Re: [PATCH v6 3/5] rust: debugfs: Support arbitrary owned backing for File

[Alice Ryhl <aliceryhl@google.com>]

On Wed, Jun 18, 2025 at 02:28:15AM +0000, Matthew Maurer wrote:
> This allows `File`s to be backed by `Deref<Target=T>` rather than just
> `&'static T`. This means that dynamically allocated objects can be
> attached to `File`s without needing to take extra steps to create a
> pinned reference that's guaranteed to live long enough.
> 
> Signed-off-by: Matthew Maurer <mmaurer@google.com>
> ---
>  rust/kernel/debugfs.rs | 51 ++++++++++++++++++++++++++++++++++++++------------
>  1 file changed, 39 insertions(+), 12 deletions(-)
> 
> diff --git a/rust/kernel/debugfs.rs b/rust/kernel/debugfs.rs
> index 6a89557d8cf49327d2984d15741ffb6640defd70..cd83f21cf2818f406575941ebbc6c426575643e4 100644
> --- a/rust/kernel/debugfs.rs
> +++ b/rust/kernel/debugfs.rs
> @@ -5,12 +5,13 @@
>  //!
>  //! C header: [`include/linux/debugfs.h`](srctree/include/linux/debugfs.h)
>  
> -#[cfg(CONFIG_DEBUG_FS)]
> +use crate::alloc::KBox;
>  use crate::prelude::GFP_KERNEL;
>  use crate::str::CStr;
>  #[cfg(CONFIG_DEBUG_FS)]
>  use crate::sync::Arc;
>  use core::fmt::Display;
> +use core::ops::Deref;
>  
>  #[cfg(CONFIG_DEBUG_FS)]
>  mod display_file;
> @@ -61,40 +62,59 @@ fn create(_name: &CStr, _parent: Option<&Dir>) -> Self {
>      }
>  
>      #[cfg(CONFIG_DEBUG_FS)]
> -    fn create_file<T: Display + Sized>(&self, name: &CStr, data: &'static T) -> File {
> +    fn create_file<D: Deref<Target = T> + 'static + Send + Sync, T: Display>(
> +        &self,
> +        name: &CStr,
> +        data: D,
> +    ) -> File {
> +        let mut file = File {
> +            _entry: entry::Entry::empty(),
> +            _data: None,
> +        };
> +        let Some(data) = KBox::new(data, GFP_KERNEL).ok() else {
> +            return file;
> +        };

We may want to consider using the ForeignOwnable trait here instead. The
trait is implemented by anything that can be converted to/from a void
pointer, so you can:

* When creating the file, convert it to a void pointer that you store in
  File and pass to debugfs_create_file_full.
* When displaying the file, create a borrowed version of the void
  pointer and display that.
* When freeing the File, convert the void pointer back into an owned
  value and drop it.

For cases where a box really is necessary, the user can create a box and
pass it themselves. But if the user already has a pointer type (e.g. and
Arc<T> or &'static T) then they can pass that pointer directly and the
pointer is stored as a void pointer without the Box indirection.

Alice