(Cc: Alice) Alice, for context, this is used when exporting a DMA buffer through debugfs, while the DMA buffer may be in use by the device, i.e. no slice can be created.
On Fri Jan 16, 2026 at 10:49 PM CET, Timur Tabi wrote: > Add a new method to UserSliceWriter that copies data from a raw kernel > pointer to userspace, without requiring a Rust slice reference. > > The method takes: > - data: raw pointer to the source buffer > - len: total size of the source buffer (for bounds checking) > - offset: byte offset into the source buffer to start copying from > - count: number of bytes to copy > > The method is marked unsafe because the caller must ensure the pointer > is valid for the specified length and that the memory is not mutated > during the call. > > Signed-off-by: Timur Tabi <[email protected]> > --- > rust/kernel/uaccess.rs | 50 ++++++++++++++++++++++++++++++++++++++++++ > 1 file changed, 50 insertions(+) > > diff --git a/rust/kernel/uaccess.rs b/rust/kernel/uaccess.rs > index f989539a31b4..8bbb0084abb1 100644 > --- a/rust/kernel/uaccess.rs > +++ b/rust/kernel/uaccess.rs > @@ -481,6 +481,56 @@ pub fn write_slice(&mut self, data: &[u8]) -> Result { > Ok(()) > } > > + /// Writes raw data to this user pointer from a raw kernel pointer. > + /// > + /// This is similar to [`Self::write_slice`] but takes a raw pointer > instead of a slice, > + /// along with a total buffer length, an offset into the that buffer, > and a count of bytes > + /// to copy. > + /// > + /// Returns error if the offset+count exceeds the buffer 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. > + /// > + /// # Safety > + /// > + /// - `data` must point to a valid memory region of at least `len` bytes > that remains allocated > + /// for the duration of this call. > + /// > + /// Note: Unlike [`Self::write_slice`], this method does not require > exclusive access to the > + /// source memory. The memory may be concurrently modified by other > threads or hardware (e.g., > + /// DMA buffers). In such cases, the copied data may be inconsistent, > but this does not cause > + /// undefined behavior. > + pub unsafe fn write_buffer( > + &mut self, > + data: *const u8, > + len: usize, > + offset: usize, > + count: usize, > + ) -> Result { Instead of this we could probably also add a safe method write_dma() that takes a dma::CoherentAllocation<u8> instead. Once we have generic I/O in place, this could be replaced with a generic write_io() method.
