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

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Message-Id: <DDPNGUVNJR6K.SX999PDIF1N2@kernel.org>
Date: Thu, 23 Oct 2025 13:03:35 +0200
From: "Danilo Krummrich" <dakr@...nel.org>
To: "Alice Ryhl" <aliceryhl@...gle.com>
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 12:37 PM CEST, Alice Ryhl wrote:
> 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

Yeah, I'm aware -- I expressed this badly.

What I meant is that for the kernel we obviously know that there's no
architecture where isize::MAX > file::Offset::MAX.

However, in the core API the conversion from usize to u128 is considered
fallible. So, applying the assumption that isize::MAX <= file::Offset::MAX is at
least inconsistent.