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Fix 32-bit decoding with large dictionary #3472

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Feb 2, 2023
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Fix 32-bit decoding with large dictionary #3472

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125 changes: 96 additions & 29 deletions lib/decompress/zstd_decompress_block.c
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Original file line number Diff line number Diff line change
Expand Up @@ -1170,7 +1170,7 @@ ZSTD_updateFseStateWithDInfo(ZSTD_fseState* DStatePtr, BIT_DStream_t* bitD, U16
}

/* We need to add at most (ZSTD_WINDOWLOG_MAX_32 - 1) bits to read the maximum
* offset bits. But we can only read at most (STREAM_ACCUMULATOR_MIN_32 - 1)
* offset bits. But we can only read at most STREAM_ACCUMULATOR_MIN_32
* bits before reloading. This value is the maximum number of bytes we read
* after reloading when we are decoding long offsets.
*/
Expand Down Expand Up @@ -1986,48 +1986,81 @@ ZSTD_decompressSequencesLong(ZSTD_DCtx* dctx,
#endif /* ZSTD_FORCE_DECOMPRESS_SEQUENCES_SHORT */


/**
* @returns The total size of the history referencable by zstd, including
* both the prefix and the extDict. At @p op any offset larger than this
* is invalid.
*/
static size_t ZSTD_totalHistorySize(BYTE* op, BYTE const* virtualStart)
{
return (size_t)(op - virtualStart);
}

#if !defined(ZSTD_FORCE_DECOMPRESS_SEQUENCES_SHORT) && \
!defined(ZSTD_FORCE_DECOMPRESS_SEQUENCES_LONG)
/* ZSTD_getLongOffsetsShare() :
typedef struct {
unsigned longOffsetShare;
unsigned maxNbAdditionalBits;
} ZSTD_OffsetInfo;

/* ZSTD_getOffsetInfo() :
* condition : offTable must be valid
* @return : "share" of long offsets (arbitrarily defined as > (1<<23))
* compared to maximum possible of (1<<OffFSELog) */
static unsigned
ZSTD_getLongOffsetsShare(const ZSTD_seqSymbol* offTable)
* compared to maximum possible of (1<<OffFSELog),
* as well as the maximum number additional bits required.
*/
static ZSTD_OffsetInfo
ZSTD_getOffsetInfo(const ZSTD_seqSymbol* offTable)
{
const void* ptr = offTable;
U32 const tableLog = ((const ZSTD_seqSymbol_header*)ptr)[0].tableLog;
const ZSTD_seqSymbol* table = offTable + 1;
U32 const max = 1 << tableLog;
U32 u, total = 0;
U32 u;
ZSTD_OffsetInfo info = {0, 0};
DEBUGLOG(5, "ZSTD_getLongOffsetsShare: (tableLog=%u)", tableLog);

assert(max <= (1 << OffFSELog)); /* max not too large */
for (u=0; u<max; u++) {
if (table[u].nbAdditionalBits > 22) total += 1;
info.maxNbAdditionalBits = MAX(info.maxNbAdditionalBits, table[u].nbAdditionalBits);
if (table[u].nbAdditionalBits > 22) info.longOffsetShare += 1;
}

assert(tableLog <= OffFSELog);
total <<= (OffFSELog - tableLog); /* scale to OffFSELog */
info.longOffsetShare <<= (OffFSELog - tableLog); /* scale to OffFSELog */

return total;
return info;
}

/**
* @returns The maximum offset we can decode in one read of our bitstream, without
* reloading more bits in the middle of the offset bits read. Any offsets larger
* than this must use the long offset decoder.
*/
static size_t ZSTD_maxShortOffset(void)
{
if (MEM_64bits()) {
/* We can decode any offset without reloading bits.
* This might change if the max window size grows.
*/
ZSTD_STATIC_ASSERT(ZSTD_WINDOWLOG_MAX <= 31);
return (size_t)-1;
} else {
/* The maximum offBase is (1 << (STREAM_ACCUMULATOR_MIN + 1)) - 1.
* This offBase would require STREAM_ACCUMULATOR_MIN extra bits.
* Then we have to subtract ZSTD_REP_NUM to get the maximum possible offset.
*/
size_t const maxOffbase = ((size_t)1 << (STREAM_ACCUMULATOR_MIN + 1)) - 1;
size_t const maxOffset = maxOffbase - ZSTD_REP_NUM;
assert(ZSTD_highbit32((U32)maxOffbase) == STREAM_ACCUMULATOR_MIN);
return maxOffset;
}
}
#endif

size_t
ZSTD_decompressBlock_internal(ZSTD_DCtx* dctx,
void* dst, size_t dstCapacity,
const void* src, size_t srcSize, const int frame, const streaming_operation streaming)
{ /* blockType == blockCompressed */
const BYTE* ip = (const BYTE*)src;
/* isLongOffset must be true if there are long offsets.
* Offsets are long if they are larger than 2^STREAM_ACCUMULATOR_MIN.
* We don't expect that to be the case in 64-bit mode.
* In block mode, window size is not known, so we have to be conservative.
* (note: it could possibly be evaluated from current-lowLimit)
*/
ZSTD_longOffset_e const isLongOffset = (ZSTD_longOffset_e)(MEM_32bits() && (!frame || (dctx->fParams.windowSize > (1ULL << STREAM_ACCUMULATOR_MIN))));
DEBUGLOG(5, "ZSTD_decompressBlock_internal (size : %u)", (U32)srcSize);

/* Note : the wording of the specification
Expand All @@ -2050,13 +2083,35 @@ ZSTD_decompressBlock_internal(ZSTD_DCtx* dctx,

/* Build Decoding Tables */
{
/* Compute the maximum block size, which must also work when !frame and fParams are unset.
* Additionally, take the min with dstCapacity to ensure that the totalHistorySize fits in a size_t.
*/
size_t const blockSizeMax = MIN(dstCapacity, (frame ? dctx->fParams.blockSizeMax : ZSTD_BLOCKSIZE_MAX));
size_t const totalHistorySize = ZSTD_totalHistorySize((BYTE*)dst + blockSizeMax, (BYTE const*)dctx->virtualStart);
/* isLongOffset must be true if there are long offsets.
* Offsets are long if they are larger than ZSTD_maxShortOffset().
* We don't expect that to be the case in 64-bit mode.
*
* We check here to see if our history is large enough to allow long offsets.
* If it isn't, then we can't possible have (valid) long offsets. If the offset
* is invalid, then it is okay to read it incorrectly.
*
* If isLongOffsets is true, then we will later check our decoding table to see
* if it is even possible to generate long offsets.
*/
ZSTD_longOffset_e isLongOffset = (ZSTD_longOffset_e)(MEM_32bits() && (totalHistorySize > ZSTD_maxShortOffset()));
/* These macros control at build-time which decompressor implementation
* we use. If neither is defined, we do some inspection and dispatch at
* runtime.
*/
#if !defined(ZSTD_FORCE_DECOMPRESS_SEQUENCES_SHORT) && \
!defined(ZSTD_FORCE_DECOMPRESS_SEQUENCES_LONG)
int usePrefetchDecoder = dctx->ddictIsCold;
#else
/* Set to 1 to avoid computing offset info if we don't need to.
* Otherwise this value is ignored.
*/
int usePrefetchDecoder = 1;
#endif
int nbSeq;
size_t const seqHSize = ZSTD_decodeSeqHeaders(dctx, &nbSeq, ip, srcSize);
Expand All @@ -2066,26 +2121,38 @@ ZSTD_decompressBlock_internal(ZSTD_DCtx* dctx,

RETURN_ERROR_IF(dst == NULL && nbSeq > 0, dstSize_tooSmall, "NULL not handled");

#if !defined(ZSTD_FORCE_DECOMPRESS_SEQUENCES_SHORT) && \
!defined(ZSTD_FORCE_DECOMPRESS_SEQUENCES_LONG)
if ( !usePrefetchDecoder
&& (!frame || (dctx->fParams.windowSize > (1<<24)))
&& (nbSeq>ADVANCED_SEQS) ) { /* could probably use a larger nbSeq limit */
U32 const shareLongOffsets = ZSTD_getLongOffsetsShare(dctx->OFTptr);
U32 const minShare = MEM_64bits() ? 7 : 20; /* heuristic values, correspond to 2.73% and 7.81% */
usePrefetchDecoder = (shareLongOffsets >= minShare);
/* If we could potentially have long offsets, or we might want to use the prefetch decoder,
* compute information about the share of long offsets, and the maximum nbAdditionalBits.
* NOTE: could probably use a larger nbSeq limit
*/
if (isLongOffset || (!usePrefetchDecoder && (totalHistorySize > (1u << 24)) && (nbSeq > 8))) {
ZSTD_OffsetInfo const info = ZSTD_getOffsetInfo(dctx->OFTptr);
if (isLongOffset && info.maxNbAdditionalBits <= STREAM_ACCUMULATOR_MIN) {
/* If isLongOffset, but the maximum number of additional bits that we see in our table is small
* enough, then we know it is impossible to have too long an offset in this block, so we can
* use the regular offset decoder.
*/
isLongOffset = ZSTD_lo_isRegularOffset;
}
if (!usePrefetchDecoder) {
U32 const minShare = MEM_64bits() ? 7 : 20; /* heuristic values, correspond to 2.73% and 7.81% */
usePrefetchDecoder = (info.longOffsetShare >= minShare);
}
}
#endif

dctx->ddictIsCold = 0;

#if !defined(ZSTD_FORCE_DECOMPRESS_SEQUENCES_SHORT) && \
!defined(ZSTD_FORCE_DECOMPRESS_SEQUENCES_LONG)
if (usePrefetchDecoder)
if (usePrefetchDecoder) {
#else
(void)usePrefetchDecoder;
{
#endif
#ifndef ZSTD_FORCE_DECOMPRESS_SEQUENCES_SHORT
return ZSTD_decompressSequencesLong(dctx, dst, dstCapacity, ip, srcSize, nbSeq, isLongOffset, frame);
#endif
}

#ifndef ZSTD_FORCE_DECOMPRESS_SEQUENCES_LONG
/* else */
Expand Down