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

[Martin Rodriguez Reboredo <yakoyoku@gmail.com>]

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.

> +    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.

> +            Ok(())
> +        })
> +    }
> +
> +    /// Maps the page and writes into it from 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 `src` is valid for reading `len` bytes.
> +    /// * Callers must ensure that this call does not race with a read or write
> +    ///   to the same page that overlaps with this write.
> +    pub unsafe fn write_raw(&self, src: *const u8, offset: usize, len: usize) -> Result {
> +        self.with_pointer_into_page(offset, len, move |dst| {
> +            // SAFETY: If `with_pointer_into_page` calls into this closure, then
> +            // it has performed a bounds check and guarantees that `dst` is
> +            // valid for `len` bytes.
> +            //
> +            // There caller guarantees that there is no data race.
> +            unsafe { ptr::copy(src, dst, len) };

Same as above

> +            Ok(())
> +        })
> +    }
> [...]