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Avoid integer overflow when calculating reduced image size #9904

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5220c53
Avoid integer overflow when calculating reduced image size
lazerg Aug 25, 2026
bf60f3c
Explain how the reduced size is rounded up
lazerg Aug 25, 2026
4d40d5d
Test reducing I and F images by a large factor
lazerg Aug 25, 2026
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16 changes: 16 additions & 0 deletions Tests/test_image_reduce.py
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Original file line number Diff line number Diff line change
Expand Up @@ -53,6 +53,22 @@ def test_args_factor(size: int | tuple[int, int], expected: tuple[int, int]) ->
assert expected == im.reduce(size).size


@pytest.mark.parametrize("mode", ("L", "I", "F"))
@pytest.mark.parametrize(
"size, expected",
(
(2**31 - 1, (1, 1)),
((2**31 - 1, 1), (1, 10)),
((1, 2**31 - 1), (10, 1)),
),
)
def test_args_factor_large(
size: int | tuple[int, int], expected: tuple[int, int], mode: str
) -> None:
im = Image.new(mode, (10, 10))
assert im.reduce(size).size == expected


@pytest.mark.parametrize(
"size, expected_error", ((0, ValueError), (2.0, TypeError), ((0, 10), ValueError))
)
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5 changes: 4 additions & 1 deletion src/libImaging/Reduce.c
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Original file line number Diff line number Diff line change
Expand Up @@ -1460,8 +1460,11 @@ ImagingReduce(Imaging imIn, int xscale, int yscale, int box[4]) {
return (Imaging)ImagingError_ModeError();
}

/* Round the size up. Dividing before adding avoids overflowing for a
large scale, and guarantees a size of at least 1x1.
*/
imOut = ImagingNewDirty(
imIn->mode, (box[2] + xscale - 1) / xscale, (box[3] + yscale - 1) / yscale
imIn->mode, (box[2] - 1) / xscale + 1, (box[3] - 1) / yscale + 1

@akx akx Aug 25, 2026

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I'd maybe add a comment here about the operation order, and that it guarantees the image to have at least size 1x1.

@lazerg lazerg Aug 25, 2026

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Added in bf60f3c.

While checking this I noticed that xscale * yscale in ImagingReduceNxN and (box[2] % xscale) * yscale in ImagingReduceCorners still overflow with a factor this large. Those values only feed loops that run zero iterations, so the output pixels are correct and no bad memory access happens, but an instrumented build will still report the overflow on the same input. Clamping both scales to the box size after the size calculation would remove it.

I am happy to add that here, or to keep this PR to the crash and raise it separately.

@akx akx Aug 25, 2026

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What do you mean with "Those values only feed loops that run zero iterations"? Those functions probably shouldn't end up being called...

In fact, it could be a good idea to add separate paths when the reduction ends up being one-dimensional in one dimension or the other, or both, since isn't the end result then just an average over rows, columns or both (separable)?

);
if (!imOut) {
return NULL;
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