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fix(gif): avoid int32 overflow in palette-split pixel-count math #5291

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fix(gif): avoid int32 overflow in palette-split pixel-count math #5291

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6 changes: 5 additions & 1 deletion src/gif.imageio/gif.h
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Original file line number Diff line number Diff line change
Expand Up @@ -340,7 +340,11 @@ void GifSplitPalette(uint8_t* image, int numPixels, int firstElt, int lastElt, i
if(bRange > gRange) splitCom = 2;
if(rRange > bRange && rRange > gRange) splitCom = 0;

int subPixelsA = numPixels * (splitElt - firstElt) / (lastElt - firstElt);
// numPixels*(splitElt-firstElt) can overflow int32 for images beyond
// about 16.9M pixels (e.g. 4117x4117); do the multiply in 64 bits. The
// final quotient is always <= numPixels, so it safely narrows back.
int subPixelsA = (int)((int64_t)numPixels * (splitElt - firstElt)
/ (lastElt - firstElt));
int subPixelsB = numPixels-subPixelsA;

GifPartitionByMedian(image, 0, numPixels, splitCom, subPixelsA);
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