linux-kernel - Re: [PATCH v16 2/4] rust: types: add `ForeignOwnable::PointedTo`

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Message-ID: <CAJ-ks9nemf0XbVhEMeDVRq0SBcdMRUJn3Pmfotbx5CJBj7QTTQ@mail.gmail.com>
Date: Sun, 16 Feb 2025 20:58:27 -0500
From: Tamir Duberstein <tamird@...il.com>
To: Benno Lossin <benno.lossin@...ton.me>
Cc: Danilo Krummrich <dakr@...nel.org>, 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>, Alice Ryhl <aliceryhl@...gle.com>, 
	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, Fiona Behrens <me@...enk.dev>
Subject: Re: [PATCH v16 2/4] rust: types: add `ForeignOwnable::PointedTo`

On Sun, Feb 16, 2025 at 8:39 PM Benno Lossin <benno.lossin@...ton.me> wrote:
>
> On 07.02.25 14:58, Tamir Duberstein wrote:
> > Allow implementors to specify the foreign pointer type; this exposes
> > information about the pointed-to type such as its alignment.
> >
> > This requires the trait to be `unsafe` since it is now possible for
> > implementors to break soundness by returning a misaligned pointer.
> >
> > Encoding the pointer type in the trait (and avoiding pointer casts)
> > allows the compiler to check that implementors return the correct
> > pointer type. This is preferable to directly encoding the alignment in
> > the trait using a constant as the compiler would be unable to check it.
> >
> > Reviewed-by: Alice Ryhl <aliceryhl@...gle.com>
> > Reviewed-by: Andreas Hindborg <a.hindborg@...nel.org>
> > Reviewed-by: Fiona Behrens <me@...enk.dev>
> > Signed-off-by: Tamir Duberstein <tamird@...il.com>
> > ---
> >  rust/kernel/alloc/kbox.rs | 38 ++++++++++++++++++++------------------
> >  rust/kernel/miscdevice.rs |  7 ++++++-
> >  rust/kernel/pci.rs        |  2 ++
> >  rust/kernel/platform.rs   |  2 ++
> >  rust/kernel/sync/arc.rs   | 21 ++++++++++++---------
> >  rust/kernel/types.rs      | 46 +++++++++++++++++++++++++++++++---------------
> >  6 files changed, 73 insertions(+), 43 deletions(-)
>
> When compiling this (on top of rust-next), I get the following error:
>
>     error[E0308]: mismatched types
>        --> rust/kernel/miscdevice.rs:300:62
>         |
>     300 |     let device = unsafe { <T::Ptr as ForeignOwnable>::borrow(private) };
>         |                           ---------------------------------- ^^^^^^^ expected `*mut <<T as MiscDevice>::Ptr as ForeignOwnable>::PointedTo`, found `*mut c_void`
>         |                           |
>         |                           arguments to this function are incorrect
>         |
>         = note: expected raw pointer `*mut <<T as MiscDevice>::Ptr as ForeignOwnable>::PointedTo`
>                    found raw pointer `*mut c_void`
>         = help: consider constraining the associated type `<<T as MiscDevice>::Ptr as ForeignOwnable>::PointedTo` to `c_void` or calling a method that returns `<<T as MiscDevice>::Ptr as ForeignOwnable>::PointedTo`
>         = note: for more information, visit https://doc.rust-lang.org/book/ch19-03-advanced-traits.html
>     note: associated function defined here
>        --> rust/kernel/types.rs:98:15
>         |
>     98  |     unsafe fn borrow<'a>(ptr: *mut Self::PointedTo) -> Self::Borrowed<'a>;
>         |               ^^^^^^
>
>     error: aborting due to 1 previous error
>
> Can anyone reproduce?
>
> ---
> Cheers,
> Benno
>

Looks like this code is behind #[cfg(CONFIG_COMPAT)] and I missed
updating it. It is fixed by

diff --git a/rust/kernel/miscdevice.rs b/rust/kernel/miscdevice.rs
index f1a081dd64c7..004dc87f441f 100644
--- a/rust/kernel/miscdevice.rs
+++ b/rust/kernel/miscdevice.rs
@@ -296,6 +296,7 @@ impl<T: MiscDevice> VtableHelper<T> {
 ) -> c_long {
     // SAFETY: The compat ioctl call of a file can access the private data.
     let private = unsafe { (*file).private_data };
+    let private = private.cast();
     // SAFETY: Ioctl calls can borrow the private data of the file.
     let device = unsafe { <T::Ptr as ForeignOwnable>::borrow(private) };

I'll include that in the next version.