Re: [PATCH v12 4/5] rust: drm: gem: Introduce shmem::SGTable

["Alexandre Courbot" <acourbot@nvidia.com>]

On Wed Apr 22, 2026 at 8:52 AM JST, Lyude Paul wrote:
<snip>
> @@ -176,6 +195,100 @@ extern "C" fn free_callback(obj: *mut bindings::drm_gem_object) {
>          // SAFETY: We're recovering the Kbox<> we created in gem_create_object()
>          let _ = unsafe { KBox::from_raw(this) };
>      }
> +
> +    // If necessary, create an SGTable for the gem object and register a Devres for it to ensure
> +    // that it is unmapped on driver unbind.
> +    fn get_sg_table<'a>(
> +        &'a self,
> +        dev: &'a device::Device<Bound>,

Just noticed that the caller can technically pass a different `dev` from
the one the buffer will be mapped onto, potentially linking the `Devres`
(and thus the device mapping) to a completely unrelated lifetime.

I.e. `dev` is only used to create the `Devres`, while
`drm_gem_shmem_get_pages_sgt_locked` uses the object's device pointer to
create the mapping. We need to add the check ensuring that the two point
to the same device:

    let expected = self.dev().as_ref().as_raw();
    if dev.as_raw() != expected {
        return Err(EINVAL);
    }

Ideally we could do without the `dev` argument altogether, but that's
our only guarantee that the device is bound, so I don't think we can do
without this runtime check.

I think it's also worth mentioning in the documentations of `sg_table`
and `owned_sg_table` which device is expected.