linux-kernel - Re: [PATCH v3 04/10] rust: uaccess: add UserSliceWriter::write_slice

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Message-Id: <DDXF9GTWLHAV.2XR669Q44I0NX@nvidia.com>
Date: Sat, 01 Nov 2025 23:19:06 +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 04/10] rust: uaccess: add
 UserSliceWriter::write_slice_partial()

On Wed Oct 22, 2025 at 11:30 PM JST, Danilo Krummrich wrote:
> The existing write_slice() method is a wrapper around copy_to_user() and
> expects the user buffer to be larger than the source buffer.
>
> However, userspace may split up reads in multiple partial operations
> providing an offset into the source buffer and a smaller user buffer.
>
> In order to support this common case, provide a helper for partial
> writes.
>
> 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 c2d3dfee8934..539e77a09cbc 100644
> --- a/rust/kernel/uaccess.rs
> +++ b/rust/kernel/uaccess.rs
> @@ -479,6 +479,22 @@ pub fn write_slice(&mut self, data: &[u8]) -> Result {
>          Ok(())
>      }
>  
> +    /// Writes raw data to this user pointer from a kernel buffer partially.
> +    ///
> +    /// This is the same as [`Self::write_slice`] but considers the given `offset` into `data` and
> +    /// truncates the write to the boundaries of `self` and `data`.
> +    ///
> +    /// On success, returns the number of bytes written.
> +    pub fn write_slice_partial(&mut self, data: &[u8], offset: usize) -> Result<usize> {
> +        let end = offset
> +            .checked_add(self.len())
> +            .unwrap_or(data.len())
> +            .min(data.len());

Same suggestion as the read counterpart about `saturating_add`.

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