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authorValentin Popov <valentin@popov.link>2024-07-19 15:37:58 +0300
committerValentin Popov <valentin@popov.link>2024-07-19 15:37:58 +0300
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-# Advisory about potential memory unsafety issues
-
-[While reviewing][i885] some `unsafe Vec::from_raw_parts` operations within the
-library, trying to justify their existence with stronger reasoning, we noticed
-that they instead did not meet the required conditions set by the standard
-library. This unsoundness was quickly removed, but we noted that the same
-unjustified reasoning had been applied by a dependency introduced in `0.21`.
-
-For efficiency reasons, we had tried to reuse the allocations made by decoders
-for the buffer of the final image. However, that process is error prone. Most
-image decoding algorithms change the representation type of color samples to
-some degree. Notably, the output pixel type may have a different size and
-alignment than the type used in the temporary decoding buffer. In this specific
-instance, the `ImageBuffer` of the output expects a linear arrangement of `u8`
-samples while the implementation of the `hdr` decoder uses a pixel
-representation of `Rgb<u8>`, which has three times the size. One of the
-requirements of `Vec::from_raw_parts` reads:
-
-> ptr's T needs to have the same size and alignment as it was allocated with.
-
-This requirement is not present on slices `[T]`, as it is motivated by the
-allocator interface. The validity invariant of a reference and slice only
-requires the correct alignment here, which was considered in the design of
-`Rgb<_>` by giving it a well-defined representation, `#[repr(C)]`. But
-critically, this does not guarantee that we can reuse the existing allocation
-through effectively transmuting a `Vec<_>`!
-
-The actual impact of this issue is, in real world implementations, limited to
-allocators which handle allocations for types of size `1` and `3`/`4`
-differently. To the best of my knowledge, this does not apply to `jemalloc` and
-the `libc` allocator. However, we decided to proceed with caution here.
-
-## Lessons for the future
-
-New library dependencies will be under a stricter policy. Not only would they
-need to be justified by functionality but also require at least some level of
-reasoning how they solve that problem better than alternatives. Some appearance
-of maintenance, or the existence of `#[deny(unsafe)]`, will help. We'll
-additionally look into existing dependencies trying to identify similar issues
-and minimizing the potential surface for implementation risks.
-
-## Sound and safe buffer reuse
-
-It seems that the `Vec` representation is entirely unfit for buffer reuse in
-the style which an image library requires. In particular, using pixel types of
-different sizes is likely common to handle either whole (encoded) pixels or
-individual samples. Thus, we started a new sub-project to address this use
-case, [image-canvas][image-canvas]. Contributions and review of its safety are
-very welcome, we ask for the communities help here. The release of `v0.1` will
-not occur until at least one such review has occurred.
-
-
-[i885]: https://github.com/image-rs/image/pull/885
-[image-canvas]: https://github.com/image-rs/canvas