fix(gif): avoid int32 overflow in palette-split pixel-count math#5292
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lgritz merged 3 commits intoJul 6, 2026
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GifSplitPalette() computed `numPixels * (splitElt - firstElt)` as a 32-bit int before dividing. For any image over roughly 16.9M pixels (e.g. 4117x4117), that intermediate multiplication overflows INT_MAX and is undefined behavior; in practice it produced a bogus negative/garbage split count that could drive later indexing into `image` out of its bounds. Do the multiply in 64 bits and narrow back afterward -- the quotient is always <= numPixels, so the narrowing itself is safe. Assisted-by: Claude Code / Sonnet 5 Signed-off-by: Larry Gritz <lg@larrygritz.com>
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Given the max dimensions of 32768x32768x4, there are 4 other int overflows related to gif input, might as well do them in the same PR:
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Signed-off-by: Larry Gritz <lg@larrygritz.com>
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So amended. |
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Looks good, but watch out for clang-format.
Signed-off-by: Larry Gritz <lg@larrygritz.com>
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GifSplitPalette() computed
numPixels * (splitElt - firstElt)as a 32-bit int before dividing. For any image over roughly 16.9M pixels (e.g. 4117x4117), that intermediate multiplication overflows INT_MAX and is undefined behavior; in practice it produced a bogus negative/garbage split count that could drive later indexing intoimageout of its bounds.Do the multiply in 64 bits and narrow back afterward -- the quotient is always <= numPixels, so the narrowing itself is safe.
Assisted-by: Claude Code / Sonnet 5