linux-kernel - Re: [PATCH v19 1/3] rust: types: add `ForeignOwnable::PointedTo`

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Message-ID: <CAH5fLghKLZR7i6YQk8cQrvfOr11xEKia5LHtj1fn8dD3Stv0dQ@mail.gmail.com>
Date: Thu, 1 May 2025 09:17:27 +0200
From: Alice Ryhl <aliceryhl@...gle.com>
To: Tamir Duberstein <tamird@...il.com>
Cc: Gary Guo <gary@...yguo.net>, Danilo Krummrich <dakr@...nel.org>, Miguel Ojeda <ojeda@...nel.org>, 
	Alex Gaynor <alex.gaynor@...il.com>, Boqun Feng <boqun.feng@...il.com>, 
	Björn Roy Baron <bjorn3_gh@...tonmail.com>, 
	Benno Lossin <benno.lossin@...ton.me>, Andreas Hindborg <a.hindborg@...nel.org>, 
	Trevor Gross <tmgross@...ch.edu>, Matthew Wilcox <willy@...radead.org>, 
	Bjorn Helgaas <bhelgaas@...gle.com>, Greg Kroah-Hartman <gregkh@...uxfoundation.org>, 
	"Rafael J. Wysocki" <rafael@...nel.org>, FUJITA Tomonori <fujita.tomonori@...il.com>, 
	"Rob Herring (Arm)" <robh@...nel.org>, Maíra Canal <mcanal@...lia.com>, 
	Asahi Lina <lina@...hilina.net>, rust-for-linux@...r.kernel.org, 
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org, 
	linux-pci@...r.kernel.org
Subject: Re: [PATCH v19 1/3] rust: types: add `ForeignOwnable::PointedTo`

On Wed, Apr 30, 2025 at 8:57 PM Tamir Duberstein <tamird@...il.com> wrote:
>
> On Wed, Apr 30, 2025 at 11:31 AM Gary Guo <gary@...yguo.net> wrote:
> >
> > On Wed, 23 Apr 2025 09:54:37 -0400
> > Tamir Duberstein <tamird@...il.com> wrote:
> > > -impl<T: 'static, A> ForeignOwnable for Box<T, A>
> > > +// SAFETY: The `into_foreign` function returns a pointer that is well-aligned.
> > > +unsafe impl<T: 'static, A> ForeignOwnable for Box<T, A>
> > >  where
> > >      A: Allocator,
> > >  {
> > > +    type PointedTo = T;
> >
> > I don't think this is the correct solution for this. The returned
> > pointer is supposed to opaque, and exposing this type may encourage
> > this is to be wrongly used.
>
> Can you give an example?

This came up when we discussed this patch in the meeting yesterday:
https://lore.kernel.org/all/20250227-configfs-v5-1-c40e8dc3b9cd@kernel.org/

This is incorrect use of the trait. The pointer is supposed to be
opaque, and you can't dereference it. See my reply to that patch as
well:
https://lore.kernel.org/all/CAH5fLggDwPBzMO2Z48oMjDm4qgoNM0NQs_63TxmVEGy+gtMpOA@mail.gmail.com/

Alice