Re: [PATCH v2 3/8] rust: uaccess: add UserSliceWriter::write_slice_partial()

[Alice Ryhl <aliceryhl@google.com>]

On Tue, Oct 21, 2025 at 12:26:15AM +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.
> 
> Signed-off-by: Danilo Krummrich <dakr@kernel.org>
>  rust/kernel/uaccess.rs | 24 ++++++++++++++++++++++++
>  1 file changed, 24 insertions(+)
> 
> diff --git a/rust/kernel/uaccess.rs b/rust/kernel/uaccess.rs
> index 2061a7e10c65..40d47e94b54f 100644
> --- a/rust/kernel/uaccess.rs
> +++ b/rust/kernel/uaccess.rs
> @@ -463,6 +463,30 @@ 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: file::Offset) -> Result<usize> {

I think for the current function signature, it's kind of weird to take a
file::Offset parameter

On one hand, it is described like a generic function for writing a
partial slice, and if that's what it is, then I would argue it should
take usize because it's an offset into the slice.

On another hand, I think what you're actually trying to do is implement
the simple_[read_from|write_to]_buffer utilities for user slices, but
it's only a "partial" version of those utilities. The full utility takes
a `&mut loff_t` so that it can also perform the required modification to
the offset.

Alice