Re: [PATCH] rust: dma: update safety comments for volatile memory access

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

On Fri Jan 30, 2026 at 2:59 PM GMT, Andreas Hindborg wrote:
> At the time `CoherentAllocation::read_field` and
> `CoherentAllocation::write_field` was merged, `ptr::{read,write}_volatile`
> was under specified. The documentation for these functions have been
> updated and we can now formulate a proper safety comment for the calls.
>
> Update safety comments in `CoherentAllocation::{read,write}_field`.
>
> Link: https://doc.rust-lang.org/stable/core/ptr/fn.read_volatile.html
> Link: https://doc.rust-lang.org/stable/core/ptr/fn.write_volatile.html
> Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>
> ---
>  rust/kernel/dma.rs | 25 +++++++++----------------
>  1 file changed, 9 insertions(+), 16 deletions(-)
>
> diff --git a/rust/kernel/dma.rs b/rust/kernel/dma.rs
> index acc65b1e0f245..0b55671a94faf 100644
> --- a/rust/kernel/dma.rs
> +++ b/rust/kernel/dma.rs
> @@ -593,14 +593,12 @@ pub fn item_from_index(&self, offset: usize) -> Result<*mut T> {
>      pub unsafe fn field_read<F: FromBytes>(&self, field: *const F) -> F {
>          // SAFETY:
>          // - By the safety requirements field is valid.
> -        // - Using read_volatile() here is not sound as per the usual rules, the usage here is
> -        // a special exception with the following notes in place. When dealing with a potential
> -        // race from a hardware or code outside kernel (e.g. user-space program), we need that
> -        // read on a valid memory is not UB. Currently read_volatile() is used for this, and the
> -        // rationale behind is that it should generate the same code as READ_ONCE() which the
> -        // kernel already relies on to avoid UB on data races. Note that the usage of
> -        // read_volatile() is limited to this particular case, it cannot be used to prevent
> -        // the UB caused by racing between two kernel functions nor do they provide atomicity.
> +        // - `field` points to memory outside any Rust allocation.

Hmm, this isn't actually correct, as the memory behind `CoherentAllocation`
isn't MMIO (they're just also accessible by device). We provide `as_slice()`
which exposes these memory directly to the Rust memory model.

Best,
Gary

> +        // - As `field` points to readable memory:
> +        //   - Reading `field` will not trap.
> +        //   - Reading `field` will not change any memory inside a Rust allocation.
> +        // - As `F: FromBytes` any bit pattern is valid for `F` and the read
> +        //   will produce a properly initialized F.
>          unsafe { field.read_volatile() }
>      }
>  
> @@ -616,14 +614,9 @@ pub unsafe fn field_read<F: FromBytes>(&self, field: *const F) -> F {
>      pub unsafe fn field_write<F: AsBytes>(&self, field: *mut F, val: F) {
>          // SAFETY:
>          // - By the safety requirements field is valid.
> -        // - Using write_volatile() here is not sound as per the usual rules, the usage here is
> -        // a special exception with the following notes in place. When dealing with a potential
> -        // race from a hardware or code outside kernel (e.g. user-space program), we need that
> -        // write on a valid memory is not UB. Currently write_volatile() is used for this, and the
> -        // rationale behind is that it should generate the same code as WRITE_ONCE() which the
> -        // kernel already relies on to avoid UB on data races. Note that the usage of
> -        // write_volatile() is limited to this particular case, it cannot be used to prevent
> -        // the UB caused by racing between two kernel functions nor do they provide atomicity.
> +        // - As `field` points to readable memory:
> +        //   - Reading `field` will not trap.
> +        //   - Reading `field` will not change any memory inside a Rust allocation.
>          unsafe { field.write_volatile(val) }
>      }
>  }
>
> ---
> base-commit: 63804fed149a6750ffd28610c5c1c98cce6bd377
> change-id: 20260130-dma-doc-update-a8a0548045e2
>
> Best regards,