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

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Message-ID: <aPnoTV4kPz5NHGBE@google.com>
Date: Thu, 23 Oct 2025 08:33:17 +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 04/10] rust: uaccess: add UserSliceWriter::write_slice_partial()

On Wed, Oct 22, 2025 at 04:30:38PM +0200, 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>

This code is ok

Reviewed-by: Alice Ryhl <aliceryhl@...gle.com>

but:

> +    /// 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());
> +
> +        data.get(offset..end)
> +            .map_or(Ok(0), |src| self.write_slice(src).map(|()| src.len()))

Isn't it more readable to write it like this?

	let Some(src) = data.get(offset..end) else {
	    return Ok(0);
	};
	
	self.write_slice(src)?;
	Ok(src.len())

Alice