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 <[email protected]> >> --- >> 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
