linux-kernel - Re: [PATCH v3 02/10] rust: uaccess: add UserSliceReader::read_slice

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Message-Id: <DDXF7OC6245W.20JRVUD4LCPFP@nvidia.com>
Date: Sat, 01 Nov 2025 23:16:46 +0900
From: "Alexandre Courbot" <acourbot@...dia.com>
To: "Danilo Krummrich" <dakr@...nel.org>, <gregkh@...uxfoundation.org>,
 <rafael@...nel.org>, <ojeda@...nel.org>, <alex.gaynor@...il.com>,
 <boqun.feng@...il.com>, <gary@...yguo.net>, <bjorn3_gh@...tonmail.com>,
 <lossin@...nel.org>, <a.hindborg@...nel.org>, <aliceryhl@...gle.com>,
 <tmgross@...ch.edu>, <mmaurer@...gle.com>
Cc: <rust-for-linux@...r.kernel.org>, <linux-fsdevel@...r.kernel.org>,
 <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 02/10] rust: uaccess: add
 UserSliceReader::read_slice_partial()

On Wed Oct 22, 2025 at 11:30 PM JST, 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.
>
> Reviewed-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> Reviewed-by: Matthew Maurer <mmaurer@...gle.com>
> Signed-off-by: Danilo Krummrich <dakr@...nel.org>
> ---
>  rust/kernel/uaccess.rs | 16 ++++++++++++++++
>  1 file changed, 16 insertions(+)
>
> diff --git a/rust/kernel/uaccess.rs b/rust/kernel/uaccess.rs
> index a8fb4764185a..c1cd3a76cff8 100644
> --- a/rust/kernel/uaccess.rs
> +++ b/rust/kernel/uaccess.rs
> @@ -287,6 +287,22 @@ 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())
> +            .unwrap_or(out.len())
> +            .min(out.len());

IIUC you should be able to replace the `checked_add().unwrap_or()` with
`saturating_add()` and end up with the same result.

Reviewed-by: Alexandre Courbot <acourbot@...dia.com>