Re: [PATCH v7 2/7] rust: uaccess: add write_dma() for copying from DMA buffers to userspace

[Alice Ryhl <aliceryhl@google.com>]

On Tue, Mar 10, 2026 at 07:52:47PM +0000, Timur Tabi wrote:
> On Mon, 2026-03-09 at 20:59 +0100, Danilo Krummrich wrote:
> 
> > > +    /// Low-level write from a raw pointer. Caller must ensure ptr is valid for `len` bytes.
> > > +    fn write_raw(&mut self, ptr: *const u8, len: usize) -> Result {
> > 
> > The method has to be unsafe as the caller has to promise that ptr is indeed a
> > slice with len elements.
> 
> Ok.
> 
> > Another option would be to pass a fat pointer, i.e. *const [u8]. write_dma()
> > would then need to use ptr::slice_from_raw_parts() and the safety requirement of
> > this function becomes that ptr simply has to be valid.
> 
> So I tried this approach, but the end result was that write_raw() and write_slice() were practically
> identical.  At this point, why bother with write_raw() -- just have write_dma() call write_slice():
> 
>         let src_ptr = unsafe { alloc.start_ptr().add(offset) };
>         let slice = unsafe { core::slice::from_raw_parts(src_ptr, count) };
>         self.write_slice(slice)
> 
> I think this is better, but I wanted to get your opinion before I posted a v8 with this change.

Unfortunately, the creation of a reference into volatile memory is not
legal. Here, 'reference' includes slices. You can implement
`write_slice` in terms of `write_raw`, but the reverse will not work.

Alice