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2023-02-23liblzma: Avoid null pointer + 0 (undefined behavior in C).Lasse Collin1-1/+5
In the C99 and C17 standards, section 6.5.6 paragraph 8 means that adding 0 to a null pointer is undefined behavior. As of writing, "clang -fsanitize=undefined" (Clang 15) diagnoses this. However, I'm not aware of any compiler that would take advantage of this when optimizing (Clang 15 included). It's good to avoid this anyway since compilers might some day infer that pointer arithmetic implies that the pointer is not NULL. That is, the following foo() would then unconditionally return 0, even for foo(NULL, 0): void bar(char *a, char *b); int foo(char *a, size_t n) { bar(a, a + n); return a == NULL; } In contrast to C, C++ explicitly allows null pointer + 0. So if the above is compiled as C++ then there is no undefined behavior in the foo(NULL, 0) call. To me it seems that changing the C standard would be the sane thing to do (just add one sentence) as it would ensure that a huge amount of old code won't break in the future. Based on web searches it seems that a large number of codebases (where null pointer + 0 occurs) are being fixed instead to be future-proof in case compilers will some day optimize based on it (like making the above foo(NULL, 0) return 0) which in the worst case will cause security bugs. Some projects don't plan to change it. For example, gnulib and thus many GNU tools currently require that null pointer + 0 is defined: https://lists.gnu.org/archive/html/bug-gnulib/2021-11/msg00000.html https://www.gnu.org/software/gnulib/manual/html_node/Other-portability-assumptions.html In XZ Utils null pointer + 0 issue should be fixed after this commit. This adds a few if-statements and thus branches to avoid null pointer + 0. These check for size > 0 instead of ptr != NULL because this way bugs where size > 0 && ptr == NULL will likely get caught quickly. None of them are in hot spots so it shouldn't matter for performance. A little less readable version would be replacing ptr + offset with offset != 0 ? ptr + offset : ptr or creating a macro for it: #define my_ptr_add(ptr, offset) \ ((offset) != 0 ? ((ptr) + (offset)) : (ptr)) Checking for offset != 0 instead of ptr != NULL allows GCC >= 8.1, Clang >= 7, and Clang-based ICX to optimize it to the very same code as ptr + offset. That is, it won't create a branch. So for hot code this could be a good solution to avoid null pointer + 0. Unfortunately other compilers like ICC 2021 or MSVC 19.33 (VS2022) will create a branch from my_ptr_add(). Thanks to Marcin Kowalczyk for reporting the problem: https://github.com/tukaani-project/xz/issues/36
2022-11-27liblzma: Pass the Filter ID to LZ encoder and decoder.Lasse Collin1-0/+1
This allows using two Filter IDs with the same initialization function and data structures.
2022-11-09liblzma: Add .lz (lzip) decompression support (format versions 0 and 1).Lasse Collin1-0/+413
Support for format version 0 was removed from lzip 1.18 for some reason. .lz format version 0 files are rare (and old) but some source packages were released in this format, and some people might have personal files in this format too. It's very little extra code to support it along side format version 1 so this commits adds support for both. The Sync Flush marker extentension to the original .lz format version 1 isn't supported. It would require changes to the LZMA decoder itself. Such files are very rare anyway. See the API doc for lzma_lzip_decoder() for more details about the .lz format support. Thanks to Michał Górny for the original patch.