zstd: reject snappy copy shorter than minimum match by Raghuvardhan-Reddy78 · Pull Request #1235 · klauspost/compress

Raghuvardhan-Reddy78 · 2026-09-27T02:06:35Z

Out-of-range match length in the Snappy converter

A Snappy Copy2/Copy4 op can carry a length of 1 or 2, and decodeSnappy turns that into a sequence with matchLen = uint32(length) - zstdMinMatch, which underflows. mlCode then returns match symbol 67, past maxMatchLengthSymbol, so encoding the block slices the 53-entry mlBitsTable at [:68] and Convert panics on the crafted stream.

A real Snappy encoder never emits a copy shorter than 4, so rejecting length < zstdMinMatch as corrupt matches the other validation in decodeSnappy and leaves valid streams unchanged. Added a test that builds such a stream and expects ErrSnappyCorrupt instead of a panic.

Summary by CodeRabbit

Bug Fixes
- Snappy streams containing copy commands shorter than the supported minimum are now rejected as corrupt, preventing invalid match lengths from being processed. Other Snappy streams are unaffected.
Documentation
- SnappyConverter is now documented as deprecated. For decoding, use s2.NewReader; for compression, use an Encoder.

coderabbitai · 2026-09-27T02:06:54Z

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Reviewing files that changed from the base of the PR and between 4d250f7 and b298e2c.

📒 Files selected for processing (2)

zstd/snappy.go
zstd/snappy_test.go

🚧 Files skipped from review as they are similar to previous changes (1)

zstd/snappy.go

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📝 Walkthrough

Walkthrough

decodeSnappy now returns ErrSnappyCorrupt for copy commands shorter than zstdMinMatch. A regression test covers a framed stream with a one-byte copy. The SnappyConverter documentation now marks the converter as deprecated and recommends an alternative.

Changes

Snappy copy validation

Layer / File(s)	Summary
Reject short copies and test conversion `zstd/snappy.go`, `zstd/snappy_test.go`	The converter documentation marks `SnappyConverter` as deprecated and recommends using `s2.NewReader` followed by an Encoder. `decodeSnappy` returns `ErrSnappyCorrupt` when a copy length is below `zstdMinMatch`. A regression test checks rejection of a framed stream with a one-byte copy.

Priority: ⬇️ Low

Estimated code review effort: 2 (Simple) | ~10 minutes

Change: Bug fix

Suggested reviewers: klauspost

Merge Risk: 🟡 Moderate · up to b298e

Valid, uncommon Snappy streams with one- or two-byte copies still cannot be converted directly. The documented decode-and-encode alternative is available, but the compatibility limitation should be explicitly accepted before merging.

Security Architecture Review

Security architecture risk: 🔵 Low · up to b298e

The change replaces a potential panic on short copies with a returned error and does not appear to broaden access to the converter. A format-valid short copy is still rejected, so callers that require full Snappy compatibility need to account for that limitation.

Retained concerns
No architecture-level concerns identified.

Security review details

Security Blast Radius

inferred — An independently supplied Snappy stream reaches an existing Convert invocation; the reviewed change adds no entrypoint or established downstream service path. External deployment exposure is not evidenced.

Trust Boundaries and Controls

observed — The converter checks the input-derived copy length before appending a sequence; Convert propagates rejection before encoding that block.

Resilience and Maintainability Implications

inferred — The error path leaves no partially encoded affected block in the writer. A frame header or completed earlier blocks can remain, consistent with the converter's existing partial-write-and-error behavior.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 66.67% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 3 functions across 2 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main change: rejecting Snappy copies shorter than the minimum match length.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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Actionable comments posted: 1

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Inline comments:
In @zstd/snappy.go:
- Around line 380-382: Preserve valid short copies in the conversion path
guarded by zstdMinMatch: route blocks containing Copy2 or legacy Copy4 lengths
below zstdMinMatch through the existing snappy.Decode literal fallback, or
materialize their copied bytes as literals, without allowing matchLen to
underflow. Update the regression test to expect io.EOF and matching decoded
output, and correct the fixture’s masked CRC32C.

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Configuration used: Repository UI

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📥 Commits

Reviewing files that changed from the base of the PR and between e4086c1 and 327dfb6.

📒 Files selected for processing (2)

zstd/snappy.go
zstd/snappy_test.go

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klauspost · 2026-09-28T10:24:05Z

Honestly, the converted should just be marked deprecated. 99.9% pointless.

Raghuvardhan-Reddy78 · 2026-09-29T04:48:44Z

Makes sense on deprecating it. This is just a small guard so a crafted stream returns ErrSnappyCorrupt instead of panicking the block encoder, so it seems worth keeping either way, but happy to add a Deprecated note in the same PR if you'd rather flag the converter here. Also pushed a commit dropping a stray debug println I'd left in the guard.

zstd: reject snappy copy shorter than minimum match

327dfb6

coderabbitai Bot reviewed Sep 27, 2026

View reviewed changes

Comment thread zstd/snappy.go

zstd: drop stray debug println from snappy copy guard

df409ae

Merge branch 'master' into snappy-min-match-copy

4d250f7

klauspost approved these changes Sep 29, 2026

View reviewed changes

klauspost added 3 commits September 29, 2026 10:41

Merge branch 'master' into snappy-min-match-copy

68a77d1

go fix

6f65fac

zstd: deprecate SnappyConverter

b298e2c

klauspost merged commit 699c618 into klauspost:master Sep 29, 2026
94 checks passed

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zstd: reject snappy copy shorter than minimum match - #1235

zstd: reject snappy copy shorter than minimum match#1235
klauspost merged 6 commits into
klauspost:masterfrom
Raghuvardhan-Reddy78:snappy-min-match-copy

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