Re: [PATCH v12 2/3] rust: add dma coherent allocator abstraction.

[Alice Ryhl]

On Mon, Mar 3, 2025 at 2:00 PM Andreas Hindborg <a.hindborg@xxxxxxxxxx> wrote:
>
> "Daniel Almeida" <daniel.almeida@xxxxxxxxxxxxx> writes:
>
> > Hi Benno,
> >
>
> [...]
>
> >>> +    /// Writes data to the region starting from `offset`. `offset` is in units of `T`, not the
> >>> +    /// number of bytes.
> >>> +    ///
> >>> +    /// # Examples
> >>> +    ///
> >>> +    /// ```
> >>> +    /// # fn test(alloc: &mut kernel::dma::CoherentAllocation<u8>) -> Result {
> >>> +    /// let somedata: [u8; 4] = [0xf; 4];
> >>> +    /// let buf: &[u8] = &somedata;
> >>> +    /// alloc.write(buf, 0)?;
> >>> +    /// # Ok::<(), Error>(()) }
> >>> +    /// ```
> >>> +    pub fn write(&self, src: &[T], offset: usize) -> Result {
> >>> +        let end = offset.checked_add(src.len()).ok_or(EOVERFLOW)?;
> >>> +        if end >= self.count {
> >>> +            return Err(EINVAL);
> >>> +        }
> >>> +        // SAFETY:
> >>> +        // - The pointer is valid due to type invariant on `CoherentAllocation`
> >>> +        // and we've just checked that the range and index is within bounds.
> >>> +        // - `offset` can't overflow since it is smaller than `self.count` and we've checked
> >>> +        // that `self.count` won't overflow early in the constructor.
> >>> +        unsafe {
> >>> +            core::ptr::copy_nonoverlapping(src.as_ptr(), self.cpu_addr.add(offset), src.len())
> >>
> >> Why are there no concurrent write or read operations on `cpu_addr`?
> >
> > Sorry, can you rephrase this question?
>
> This write is suffering the same complications as discussed here [1].
> There are multiple issues with this implementation.
>
> 1) `write` takes a shared reference and thus may be called concurrently.
> There is no synchronization, so `copy_nonoverlapping` could be called
> concurrently on the same address. The safety requirements for
> `copy_nonoverlapping` state that the destination must be valid for
> write. Alice claims in [1] that any memory area that experience data
> races are not valid for writes. So the safety requirement of
> `copy_nonoverlapping` is violated and this call is potential UB.
>
> 2) The destination of this write is DMA memory. It could be concurrently
> modified by hardware, leading to the same issues as 1). Thus the
> function cannot be safe if we cannot guarantee hardware will not write
> to the region while this function is executing.
>
> Now, I don't think that these _should_ be issues, but according to our
> Rust language experts they _are_.
>
> I really think that copying data through a raw pointer to or from a
> place that experiences data races, should _not_ be UB if the data is not
> interpreted in any way, other than moving it.
>
>
> Best regards,
> Andreas Hindborg

We need to make progress on this series, and it's starting to get late
in the cycle. I suggest we:

1. Delete as_slice, as_slice_mut, write, and skip_drop.
2. Change field_read/field_write to use a volatile read/write.

This will let us make progress now and sidestep this discussion. The
deleted methods can happen in a follow-up.

Similarly for the dma mask methods, let's either drop them to a
follow-up patch or just put them anywhere and move them later.

Alice