Re: [PATCH v2] rust: dma: return zero for Coherent reads past EOF

On Thu, 30 Jul 2026 18:34:38 +0200
Younes Akhouayri via B4 Relay <[email protected]> wrote:

> From: Younes Akhouayri <[email protected]>
> 
> 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]>")
> Cc: [email protected]
> Link: https://rust-for-linux.zulipchat.com/#narrow/channel/291566-Library/topic/.E2.9C.94.20Possible.20past-EOF.20bug.20in.20Coherent.3CT.3E.3A.3Awrite_to_slice/near/611677095
> Signed-off-by: Younes Akhouayri <[email protected]>
> ---
> Changes in v2:
> - Use ordinary subtraction after the explicit EOF check, which makes
>   underflow impossible.
> - Link to v1: https://patch.msgid.link/[email protected]
> ---
>  rust/kernel/dma.rs | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/rust/kernel/dma.rs b/rust/kernel/dma.rs
> index 200def84fb69..556424b88fb9 100644
> --- a/rust/kernel/dma.rs
> +++ b/rust/kernel/dma.rs
> @@ -1005,7 +1005,11 @@ fn write_to_slice(
>              return Ok(0);
>          };
>  
> -        let count = self.size().saturating_sub(offset_val).min(writer.len());

Nit: I would probably just check if `count` is zero next to this line.

> +        if offset_val >= self.size() {
> +            return Ok(0);
> +        }
> +
> +        let count = (self.size() - offset_val).min(writer.len());
>  
>          writer.write_dma(self, offset_val, count)?;
>  
> 
> ---
> base-commit: 667d0fb32149f023b8b34a1f6f3d384556eafb5a
> change-id: 20260720-fix-dma-coherent-eof-d0d08c91e013
> 
> Best regards,
> --  
> Younes Akhouayri <[email protected]>
> 
>