Fix mutable aliasing UB: Replace Box<Entry> with NonNull<Entry>#302
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matteoldani wants to merge 1 commit into
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Fix mutable aliasing UB: Replace Box<Entry> with NonNull<Entry>#302matteoldani wants to merge 1 commit into
matteoldani wants to merge 1 commit into
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In Rust's Stacked Borrows and Tree Borrows memory models, moving a `Box<T>` asserts unique access and invalidates all existing raw pointers to its contents. In `string-cache`, dynamic string entries are allocated via `Box<Entry>` and immediately handed out as raw pointers (`*mut Entry`) to be held by `Atom`. However, because these `Box`es are moved into the global linked list (`*linked_list = Some(entry)`), and moved again whenever a hash collision occurs (`next_in_bucket: linked_list.take()`), the raw pointers held by active `Atom`s were being continuously invalidated. This led to pervasive UB when `Atom` subsequently dereferenced them in `clone` and `drop`. This commit re-architects the linked list to use `Option<NonNull<Entry>>`. By manually allocating (`Box::into_raw`) and destroying (`Box::from_raw`) the entries, we sever the compiler's strict aliasing assumptions over the pointers, preserving their provenance. We also provide a custom `Drop` implementation for `Set` to prevent memory leaks during tests, manually re-implement `Send` and `Sync`, and include the missing integer overflow guard inside `Set::insert`.
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Fixes #264
This PR resolves Undefined Behavior caused by mutable aliasing in the
dynamic_set.rsimplementation.Previously, the
Settraversal logic temporarily dropped and re-boxed theOption<Box<Entry>>links, which violated Stacked Borrows and invalidated raw pointers pointing to the entry duringinsertandremoveoperations.This has been fixed by replacing
Option<Box<Entry>>withOption<NonNull<Entry>>, adding proper manualDropsemantics to prevent memory leaks, and implementing explicitSendandSyncbounds.