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Determinism: Avoid Mapping Window into Reserved Indices during Reduction #2869

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Nov 18, 2021
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Determinism: Avoid Mapping Window into Reserved Indices during Reduction #2869

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32 changes: 22 additions & 10 deletions lib/compress/zstd_compress_internal.h
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Original file line number Diff line number Diff line change
Expand Up @@ -982,7 +982,9 @@ MEM_STATIC U32 ZSTD_window_canOverflowCorrect(ZSTD_window_t const window,
{
U32 const cycleSize = 1u << cycleLog;
U32 const curr = (U32)((BYTE const*)src - window.base);
U32 const minIndexToOverflowCorrect = cycleSize + MAX(maxDist, cycleSize);
U32 const minIndexToOverflowCorrect = cycleSize
+ MAX(maxDist, cycleSize)
+ ZSTD_WINDOW_START_INDEX;

/* Adjust the min index to backoff the overflow correction frequency,
* so we don't waste too much CPU in overflow correction. If this
Expand Down Expand Up @@ -1057,10 +1059,14 @@ MEM_STATIC U32 ZSTD_window_correctOverflow(ZSTD_window_t* window, U32 cycleLog,
U32 const cycleSize = 1u << cycleLog;
U32 const cycleMask = cycleSize - 1;
U32 const curr = (U32)((BYTE const*)src - window->base);
U32 const currentCycle0 = curr & cycleMask;
/* Exclude zero so that newCurrent - maxDist >= 1. */
U32 const currentCycle1 = currentCycle0 == 0 ? cycleSize : currentCycle0;
U32 const newCurrent = currentCycle1 + MAX(maxDist, cycleSize);
U32 const currentCycle = curr & cycleMask;
/* Ensure newCurrent - maxDist >= ZSTD_WINDOW_START_INDEX. */
U32 const currentCycleCorrection = currentCycle < ZSTD_WINDOW_START_INDEX
? MAX(cycleSize, ZSTD_WINDOW_START_INDEX)
: 0;
U32 const newCurrent = currentCycle
+ currentCycleCorrection
+ MAX(maxDist, cycleSize);
U32 const correction = curr - newCurrent;
/* maxDist must be a power of two so that:
* (newCurrent & cycleMask) == (curr & cycleMask)
Expand All @@ -1076,14 +1082,20 @@ MEM_STATIC U32 ZSTD_window_correctOverflow(ZSTD_window_t* window, U32 cycleLog,

window->base += correction;
window->dictBase += correction;
if (window->lowLimit <= correction) window->lowLimit = 1;
else window->lowLimit -= correction;
if (window->dictLimit <= correction) window->dictLimit = 1;
else window->dictLimit -= correction;
if (window->lowLimit < correction + ZSTD_WINDOW_START_INDEX) {
window->lowLimit = ZSTD_WINDOW_START_INDEX;
} else {
window->lowLimit -= correction;
}
if (window->dictLimit < correction + ZSTD_WINDOW_START_INDEX) {
window->dictLimit = ZSTD_WINDOW_START_INDEX;
} else {
window->dictLimit -= correction;
}

/* Ensure we can still reference the full window. */
assert(newCurrent >= maxDist);
assert(newCurrent - maxDist >= 1);
assert(newCurrent - maxDist >= ZSTD_WINDOW_START_INDEX);
/* Ensure that lowLimit and dictLimit didn't underflow. */
assert(window->lowLimit <= newCurrent);
assert(window->dictLimit <= newCurrent);
Expand Down