cksum: do not abort on an over-length BLAKE2b digest in a check file

[Dijaa]

A check line whose digest is longer than the 64 bytes BLAKE2b allows takes
down the whole run instead of being skipped:

$ echo data > f1
$ printf '%s f1\n' "$(printf 'ab%.0s' $(seq 65))" > sums # 65-byte digest
$ ./target/release/coreutils b2sum -c sums
thread 'main' panicked at blake2b_simd-1.0.5/src/lib.rs:241:9:
Bad hash length: 65
Aborted (core dumped)
$ echo $?
134

GNU coreutils 9.4 skips the line and says so:

$ b2sum -c sums
b2sum: sums: no properly formatted checksum lines found
$ echo $?
1

cksum -a blake2b --check goes down the same path.

The --length flag and tagged lines (BLAKE2b-520 (f) = ...) both run their
length through parse_blake_length, which rejects anything above 512 bits.
The untagged path does not: process_non_algo_based_line takes the length
straight from the digest it just decoded and hands it on, so
Blake2b::with_output_bytes reaches blake2b_simd's hash_length with 65
and trips its assertion.

This sends that inferred length through parse_blake_length too, and turns a
rejected length into LineCheckError::ImproperlyFormatted — the same "skip
the line" path GNU takes. HashLength::from_bytes had no caller left
afterwards, so it goes.

Worth noting for anyone reproducing it: the exit code depends on the profile.
In a debug build our own debug_assert! in Blake2b::with_output_bytes fires
first and you get 101; in release it is compiled out, blake2b_simd aborts,
and panic = "abort" turns that into SIGABRT, 134. Same bug either way.

Fixes #14487

[Copilot]

🟢 Approval recommended

The supplied review found no unresolved approval blockers.

Pull request overview

Fixes checksum validation so over-length BLAKE2b digests are rejected as malformed instead of aborting.

Changes:

• Validate inferred BLAKE2b digest lengths.
• Remove the unused HashLength::from_bytes helper.
• Add regression coverage.

File summaries

File	Description
tests/by-util/test_cksum.rs	Adds oversized BLAKE2b digest coverage.
src/uucore/src/lib/features/checksum/validate.rs	Validates inferred digest lengths safely.
src/uucore/src/lib/features/checksum/mod.rs	Removes the unused conversion helper.

Review details

• Files reviewed: 3/3 changed files
• Comments generated: 0
• Review effort level: Lite

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[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/cut/bounded-memory (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/tail/tail-n0f (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/date/date-locale-hour (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/pr/bounded-memory (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/cut/cut-huge-range is now passing!
Congrats! The gnu test tests/rm/many-dir-entries-vs-OOM is now passing!
Note: The gnu test tests/misc/write-errors was skipped on 'main' but is now failing.