linux-kernel - Re: [PATCH v2] rust: irq: add &Device<Bound> argument to irq callbacks

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Message-ID: <CAH5fLghitfmSOByu4ZRmhwdsOadzJOLei_qrAjNM+V15spq44w@mail.gmail.com>
Date: Mon, 11 Aug 2025 16:05:31 +0200
From: Alice Ryhl <aliceryhl@...gle.com>
To: Boqun Feng <boqun.feng@...il.com>
Cc: Daniel Almeida <daniel.almeida@...labora.com>, Miguel Ojeda <ojeda@...nel.org>, 
	Gary Guo <gary@...yguo.net>, Björn Roy Baron <bjorn3_gh@...tonmail.com>, 
	Andreas Hindborg <a.hindborg@...nel.org>, Trevor Gross <tmgross@...ch.edu>, 
	Danilo Krummrich <dakr@...nel.org>, Greg Kroah-Hartman <gregkh@...uxfoundation.org>, 
	"Rafael J. Wysocki" <rafael@...nel.org>, Thomas Gleixner <tglx@...utronix.de>, 
	Bjorn Helgaas <bhelgaas@...gle.com>, Krzysztof Wilczy´nski <kwilczynski@...nel.org>, 
	Benno Lossin <lossin@...nel.org>, Dirk Behme <dirk.behme@...il.com>, linux-kernel@...r.kernel.org, 
	rust-for-linux@...r.kernel.org, linux-pci@...r.kernel.org
Subject: Re: [PATCH v2] rust: irq: add &Device<Bound> argument to irq callbacks

On Mon, Aug 11, 2025 at 3:56 PM Boqun Feng <boqun.feng@...il.com> wrote:
>
> On Mon, Aug 11, 2025 at 12:33:51PM +0000, Alice Ryhl wrote:
> [...]
> > @@ -207,8 +207,8 @@ pub fn new<'a>(
> >              inner <- Devres::new(
> >                  request.dev,
> >                  try_pin_init!(RegistrationInner {
> > -                    // SAFETY: `this` is a valid pointer to the `Registration` instance
> > -                    cookie: unsafe { &raw mut (*this.as_ptr()).handler }.cast(),
> > +                    // INVARIANT: `this` is a valid pointer to the `Registration` instance
> > +                    cookie: this.as_ptr().cast::<c_void>(),
>
> At this moment the `Regstration` is not fully initialized...
>
> >                      irq: {
> >                          // SAFETY:
> >                          // - The callbacks are valid for use with request_irq.
> > @@ -221,7 +221,7 @@ pub fn new<'a>(
> >                                  Some(handle_irq_callback::<T>),
> >                                  flags.into_inner(),
> >                                  name.as_char_ptr(),
> > -                                (&raw mut (*this.as_ptr()).handler).cast(),
> > +                                this.as_ptr().cast::<c_void>(),
> >                              )
>
> ... and interrupt can happen right after request_irq() ...
>
> >                          })?;
> >                          request.irq
> > @@ -258,9 +258,13 @@ pub fn synchronize(&self, dev: &Device<Bound>) -> Result {
> >  ///
> >  /// This function should be only used as the callback in `request_irq`.
> >  unsafe extern "C" fn handle_irq_callback<T: Handler>(_irq: i32, ptr: *mut c_void) -> c_uint {
> > -    // SAFETY: `ptr` is a pointer to T set in `Registration::new`
> > -    let handler = unsafe { &*(ptr as *const T) };
> > -    T::handle(handler) as c_uint
> > +    // SAFETY: `ptr` is a pointer to `Registration<T>` set in `Registration::new`
> > +    let registration = unsafe { &*(ptr as *const Registration<T>) };
>
> ... hence it's not correct to construct a reference to `Registration`
> here, but yes, both `handler` and the `device` part of `inner` has been
> properly initialized. So
>
>         let registration = ptr.cast::<Registration<T>>();
>
>         // SAFETY: The `data` part of `Devres` is `Opaque` and here we
>         // only access `.device()`, which has been properly initialized
>         // before `request_irq()`.
>         let device = unsafe { (*registration).inner.device() };
>
>         // SAFETY: The irq callback is removed before the device is
>         // unbound, so the fact that the irq callback is running implies
>         // that the device has not yet been unbound.
>         let device = unsafe { device.as_bound() };
>
>         // SAFETY: `.handler` has been properly initialized before
>         // `request_irq()`.
>         T::handle(unsafe { &(*registration).handler }, device) as c_uint
>
> Thoughts? Similar for the threaded one.

This code is no different. It creates a reference to `inner` before
the `irq` field is written. Of course, it's also no different in that
since data of a `Devres` is in `Opaque`, this is not actually UB.

What I can offer you is to use the closure form of pin-init to call
request_irq after initialization has fully completed.

Alice