linux-kernel - Re: [PATCH v4 2/4] rust: iov: add iov_iter abstractions for ITER

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Message-Id: <DC19O2AXINJW.3DS7GDDU3O3E1@kernel.org>
Date: Wed, 13 Aug 2025 13:34:46 +0200
From: "Danilo Krummrich" <dakr@...nel.org>
To: "Alice Ryhl" <aliceryhl@...gle.com>
Cc: "Greg Kroah-Hartman" <gregkh@...uxfoundation.org>, "Alexander Viro"
 <viro@...iv.linux.org.uk>, "Arnd Bergmann" <arnd@...db.de>, "Miguel Ojeda"
 <ojeda@...nel.org>, "Boqun Feng" <boqun.feng@...il.com>, "Gary Guo"
 <gary@...yguo.net>, Björn Roy Baron
 <bjorn3_gh@...tonmail.com>, "Andreas Hindborg" <a.hindborg@...nel.org>,
 "Trevor Gross" <tmgross@...ch.edu>, "Matthew Maurer" <mmaurer@...gle.com>,
 "Lee Jones" <lee@...nel.org>, <linux-kernel@...r.kernel.org>,
 <rust-for-linux@...r.kernel.org>, "Benno Lossin" <lossin@...nel.org>
Subject: Re: [PATCH v4 2/4] rust: iov: add iov_iter abstractions for
 ITER_DEST

On Wed Aug 13, 2025 at 10:27 AM CEST, Alice Ryhl wrote:
> +    /// Write data to this IO vector.
> +    ///
> +    /// Returns the number of bytes that were written. If this is shorter than the provided slice,
> +    /// then no more bytes can be written.
> +    #[inline]
> +    pub fn copy_to_iter(&mut self, input: &[u8]) -> usize {
> +        // SAFETY:
> +        // * By the struct invariants, it is still valid to write to this IO vector.

NIT: I think we usually say type invariants.

> +        // * `input` is valid for `input.len()` bytes.
> +        unsafe { bindings::_copy_to_iter(input.as_ptr().cast(), input.len(), self.as_raw()) }
> +    }
> +
> +    /// Utility for implementing `read_iter` given the full contents of the file.
> +    ///
> +    /// The full contents of the file being read from is represented by `contents`. This call will
> +    /// write the appropriate sub-slice of `contents` and update the file position in `ppos` so
> +    /// that the file will appear to contain `contents` even if takes multiple reads to read the
> +    /// entire file.
> +    #[inline]
> +    pub fn simple_read_from_buffer(&mut self, ppos: &mut i64, contents: &[u8]) -> Result<usize> {
> +        if *ppos < 0 {
> +            return Err(EINVAL);
> +        }
> +        let Ok(pos) = usize::try_from(*ppos) else {
> +            return Ok(0);
> +        };
> +        if pos >= contents.len() {
> +            return Ok(0);
> +        }
> +
> +        // BOUNDS: We just checked that `pos < contents.len()` above.

I like this one (and the one below). It would be nice to have a lint asking for such
comments, such as I'd like to have a "FORGET" one. :)

> +        let num_written = self.copy_to_iter(&contents[pos..]);
> +
> +        // OVERFLOW: `pos+num_written <= contents.len() <= isize::MAX <= i64::MAX`.
> +        *ppos = (pos + num_written) as i64;
> +
> +        Ok(num_written)
> +    }
> +}

Reviewed-by: Danilo Krummrich <dakr@...nel.org>