Re: [PATCH 1/3] rust: revocable: implement Revocable::access()

[Benno Lossin <benno.lossin@proton.me>]

On Sat Apr 26, 2025 at 11:18 PM CEST, Danilo Krummrich wrote:
> On Sat, Apr 26, 2025 at 08:24:14PM +0000, Benno Lossin wrote:
>> On Sat Apr 26, 2025 at 3:30 PM CEST, Danilo Krummrich wrote:
>> > Implement an unsafe direct accessor for the data stored within the
>> > Revocable.
>> >
>> > This is useful for cases where we can proof that the data stored within
>> > the Revocable is not and cannot be revoked for the duration of the
>> > lifetime of the returned reference.
>> >
>> > Signed-off-by: Danilo Krummrich <dakr@kernel.org>
>> > ---
>> > The explicit lifetimes in access() probably don't serve a practical
>> > purpose, but I found them to be useful for documentation purposes.
>> > ---
>> >  rust/kernel/revocable.rs | 12 ++++++++++++
>> >  1 file changed, 12 insertions(+)
>> >
>> > diff --git a/rust/kernel/revocable.rs b/rust/kernel/revocable.rs
>> > index 971d0dc38d83..33535de141ce 100644
>> > --- a/rust/kernel/revocable.rs
>> > +++ b/rust/kernel/revocable.rs
>> > @@ -139,6 +139,18 @@ pub fn try_access_with<R, F: FnOnce(&T) -> R>(&self, f: F) -> Option<R> {
>> >          self.try_access().map(|t| f(&*t))
>> >      }
>> >  
>> > +    /// Directly access the revocable wrapped object.
>> > +    ///
>> > +    /// # Safety
>> > +    ///
>> > +    /// The caller must ensure this [`Revocable`] instance hasn't been revoked and won't be revoked
>> > +    /// for the duration of `'a`.
>> 
>> Ah I missed this in my other email, in case you want to directly refer
>> to the lifetime, you should keep it defined. I would still remove the
>> `'s` lifetime though.
>> > +    pub unsafe fn access<'a, 's: 'a>(&'s self) -> &'a T {
>> > +        // SAFETY: By the safety requirement of this function it is guaranteed that
>> > +        // `self.data.get()` is a valid pointer to an instance of `T`.
>> 
>> I don't see how the "not-being revoked" state makes the `data` ptr be
>> valid. Is that an invariant of `Revocable`? (it's not documented to have
>> any invariants)
>
> What else makes it valid?

IMO an `# Invariants` section with the corresponding invariant that
`data` is valid when `is_available` is true.

> AFAICS, try_access() and try_access_with_guard() argue the exact same way,
> except that the reason for not being revoked is the atomic check and the RCU
> read lock.

Just because other code is doing the same mistake doesn't make it
correct. If I had reviewed the patch at that time I'm sure I would have
pointed this out.

I opened an issue about this:

    https://github.com/Rust-for-Linux/linux/issues/1160

Feel free to comment any additional information.

---
Cheers,
Benno