // and no multi byte which might be split at the edge of a buffer!
          print!(
            "{}",
            std::str::from_utf8(&decompressed_data_buffer[..output_produced]).unwrap()
          );
        }
      }
      Ok(XzNextBlockResult::EndOfStream(_, output_produced)) => {
        if output_produced > 0 {
          // Note: We know this input file contains only ascii characters
          // and no multi byte which might be split at the edge of a buffer!
          print!(
            "{}",
            std::str::from_utf8(&decompressed_data_buffer[..output_produced]).unwrap()
          );
        }
        println!();
        println!("Finished!");
        break;
      }
      Err(err) => panic!("Decompression failed {}", err),
    };
  }
}

no_std + no allocator + minimal stack space

use spin::mutex::SpinMutex;
use xz4rust::{XzNextBlockResult, XzStaticDecoder};

// 65536 is the size of the dictionary!
// This entire variable is about 100k in size, which will be placed in your binary.
// If you are willing to use unsafe code then you can also use zeroed memory.
// This will work as long as you reset the decoder before using it.
// In this case we use no unsafe code and just accept that the binary gets bigger.
static DECODER: SpinMutex<XzStaticDecoder<65536>> = SpinMutex::new(XzStaticDecoder::new());
fn main() {
  //This file contains Hello\nWorld!
  let compressed_data = include_bytes!("../test_files/good-1-block_header-1.xz");

  let mut decompressed_data_buffer = [0u8; 16];

  let mut decoder = DECODER.lock();
  let mut input_position = 0usize;
  loop {
    match decoder.decode(
      &compressed_data[input_position..],
      &mut decompressed_data_buffer,
    ) {
      Ok(XzNextBlockResult::NeedMoreData(input_consumed, output_produced)) => {
        input_position += input_consumed;
        if output_produced > 0 {
          // Note: We know this input file contains only ascii characters
          // and no multibyte which might be split at the edge of a buffer!
          print!(
            "{}",
            std::str::from_utf8(&decompressed_data_buffer[..output_produced]).unwrap()
          );
        }
      }
      Ok(XzNextBlockResult::EndOfStream(_, output_produced)) => {
        if output_produced > 0 {
          // Note: We know this input file contains only ascii characters
          // and no multibyte which might be split at the edge of a buffer!
          print!(
            "{}",
            std::str::from_utf8(&decompressed_data_buffer[..output_produced]).unwrap()
          );
        }
        println!();
        println!("Finished!");
        break;
      }
      Err(err) => panic!("Decompression failed {}", err),
    };
  }
}

no_std + alloc

use xz4rust::{XzDecoder, XzNextBlockResult};

fn main() {
  //This file contains Hello\nWorld!
  let compressed_data = include_bytes!("../test_files/good-1-block_header-1.xz");
  let mut decompressed_data = Vec::new();

  let initial_alloc_size = 4096;
  let max_alloc_size = 0x10000;
  let mut decoder = XzDecoder::in_heap_with_alloc_dict_size(initial_alloc_size, max_alloc_size);

  let mut input_position = 0usize;
  loop {
    let mut temp_buffer = [0u8; 4096];
    match decoder.decode(&compressed_data[input_position..], &mut temp_buffer) {
      Ok(XzNextBlockResult::NeedMoreData(input_consumed, output_produced)) => {
        input_position += input_consumed;
        decompressed_data.extend_from_slice(&temp_buffer[..output_produced]);
      }
      Ok(XzNextBlockResult::EndOfStream(_, output_produced)) => {
        decompressed_data.extend_from_slice(&temp_buffer[..output_produced]);
        break;
      }
      Err(err) => panic!("Decompression failed {}", err),
    };
  }

  //This obviously requires std, but is just for illustrative purposes, you can do something else with the data...
  println!(
    "Decompressed contents: {}",
    String::from_utf8_lossy(&decompressed_data)
  );
  println!("Finished!");
}

Comparison to other XZ decoders available for Rust

(*1) It would probably be trivial to patch the crate.

Benchmarks

Speed values are based on the b2 benchmark in the benches' directory. This benchmark decodes a ~1mb amd64 .so file. IO delay is not included as the entire 1mb xz file is loaded into memory before the benchmark begins.

The benchmarks uses rust 1.86

The "Steam Deck" column refers to a baseline Steam Deck so it should be reasonably reproducible.

The "I7 8700k" Column refers to an ordinary desktop computer with a non overclocked Intel I7 8700k CPU running debian linux. This benchmark is probably not reproducible on a different computer.

Features

The default features assume you are using the rust standard library. For no_std disable the default features and enable them as needed!

• bcj - enables support for decoding BCJ xz files.
  • Enabled by default
  • BCJ improves the compression of compiled executable code. This is usually present in .xz packages bundled by some linux distributions.
  • If you only need to decode .xz files that you create yourself then you probably do not need this feature unless you explicitly enable it during compression.
  • If this feature is disabled then upon decoding of the header of a xz file with bcj the implementation will return an Err.
• crc64
  • Enabled by default
  • Support for crc64 checksums in xz files
  • Note: the xz command line application will use crc64 checksums by default, disabling this feature will prevent you from decoding those
  • If this feature is disabled then upon decoding of the header of a xz file with crc64 the implementation will return an Err.
• sha256
  • Enabled by default
  • Adds a dependency to the sha2 crate
  • If this feature is disabled then upon decoding of the header of a xz file with sha256 the implementation will return an Err.
• alloc
  • Enabled by default
  • Requires you to have an allocator present in your binary. (If you use the stdlib then you have an allocator)
  • When creating the decoder you will have to decide how the decoder allocates the dictionary.
    • If you disable this feature then you cannot choose the option to let the decoder allocate the dictionary on the heap.
• std
  • Enabled by default
  • Requires the standard library
  • Adds support for decoding transparently from a std::io::Read
• no_unsafe
  • Not enabled by default
  • Disables all unsafe code in this crate.
  • Read below for more info.

How was this crate implemented?

This implementation is a port of the C library xz-embedded to rust.

Only features present in xz-embedded are present in this implementation.

This implementation has the same limitations as xz-embedded (3GiB dictionary size)

A memory allocator is optional for this implementation.

The C code of xz-embedded has been translated using c2rust and then manually refactored until no unsafe code remained and the rust code looked sane.

License

The rust source code in this project is released under the MIT License. The rust source code is a port/translation of xz-embedded as permitted by the license of xz-embedded. For more information regarding xz-embedded see here: https://github.com/tukaani-project/xz-embedded

The MIT License does NOT apply to the test files in the test_files directory of this repository. It contains compressed binaries that are released under different licenses (such as LGPLv3). This makes any binary builds of the test code non trivially redistributable. As should be obvious, this has no effect on any non-test builds of this library.

Some of the files in the test_files directory appear to also be in the public domain. They are sourced from the xz-utils git repo. If you are looking to re-use or redistribute only those test files then I recommend sourcing them from the xz-utils git repo directly.

Tests

This implementation can decode all test files from the xz-repo that xz-embedded can also decode. The only test files that cannot be decoded are those requiring the delta filter or use a custom offset address for a BCJ filter. Both of which is not implemented in the native xz-embedded.

Unsafe code

This crate features two optional unsafe blocks. Both are only related to allocation of the memory for the decoder. Once the decoder is allocated, no unsafe code is needed to perform the actual decoding.

One in the alloc feature to allocate a 32kb large struct in the heap using Box::new_uninit. This unsafe block is trivially verifiable and has been tested with miri.

One function to allocate the decoder at an arbitrary address. This can be useful if you want to place the decoder in a union. You can just ignore this function if you don't need it. This unsafe block is trivially verifiable and has been tested with miri.

Why?

This is unfortunately needed because rust has no other guaranteed way to allocate a structure on the heap. You can move a struct to the heap, but you cant reliably allocate it there without unsafe. On small stack sizes stack allocation of 32k (like with libc-musl which has only 90k stack) may blow the stack. There is a test that ensures that .xz files can be decoded with as little as 8k of stack. Depending on the optimization level and cpu architecture less than 4k of stack are also sufficient.

Disabling unsafe code

If your target system has enough stack (at least ~40k) you can disable all unsafe code in this crate (enforced by deny(unsafe_code)) if you enable the no_unsafe feature without having to worry about any side effects. I do not recommend enabling the no_unsafe feature on libc-musl targets due to stack overflows that might occur then.

Is this library panic free?

Probably not. There are a lot of operations that could potentially panic given a malicious input file. Until the entire codebase is fully fuzzed I cannot rule out that this implementation panics. Since this implementation has effectively no unsafe code as mentioned above a panic is the worst that could happen tho. There is also uncertainty about some places regarding unsigned integer overflows. In debug mode these panic, in release mode they just act like they do in C. If you have any test files that cause panics or overflows (and therefore panics in debug mode) then those would be greatly appreciated.

xzcheck program

This library is used by me to decode patches/releases for a different application as part an installer application. To reduce risk and ensure that patches can be decoded without issue by the installer, I have created a small program called xzcheck which verifies that a .xz file is valid and can be decoded by this library. I run xzcheck as part of my build process for my patches. If your use-case allows for this then I recommend you to do the same if you are considering this library.

You can install it normally via cargo install xzcheck It fully decodes the entire .xz file and checks the content hash. If it succeeds in decoding it exits with code 0 otherwise it exits with code 255 and prints an error to stderr. This is useful if you wish to automatically test created xz files to see if your application will later be able to decode them.

Unsupported targets

Any rust target where usize/pointer size is 16 bit is not supported. Currently, rust only has one tier-3 target for some microcontroller where this is the case. This crate will emit a compiler error for such targets.

Future work

• Finish refactoring existing code.
• Implement the "Delta" filter.
• Implement offsets for the BCJ filters.
• Implement filter chains.
• Optimize the current implementation using perf.
• Port/Implement an XZ Encoder. (A lot of work, that I currently do not need myself...)