linux-kernel - Re: [PATCH v3 09/10] rust: list: support heterogeneous lists

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Message-ID: <CAH5fLghkPgj560a2b_1oRnvuurEugT4TNj+o-fYhGsuSGpywPg@mail.gmail.com>
Date: Thu, 1 Aug 2024 11:38:50 +0200
From: Alice Ryhl <aliceryhl@...gle.com>
To: Benno Lossin <benno.lossin@...ton.me>
Cc: Miguel Ojeda <ojeda@...nel.org>, Andrew Morton <akpm@...ux-foundation.org>, 
	Alex Gaynor <alex.gaynor@...il.com>, Wedson Almeida Filho <wedsonaf@...il.com>, 
	Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>, 
	Björn Roy Baron <bjorn3_gh@...tonmail.com>, 
	Andreas Hindborg <a.hindborg@...sung.com>, Marco Elver <elver@...gle.com>, Coly Li <colyli@...e.de>, 
	Paolo Abeni <pabeni@...hat.com>, Pierre Gondois <pierre.gondois@....com>, 
	Ingo Molnar <mingo@...nel.org>, Jakub Kicinski <kuba@...nel.org>, Wei Yang <richard.weiyang@...il.com>, 
	Matthew Wilcox <willy@...radead.org>, linux-kernel@...r.kernel.org, 
	rust-for-linux@...r.kernel.org, Kees Cook <kees@...nel.org>
Subject: Re: [PATCH v3 09/10] rust: list: support heterogeneous lists

On Thu, Aug 1, 2024 at 11:24 AM Benno Lossin <benno.lossin@...ton.me> wrote:
>
> On 23.07.24 10:22, Alice Ryhl wrote:
> > @@ -181,6 +185,47 @@ unsafe fn from_fields(me: *mut ListLinksFields) -> *mut Self {
> >      }
> >  }
> >
> > +/// Similar to [`ListLinks`], but also contains a pointer to the full value.
> > +///
> > +/// This type can be used instead of [`ListLinks`] to support lists with trait objects.
> > +#[repr(C)]
> > +pub struct ListLinksSelfPtr<T: ?Sized, const ID: u64 = 0> {
> > +    /// The `ListLinks` field inside this value.
> > +    ///
> > +    /// This is public so that it can be used with `impl_has_list_links!`.
> > +    pub inner: ListLinks<ID>,
> > +    self_ptr: UnsafeCell<MaybeUninit<*const T>>,
>
> Why do you need `MaybeUninit`?

Right now the constructor initializes it to MaybeUninit::zeroed().
What would you initialize it to without MaybeUninit? Remember that the
vtable pointer in a fat pointer has strict validity requirements.

> > +}
> > +
> > +// SAFETY: The fields of a ListLinksSelfPtr can be moved across thread boundaries.
> > +unsafe impl<T: ?Sized + Send, const ID: u64> Send for ListLinksSelfPtr<T, ID> {}
> > +// SAFETY: The type is opaque so immutable references to a ListLinksSelfPtr are useless. Therefore,
> > +// it's okay to have immutable access to a ListLinks from several threads at once.
> > +//
> > +// Note that `inner` being a public field does not prevent this type from being opaque, since
> > +// `inner` is a opaque type.
> > +unsafe impl<T: ?Sized + Sync, const ID: u64> Sync for ListLinksSelfPtr<T, ID> {}
>
> [...]
>
> > @@ -135,5 +178,91 @@ unsafe fn post_remove(me: *mut $crate::list::ListLinks<$num>) -> *const Self {
> >              }
> >          }
> >      };
> > +
> > +    (
> > +        impl$({$($generics:tt)*})? ListItem<$num:tt> for $t:ty {
> > +            using ListLinksSelfPtr;
> > +        } $($rest:tt)*
> > +    ) => {
> > +        // SAFETY: See GUARANTEES comment on each method.
> > +        unsafe impl$(<$($generics)*>)? $crate::list::ListItem<$num> for $t {
> > +            // GUARANTEES:
> > +            // This implementation of `ListItem` will not give out exclusive access to the same
> > +            // `ListLinks` several times because calls to `prepare_to_insert` and `post_remove`
> > +            // must alternate and exclusive access is given up when `post_remove` is called.
> > +            //
> > +            // Other invocations of `impl_list_item!` also cannot give out exclusive access to the
> > +            // same `ListLinks` because you can only implement `ListItem` once for each value of
> > +            // `ID`, and the `ListLinks` fields only work with the specified `ID`.
> > +            unsafe fn prepare_to_insert(me: *const Self) -> *mut $crate::list::ListLinks<$num> {
> > +                // SAFETY: The caller promises that `me` points at a valid value of type `Self`.
> > +                let links_field = unsafe { <Self as $crate::list::ListItem<$num>>::view_links(me) };
> > +
> > +                let spoff = $crate::list::ListLinksSelfPtr::<Self, $num>::LIST_LINKS_SELF_PTR_OFFSET;
> > +                // SAFETY: The constant is equal to `offset_of!(ListLinksSelfPtr, self_ptr)`, so
> > +                // the pointer stays in bounds of the allocation.
> > +                let self_ptr = unsafe { (links_field as *const u8).add(spoff) }
> > +                    as *const ::core::cell::UnsafeCell<*const Self>;
>
> A bit confused why you need to do it this way, can't you just do this?:
>
>     let links_self_field = links_field.cast::<$crate::list::ListLinksSelfPtr>();
>     // SAFETY: ...
>     let self_ptr = unsafe { ::core::ptr::addr_of_mut!((*links_self_field).self_ptr) };

If nothing else, the field is not public. I can't remember if there
was another reason.

Alice