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

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Message-ID: <96d799a7-ec70-4d3f-951b-52c56b545255@proton.me>
Date: Mon, 17 Feb 2025 01:39:40 +0000
From: Benno Lossin <benno.lossin@...ton.me>
To: Tamir Duberstein <tamird@...il.com>, 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>
Cc: 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 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