Re: [PATCH v6 2/7] rust: uaccess: add write_dma() for copying from DMA buffers to userspace

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

On Thu Jan 29, 2026 at 3:28 AM CET, Timur Tabi wrote:
> +    /// Writes raw data to this user pointer from a DMA coherent allocation.
> +    ///
> +    /// Returns error if the offset+count exceeds the allocation size.
> +    ///
> +    /// Fails with [`EFAULT`] if the write happens on a bad address, or if the write goes out of
> +    /// bounds of this [`UserSliceWriter`]. This call may modify the associated userspace slice
> +    /// even if it returns an error.
> +    ///
> +    /// Note: The memory may be concurrently modified by hardware (e.g., DMA). In such cases,
> +    /// the copied data may be inconsistent, but this does not cause undefined behavior.
> +    pub fn write_dma(
> +        &mut self,
> +        alloc: &CoherentAllocation<u8>,
> +        offset: usize,
> +        count: usize,
> +    ) -> Result {
> +        let len = alloc.count();
> +        if offset.checked_add(count).ok_or(EOVERFLOW)? > len {
> +            return Err(ERANGE);
> +        }
> +
> +        // SAFETY: `start_ptr()` returns a valid pointer to a memory region of `count()` bytes,
> +        // as guaranteed by the `CoherentAllocation` invariants. The check above ensures
> +        // `offset + count <= len`.
> +        let src_ptr = unsafe { alloc.start_ptr().add(offset) };
> +
> +        // SAFETY: `src_ptr` is valid for reads of `count` bytes per the above.
> +        let res = unsafe {
> +            bindings::copy_to_user(self.ptr.as_mut_ptr(), src_ptr.cast::<c_void>(), count)
> +        };
> +        if res != 0 {
> +            return Err(EFAULT);
> +        }
> +
> +        self.ptr = self.ptr.wrapping_byte_add(count);
> +        self.length -= count;
> +
> +        Ok(())
> +    }

I think the idea was to have a common read_slice_raw() method that can be shared
between write_dma() and write_slice(). Can you please factor this out?