Re: [PATCH v2] rust: num: seal Integer

[Miguel Ojeda]

On Sat, Sep 5, 2026 at 5:17 PM Younes Akhouayri via B4 Relay
<devnull+git.younes.io@xxxxxxxxxx> wrote:
>
> From: Younes Akhouayri <git@xxxxxxxxx>
>
> Bounded relies on Integer implementations to describe primitive integer
> semantics correctly. In particular, it uses Integer::BITS and Signedness
> to justify unchecked operations.
>
> Integer is currently safe and externally implementable, so an
> implementation can violate those assumptions and make safe Bounded
> operations reach undefined behavior.
>
> For example, an Integer implementation for a u8 wrapper can report
> BITS = 16. Safe code can then cast a Bounded<u16, 9> containing 256
> to that wrapper. Its TryFrom<u16> implementation returns Err, and
> Bounded::cast() calls unwrap_unchecked() on it, causing undefined
> behavior.
>
> Seal Integer so only the primitive implementations provided by the
> kernel crate can satisfy it.
>
> Fixes: 01e345e82ec3 ("rust: num: add Bounded integer wrapping type")
> Reported-by: Miguel Ojeda <ojeda@xxxxxxxxxx>
> Closes: https://lore.kernel.org/rust-for-linux/CANiq72mOfR33s4y+Ueivd5NrC5yre+Pcp57ZOBz0msw9A4AP1Q@xxxxxxxxxxxxxx/
> Cc: stable@xxxxxxxxxxxxxxx
> Suggested-by: Miguel Ojeda <ojeda@xxxxxxxxxx>
> Signed-off-by: Younes Akhouayri <git@xxxxxxxxx>

Applied to `rust-fixes` -- thanks everyone!

It would be nice to explain somewhere in the code why the trait is
sealed, especially if we rely on it for safety, but that can be
improved later.

Cheers,
Miguel