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Soundness hole in Ptr::read_unaligned #1892

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Soundness hole in Ptr::read_unaligned#1892
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joshlf
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@joshlf
joshlf
opened on Oct 12, 2024

From what I can tell, it's not exercised anywhere in the codebase.

Ptr::read_unaligned permits reading from a shared pointer through UnsafeCells:

zerocopy/src/pointer/mod.rs

Lines 35 to 53 in 7349fe0

impl<'a, T, Aliasing, Alignment> MaybeAligned<'a, T, Aliasing, Alignment>
where
T: 'a + ?Sized,
Aliasing: invariant::Aliasing + invariant::AtLeast<invariant::Shared>,
Alignment: invariant::Alignment,
{
/// Reads the value from `MaybeAligned`.
#[must_use]
#[inline]
pub fn read_unaligned(self) -> T
where
T: Copy,
{
let raw = self.as_non_null().as_ptr();
// SAFETY: By invariant on `MaybeAligned`, `raw` contains
// validly-initialized data for `T`. The value is safe to read and
// return, because `T` is copy.
unsafe { core::ptr::read_unaligned(raw) }
}

We should either add an Immutable bound or an AliasingSafe bound.

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