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Message-ID: <20230725115759.424300-1-aliceryhl@google.com>
Date: Tue, 25 Jul 2023 11:57:59 +0000
From: Alice Ryhl <aliceryhl@...gle.com>
To: benno.lossin@...ton.me
Cc: alex.gaynor@...il.com, aliceryhl@...gle.com,
bjorn3_gh@...tonmail.com, boqun.feng@...il.com, gary@...yguo.net,
linux-kernel@...r.kernel.org, nmi@...aspace.dk, ojeda@...nel.org,
rust-for-linux@...r.kernel.org, wedsonaf@...il.com
Subject: Re: [PATCH v2 09/12] rust: init: implement Zeroable for Opaque<T>
Benno Lossin <benno.lossin@...ton.me> writes:
> On 20.07.23 15:34, Alice Ryhl wrote:
>> Benno Lossin <benno.lossin@...ton.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@...ton.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
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