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1 | Decompressor Errata |
2 | =================== |
3 | |
4 | This document captures known decompressor bugs, where the decompressor rejects a valid zstd frame. |
5 | Each entry will contain: |
6 | 1. The last affected decompressor versions. |
7 | 2. The decompressor components affected. |
8 | 2. Whether the compressed frame could ever be produced by the reference compressor. |
9 | 3. An example frame. |
10 | 4. A description of the bug. |
11 | |
12 | The document is in reverse chronological order, with the bugs that affect the most recent zstd decompressor versions listed first. |
13 | |
14 | |
15 | Compressed block with 0 literals and 0 sequences |
16 | ------------------------------------------------ |
17 | |
18 | **Last affected version**: v1.5.2 |
19 | |
20 | **Affected decompressor component(s)**: Library & CLI |
21 | |
22 | **Produced by the reference compressor**: No |
23 | |
24 | **Example Frame**: `28b5 2ffd 2000 1500 0000 00` |
25 | |
26 | The zstd decoder incorrectly rejected blocks of type `Compressed_Block` that encodes literals as `Raw_Literals_Block` with no literals, and has no sequences. |
27 | |
28 | This type of block was never generated by the reference compressor. |
29 | |
30 | Additionally, these blocks were disallowed by the spec up until spec version 0.3.2 when the restriction was lifted by [PR#1689](https://github.com/facebook/zstd/pull/1689). |
31 | |
32 | > A Compressed_Block has the extra restriction that Block_Size is always strictly less than the decompressed size. If this condition cannot be respected, the block must be sent uncompressed instead (Raw_Block). |
33 | |
34 | First block is RLE block |
35 | ------------------------ |
36 | |
37 | **Last affected version**: v1.4.3 |
38 | |
39 | **Affected decompressor component(s)**: CLI only |
40 | |
41 | **Produced by the reference compressor**: No |
42 | |
43 | **Example Frame**: `28b5 2ffd a001 0002 0002 0010 000b 0000 00` |
44 | |
45 | The zstd CLI decompressor rejected cases where the first block was an RLE block whose `Block_Size` is 131072, and the frame contains more than one block. |
46 | This only affected the zstd CLI, and not the library. |
47 | |
48 | The example is an RLE block with 131072 bytes, followed by a second RLE block with 1 byte. |
49 | |
50 | The compressor currently works around this limitation by explicitly avoiding producing RLE blocks as the first |
51 | block. |
52 | |
53 | https://github.com/facebook/zstd/blob/8814aa5bfa74f05a86e55e9d508da177a893ceeb/lib/compress/zstd_compress.c#L3527-L3535 |
54 | |
55 | Tiny FSE Table & Block |
56 | ---------------------- |
57 | |
58 | **Last affected version**: v1.3.4 |
59 | |
60 | **Affected decompressor component(s)**: Library & CLI |
61 | |
62 | **Produced by the reference compressor**: Possibly until version v1.3.4, but probably never |
63 | |
64 | **Example Frame**: `28b5 2ffd 2027 c500 0080 f3f1 f0ec ebc6 c5c7 f09d 4300 0000 e0e0 0658 0100 603e 52` |
65 | |
66 | The zstd library rejected blocks of type `Compressed_Block` whose offset of the last table with type `FSE_Compressed_Mode` was less than 4 bytes from the end of the block. |
67 | |
68 | In more depth, let `Last_Table_Offset` be the offset in the compressed block (excluding the header) that |
69 | the last table with type `FSE_Compressed_Mode` started. If `Block_Content - Last_Table_Offset < 4` then |
70 | the buggy zstd decompressor would reject the block. This occurs when the last serialized table is 2 bytes |
71 | and the bitstream size is 1 byte. |
72 | |
73 | For example: |
74 | * There is 1 sequence in the block |
75 | * `Literals_Lengths_Mode` is `FSE_Compressed_Mode` & the serialized table size is 2 bytes |
76 | * `Offsets_Mode` is `Predefined_Mode` |
77 | * `Match_Lengths_Mode` is `Predefined_Mode` |
78 | * The bitstream is 1 byte. E.g. there is only one sequence and it fits in 1 byte. |
79 | |
80 | The total `Block_Content` is `5` bytes, and `Last_Table_Offset` is `2`. |
81 | |
82 | See the compressor workaround code: |
83 | |
84 | https://github.com/facebook/zstd/blob/8814aa5bfa74f05a86e55e9d508da177a893ceeb/lib/compress/zstd_compress.c#L2667-L2682 |