linux-kernel - Re: [PATCH] rust: miscdevice: Export vtable testing

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Message-ID: <CAGSQo03_FTLyYpvLRO8pFzZ1_aCu+i2-Xp5GVYVDzEg5MKGyPQ@mail.gmail.com>
Date: Fri, 27 Jun 2025 16:46:04 -0700
From: Matthew Maurer <mmaurer@...gle.com>
To: 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>, 
	Benno Lossin <lossin@...nel.org>, Andreas Hindborg <a.hindborg@...nel.org>, 
	Alice Ryhl <aliceryhl@...gle.com>, Trevor Gross <tmgross@...ch.edu>, 
	Danilo Krummrich <dakr@...nel.org>, "Isaac J. Manjarres" <isaacmanjarres@...gle.com>
Cc: rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] rust: miscdevice: Export vtable testing

On Fri, Jun 27, 2025 at 4:42 PM Matthew Maurer <mmaurer@...gle.com> wrote:
>
> A common pattern in the kernel is to test whether a file belongs to a
> particular driver by checking its `f_op` struct against an expected
> value. This provides a safe way to perform that test for `MiscDevice`
> implementations without needing to directly expose the vtable.
>
> Signed-off-by: Matthew Maurer <mmaurer@...gle.com>

Additionally, we have a sample user[1] of this in the Android ashmem
wrapper. They're currently working around it by grabbing the vtable
out of the registration and testing manually.

[1]: https://android-review.git.corp.google.com/c/kernel/common/+/3477511

> ---
>  rust/kernel/miscdevice.rs | 8 ++++++++
>  1 file changed, 8 insertions(+)
>
> diff --git a/rust/kernel/miscdevice.rs b/rust/kernel/miscdevice.rs
> index 939278bc7b03489a647b697012e09223871c90cd..5f59eda57c38be5f0d54fa9692fe5b2819e31480 100644
> --- a/rust/kernel/miscdevice.rs
> +++ b/rust/kernel/miscdevice.rs
> @@ -177,6 +177,14 @@ fn show_fdinfo(
>      }
>  }
>
> +/// Determines whether a given `File` is backed by the `T` `MiscDevice` based on vtable matching.
> +pub fn is_miscdevice_file<T: MiscDevice>(file: &File) -> bool {
> +    let vtable = core::ptr::from_ref(&MiscdeviceVTable::<T>::VTABLE);
> +    // SAFETY: `f_op` is not mutated after file creation
> +    let file_vtable = unsafe { (*file.as_ptr()).f_op };
> +    vtable == file_vtable
> +}
> +
>  /// A vtable for the file operations of a Rust miscdevice.
>  struct MiscdeviceVTable<T: MiscDevice>(PhantomData<T>);
>
>
> ---
> base-commit: 86731a2a651e58953fc949573895f2fa6d456841
> change-id: 20250627-linux-miscident-7b67db234a5c
>
> Best regards,
> --
> Matthew Maurer <mmaurer@...gle.com>
>