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diff --git a/vendor/image/docs/2019-04-23-memory-unsafety.md b/vendor/image/docs/2019-04-23-memory-unsafety.md deleted file mode 100644 index 3989eb2..0000000 --- a/vendor/image/docs/2019-04-23-memory-unsafety.md +++ /dev/null @@ -1,54 +0,0 @@ -# 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 |