Fix UB in Allocator::grow_zeroed#321
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fitzgen merged 1 commit intofitzgen:mainfrom Apr 27, 2026
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I'm surprised Miri didn't catch this already. Perhaps there isn't test coverage for this method. |
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Pull request overview
Fixes undefined behavior in Allocator::grow_zeroed by avoiding creation of a &mut [u8] that would span uninitialized bytes in the grown allocation.
Changes:
- Replace slice-based tail zeroing with raw-pointer-based zeroing in
Allocator::grow_zeroed. - Add safety commentary explaining why constructing a full
&mut [u8]is UB in this context.
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Apr 25, 2026
Implementation of `grow_zeroed` method of `allocator_api2::alloc::Allocator` trait for `Arena` created a `&mut [u8]` covering the tail end of a new allocation which may contain uninitialized bytes. This is UB. Fix this by using `NonNull::write_bytes` to zero the bytes instead. Same fix submitted to `bumpalo`: fitzgen/bumpalo#321
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Allocator::grow_zeroedcreated a&mut [u8]covering the tail end of a new allocation which may contain uninitialized bytes. This is UB.Use
ptr::write_bytesto zero the bytes instead.