multiboot2: massive refactoring, removed UB, Miri passes all tests #226
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The goal of this commit is to prepare 100% memory safety (using Miri passing tests as reference). For that, we need a significant under-the-hood changes. `GenericTag` is the generalization of all tags as DST that owns all memory of the tag. This tag can be created from bytes, thus, we can ensure a lifetime and a valid memory range. This tag then can be casted to specific implementations of `TagTrait`. We now never have to increase or decrease the size of the referenced memory during a Tag cast, as the GenericTag already holds all bytes that the more specific type needs. Assertions and the memory checks along the way ensure that nothing can co wrong. Further, the creation of various test data structures was modified to fulfill the new guarantees, that are given in real-world scenarios and are also what the compiler expects.
This was already transitively the case as soon as they have a `header: TagHeader` field, however, for perfection, this can now safely be added.
This will replace the existing BoxedDst typ, which has UB.
Note that Miri runs significantly longer with this change. More memory accesses that need to be tracked.
Miri still outputs a warning tho: "warning: integer-to-pointer cast" Nevertheless, no we are UB free!
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- all public fields -> methods - Added missing InformationBuilder support for VBEInfoTag
This reverts commit 484160e
This way, I don't have to increase the MSRV for the next release. I want as many users as possible to adopt the latest version, so I want to make it less breaking.
The same changes made in this crate, also needs to be done in multiboot2-header.
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About
This PR contains a massive refactoring of various internals. Now, all
unit tests pass Miri, thus we removed lots of undefined behaviour and
increased the memory safety! 🎉 Only a small part of these internal refactorings
leak to the public interface. If you don't provide external custom tags, you
should be fine.
Impact of Release
Please note that all previous releases must be considered unsafe, as they
contain UB. However, it is never clear how UB results in immediate incorrect
behaviour and it might work. Nevertheless, users should migrate to the latest
release and they will be fine!
All previous releases on crates.io will be yanked, once this is released.
Code Changes
It was not really possible to split this into smaller commits or smaller PRs. At least not without massive time investment. The main changes are that there are now the internal types
GenericTagandTagBytesRef, the refactoring ofTagTrait, and thatBoxedDstwas replaced bynew_boxed()which returns a normal Box.Miri is now 100% happy - This is great success and a big improvement. #160 is entirely closed.
Technical Insights
Memory Properties of this crate:
[u8]field as this is the best Rusty representation for tags. This needs fat pointers which are complex for itself. A solution is to use theptr_metacrateAll these things are now solved. This is also done in a nice, non-hacky way.