Re: [PATCH v2 4/4] rust: add abstraction for `struct page`

[Alice Ryhl <aliceryhl@google.com>]

On Sat, Feb 10, 2024 at 5:23 AM Martin Rodriguez Reboredo
<yakoyoku@gmail.com> wrote:
>
> On 2/8/24 12:47, Alice Ryhl wrote:
> > [...]
> > +    /// Maps the page and reads from it into the given buffer.
> > +    ///
> > +    /// This method will perform bounds checks on the page offset. If `offset ..
> > +    /// offset+len` goes outside ot the page, then this call returns `EINVAL`.
> > +    ///
> > +    /// # Safety
> > +    ///
> > +    /// * Callers must ensure that `dst` is valid for writing `len` bytes.
> > +    /// * Callers must ensure that this call does not race with a write to the
> > +    ///   same page that overlaps with this read.
>
> This safety section says that a call mustn't race with a page that
> overlaps this read, hmmmmm.

Is there a question here?

> > +    pub unsafe fn read_raw(&self, dst: *mut u8, offset: usize, len: usize) -> Result {
> > +        self.with_pointer_into_page(offset, len, move |src| {
> > +            // SAFETY: If `with_pointer_into_page` calls into this closure, then
> > +            // it has performed a bounds check and guarantees that `src` is
> > +            // valid for `len` bytes.
> > +            //
> > +            // There caller guarantees that there is no data race.
> > +            unsafe { ptr::copy(src, dst, len) };
>
> If `src` and `dst` overlap then wouldn't that be a bad idea? If so then
> how about mentioning that callers have to ensure that `dst` does not
> overlap with the page that's being read and use
> `core::ptr::copy_nonoverlapping` instead, otherwise the doc comment
> could mention that `dst` can overlap.

I'll use copy_nonoverlapping. Thanks for the suggestion.

Alice