[Linaro-mm-sig] Re: [PATCH v9 5/7] rust: drm: gem: shmem: Add DRM shmem helper abstraction

On Mon Mar 16, 2026 at 10:16 PM CET, Lyude Paul wrote:

• /// Creates (if necessary) and returns an immutable reference to a scatter-gather table of DMA
• /// pages for this object.
• ///
• /// This will pin the object in memory.
• #[inline]
• pub fn sg_table(&self) -> Result<&scatterlist::SGTable> {

•    // SAFETY:
  

•    // - drm_gem_shmem_get_pages_sgt is thread-safe.
  

•    // - drm_gem_shmem_get_pages_sgt returns either a valid pointer to a scatterlist, or an
  

•    //   error pointer.
  

•    let sgt =
  

•        from_err_ptr(unsafe { bindings::drm_gem_shmem_get_pages_sgt(self.as_raw_shmem()) })?;
  

This is unsound as nothing guarantees that the device used by drm_gem_shmem_get_pages_sgt() is actually bound to the calling driver. It is also not guaranteed that the DMA mapping within the returned &SGTable does not out-live driver unbind.

There are two possible solutions.

(1) Change drm_gem_shmem_get_pages_sgt() to provide this guarantee.

(2) Don't use drm_gem_shmem_get_pages_sgt() in the first place and instead use SGTable::new(), which guarantees to destroy the backing DMA mapping on driver unbind.

In any case, this function needs to take a &Device<Bound> argument that matches the bus devices stored in the backing GEM object.

•    // SAFETY: We checked above that `sgt` is not an error pointer, so it must be a valid
  

•    // pointer to a scatterlist
  

•    Ok(unsafe { scatterlist::SGTable::from_raw(sgt) })
  

• }

+}