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

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Message-ID: <20250627-linux-miscident-v1-1-d37c870550ef@google.com>
Date: Fri, 27 Jun 2025 23:42:38 +0000
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, 
	Matthew Maurer <mmaurer@...gle.com>
Subject: [PATCH] rust: miscdevice: Export vtable testing

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>
---
 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>