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

On Thu, 30 Jul 2026 18:34:38 +0200=0D
Younes Akhouayri via B4 Relay <[email protected]> wrote:=0D
=0D
> From: Younes Akhouayri <[email protected]>=0D
> =0D
> Coherent<T>::write_to_slice() calculates a zero-byte copy when the file=0D
> offset is beyond the allocation, but still calls=0D
> UserSliceWriter::write_dma(). The latter rejects offsets beyond the=0D
> allocation even when the copy length is zero, so a debugfs read past EOF=
=0D
> returns -ERANGE.=0D
> =0D
> Return before calling write_dma() when the offset is at or beyond the=0D
> allocation, matching simple_read_from_buffer() EOF semantics.=0D
> =0D
> Fixes: 016818513936 ("rust: dma: implement BinaryWriter for Coherent<[u8]=
>")=0D
> Cc: [email protected]=0D
> Link: https://rust-for-linux.zulipchat.com/#narrow/channel/291566-Library=
/topic/.E2.9C.94.20Possible.20past-EOF.20bug.20in.20Coherent.3CT.3E.3A.3Awr=
ite_to_slice/near/611677095=0D
> Signed-off-by: Younes Akhouayri <[email protected]>=0D
> ---=0D
> Changes in v2:=0D
> - Use ordinary subtraction after the explicit EOF check, which makes=0D
>   underflow impossible.=0D
> - Link to v1: https://patch.msgid.link/20260720-fix-dma-coherent-eof-v1-1=
[email protected]=0D
> ---=0D
>  rust/kernel/dma.rs | 6 +++++-=0D
>  1 file changed, 5 insertions(+), 1 deletion(-)=0D
> =0D
> diff --git a/rust/kernel/dma.rs b/rust/kernel/dma.rs=0D
> index 200def84fb69..556424b88fb9 100644=0D
> --- a/rust/kernel/dma.rs=0D
> +++ b/rust/kernel/dma.rs=0D
> @@ -1005,7 +1005,11 @@ fn write_to_slice(=0D
>              return Ok(0);=0D
>          };=0D
>  =0D
> -        let count =3D self.size().saturating_sub(offset_val).min(writer.=
len());=0D
=0D
Nit: I would probably just check if `count` is zero next to this line.=0D
=0D
> +        if offset_val >=3D self.size() {=0D
> +            return Ok(0);=0D
> +        }=0D
> +=0D
> +        let count =3D (self.size() - offset_val).min(writer.len());=0D
>  =0D
>          writer.write_dma(self, offset_val, count)?;=0D
>  =0D
> =0D
> ---=0D
> base-commit: 667d0fb32149f023b8b34a1f6f3d384556eafb5a=0D
> change-id: 20260720-fix-dma-coherent-eof-d0d08c91e013=0D
> =0D
> Best regards,=0D
> --  =0D
> Younes Akhouayri <[email protected]>=0D
> =0D
> =0D