rust: num: seal Integer

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: 01e345e82e ("rust: num: add Bounded integer wrapping type")
Reported-by: Miguel Ojeda <ojeda@kernel.org>
Closes: https://lore.kernel.org/rust-for-linux/CANiq72mOfR33s4y+Ueivd5NrC5yre+Pcp57ZOBz0msw9A4AP1Q@mail.gmail.com/
Cc: stable@vger.kernel.org
Suggested-by: Miguel Ojeda <ojeda@kernel.org>
Signed-off-by: Younes Akhouayri <git@younes.io>
Acked-by: Alexandre Courbot <acourbot@nvidia.com>
Link: https://patch.msgid.link/20260905-feature-rust-num-seal-integer-v2-1-f1311ffbe6e7@younes.io
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

rust/kernel/num.rs

@@ -15,9 +15,14 @@ pub enum Unsigned {}
 /// Designates signed primitive types.
 pub enum Signed {}
 
+mod private {
+    pub trait Sealed {}
+}
+
 /// Describes core properties of integer types.
 pub trait Integer:
-    Sized
+    private::Sealed
+    + Sized
     + Copy
     + Clone
     + PartialEq
@@ -56,6 +61,8 @@ pub trait Integer:
 macro_rules! impl_integer {
     ($($type:ty: $signedness:ty), *) => {
         $(
+        impl private::Sealed for $type {}
+
         impl Integer for $type {
             type Signedness = $signedness;