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rust: num: reject Bounded::shr overshifts at build time

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Make `shr` reject shifts of at least the type's bit width at build
time, instead of panicking or masking the shift amount at runtime.

[ This implies we can break the type invariant, which in turn means
  we can trigger UB via `Deref`, e.g.:

      rust_kernel: panicked at rust/kernel/num/bounded.rs:528:22:
      unsafe precondition(s) violated: hint::unreachable_unchecked must never be reached

    - Miguel ]

Signed-off-by: Eliot Courtney <ecourtney@nvidia.com>
Acked-by: Alexandre Courbot <acourbot@nvidia.com>
Reviewed-by: Gary Guo <gary@garyguo.net>
Reviewed-by: Danilo Krummrich <dakr@kernel.org>
Cc: stable@vger.kernel.org
Fixes: c59a2d14cd ("rust: num: add `shr` and `shl` methods to `Bounded`")
Link: https://patch.msgid.link/20260810-pramin-split-v2-2-65a00b3c7309@nvidia.com
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
This commit is contained in:
Eliot Courtney 2026-08-10 22:55:24 +09:00 committed by Miguel Ojeda
parent 119b598467
commit 223aa25aee
1 changed files with 1 additions and 0 deletions
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rust/kernel/num/bounded.rs
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@ -485,6 +485,7 @@ pub fn cast<U>(self) -> Bounded<U, N>
/// assert_eq!(v_shifted.get(), 0xff);
/// ```
pub fn shr<const SHIFT: u32, const RES: u32>(self) -> Bounded<T, RES> {
const_assert!(SHIFT < T::BITS);
const_assert!(RES + SHIFT >= N);
// SAFETY: We shift the value right by `SHIFT`, reducing the number of bits needed to