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This is an archive of xz repository, as it stood @ 2024-04-04T10:36:00Z. See https://tukaani.org/xz-backdoor/ as for what happened. Repository is frozen and will not be updated.
https://tukaani.org/xz-backdoor/
31d80c6b26
RHEL/CentOS 7 shipped with 5.1.2alpha, including the threaded encoder that is behind #ifdef LZMA_UNSTABLE in the API headers. In 5.1.2alpha these symbols are under XZ_5.1.2alpha in liblzma.map. API/ABI compatibility tracking isn't done between development releases so newer releases didn't have XZ_5.1.2alpha anymore. Later RHEL/CentOS 7 updated xz to 5.2.2 but they wanted to keep the exported symbols compatible with 5.1.2alpha. After checking the ABI changes it turned out that >= 5.2.0 ABI is backward compatible with the threaded encoder functions from 5.1.2alpha (but not vice versa as fixes and extensions to these functions were made between 5.1.2alpha and 5.2.0). In RHEL/CentOS 7, XZ Utils 5.2.2 was patched with xz-5.2.2-compat-libs.patch to modify liblzma.map: - XZ_5.1.2alpha was added with lzma_stream_encoder_mt and lzma_stream_encoder_mt_memusage. This matched XZ Utils 5.1.2alpha. - XZ_5.2 was replaced with XZ_5.2.2. It is clear that this was an error; the intention was to keep using XZ_5.2 (XZ_5.2.2 has never been used in XZ Utils). So XZ_5.2.2 lists all symbols that were listed under XZ_5.2 before the patch. lzma_stream_encoder_mt and _mt_memusage are included too so they are listed both here and under XZ_5.1.2alpha. The patch didn't add any __asm__(".symver ...") lines to the .c files. Thus the resulting liblzma.so exports the threaded encoder functions under XZ_5.1.2alpha only. Listing the two functions also under XZ_5.2.2 in liblzma.map has no effect without matching .symver lines. The lack of XZ_5.2 in RHEL/CentOS 7 means that binaries linked against unpatched XZ Utils 5.2.x won't run on RHEL/CentOS 7. This is unfortunate but this alone isn't too bad as the problem is contained within RHEL/CentOS 7 and doesn't affect users of other distributions. It could also be fixed internally in RHEL/CentOS 7. The second problem is more serious: In XZ Utils 5.2.2 the API headers don't have #ifdef LZMA_UNSTABLE for obvious reasons. This is true in RHEL/CentOS 7 version too. Thus now programs using new APIs can be compiled without an extra #define. However, the programs end up depending on symbol version XZ_5.1.2alpha (and possibly also XZ_5.2.2) instead of XZ_5.2 as they would with an unpatched XZ Utils 5.2.2. This means that such binaries won't run on other distributions shipping XZ Utils >= 5.2.0 as they don't provide XZ_5.1.2alpha or XZ_5.2.2; they only provide XZ_5.2 (and XZ_5.0). (This includes RHEL/CentOS 8 as the patch luckily isn't included there anymore with XZ Utils 5.2.4.) Binaries built by RHEL/CentOS 7 users get distributed and then people wonder why they don't run on some other distribution. Seems that people have found out about the patch and been copying it to some build scripts, seemingly curing the symptoms but actually spreading the illness further and outside RHEL/CentOS 7. The ill patch seems to be from late 2016 (RHEL 7.3) and in 2017 it had spread at least to EasyBuild. I heard about the events only recently. :-( This commit splits liblzma.map into two versions: one for GNU/Linux and another for other OSes that can use symbol versioning (FreeBSD, Solaris, maybe others). The Linux-specific file and the matching additions to .c files add full compatibility with binaries that have been built against a RHEL/CentOS-patched liblzma. Builds for OSes other than GNU/Linux won't get the vaccine as they should be immune to the problem (I really hope that no build script uses the RHEL/CentOS 7 patch outside GNU/Linux). The RHEL/CentOS compatibility symbols XZ_5.1.2alpha and XZ_5.2.2 are intentionally put *after* XZ_5.2 in liblzma_linux.map. This way if one forgets to #define HAVE_SYMBOL_VERSIONS_LINUX when building, the resulting liblzma.so.5 will have lzma_stream_encoder_mt@@XZ_5.2 since XZ_5.2 {...} is the first one that lists that function. Without HAVE_SYMBOL_VERSIONS_LINUX @XZ_5.1.2alpha and @XZ_5.2.2 will be missing but that's still a minor problem compared to only having lzma_stream_encoder_mt@@XZ_5.1.2alpha! The "local: *;" line was moved to XZ_5.0 so that it doesn't need to be moved around. It doesn't matter where it is put. Having two similar liblzma_*.map files is a bit silly as it is, at least for now, easily possible to generate the generic one from the Linux-specific file. But that adds extra steps and increases the risk of mistakes when supporting more than one build system. So I rather maintain two files in parallel and let validate_map.sh check that they are in sync when "make mydist" is run. This adds .symver lines for lzma_stream_encoder_mt@XZ_5.2.2 and lzma_stream_encoder_mt_memusage@XZ_5.2.2 even though these weren't exported by RHEL/CentOS 7 (only @@XZ_5.1.2alpha was for these two). I added these anyway because someone might misunderstand the RHEL/CentOS 7 patch and think that @XZ_5.2.2 (@@XZ_5.2.2) versions were exported too. At glance one could suggest using __typeof__ to copy the function prototypes when making aliases. However, this doesn't work trivially because __typeof__ won't copy attributes (lzma_nothrow, lzma_pure) and it won't change symbol visibility from hidden to default (done by LZMA_API()). Attributes could be copied with __copy__ attribute but that needs GCC 9 and a fallback method would be needed anyway. This uses __symver__ attribute with GCC >= 10 and __asm__(".symver ...") with everything else. The attribute method is required for LTO (-flto) support with GCC. Using -flto with GCC older than 10 is now broken on GNU/Linux and will not be fixed (can silently result in a broken liblzma build that has dangerously incorrect symbol versions). LTO builds with Clang seem to work with the traditional __asm__(".symver ...") method. Thanks to Boud Roukema for reporting the problem and discussing the details and testing the fix. |
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XZ Utils ======== 0. Overview 1. Documentation 1.1. Overall documentation 1.2. Documentation for command-line tools 1.3. Documentation for liblzma 2. Version numbering 3. Reporting bugs 4. Translations 5. Other implementations of the .xz format 6. Contact information 0. Overview ----------- XZ Utils provide a general-purpose data-compression library plus command-line tools. The native file format is the .xz format, but also the legacy .lzma format is supported. The .xz format supports multiple compression algorithms, which are called "filters" in the context of XZ Utils. The primary filter is currently LZMA2. With typical files, XZ Utils create about 30 % smaller files than gzip. To ease adapting support for the .xz format into existing applications and scripts, the API of liblzma is somewhat similar to the API of the popular zlib library. For the same reason, the command-line tool xz has a command-line syntax similar to that of gzip. When aiming for the highest compression ratio, the LZMA2 encoder uses a lot of CPU time and may use, depending on the settings, even hundreds of megabytes of RAM. However, in fast modes, the LZMA2 encoder competes with bzip2 in compression speed, RAM usage, and compression ratio. LZMA2 is reasonably fast to decompress. It is a little slower than gzip, but a lot faster than bzip2. Being fast to decompress means that the .xz format is especially nice when the same file will be decompressed very many times (usually on different computers), which is the case e.g. when distributing software packages. In such situations, it's not too bad if the compression takes some time, since that needs to be done only once to benefit many people. With some file types, combining (or "chaining") LZMA2 with an additional filter can improve the compression ratio. A filter chain may contain up to four filters, although usually only one or two are used. For example, putting a BCJ (Branch/Call/Jump) filter before LZMA2 in the filter chain can improve compression ratio of executable files. Since the .xz format allows adding new filter IDs, it is possible that some day there will be a filter that is, for example, much faster to compress than LZMA2 (but probably with worse compression ratio). Similarly, it is possible that some day there is a filter that will compress better than LZMA2. XZ Utils supports multithreaded compression. XZ Utils doesn't support multithreaded decompression yet. It has been planned though and taken into account when designing the .xz file format. In the future, files that were created in threaded mode can be decompressed in threaded mode too. 1. Documentation ---------------- 1.1. Overall documentation README This file INSTALL.generic Generic install instructions for those not familiar with packages using GNU Autotools INSTALL Installation instructions specific to XZ Utils PACKAGERS Information to packagers of XZ Utils COPYING XZ Utils copyright and license information COPYING.GPLv2 GNU General Public License version 2 COPYING.GPLv3 GNU General Public License version 3 COPYING.LGPLv2.1 GNU Lesser General Public License version 2.1 AUTHORS The main authors of XZ Utils THANKS Incomplete list of people who have helped making this software NEWS User-visible changes between XZ Utils releases ChangeLog Detailed list of changes (commit log) TODO Known bugs and some sort of to-do list Note that only some of the above files are included in binary packages. 1.2. Documentation for command-line tools The command-line tools are documented as man pages. In source code releases (and possibly also in some binary packages), the man pages are also provided in plain text (ASCII only) and PDF formats in the directory "doc/man" to make the man pages more accessible to those whose operating system doesn't provide an easy way to view man pages. 1.3. Documentation for liblzma The liblzma API headers include short docs about each function and data type as Doxygen tags. These docs should be quite OK as a quick reference. There are a few example/tutorial programs that should help in getting started with liblzma. In the source package the examples are in "doc/examples" and in binary packages they may be under "examples" in the same directory as this README. Since the liblzma API has similarities to the zlib API, some people may find it useful to read the zlib docs and tutorial too: http://zlib.net/manual.html http://zlib.net/zlib_how.html 2. Version numbering -------------------- The version number format of XZ Utils is X.Y.ZS: - X is the major version. When this is incremented, the library API and ABI break. - Y is the minor version. It is incremented when new features are added without breaking the existing API or ABI. An even Y indicates a stable release and an odd Y indicates unstable (alpha or beta version). - Z is the revision. This has a different meaning for stable and unstable releases: * Stable: Z is incremented when bugs get fixed without adding any new features. This is intended to be convenient for downstream distributors that want bug fixes but don't want any new features to minimize the risk of introducing new bugs. * Unstable: Z is just a counter. API or ABI of features added in earlier unstable releases having the same X.Y may break. - S indicates stability of the release. It is missing from the stable releases, where Y is an even number. When Y is odd, S is either "alpha" or "beta" to make it very clear that such versions are not stable releases. The same X.Y.Z combination is not used for more than one stability level, i.e. after X.Y.Zalpha, the next version can be X.Y.(Z+1)beta but not X.Y.Zbeta. 3. Reporting bugs ----------------- Naturally it is easiest for me if you already know what causes the unexpected behavior. Even better if you have a patch to propose. However, quite often the reason for unexpected behavior is unknown, so here are a few things to do before sending a bug report: 1. Try to create a small example how to reproduce the issue. 2. Compile XZ Utils with debugging code using configure switches --enable-debug and, if possible, --disable-shared. If you are using GCC, use CFLAGS='-O0 -ggdb3'. Don't strip the resulting binaries. 3. Turn on core dumps. The exact command depends on your shell; for example in GNU bash it is done with "ulimit -c unlimited", and in tcsh with "limit coredumpsize unlimited". 4. Try to reproduce the suspected bug. If you get "assertion failed" message, be sure to include the complete message in your bug report. If the application leaves a coredump, get a backtrace using gdb: $ gdb /path/to/app-binary # Load the app to the debugger. (gdb) core core # Open the coredump. (gdb) bt # Print the backtrace. Copy & paste to bug report. (gdb) quit # Quit gdb. Report your bug via email or IRC (see Contact information below). Don't send core dump files or any executables. If you have a small example file(s) (total size less than 256 KiB), please include it/them as an attachment. If you have bigger test files, put them online somewhere and include a URL to the file(s) in the bug report. Always include the exact version number of XZ Utils in the bug report. If you are using a snapshot from the git repository, use "git describe" to get the exact snapshot version. If you are using XZ Utils shipped in an operating system distribution, mention the distribution name, distribution version, and exact xz package version; if you cannot repeat the bug with the code compiled from unpatched source code, you probably need to report a bug to your distribution's bug tracking system. 4. Translations --------------- The xz command line tool and all man pages can be translated. The translations are handled via the Translation Project. If you wish to help translating xz, please join the Translation Project: https://translationproject.org/html/translators.html Several strings will change in a future version of xz so if you wish to start a new translation, look at the code in the xz git repository instead of a 5.2.x release. 5. Other implementations of the .xz format ------------------------------------------ 7-Zip and the p7zip port of 7-Zip support the .xz format starting from the version 9.00alpha. http://7-zip.org/ http://p7zip.sourceforge.net/ XZ Embedded is a limited implementation written for use in the Linux kernel, but it is also suitable for other embedded use. https://tukaani.org/xz/embedded.html 6. Contact information ---------------------- If you have questions, bug reports, patches etc. related to XZ Utils, contact Lasse Collin <lasse.collin@tukaani.org> (in Finnish or English). I'm sometimes slow at replying. If you haven't got a reply within two weeks, assume that your email has got lost and resend it or use IRC. You can find me also from #tukaani on Freenode; my nick is Larhzu. The channel tends to be pretty quiet, so just ask your question and someone may wake up.