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fixed decoder behavior when nbSeqs==0 is encoded using 2 bytes

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The sequence section starts with a number, which tells how sequences are present in the section.
If this number if 0, the section automatically ends.

The number 0 can be represented using the 1 byte or the 2 bytes formats.
That's because the 2-bytes formats fully overlaps the 1 byte format.

However, when 0 is represented using the 2-bytes format,
the decoder was expecting the sequence section to continue,
and was looking for FSE tables, which is incorrect.

Fixed this behavior, in both the reference decoder and the educational behavior.

In practice, this behavior never happens,
because the encoder will always select the 1-byte format to represent 0,
since this is more efficient.

Completed the fix with a new golden sample for tests,
a clarification of the specification,
and a decoder errata paragraph.
This commit is contained in:
Yann Collet
2023-06-05 16:03:00 -07:00
parent 3e815f5b3a
commit 3732a08f5b
6 changed files with 44 additions and 18 deletions
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lib/common/zstd_internal.h
+2 -2
View File
@@ -366,13 +366,13 @@ typedef struct {
/*! ZSTD_getcBlockSize() :
* Provides the size of compressed block from block header `src` */
/* Used by: decompress, fullbench (does not get its definition from here) */
/* Used by: decompress, fullbench */
size_t ZSTD_getcBlockSize(const void* src, size_t srcSize,
blockProperties_t* bpPtr);
/*! ZSTD_decodeSeqHeaders() :
* decode sequence header from src */
/* Used by: decompress, fullbench (does not get its definition from here) */
/* Used by: zstd_decompress_block, fullbench */
size_t ZSTD_decodeSeqHeaders(ZSTD_DCtx* dctx, int* nbSeqPtr,
const void* src, size_t srcSize);
lib/decompress/zstd_decompress_block.c
+5 -5
View File
@@ -706,11 +706,6 @@ size_t ZSTD_decodeSeqHeaders(ZSTD_DCtx* dctx, int* nbSeqPtr,
/* SeqHead */
nbSeq = *ip++;
if (!nbSeq) {
*nbSeqPtr=0;
RETURN_ERROR_IF(srcSize != 1, srcSize_wrong, "");
return 1;
}
if (nbSeq > 0x7F) {
if (nbSeq == 0xFF) {
RETURN_ERROR_IF(ip+2 > iend, srcSize_wrong, "");
@@ -723,6 +718,11 @@ size_t ZSTD_decodeSeqHeaders(ZSTD_DCtx* dctx, int* nbSeqPtr,
}
*nbSeqPtr = nbSeq;
if (nbSeq == 0) {
/* No sequence : section ends immediately */
return (size_t)(ip - istart);
}
/* FSE table descriptors */
RETURN_ERROR_IF(ip+1 > iend, srcSize_wrong, ""); /* minimum possible size: 1 byte for symbol encoding types */
{ symbolEncodingType_e const LLtype = (symbolEncodingType_e)(*ip >> 6);