tac: use MAP_PRIVATE to prevent SIGBUS on file truncation#11464
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ctonneslan wants to merge 1 commit intouutils:mainfrom
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tac: use MAP_PRIVATE to prevent SIGBUS on file truncation#11464ctonneslan wants to merge 1 commit intouutils:mainfrom
ctonneslan wants to merge 1 commit intouutils:mainfrom
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When tac reads a file via mmap and that file gets truncated concurrently (e.g. log rotation), the process receives SIGBUS because the mapped pages are no longer backed by the file. Switch from Mmap::map() (MAP_SHARED) to map_copy_read_only() (MAP_PRIVATE). With MAP_PRIVATE, the kernel returns zero-filled pages for truncated regions instead of raising SIGBUS, matching GNU tac's behavior of graceful handling. Fixes uutils#9748
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could you please add a test to make sure we don't regress in the future? thanks |
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GNU testsuite comparison: |
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Fixes #9748
When `tac` reads a file via `mmap` and that file gets truncated concurrently (e.g. during log rotation), the process gets SIGBUS because the mapped pages are no longer backed by the file.
The fix switches from `Mmap::map()` (which uses MAP_SHARED) to `map_copy_read_only()` (which uses MAP_PRIVATE). With MAP_PRIVATE, the kernel returns zero-filled pages for truncated regions instead of raising SIGBUS. The performance characteristics are the same for read-only access since no actual copy is made until a write occurs (which we never do).
This matches GNU `tac`'s approach of not using direct shared memory mapping on untrusted inputs.
All 29 tac integration tests pass.