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Thu, 05 Jun 2025 09:52:06 -0700 (PDT) Message-ID: <89066f83-db7f-405c-b3b5-ce553f8e6b48@gmail.com> Date: Thu, 5 Jun 2025 18:52:05 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v4 2/3] rust: miscdevice: add additional data to MiscDeviceRegistration To: Benno Lossin , Miguel Ojeda , Danilo Krummrich , Alex Gaynor , Boqun Feng , Gary Guo , =?UTF-8?Q?Bj=C3=B6rn_Roy_Baron?= , Andreas Hindborg , Alice Ryhl , Trevor Gross , Arnd Bergmann , Greg Kroah-Hartman , Lee Jones , Daniel Almeida Cc: =?UTF-8?Q?Gerald_Wisb=C3=B6ck?= , rust-for-linux@vger.kernel.org, linux-kernel@vger.kernel.org References: <20250530-b4-rust_miscdevice_registrationdata-v4-0-d313aafd7e59@gmail.com> <20250530-b4-rust_miscdevice_registrationdata-v4-2-d313aafd7e59@gmail.com> <3eef5777-9190-4782-8433-7b6ad4b9acd3@gmail.com> <3c1c0563-7f48-4222-a28d-316f885bcad4@gmail.com> Content-Language: en-US, de-DE From: Christian Schrefl In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 05.06.25 6:05 PM, Benno Lossin wrote: > On Thu Jun 5, 2025 at 4:57 PM CEST, Christian Schrefl wrote: >> On 04.06.25 1:29 AM, Benno Lossin wrote: >>> On Mon Jun 2, 2025 at 11:16 PM CEST, Christian Schrefl wrote: >>>> On 31.05.25 2:23 PM, Benno Lossin wrote: >>>>> On Fri May 30, 2025 at 10:46 PM CEST, Christian Schrefl wrote: >>>>>> +// SAFETY: >>>>>> +// - All `&self` methods on this type are written to ensure that it is safe to call them in >>>>>> +// parallel. >>>>>> +// - `MiscDevice::RegistrationData` is always `Sync`. >>>>>> +unsafe impl Sync for MiscDeviceRegistration {} >>>>> >>>>> I would feel better if we still add the `T::RegistrationData: Sync` >>>>> bound here even if it is vacuous today. >>>> >>>> Since a reference the `MiscDeviceRegistration` struct is an >>>> argument to the open function this struct must always be Sync, >>>> so adding bounds here doesn't make much sense. >>> >>> Well yes, but this statement makes `MiscDeviceRegistration` be `Sync` >>> even if `T::RegistrationData` is not `Sync` if that bound got removed >>> at some point. And this "instability" is what I'm worried about. >>> >>>> I'll add this a safety comment in `MiscdeviceVTable::open` >>>> about this. >>>> >>>> Is there a good way to assert this at build to avoid regessions? >>> >>> const _: () = { >>> fn assert_sync() {} >>> fn ctx() { >>> assert_sync::(); >>> } >>> }; >>> >> >> I'll add the bound and a TODO about `assert_sync`, in `open` >> where `Send` is required. >> >> I intend to write a patch for `assert_sync` later. > > Great :) > >>> That would also be fine with me if you insist on not adding the bound. >>> >>> (the `assert_sync` function should maybe be somewhere where everyone can >>> use it) >>> >>>>>> impl MiscDeviceRegistration { >>>>>> /// Register a misc device. >>>>>> - pub fn register(opts: MiscDeviceOptions) -> impl PinInit { >>>>>> + pub fn register( >>>>>> + opts: MiscDeviceOptions, >>>>>> + data: impl PinInit, >>>>>> + ) -> impl PinInit { >>>>>> 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::()) }; >>>>>> >>>>>> - // 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) }) >>>>>> @@ -93,13 +108,24 @@ 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::RegistrationData { >>>>>> + // 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`. >>>>> >>>>> Please add type invariants for these two requirements. >>>>> >>>>>> + unsafe { &*self.data.get() } >>>>>> + } >>>>>> } >>>>>> >>>>>> #[pinned_drop] >>>>>> -impl PinnedDrop for MiscDeviceRegistration { >>>>>> +impl PinnedDrop for MiscDeviceRegistration { >>>>>> 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` is valid for dropping and nothing uses it anymore. >>>>> >>>>> Ditto. >>>> >>>> I'm not quite sure how to formulate these, what do you think of: >>>> >>>> /// - `inner` is a registered misc device. >>> >>> This doesn't really mean something to me, maybe it's better to reference >>> the registering function? >> >> That is from previous code so this should probably not be changed >> in this series. > > I personally wouldn't mind a commit that fixes this up, but if you don't > want to do it, let me know then we can make this a good-first-issue. I can do it, but I think it would make a good-first-issue so lets go with that for now. > >>>> /// - `data` contains a valid `T::RegistrationData` for the whole lifetime of [`MiscDeviceRegistration`] >>> >>> This sounds good. But help me understand, why do we need `Opaque` / >>> `UnsafePinned` again? If we're only using shared references, then we >>> could also just store the object by value? >> >> Since the Module owns the `MiscDeviceRegistration` it may create `&mut MiscDeviceRegistration`, >> so from what I understand having a `& RegistrationData` reference into that is UB without >> `UnsafePinned` (or `Opaque` since that includes `UnsafePinned` semantics). > > And the stored `T::RegistrationData` is shared as read-only with the C > side? Yes in that case we want `UnsafePinned>` (or for the > moment `Opaque`). Not really shared with the C side, but with the `open` implementation in `MiscDevice` that is (indirectly) called by C. (`UnsafeCell` will probably not be needed, as `UnsafePinned` will almost certainly have `UnsafeCell` semantics in upstream). Thinking about this has made me realize that the current code already is a bit iffy, since `MiscDevice::open` gets `&MiscDeviceRegistration` as an argument. (It should be fine since `UnsafeCell` and `UnsafePinned` semantics also apply to "parrent" types i.e. `&MiscDeviceRegistration` also has the semantics of `Opaque`). > >>>> /// - `data` must be usable until `misc_deregister` (called when dropped) has returned. >>> >>> What does "usable" mean? >> >> I guess valid / alive might be better wording? >> >> I meant to say that the `fops` functions might use the `RegistrationData` until >> `misc_deregister` has returned so we must ensure that these accesses are allowed. > > Then use `valid`. Alright. > >>>> /// - no mutable references to `data` may be created. >>> >>>>>> + unsafe { core::ptr::drop_in_place(self.data.get()) }; >>>>>> } >>>>>> } >>>>>> >>>>>> @@ -109,6 +135,13 @@ pub trait MiscDevice: Sized { >>>>>> /// What kind of pointer should `Self` be wrapped in. >>>>>> type Ptr: ForeignOwnable + Send + Sync; >>>>>> >>>>>> + /// The additional data carried by the [`MiscDeviceRegistration`] for this [`MiscDevice`]. >>>>>> + /// If no additional data is required than the unit type `()` should be used. >>>>>> + /// >>>>>> + /// This data can be accessed in [`MiscDevice::open()`] using >>>>>> + /// [`MiscDeviceRegistration::data()`]. >>>>>> + type RegistrationData: Sync; >>>>> >>>>> Why do we require `Sync` here? >>>> >>>> Needed for `MiscDeviceRegistration` to be `Send`, see response above. >>> >>> You could also just ask the type there to be `Sync`, then users will get >>> an error when they try to use `MiscDevice` in a way where >>> `RegistrationData` is required to be `Sync`. >> >> I don't think there is any point to allow defining a `MiscDevice` implementation >> that cant actually be used/registered. > > Sure, but the bound asserting that it is `Sync` doesn't need to be here, > having it just on the `impl Sync for MiscDeviceRegistration` is good > enough. (though one could argue that people would get an earlier error > if it is already asserted here. I think we should have some general > guidelines here :) That would require a `Send` bound in the `register` function, since a `MiscDevice` with `!Sync` `Data` would be valid now (meaning that `MiscDeviceRegistration` may also be `!Sync`). If you want I can go with that. I'm not really sure if its really better (tough I don't feel that strongly either way). > >>>>> We might want to give this a shorter name? >>>> >>>> I think its fine, but I am open to Ideas. >>> >>> `Data`? >> >> I feel that `Data` is just very ambiguous, especially since it is associated with >> `MiscDevice` not the `MiscDeviceRegistration` in which its used. > > But it is the data of the MiscDevice, no? > >> One Idea I've had was `AssociatedData` but that's less clear and not much shorter >> than `RegistrationData`. > > Of the two, I'd prefer `RegistrationData`. > >> But I'd be alright to just with `Data` if that is wanted. > > If you think that `RegistrationData` is more clear then go with that. > But I honestly don't derive much meaning from that over just `Data`. You > can still of course mention in the docs that this data is stored in the > registration. > > But since there is no other way to associate data to a `MiscDevice`, I > think it makes sense to call it `Data`. > Alright I'll go with `Data` then. > --- > Cheers, > Benno