tail: don't panic on '-c -N' with a huge N - #14372
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`bounded_tail` computed the backwards seek offset for `-c -N` as `-(count as i64)`. For N of 2^63 that cast yields `i64::MIN`, whose negation overflows: a build with overflow checks aborts with exit 101, and a release build wraps around and seeks to a bogus position. Convert the count to an `i64` first and, when it does not fit, take the existing path for an offset that lands before the start of the file.
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sylvestre
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Sep 4, 2026
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| // A count above `i64::MAX` has no negative counterpart to seek by, | ||
| // and reaches further back than any file can be long, so treat it | ||
| // like any other offset landing before the start of the file. |
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Fixes #14353.
tail -c -9223372036854775808 FILEon a seekable file larger than the block size aborts withattempt to negate with overflow(exit 101) in a build with overflow checks, and wraps silently in a release build.bounded_tailbuilt the backwards seek offset for-c -Nas-(*count as i64).countis au64, so2^63casts toi64::MIN, and negatingi64::MINoverflows.I now convert the count with
i64::try_frombefore negating it. A count that doesn't fit in ani64reaches further back than any file can be long, so it falls into the branch the arm already had for an offset landing before the start of the file: seek to 0 and print the whole file. That matches what-c -Nalready does for any other N larger than the file.Tested with a new test in
tests/by-util/test_tail.rsthat runs-c -9223372036854775808against an 8 KiB file (larger thansane_blksize, so the seek path is taken) and expects the whole file back. It fails with exit 101 before the change and passes after. I also ran the rest oftest_tailandcargo clippy -p uu_tail. Everything was run on macOS only;test_follow_detects_file_truncationis flaky here under load, but it fails the same way on an unmodified checkout.