Re: [PATCH] rust: dma: return zero for Coherent reads past EOF

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

On Mon Jul 20, 2026 at 8:21 PM BST, Younes Akhouayri via B4 Relay wrote:
> From: Younes Akhouayri <git@younes.io>
>
> Coherent<T>::write_to_slice() calculates a zero-byte copy when the file
> offset is beyond the allocation, but still calls
> UserSliceWriter::write_dma(). The latter rejects offsets beyond the
> allocation even when the copy length is zero, so a debugfs read past EOF
> returns -ERANGE.
>
> Return before calling write_dma() when the offset is at or beyond the
> allocation, matching simple_read_from_buffer() EOF semantics.
>
> Fixes: 016818513936 ("rust: dma: implement BinaryWriter for Coherent<[u8]>")
> Signed-off-by: Younes Akhouayri <git@younes.io>
> ---
>  rust/kernel/dma.rs | 4 ++++
>  1 file changed, 4 insertions(+)
>
> diff --git a/rust/kernel/dma.rs b/rust/kernel/dma.rs
> index 200def84fb69..36b72fb9ab46 100644
> --- a/rust/kernel/dma.rs
> +++ b/rust/kernel/dma.rs
> @@ -1005,6 +1005,10 @@ fn write_to_slice(
>              return Ok(0);
>          };
>  
> +        if offset_val >= self.size() {
> +            return Ok(0);
> +        }
> +
>          let count = self.size().saturating_sub(offset_val).min(writer.len());

This saturating_sub is redundant now and should just use -.

Best,
Gary

>  
>          writer.write_dma(self, offset_val, count)?;
>
> ---
> base-commit: 880c43b185ca52239e75bc546cc4f4d9154d0fed
> change-id: 20260720-fix-dma-coherent-eof-d0d08c91e013
>
> Best regards,
> --  
> Younes Akhouayri <git@younes.io>