frombytes() and in-place ImageCms transforms write into read-only image memory #10235

What did you do?

Called frombytes() on an image opened from an uncompressed TIFF, which Pillow maps into memory read-only:

from PIL import Image

Image.new("RGBA", (64, 64)).save("test.tif")

with Image.open("test.tif") as im:
    im.load()
    im.frombytes(b"\x01" * 64 * 64 * 4)

An in-place colour transform does the same, even without load():

from PIL import Image, ImageCms

srgb = ImageCms.createProfile("sRGB")
with Image.open("test.tif") as im:
    ImageCms.profileToProfile(im, srgb, srgb, inPlace=True)

And frombytes() on an image made by frombuffer() from bytes:

from PIL import Image

data = bytes(64)
im = Image.frombuffer("L", (8, 8), data, "raw", "L", 0, 1)
im.frombytes(b"\x01" * 64)
print(data[:4])

What did you expect to happen?

The same as with paste(), putpixel(), putdata() or putalpha(): the image is copied first and the copy is changed, while the file and data stay as they were.

What actually happened?

The first two crash, with a bus error on macOS and a segmentation fault on Linux. The third prints b'\x01\x01\x01\x01': the bytes object was changed. An image from Image.fromarrow() behaves the same way, the new values end up in the Arrow array.

frombytes() gives self.im to the decoder without looking at readonly, and ImageCmsTransform.apply(im, imOut), which inPlace=True uses, writes into imOut the same way. The methods above call _ensure_mutable() first.

This fixes it for me, and the test suite still passes:

# Image.frombytes(), after the zero-size check
if self.im.readonly:
    self._copy()

# ImageCmsTransform.apply()
if imOut is None:
    imOut = Image.new(self.output_mode, im.size)
else:
    imOut._ensure_mutable()

I did not use _ensure_mutable() in frombytes() because plugins call frombytes() from their own load(), while ImageFile.readonly is still 1. Every GbrImagePlugin and WalImageFile image would be copied once more, and a plugin that marks itself loaded only after frombytes() (pillow-heif 1.8.1, for example) ends in RecursionError.

We ran into this while moving pillow-heif to Image.fromarrow() (bigcat88/pillow_heif#463, after #9896).

What are your OS, Python and Pillow versions?

• OS: macOS 26.5 arm64, Linux aarch64 (Docker python:3.11-slim)
• Python: 3.11
• Pillow: 12.3.0, 13.0.0 and main (6085656)