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

["Gary Guo" <gary@garyguo.net>]

On Mon Jun 15, 2026 at 5:28 AM BST, Alexandre Courbot wrote:
> 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?

Adding a trait invariant would require marking the trait as `unsafe`, which I
don't want to do because the `addr()` method is removed later anyway.

One argument is that it's `Io` implementation causing issue for its own if its
`addr()` is not aligned. This is later redefined using projection and views,
which further shifts responsiblity of upholding invariants to the `Io` type
implementator itself.

Best,
Gary