Re: [RFC PATCH V2 1/8] rust: Add initial bindings for OPP framework

[Manos Pitsidianakis]

On Fri, 07 Jun 2024 13:51, Alice Ryhl <aliceryhl@xxxxxxxxxx> wrote:
> On Fri, Jun 7, 2024 at 11:12 AM Viresh Kumar <viresh.kumar@xxxxxxxxxx> wrote:
> > This commit adds initial Rust bindings for the Operating performance
> > points (OPP) core. This adds bindings for `struct dev_pm_opp` and
> > `struct dev_pm_opp_data` to begin with.
> >
> > Signed-off-by: Viresh Kumar <viresh.kumar@xxxxxxxxxx>
>
> > +//! Operating performance points.
> > +//!
> > +//! This module provides bindings for interacting with the OPP subsystem.
> > +//!
> > +//! C header: [`include/linux/pm_opp.h`](../../../../../../include/linux/pm_opp.h)
>
> Please use srctree links instead.
>
> C header: [`include/linux/pm_opp.h`](srctree/include/linux/pm_opp.h)
>
> > +impl OPP {
> > +    /// Creates a reference to a [`OPP`] from a valid pointer.
> > +    ///
> > +    /// # Safety
> > +    ///
> > +    /// The caller must ensure that `ptr` is valid and remains valid for the lifetime of the
> > +    /// returned [`OPP`] reference.
> > +    pub unsafe fn from_ptr_owned(ptr: *mut bindings::dev_pm_opp) -> Result<ARef<Self>> {
> > +        let ptr = ptr::NonNull::new(ptr).ok_or(ENODEV)?;
> > +
> > +        // SAFETY: The safety requirements guarantee the validity of the pointer.
> > +        //
> > +        // INVARIANT: The refcount is already incremented by the C API that returned the pointer,
> > +        // and we pass ownership of the refcount to the new `ARef<OPP>`.
> > +        Ok(unsafe { ARef::from_raw(ptr.cast()) })
> > +    }
> > +
> > +    /// Creates a reference to a [`OPP`] from a valid pointer.
> > +    ///
> > +    /// # Safety
> > +    ///
> > +    /// The caller must ensure that `ptr` is valid and remains valid for the lifetime of the
> > +    /// returned [`OPP`] reference.
> > +    pub unsafe fn from_ptr(ptr: *mut bindings::dev_pm_opp) -> Result<ARef<Self>> {
> > +        let opp = unsafe { Self::from_ptr_owned(ptr) }?;
> > +
> > +        // Take an extra reference to the OPP since the caller didn't take it.
> > +        opp.inc_ref();
> > +
> > +        Ok(opp)
> > +    }
>
> I would recommend a slightly different approach here. You can provide
> a method called `from_raw_opp` that takes a *mut bindings::dev_pm_opp
> and returns a &Self. The ARef type provides a method that converts
> &Self to ARef<Self> by taking a refcount. This way, users would also
> be able to call OPP methods without giving Rust any refcounts. You can

Wouldn't this allow for use-after-free? What if the refcount drops to 0 
before the method is called?

> As for `from_ptr_owned`, I would probably rename it to
> `from_raw_opp_owned` or similar. It's often nice to use a more
> descriptive name than just "ptr".
> I think most existing examples call this `as_raw` and mark it 
> `#[inline]`.

I think `ptr` is more idiomatic to Rust users, not that your suggestion 
is wrong. from_ptr_owned also implies the function signature.


> > +    /// Adds an OPP dynamically.
> > +    pub fn add(dev: ARef<Device>, mut data: Data) -> Result<()> {
> > +        // SAFETY: The requirements are satisfied by the existence of `Device` and its safety
> > +        // requirements.
> > +        to_result(unsafe { bindings::dev_pm_opp_add_dynamic(dev.as_raw(), &mut data.0) })
> > +    }
> > +
> > +    /// Removes a dynamically added OPP.
> > +    pub fn remove(dev: ARef<Device>, freq: u64) {
> > +        // SAFETY: The requirements are satisfied by the existence of `Device` and its safety
> > +        // requirements.
> > +        unsafe { bindings::dev_pm_opp_remove(dev.as_raw(), freq) };
> > +    }
>
> Is it intentional that these methods take ownership of a refcount to
> the device that it then drops after calling the C function?

use-after-free again? Though I'm suggesting this without actually 
examining if it can happen.