linux-kernel - Re: [PATCH v3 05/10] rust: uaccess: add UserSliceWriter::write_slice

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Message-ID: <aPnnkU3IWwgERuT3@google.com>
Date: Thu, 23 Oct 2025 08:30:09 +0000
From: Alice Ryhl <aliceryhl@...gle.com>
To: Danilo Krummrich <dakr@...nel.org>
Cc: 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, 
	tmgross@...ch.edu, mmaurer@...gle.com, rust-for-linux@...r.kernel.org, 
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 05/10] rust: uaccess: add UserSliceWriter::write_slice_file()

On Wed, Oct 22, 2025 at 04:30:39PM +0200, Danilo Krummrich wrote:
> Add UserSliceWriter::write_slice_file(), which is the same as
> UserSliceWriter::write_slice_partial() but updates the given
> file::Offset by the number of bytes written.
> 
> This is equivalent to C's `simple_read_from_buffer()` and useful when
> dealing with file offsets from file operations.
> 
> Signed-off-by: Danilo Krummrich <dakr@...nel.org>
> ---
>  rust/kernel/uaccess.rs | 24 ++++++++++++++++++++++++
>  1 file changed, 24 insertions(+)
> 
> diff --git a/rust/kernel/uaccess.rs b/rust/kernel/uaccess.rs
> index 539e77a09cbc..20ea31781efb 100644
> --- a/rust/kernel/uaccess.rs
> +++ b/rust/kernel/uaccess.rs
> @@ -495,6 +495,30 @@ pub fn write_slice_partial(&mut self, data: &[u8], offset: usize) -> Result<usiz
>              .map_or(Ok(0), |src| self.write_slice(src).map(|()| src.len()))
>      }
>  
> +    /// Writes raw data to this user pointer from a kernel buffer partially.
> +    ///
> +    /// This is the same as [`Self::write_slice_partial`] but updates the given [`file::Offset`] by
> +    /// the number of bytes written.
> +    ///
> +    /// This is equivalent to C's `simple_read_from_buffer()`.
> +    ///
> +    /// On success, returns the number of bytes written.
> +    pub fn write_slice_file(&mut self, data: &[u8], offset: &mut file::Offset) -> Result<usize> {
> +        if offset.is_negative() {
> +            return Err(EINVAL);
> +        }
> +
> +        let Ok(offset_index) = (*offset).try_into() else {
> +            return Ok(0);
> +        };
> +
> +        let written = self.write_slice_partial(data, offset_index)?;
> +
> +        *offset = offset.saturating_add_usize(written);

This addition should never overflow:

	offset + written <= data.len() <= isize::MAX <= Offset::MAX

I can't help but think that maybe this should be a + operation instead?

Alice