Re: [PATCH v4 02/20] rust: io: add missing safety requirement in `IoCapable` methods

["Alexandre Courbot" <acourbot@nvidia.com>]

On Fri Jun 12, 2026 at 1:28 AM JST, Gary Guo wrote:
> The current safety comment on `io_read`/`io_write` does not cover the topic
> about alignment. Add it so it can be relied on by implementor of
> `IoCapable`.
>
> Expand the check `Io` by taking `self.addr()` into consideration when

"the check performed by `Io`" maybe?

> checking if `offset` is aligned. For the compile-time `io_addr_assert`
> check, check using the known minimum alignment of `IO::Target` and the

typo: s/IO/Io.

> accessed type.
>
> While at it, fix the alignment check to use `align_of` instead of
> `size_of`. The values match for all primitives (including u64, given that
> we do not provide u64 accessor on 32-bit platforms), but are not
> necessarily true for custom types.
>
> Signed-off-by: Gary Guo <gary@garyguo.net>
> ---
>  rust/kernel/io.rs | 25 ++++++++++++++++---------
>  1 file changed, 16 insertions(+), 9 deletions(-)
>
> diff --git a/rust/kernel/io.rs b/rust/kernel/io.rs
> index bef571dad6eb..fa9ae39ad9d2 100644
> --- a/rust/kernel/io.rs
> +++ b/rust/kernel/io.rs
> @@ -196,13 +196,14 @@ pub fn maxsize(&self) -> usize {
>  #[repr(transparent)]
>  pub struct Mmio<const SIZE: usize = 0>(MmioRaw<SIZE>);
>  
> -/// Checks whether an access of type `U` at the given `offset`
> +/// Checks whether an access of type `U` at the given `base` and the given `offset`
>  /// is valid within this region.
> +///
> +/// The `base` is used for alignment checking only. This can be set to 0 to skip the check.
>  #[inline]
> -const fn offset_valid<U>(offset: usize, size: usize) -> bool {
> -    let type_size = core::mem::size_of::<U>();
> -    if let Some(end) = offset.checked_add(type_size) {
> -        end <= size && offset % type_size == 0
> +const fn offset_valid<U>(base: usize, offset: usize, size: usize) -> bool {
> +    if let Some(end) = offset.checked_add(size_of::<U>()) {
> +        end <= size && (base.wrapping_add(offset) % align_of::<U>() == 0)
>      } else {
>          false
>      }
> @@ -221,14 +222,16 @@ pub trait IoCapable<T> {
>      ///
>      /// # Safety
>      ///
> -    /// The range `[address..address + size_of::<T>()]` must be within the bounds of `Self`.
> +    /// - The range `[address..address + size_of::<T>()]` must be within the bounds of `Self`.
> +    /// - `address` must be aligned.
>      unsafe fn io_read(&self, address: usize) -> T;
>  
>      /// Performs an I/O write of `value` at `address`.
>      ///
>      /// # Safety
>      ///
> -    /// The range `[address..address + size_of::<T>()]` must be within the bounds of `Self`.
> +    /// - The range `[address..address + size_of::<T>()]` must be within the bounds of `Self`.
> +    /// - `address` must be aligned.
>      unsafe fn io_write(&self, value: T, address: usize);
>  }
>  
> @@ -310,7 +313,11 @@ pub trait Io {
>      // Always inline to optimize out error path of `build_assert`.
>      #[inline(always)]
>      fn io_addr_assert<U>(&self, offset: usize) -> usize {
> -        build_assert!(offset_valid::<U>(offset, Self::Target::MIN_SIZE));
> +        // We cannot check alignment with `offset_valid` using `self.addr()`. So set 0 for it and
> +        // ensure alignment by checking that the alignment of `U` is smaller or equal to the
> +        // alignment of `Self::Target`.
> +        const_assert!(Alignment::of::<U>().as_usize() <= Self::Target::MIN_ALIGN.as_usize());
> +        build_assert!(offset_valid::<U>(0, offset, Self::Target::MIN_SIZE));

IIUC this can allow unaligned accesses if `self.addr()` itself is not
properly aligned. Do we need a new `Io` invariant for that or is it
already enforced somewhere?