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

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Message-ID: <CAH5fLgiM4gFFAyOd3nvemHPg-pdYKK6ttx35pnYOAEz8ZmrubQ@mail.gmail.com>
Date: Thu, 23 Oct 2025 12:37:25 +0200
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 Thu, Oct 23, 2025 at 12:35 PM Danilo Krummrich <dakr@...nel.org> wrote:
>
> On Thu Oct 23, 2025 at 10:30 AM CEST, Alice Ryhl wrote:
> > 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:
>
> It probably never will (which is why this was a + operation in v1).
>
> >       offset + written <= data.len() <= isize::MAX <= Offset::MAX
>
> However, this would rely on implementation details you listed, i.e. the
> invariant that a slice length should be at most isize::MAX and what's the
> maximum size of file::Offset::MAX.

It's not an implementation detail. All Rust allocations are guaranteed
to fit in isize::MAX bytes:
https://doc.rust-lang.org/stable/std/ptr/index.html#allocation

Alice