Re: [PATCH v2 09/12] rust: init: implement Zeroable for Opaque<T>

[Alice Ryhl <aliceryhl@google.com>]

Benno Lossin <benno.lossin@proton.me> writes:
> On 20.07.23 15:34, Alice Ryhl wrote:
>> Benno Lossin <benno.lossin@proton.me> writes:
>>> Since `Opaque<T>` contains a `MaybeUninit<T>`, all bytes zero is a valid
>>> bit pattern for that type.
>>>
>>> Signed-off-by: Benno Lossin <benno.lossin@proton.me>
>>> ---
>>>   ///
>>>   /// This is meant to be used with FFI objects that are never interpreted by Rust code.
>>>   #[repr(transparent)]
>>> +#[derive(Zeroable)]
>>>   pub struct Opaque<T> {
>>>       value: UnsafeCell<MaybeUninit<T>>,
>>>       _pin: PhantomPinned,
>>>   }
>> 
>> Does this actually work? I don't think we implement Zeroable for
>> UnsafeCell.
> 
> Good catch, this does compile, but only because the current
> implementation of the derive expands to (modulo correct paths):
> ```
> impl<T> Zeroable for Opaque<T>
> where
>      UnsafeCell<MaybeUninit<T>>: Zeroable,
>      PhantomPinned: Zeroable,
> {}
> ```
> This implementation is of course useless, since `UnsafeCell` is never
> `Zeroable` at the moment. We could of course implement that and then this
> should work, but the question is if this is actually the desired output in
> general. I thought before that this would be a good idea, but I forgot that
> if the bounds are never satisfied it would silently compile.
> 
> Do you think that we should have this expanded output instead?
> ```
> impl<T: Zeroable> Zeroable for Foo<T> {}
> const _: () = {
>      fn assert_zeroable<T: Zeroable>() {}
>      fn ensure_zeroable<T: Zeroable>() {
>          assert_zeroable::<Field1>();
>          assert_zeroable::<Field2>();
>      }
> };
> ```
> If the input was
> ```
> #[derive(Zeroable)]
> struct Foo<T> {
>      field1: Field1,
>      field2: Field2,
> }
> ```
 
Yeah. The way that these macros usually expand is by adding `where T:
Zeroable` to the impl for each generic parameter, and failing
compilation if that is not enough to ensure that all of the fields are
`Zeroable`.

You might want to consider this expansion instead:
```
impl<T: Zeroable> Zeroable for Foo<T> {}
const _: () = {
     fn assert_zeroable<T: Zeroable>(arg: &T) {}
     fn ensure_zeroable<T: Zeroable>(arg: &Foo<T>) {
         assert_zeroable(&arg.field1);
         assert_zeroable(&arg.field2);
     }
};
```

>> I suggest you instead add Opaque to the `impl_zeroable!` macro in
>> `rust/kernel/init.rs`.
> 
> We would have to do this when using the alternative approach from
> above, since we do not want the `Zeroable` bound on `T` for `Opaque`.
> I wanted to avoid the `SAFETY` comment, since that is needed for the
> `impl_zeroable` macro (as it has an unsafe block inside).

Indeed, if we expand the derive macro in the standard way, then it
doesn't work for `Opaque` due to the extra unnecessary bound.

Alice