[PATCH v5 0/3] rust: miscdevice: add additional data to MiscDeviceRegistration

[PATCH v5 0/3] rust: miscdevice: add additional data to MiscDeviceRegistration

[Christian Schrefl]

Currently there is no good way to pass arbitrary data from the driver to
a `miscdevice` or to share data between individual handles to a 
`miscdevice` in rust.

This series adds additional (generic) data to the MiscDeviceRegistration
for this purpose.

The first patch originally comes from my `UnsafePinned` patch series [0].

The second patch implements the changes and fixes the build of the sample
without changing any functionality (this is currently the only in tree 
user).

The third patch changes the `rust_misc_device` sample to use this to 
share the same data between multiple handles to the `miscdevice`.
I have tested the sample with qemu and the C userspace example
from the doc comments.

Some discussion on Zulip about the motivation and approach [1].
Thanks a lot to everyone helping me out with this.

This patch series is based on the rust-next branch.

Link: https://lore.kernel.org/rust-for-linux/20250511-rust_unsafe_pinned-v4-2-a86c32e47e3d@gmail.com/  [0]
Link: https://rust-for-linux.zulipchat.com/#narrow/channel/288089-General/topic/Passing.20a.20DevRes.20to.20a.20miscdev/near/494553814 [1]

Signed-off-by: Christian Schrefl <chrisi.schrefl@gmail.com>
---
Changes in v5:
- Remove repr(C) and PhantomData (Benno)
- Rename `RegistrationData` to just `Data` (Benno)
- Add bound `Data: Send` bound to `impl Send for
    MiscDeviceRegistration` (Benno)
- Add safety comment about `MiscDeviceRegistration: Send` requirement
- Add Invariants to `MiscDeviceRegistration` (Benno)
- Slightly reword safety comment in drop.
- Removed spurious newline changes in sample (Benno)
- Removed spurious typo fix (Miguel)
- Add Alice's Reviewed-by from v3.
- Link to v4: https://lore.kernel.org/r/20250530-b4-rust_miscdevice_registrationdata-v4-0-d313aafd7e59@gmail.com

Changes in v4:
- Rework to use Opaque instead of `UnsafePinned`.
- Include `impl Wrapper for Opaque` patch.
- Link to v3: https://lore.kernel.org/r/20250517-b4-rust_miscdevice_registrationdata-v3-0-cdb33e228d37@gmail.com

Changes in v3:
- Rebased on top of my `UnsafePinned` series.
- Link to v2: https://lore.kernel.org/r/20250131-b4-rust_miscdevice_registrationdata-v2-0-588f1e6cfabe@gmail.com

Changes in v2:
- Don't use associated_type_bounds since the MSRV does not support
    that on stable yet (Kernel test robot)
- Doc changes and add intra-doc links (Miguel)
- Use container_of macro instead of pointer cast in `fops_open` (Greg)
- Rename `Aliased` to `UnsafePinned` (Boqun)
- Make sure Data is initialized before `misc_register` is called
- Rework the example to use an additional shared value instead of 
    replacing the unique one
- Expanded the c code for the example to use the new ioctls
- Link to v1: https://lore.kernel.org/r/20250119-b4-rust_miscdevice_registrationdata-v1-0-edbf18dde5fc@gmail.com

---
Christian Schrefl (3):
      rust: implement `Wrapper<T>` for `Opaque<T>`
      rust: miscdevice: add additional data to `MiscDeviceRegistration`
      rust: miscdevice: adjust the `rust_misc_device` sample to use `Data`.

 rust/kernel/miscdevice.rs        |  93 +++++++++++++++++++++++--------
 rust/kernel/revocable.rs         |   2 +
 rust/kernel/types.rs             |  25 +++++----
 samples/rust/rust_misc_device.rs | 116 +++++++++++++++++++++++++++++++++++++--
 4 files changed, 195 insertions(+), 41 deletions(-)
---
base-commit: 7a17bbc1d952057898cb0739e60665908fbb8c72
change-id: 20250119-b4-rust_miscdevice_registrationdata-a11d88dcb284

Best regards,
-- 
Christian Schrefl <chrisi.schrefl@gmail.com>

[PATCH v5 1/3] rust: implement `Wrapper<T>` for `Opaque<T>`

[Christian Schrefl]

Moves the implementation for `pin-init` from an associated function
to the trait function of the `Wrapper` trait and extends the
implementation to support pin-initializers with error types.

Adds a use for the `Wrapper` trait in `revocable.rs`, to use the new
`pin-init` function. This is currently the only usage in the kernel.

Reviewed-by: Danilo Krummrich <dakr@kernel.org>
Reviewed-by: Gerald Wisböck <gerald.wisboeck@feather.ink>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Benno Lossin <lossin@kernel.org>
Signed-off-by: Christian Schrefl <chrisi.schrefl@gmail.com>
---
 rust/kernel/revocable.rs |  2 ++
 rust/kernel/types.rs     | 25 +++++++++++++------------
 2 files changed, 15 insertions(+), 12 deletions(-)

diff --git a/rust/kernel/revocable.rs b/rust/kernel/revocable.rs
index 1e5a9d25c21b279b01f90b02997492aa4880d84f..4db68ea2207ebafcc09d082fdc1e281f31846a38 100644
--- a/rust/kernel/revocable.rs
+++ b/rust/kernel/revocable.rs
@@ -5,6 +5,8 @@
 //! The [`Revocable`] type wraps other types and allows access to them to be revoked. The existence
 //! of a [`RevocableGuard`] ensures that objects remain valid.
 
+use pin_init::Wrapper;
+
 use crate::{bindings, prelude::*, sync::rcu, types::Opaque};
 use core::{
     marker::PhantomData,
diff --git a/rust/kernel/types.rs b/rust/kernel/types.rs
index 86562e738eac85480a048041e979335b81c5e3c9..7ab70d5f76099c3442dce5b02c6b226fc74c851e 100644
--- a/rust/kernel/types.rs
+++ b/rust/kernel/types.rs
@@ -9,7 +9,7 @@
     ops::{Deref, DerefMut},
     ptr::NonNull,
 };
-use pin_init::{PinInit, Zeroable};
+use pin_init::{PinInit, Wrapper, Zeroable};
 
 /// Used to transfer ownership to and from foreign (non-Rust) languages.
 ///
@@ -345,17 +345,6 @@ pub const fn uninit() -> Self {
         }
     }
 
-    /// Create an opaque pin-initializer from the given pin-initializer.
-    pub fn pin_init(slot: impl PinInit<T>) -> impl PinInit<Self> {
-        Self::ffi_init(|ptr: *mut T| {
-            // SAFETY:
-            //   - `ptr` is a valid pointer to uninitialized memory,
-            //   - `slot` is not accessed on error; the call is infallible,
-            //   - `slot` is pinned in memory.
-            let _ = unsafe { PinInit::<T>::__pinned_init(slot, ptr) };
-        })
-    }
-
     /// Creates a pin-initializer from the given initializer closure.
     ///
     /// The returned initializer calls the given closure with the pointer to the inner `T` of this
@@ -406,6 +395,18 @@ pub const fn raw_get(this: *const Self) -> *mut T {
         UnsafeCell::raw_get(this.cast::<UnsafeCell<MaybeUninit<T>>>()).cast::<T>()
     }
 }
+impl<T> Wrapper<T> for Opaque<T> {
+    /// Create an opaque pin-initializer from the given pin-initializer.
+    fn pin_init<E>(slot: impl PinInit<T, E>) -> impl PinInit<Self, E> {
+        Self::try_ffi_init(|ptr: *mut T| {
+            // SAFETY:
+            //   - `ptr` is a valid pointer to uninitialized memory,
+            //   - `slot` is not accessed on error,
+            //   - `slot` is pinned in memory.
+            unsafe { PinInit::<T, E>::__pinned_init(slot, ptr) }
+        })
+    }
+}
 
 /// Types that are _always_ reference counted.
 ///

-- 
2.49.0

Re: [PATCH v5 1/3] rust: implement `Wrapper<T>` for `Opaque<T>`

[Miguel Ojeda]

On Sat, Jun 7, 2025 at 2:07 PM Christian Schrefl
<chrisi.schrefl@gmail.com> wrote:
>
> Reviewed-by: Gerald Wisböck <gerald.wisboeck@feather.ink>

Was this a privately-given tag?

Thanks!

Cheers,
Miguel

Re: [PATCH v5 1/3] rust: implement `Wrapper<T>` for `Opaque<T>`

[Christian Schrefl]

On 08.06.25 1:42 PM, Miguel Ojeda wrote:
> On Sat, Jun 7, 2025 at 2:07 PM Christian Schrefl
> <chrisi.schrefl@gmail.com> wrote:
>>
>> Reviewed-by: Gerald Wisböck <gerald.wisboeck@feather.ink>
> 
> Was this a privately-given tag?

Yes, sent it to him before sending it publicly and he gave
it to me there.

Cheers
Christian

[PATCH v5 2/3] rust: miscdevice: add additional data to `MiscDeviceRegistration`

[Christian Schrefl]

When using the Rust miscdevice bindings, you generally embed the
`MiscDeviceRegistration` within another struct:

struct MyDriverData {
    data: SomeOtherData,
    misc: MiscDeviceRegistration<MyMiscFile>
}

In the `fops->open` callback of the miscdevice, you are given a
reference to the registration, which allows you to access its fields.
For example, as of commit 284ae0be4dca ("rust: miscdevice: Provide
accessor to pull out miscdevice::this_device") you can access the
internal `struct device`. However, there is still no way to access the
`data` field in the above example, because you only have a reference to
the registration.

Using `container_of` is also not possible to do safely. For example, if
the destructor of `MyDriverData` runs, then the destructor of `data`
would run before the miscdevice is deregistered, so using `container_of`
to access `data` from `fops->open` could result in a UAF. A similar
problem can happen on initialization if `misc` is not the last field to
be initialized.

To provide a safe way to access user-defined data stored next to the
`struct miscdevice`, make `MiscDeviceRegistration` into a container that
can store a user-provided piece of data. This way, `fops->open` can
access that data via the registration, since the data is stored inside
the registration.

The container enforces that the additional user data is initialized
before the miscdevice is registered, and that the miscdevice is
deregistered before the user data is destroyed. This ensures that access
to the userdata is safe.

For the same reasons as in commit 88441d5c6d17 ("rust: miscdevice:
access the `struct miscdevice` from fops->open()"), you cannot access
the user data in any other fops callback than open. This is because a
miscdevice can be deregistered while there are still open files.

A situation where this user data might be required is when a platform
driver acquires a resource in `probe` and wants to use this resource in
the `fops` implementation of a `MiscDevice`.

This solution is similar to the approach used by the initial downstream
Rust-for-Linux/Rust branch [0].

Link: https://github.com/Rust-for-Linux/linux/blob/rust/rust/kernel/miscdev.rs#L108 [0]
Suggested-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Christian Schrefl <chrisi.schrefl@gmail.com>
---
 rust/kernel/miscdevice.rs        | 93 ++++++++++++++++++++++++++++++----------
 samples/rust/rust_misc_device.rs |  4 +-
 2 files changed, 73 insertions(+), 24 deletions(-)

diff --git a/rust/kernel/miscdevice.rs b/rust/kernel/miscdevice.rs
index b4c5f74de23d6f4fbcdebfe408d6954884609e8f..92b1b39c9728c7f18cc1ea3bd26839664600f9df 100644
--- a/rust/kernel/miscdevice.rs
+++ b/rust/kernel/miscdevice.rs
@@ -9,7 +9,7 @@
 //! Reference: <https://www.kernel.org/doc/html/latest/driver-api/misc_devices.html>
 
 use crate::{
-    bindings,
+    bindings, container_of,
     device::Device,
     error::{to_result, Error, Result, VTABLE_DEFAULT_ERROR},
     ffi::{c_int, c_long, c_uint, c_ulong},
@@ -20,6 +20,7 @@
     types::{ForeignOwnable, Opaque},
 };
 use core::{marker::PhantomData, mem::MaybeUninit, pin::Pin};
+use pin_init::Wrapper;
 
 /// Options for creating a misc device.
 #[derive(Copy, Clone)]
@@ -44,38 +45,55 @@ pub const fn into_raw<T: MiscDevice>(self) -> bindings::miscdevice {
 ///
 /// # Invariants
 ///
-/// `inner` is a registered misc device.
-#[repr(transparent)]
+/// - `inner` is a registered misc device.
+/// - `data` contains a valid `T::Data` for the whole lifetime of [`MiscDeviceRegistration`]
+/// - `data` must be valid until `misc_deregister` (called when dropped) has returned.
+/// - no mutable references to `data` may be created.
 #[pin_data(PinnedDrop)]
-pub struct MiscDeviceRegistration<T> {
+pub struct MiscDeviceRegistration<T: MiscDevice> {
     #[pin]
     inner: Opaque<bindings::miscdevice>,
-    _t: PhantomData<T>,
+    #[pin]
+    data: Opaque<T::Data>,
 }
 
-// SAFETY: It is allowed to call `misc_deregister` on a different thread from where you called
-// `misc_register`.
-unsafe impl<T> Send for MiscDeviceRegistration<T> {}
-// SAFETY: All `&self` methods on this type are written to ensure that it is safe to call them in
-// parallel.
-unsafe impl<T> Sync for MiscDeviceRegistration<T> {}
+// SAFETY:
+// - It is allowed to call `misc_deregister` on a different thread from where you called
+//   `misc_register`.
+// - Only implements `Send` if `MiscDevice::Data` is also `Send`.
+unsafe impl<T: MiscDevice> Send for MiscDeviceRegistration<T> where T::Data: Send {}
+
+// SAFETY:
+// - All `&self` methods on this type are written to ensure that it is safe to call them in
+//   parallel.
+// - Only implements `Sync` if `MiscDevice::Data` is also `Sync`.
+unsafe impl<T: MiscDevice> Sync for MiscDeviceRegistration<T> where T::Data: Sync {}
 
 impl<T: MiscDevice> MiscDeviceRegistration<T> {
     /// Register a misc device.
-    pub fn register(opts: MiscDeviceOptions) -> impl PinInit<Self, Error> {
+    pub fn register(
+        opts: MiscDeviceOptions,
+        data: impl PinInit<T::Data, Error>,
+    ) -> impl PinInit<Self, Error>
+    where
+        Self: Sync,
+    {
         try_pin_init!(Self {
+            data <- Opaque::pin_init(data),
             inner <- Opaque::try_ffi_init(move |slot: *mut bindings::miscdevice| {
                 // SAFETY: The initializer can write to the provided `slot`.
                 unsafe { slot.write(opts.into_raw::<T>()) };
 
-                // SAFETY: We just wrote the misc device options to the slot. The miscdevice will
-                // get unregistered before `slot` is deallocated because the memory is pinned and
-                // the destructor of this type deallocates the memory.
+                // SAFETY:
+                // * We just wrote the misc device options to the slot. The miscdevice will
+                //   get unregistered before `slot` is deallocated because the memory is pinned and
+                //   the destructor of this type deallocates the memory.
+                // * `data` is Initialized before `misc_register` so no race with `fops->open()`
+                //   is possible.
                 // INVARIANT: If this returns `Ok(())`, then the `slot` will contain a registered
                 // misc device.
                 to_result(unsafe { bindings::misc_register(slot) })
             }),
-            _t: PhantomData,
         })
     }
 
@@ -93,13 +111,24 @@ pub fn device(&self) -> &Device {
         // before the underlying `struct miscdevice` is destroyed.
         unsafe { Device::as_ref((*self.as_raw()).this_device) }
     }
+
+    /// Access the additional data stored in this registration.
+    pub fn data(&self) -> &T::Data {
+        // SAFETY:
+        // * No mutable reference to the value contained by `self.data` can ever be created.
+        // * The value contained by `self.data` is valid for the entire lifetime of `&self`.
+        unsafe { &*self.data.get() }
+    }
 }
 
 #[pinned_drop]
-impl<T> PinnedDrop for MiscDeviceRegistration<T> {
+impl<T: MiscDevice> PinnedDrop for MiscDeviceRegistration<T> {
     fn drop(self: Pin<&mut Self>) {
         // SAFETY: We know that the device is registered by the type invariants.
         unsafe { bindings::misc_deregister(self.inner.get()) };
+
+        // SAFETY: `self.data` contains a valid `Data` and does not need to be valid anymore.
+        unsafe { core::ptr::drop_in_place(self.data.get()) };
     }
 }
 
@@ -109,6 +138,13 @@ pub trait MiscDevice: Sized {
     /// What kind of pointer should `Self` be wrapped in.
     type Ptr: ForeignOwnable + Send + Sync;
 
+    /// Additional data carried by the [`MiscDeviceRegistration`] for this [`MiscDevice`].
+    /// If no additional data is required than the unit type `()` should be used.
+    ///
+    /// This can be accessed in [`MiscDevice::open()`] using
+    /// [`MiscDeviceRegistration::data()`].
+    type Data;
+
     /// Called when the misc device is opened.
     ///
     /// The returned pointer will be stored as the private data for the file.
@@ -178,18 +214,29 @@ impl<T: MiscDevice> MiscdeviceVTable<T> {
         // SAFETY: The open call of a file can access the private data.
         let misc_ptr = unsafe { (*raw_file).private_data };
 
-        // SAFETY: This is a miscdevice, so `misc_open()` set the private data to a pointer to the
-        // associated `struct miscdevice` before calling into this method. Furthermore,
-        // `misc_open()` ensures that the miscdevice can't be unregistered and freed during this
-        // call to `fops_open`.
-        let misc = unsafe { &*misc_ptr.cast::<MiscDeviceRegistration<T>>() };
+        // This is a miscdevice, so `misc_open()` sets the private data to a pointer to the
+        // associated `struct miscdevice` before calling into this method.
+        let misc_ptr = misc_ptr.cast::<Opaque<bindings::miscdevice>>();
+
+        // SAFETY:
+        // * `misc_open()` ensures that the `struct miscdevice` can't be unregistered and freed
+        //   during this call to `fops_open`.
+        // * The `misc_ptr` always points to the `inner` field of a `MiscDeviceRegistration<T>`.
+        // * The `MiscDeviceRegistration<T>` is valid until the `struct miscdevice` was
+        //   unregistered.
+        // * `MiscDeviceRegistration<T>` is `Send` since `MiscDeviceRegistration::register` has a
+        //   `Self: Send` bound and is the only way to create a `MiscDeviceRegistration`. This
+        //   means that a reference to it can be shared between contexts.
+        // TODO: add `assert_sync` for `MiscDeviceRegistration<T>` and
+        // `MiscDeviceRegistration<T>::Data`.
+        let registration = unsafe { &*container_of!(misc_ptr, MiscDeviceRegistration<T>, inner) };
 
         // SAFETY:
         // * This underlying file is valid for (much longer than) the duration of `T::open`.
         // * There is no active fdget_pos region on the file on this thread.
         let file = unsafe { File::from_raw_file(raw_file) };
 
-        let ptr = match T::open(file, misc) {
+        let ptr = match T::open(file, registration) {
             Ok(ptr) => ptr,
             Err(err) => return err.to_errno(),
         };
diff --git a/samples/rust/rust_misc_device.rs b/samples/rust/rust_misc_device.rs
index c881fd6dbd08cf4308fe1bd37d11d28374c1f034..c0b912920d6c4b60e747d9d298900ad64df67339 100644
--- a/samples/rust/rust_misc_device.rs
+++ b/samples/rust/rust_misc_device.rs
@@ -137,7 +137,7 @@ fn init(_module: &'static ThisModule) -> impl PinInit<Self, Error> {
         };
 
         try_pin_init!(Self {
-            _miscdev <- MiscDeviceRegistration::register(options),
+            _miscdev <- MiscDeviceRegistration::register(options, ()),
         })
     }
 }
@@ -157,6 +157,8 @@ struct RustMiscDevice {
 impl MiscDevice for RustMiscDevice {
     type Ptr = Pin<KBox<Self>>;
 
+    type Data = ();
+
     fn open(_file: &File, misc: &MiscDeviceRegistration<Self>) -> Result<Pin<KBox<Self>>> {
         let dev = ARef::from(misc.device());
 

-- 
2.49.0

Re: [PATCH v5 2/3] rust: miscdevice: add additional data to `MiscDeviceRegistration`

[Christian Schrefl]

On 07.06.25 2:07 PM, Christian Schrefl wrote:
> @@ -178,18 +214,29 @@ impl<T: MiscDevice> MiscdeviceVTable<T> {>          // SAFETY: The open call of a file can access the private data.
>          let misc_ptr = unsafe { (*raw_file).private_data };
>  
> -        // SAFETY: This is a miscdevice, so `misc_open()` set the private data to a pointer to the
> -        // associated `struct miscdevice` before calling into this method. Furthermore,
> -        // `misc_open()` ensures that the miscdevice can't be unregistered and freed during this
> -        // call to `fops_open`.
> -        let misc = unsafe { &*misc_ptr.cast::<MiscDeviceRegistration<T>>() };
> +        // This is a miscdevice, so `misc_open()` sets the private data to a pointer to the
> +        // associated `struct miscdevice` before calling into this method.
> +        let misc_ptr = misc_ptr.cast::<Opaque<bindings::miscdevice>>();
> +
> +        // SAFETY:
> +        // * `misc_open()` ensures that the `struct miscdevice` can't be unregistered and freed
> +        //   during this call to `fops_open`.
> +        // * The `misc_ptr` always points to the `inner` field of a `MiscDeviceRegistration<T>`.
> +        // * The `MiscDeviceRegistration<T>` is valid until the `struct miscdevice` was
> +        //   unregistered.
> +        // * `MiscDeviceRegistration<T>` is `Send` since `MiscDeviceRegistration::register` has a
> +        //   `Self: Send` bound and is the only way to create a `MiscDeviceRegistration`. This
> +        //   means that a reference to it can be shared between contexts.
> +        // TODO: add `assert_sync` for `MiscDeviceRegistration<T>` and
> +        // `MiscDeviceRegistration<T>::Data`.

After trying this out it this needs `: Sync` bounds on this impl block and
the `MiscDeviceOptions::into_raw` in addition to the register function.
That's not great, but could be worse.

> +        let registration = unsafe { &*container_of!(misc_ptr, MiscDeviceRegistration<T>, inner) };
>  
>          // SAFETY:
>          // * This underlying file is valid for (much longer than) the duration of `T::open`.
>          // * There is no active fdget_pos region on the file on this thread.
>          let file = unsafe { File::from_raw_file(raw_file) };
>  
> -        let ptr = match T::open(file, misc) {
> +        let ptr = match T::open(file, registration) {
>              Ok(ptr) => ptr,
>              Err(err) => return err.to_errno(),
>          };

[PATCH v5 3/3] rust: miscdevice: adjust the `rust_misc_device` sample to use `Data`.

[Christian Schrefl]

Add a second mutex to the `RustMiscDevice``, which is shared between all
instances of the device using an `Arc` and the `Data` of
`MiscDeviceRegistration`.

This is mostly to demonstrate the capability to share data in this way.

Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Christian Schrefl <chrisi.schrefl@gmail.com>
---
 samples/rust/rust_misc_device.rs | 116 ++++++++++++++++++++++++++++++++++++---
 1 file changed, 109 insertions(+), 7 deletions(-)

diff --git a/samples/rust/rust_misc_device.rs b/samples/rust/rust_misc_device.rs
index c0b912920d6c4b60e747d9d298900ad64df67339..7519ff6a79985e8bdbb7f4c79d8a6ebf160ef8cc 100644
--- a/samples/rust/rust_misc_device.rs
+++ b/samples/rust/rust_misc_device.rs
@@ -18,6 +18,8 @@
 //! #define RUST_MISC_DEV_HELLO _IO('|', 0x80)
 //! #define RUST_MISC_DEV_GET_VALUE _IOR('|', 0x81, int)
 //! #define RUST_MISC_DEV_SET_VALUE _IOW('|', 0x82, int)
+//! #define RUST_MISC_DEV_GET_SHARED_VALUE _IOR('|', 0x83, int)
+//! #define RUST_MISC_DEV_SET_SHARED_VALUE _IOW('|', 0x84, int)
 //!
 //! int main() {
 //!   int value, new_value;
@@ -86,6 +88,62 @@
 //!     return -1;
 //!   }
 //!
+//!   value++;
+//!
+//!   // Set shared value to something different
+//!   printf("Submitting new shared value (%d)\n", value);
+//!   ret = ioctl(fd, RUST_MISC_DEV_SET_SHARED_VALUE, &value);
+//!   if (ret < 0) {
+//!     perror("ioctl: Failed to submit new value");
+//!     close(fd);
+//!     return errno;
+//!   }
+//!
+//!   // Close the device file
+//!   printf("Closing /dev/rust-misc-device\n");
+//!   close(fd);
+//!
+//!   // Open the device file again
+//!   printf("Opening /dev/rust-misc-device again for reading\n");
+//!   fd = open("/dev/rust-misc-device", O_RDWR);
+//!   if (fd < 0) {
+//!     perror("open");
+//!     return errno;
+//!   }
+//!
+//!   // Ensure new value was applied
+//!   printf("Fetching new value\n");
+//!   ret = ioctl(fd, RUST_MISC_DEV_GET_SHARED_VALUE, &new_value);
+//!   if (ret < 0) {
+//!     perror("ioctl: Failed to fetch the new value");
+//!     close(fd);
+//!     return errno;
+//!   }
+//!
+//!   if (value != new_value) {
+//!     printf("Failed: Committed and retrieved values are different (%d - %d)\n",
+//!            value, new_value);
+//!     close(fd);
+//!     return -1;
+//!   }
+//!
+//!   value = 0;
+//!   // Ensure non-shared value is still 0
+//!   printf("Fetching new value\n");
+//!   ret = ioctl(fd, RUST_MISC_DEV_GET_VALUE, &new_value);
+//!   if (ret < 0) {
+//!     perror("ioctl: Failed to fetch the new value");
+//!     close(fd);
+//!     return errno;
+//!   }
+//!
+//!   if (value != new_value) {
+//!     printf("Failed: Committed and retrieved values are different (%d - %d)\n",
+//!            value, new_value);
+//!     close(fd);
+//!     return -1;
+//!   }
+//!
 //!   // Close the device file
 //!   printf("Closing /dev/rust-misc-device\n");
 //!   close(fd);
@@ -105,7 +163,7 @@
     miscdevice::{MiscDevice, MiscDeviceOptions, MiscDeviceRegistration},
     new_mutex,
     prelude::*,
-    sync::Mutex,
+    sync::{Arc, Mutex},
     types::ARef,
     uaccess::{UserSlice, UserSliceReader, UserSliceWriter},
 };
@@ -113,6 +171,8 @@
 const RUST_MISC_DEV_HELLO: u32 = _IO('|' as u32, 0x80);
 const RUST_MISC_DEV_GET_VALUE: u32 = _IOR::<i32>('|' as u32, 0x81);
 const RUST_MISC_DEV_SET_VALUE: u32 = _IOW::<i32>('|' as u32, 0x82);
+const RUST_MISC_DEV_GET_SHARED_VALUE: u32 = _IOR::<i32>('|' as u32, 0x83);
+const RUST_MISC_DEV_SET_SHARED_VALUE: u32 = _IOW::<i32>('|' as u32, 0x84);
 
 module! {
     type: RustMiscDeviceModule,
@@ -137,7 +197,10 @@ fn init(_module: &'static ThisModule) -> impl PinInit<Self, Error> {
         };
 
         try_pin_init!(Self {
-            _miscdev <- MiscDeviceRegistration::register(options, ()),
+            _miscdev <- MiscDeviceRegistration::register(
+                options,
+                Arc::pin_init(new_mutex!(Inner { value: 0_i32 }), GFP_KERNEL)?
+            ),
         })
     }
 }
@@ -148,8 +211,9 @@ struct Inner {
 
 #[pin_data(PinnedDrop)]
 struct RustMiscDevice {
+    shared: Arc<Mutex<Inner>>,
     #[pin]
-    inner: Mutex<Inner>,
+    unique: Mutex<Inner>,
     dev: ARef<Device>,
 }
 
@@ -157,7 +221,7 @@ struct RustMiscDevice {
 impl MiscDevice for RustMiscDevice {
     type Ptr = Pin<KBox<Self>>;
 
-    type Data = ();
+    type Data = Arc<Mutex<Inner>>;
 
     fn open(_file: &File, misc: &MiscDeviceRegistration<Self>) -> Result<Pin<KBox<Self>>> {
         let dev = ARef::from(misc.device());
@@ -167,7 +231,8 @@ fn open(_file: &File, misc: &MiscDeviceRegistration<Self>) -> Result<Pin<KBox<Se
         KBox::try_pin_init(
             try_pin_init! {
                 RustMiscDevice {
-                    inner <- new_mutex!( Inner{ value: 0_i32 } ),
+                    shared: misc.data().clone(),
+                    unique <- new_mutex!(Inner { value: 0_i32 }),
                     dev: dev,
                 }
             },
@@ -183,6 +248,12 @@ fn ioctl(me: Pin<&RustMiscDevice>, _file: &File, cmd: u32, arg: usize) -> Result
         match cmd {
             RUST_MISC_DEV_GET_VALUE => me.get_value(UserSlice::new(arg, size).writer())?,
             RUST_MISC_DEV_SET_VALUE => me.set_value(UserSlice::new(arg, size).reader())?,
+            RUST_MISC_DEV_GET_SHARED_VALUE => {
+                me.get_shared_value(UserSlice::new(arg, size).writer())?
+            }
+            RUST_MISC_DEV_SET_SHARED_VALUE => {
+                me.set_shared_value(UserSlice::new(arg, size).reader())?
+            }
             RUST_MISC_DEV_HELLO => me.hello()?,
             _ => {
                 dev_err!(me.dev, "-> IOCTL not recognised: {}\n", cmd);
@@ -204,7 +275,7 @@ fn drop(self: Pin<&mut Self>) {
 impl RustMiscDevice {
     fn set_value(&self, mut reader: UserSliceReader) -> Result<isize> {
         let new_value = reader.read::<i32>()?;
-        let mut guard = self.inner.lock();
+        let mut guard = self.unique.lock();
 
         dev_info!(
             self.dev,
@@ -217,7 +288,38 @@ fn set_value(&self, mut reader: UserSliceReader) -> Result<isize> {
     }
 
     fn get_value(&self, mut writer: UserSliceWriter) -> Result<isize> {
-        let guard = self.inner.lock();
+        let guard = self.unique.lock();
+        let value = guard.value;
+
+        // Free-up the lock and use our locally cached instance from here
+        drop(guard);
+
+        dev_info!(
+            self.dev,
+            "-> Copying data to userspace (value: {})\n",
+            &value
+        );
+
+        writer.write::<i32>(&value)?;
+        Ok(0)
+    }
+
+    fn set_shared_value(&self, mut reader: UserSliceReader) -> Result<isize> {
+        let new_value = reader.read::<i32>()?;
+        let mut guard = self.shared.lock();
+
+        dev_info!(
+            self.dev,
+            "-> Copying data from userspace (value: {})\n",
+            new_value
+        );
+
+        guard.value = new_value;
+        Ok(0)
+    }
+
+    fn get_shared_value(&self, mut writer: UserSliceWriter) -> Result<isize> {
+        let guard = self.shared.lock();
         let value = guard.value;
 
         // Free-up the lock and use our locally cached instance from here

-- 
2.49.0

Re: [PATCH v5 0/3] rust: miscdevice: add additional data to MiscDeviceRegistration

[Danilo Krummrich]

Greg,

On Sat, Jun 07, 2025 at 02:07:29PM +0200, Christian Schrefl wrote:
> Christian Schrefl (3):
>       rust: implement `Wrapper<T>` for `Opaque<T>`

Just a heads-up, this patch is also required for a patch series with some
improvements for devres (sent out soon), which itself in turn will be needed for
the next version of the series adding device driver support for misc device [1].

Unless there's a reason not to do so, we hence probably want to take the misc
device patches through the driver-core tree. A subsequent conflict with the
char-misc tree seems extremely unlikely.

[1] https://lore.kernel.org/lkml/20250530142447.166524-1-dakr@kernel.org/