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

["Danilo Krummrich" <dakr@kernel.org>]

On Tue Mar 10, 2026 at 8:52 PM CET, 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.

That sounds wrong, the only thing that write_slice() needs to do is to forward
to write_raw(), that's a one-liner and not different to what you have in your
patch currently?

> 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.

The problem is that creating a slice of a DMA buffer, that is used by a device
concurrently, is undefined behavior. Hence the indirection through write_raw().