s2: Fix S2 best dictionary wrong encoding (#871)

A dictionary match one byte out of dictionary range was possible. Recovery is possible. Send me a request if you find any content with this error in the [discussion](https://github.com/klauspost/compress/discussions/categories/general).
klauspost · Oct 13, 2023 · 68eabfa · 68eabfa
1 parent 85bbf19
commit 68eabfa
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Showing 3 changed files with 5 additions and 1 deletion.
diff --git a/s2/dict_test.go b/s2/dict_test.go
@@ -402,13 +402,14 @@ func TestDictSize(t *testing.T) {
 			}
 		})
 	}
-	fmt.Printf("%d files, %d -> %d (%.2f%%) - %.02f bytes saved/file\n", totalCount, totalIn, totalOut, float64(totalOut*100)/float64(totalIn), float64(totalIn-totalOut)/float64(totalCount))
+	t.Logf("%d files, %d -> %d (%.2f%%) - %.02f bytes saved/file\n", totalCount, totalIn, totalOut, float64(totalOut*100)/float64(totalIn), float64(totalIn-totalOut)/float64(totalCount))
 }
 
 func FuzzDictBlocks(f *testing.F) {
 	fuzz.AddFromZip(f, "testdata/enc_regressions.zip", fuzz.TypeRaw, false)
 	fuzz.AddFromZip(f, "testdata/fuzz/block-corpus-raw.zip", fuzz.TypeRaw, testing.Short())
 	fuzz.AddFromZip(f, "testdata/fuzz/block-corpus-enc.zip", fuzz.TypeGoFuzz, testing.Short())
+	fuzz.AddFromZip(f, "testdata/fuzz/dict-corpus-oss.zip", fuzz.TypeOSSFuzz, testing.Short())
 
 	// Fuzzing tweaks:
 	const (

diff --git a/s2/encode_best.go b/s2/encode_best.go
@@ -157,6 +157,9 @@ func encodeBlockBest(dst, src []byte, dict *Dict) (d int) {
 				return m
 			}
 			matchDict := func(candidate, s int, first uint32, rep bool) match {
+				if s >= MaxDictSrcOffset {
+					return match{offset: candidate, s: s}
+				}
 				// Calculate offset as if in continuous array with s
 				offset := -len(dict.dict) + candidate
 				if best.length != 0 && best.s-best.offset == s-offset && !rep {

diff --git a/s2/testdata/fuzz/dict-corpus-oss.zip b/s2/testdata/fuzz/dict-corpus-oss.zip