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

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Message-ID: <CAGSQo00fwu-UEi9D+Q4F5WpfUUuz562odhaDhp=F99cJyd9WyQ@mail.gmail.com>
Date: Wed, 18 Jun 2025 08:00:51 -0700
From: Matthew Maurer <mmaurer@...gle.com>
To: Alice Ryhl <aliceryhl@...gle.com>
Cc: Miguel Ojeda <ojeda@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>, 
	Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>, 
	Björn Roy Baron <bjorn3_gh@...tonmail.com>, 
	Andreas Hindborg <a.hindborg@...nel.org>, Trevor Gross <tmgross@...ch.edu>, 
	Danilo Krummrich <dakr@...nel.org>, Greg Kroah-Hartman <gregkh@...uxfoundation.org>, 
	"Rafael J. Wysocki" <rafael@...nel.org>, Sami Tolvanen <samitolvanen@...gle.com>, 
	Timur Tabi <ttabi@...dia.com>, Benno Lossin <lossin@...nel.org>, linux-kernel@...r.kernel.org, 
	rust-for-linux@...r.kernel.org
Subject: Re: [PATCH v6 3/5] rust: debugfs: Support arbitrary owned backing for File

> We may want to consider using the ForeignOwnable trait here instead. The

I was considering trying to switch over to `StableDeref`-like trait
[1] in a follow-up patchset. The core property I need is that moving
the `D` cannot result in the pointer it would `deref` to changing.

The problem with `ForeignOwnable` is that it forbids the user from
passing in a `Box<dyn Foo>`, because that doesn't fit in a `void*` A
`StableDeref` version would not have this issue. I agree that
`ForeignOwnable` would be a strict upgrade to what I have now, since a
user can still pass in a `Box<Box<dyn Foo>>` and have it work with
`ForeignOwnable`, and if we ever added `StableDeref`, then
`ForeignOwnable` would have a blanket impl for it.

I'll send a new version using `ForeignOwnable`, and we can consider
the `StableDeref` version in the future.

[1]: https://docs.rs/gimli/latest/gimli/trait.StableDeref.html

> 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