Re: [PATCH 1/7] rust: uaccess: add UserSliceReader::read_slice_partial()

[Alice Ryhl <aliceryhl@google.com>]

On Sat, Oct 04, 2025 at 12:26:38AM +0200, Danilo Krummrich wrote:
> The existing read_slice() method is a wrapper around copy_from_user()
> and expects the user buffer to be larger than the destination buffer.
> 
> However, userspace may split up writes in multiple partial operations
> providing an offset into the destination buffer and a smaller user
> buffer.
> 
> In order to support this common case, provide a helper for partial
> reads.
> 
> Signed-off-by: Danilo Krummrich <dakr@kernel.org>
> ---
>  rust/kernel/uaccess.rs | 13 +++++++++++++
>  1 file changed, 13 insertions(+)
> 
> diff --git a/rust/kernel/uaccess.rs b/rust/kernel/uaccess.rs
> index a8fb4764185a..1b0b57e855c9 100644
> --- a/rust/kernel/uaccess.rs
> +++ b/rust/kernel/uaccess.rs
> @@ -287,6 +287,19 @@ pub fn read_slice(&mut self, out: &mut [u8]) -> Result {
>          self.read_raw(out)
>      }
>  
> +    /// Reads raw data from the user slice into a kernel buffer partially.
> +    ///
> +    /// This is the same as [`Self::read_slice`] but considers the given `offset` into `out` and
> +    /// truncates the read to the boundaries of `self` and `out`.
> +    ///
> +    /// On success, returns the number of bytes read.
> +    pub fn read_slice_partial(&mut self, out: &mut [u8], offset: usize) -> Result<usize> {
> +        let end = offset.checked_add(self.len()).ok_or(EINVAL)?.min(out.len());

Should this be?
let end = offset.checked_add(self.len()).unwrap_or(out.len()).min(out.len());

> +        out.get_mut(offset..end)
> +            .map_or(Ok(0), |dst| self.read_slice(dst).map(|()| dst.len()))

So if out.len() < offset, then we return Ok(0)?

Alice