Re: [PATCH v4 2/4] rust: iov: add iov_iter abstractions for ITER_DEST

[Alice Ryhl <aliceryhl@google.com>]

On Wed, Aug 13, 2025 at 03:33:39PM +0200, Benoît du Garreau wrote:
> On Wed, 13 Aug 2025 08:27:18 +0000 Alice Ryhl <aliceryhl@google.com> wrote:
> 
> > +    /// 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.
> > +        let num_written = self.copy_to_iter(&contents[pos..]);
> 
> This should use `contents.get(..pos)` instead of doing the bound check manually.

Using contents.get(pos..) does not have the same behavior as the bounds
check, because the bounds check exits early with Ok(0) if pos is equal
to contents.len(), whereas the proposed change would pass an empty array
to copy_to_iter.

Alice