linux-kernel - Re: [PATCH v2 2/2] rust: arc: remove `ArcBorrow` in favour of `WithRef`

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Date:   Mon, 25 Sep 2023 15:02:27 -0700
From:   Boqun Feng <boqun.feng@...il.com>
To:     Alice Ryhl <alice@...l.io>
Cc:     Benno Lossin <benno.lossin@...ton.me>,
        Alice Ryhl <aliceryhl@...gle.com>,
        Wedson Almeida Filho <wedsonaf@...il.com>,
        rust-for-linux@...r.kernel.org, Miguel Ojeda <ojeda@...nel.org>,
        Alex Gaynor <alex.gaynor@...il.com>,
        Gary Guo <gary@...yguo.net>,
        Björn Roy Baron <bjorn3_gh@...tonmail.com>,
        Andreas Hindborg <a.hindborg@...sung.com>,
        linux-kernel@...r.kernel.org,
        Wedson Almeida Filho <walmeida@...rosoft.com>
Subject: Re: [PATCH v2 2/2] rust: arc: remove `ArcBorrow` in favour of
 `WithRef`

On Mon, Sep 25, 2023 at 11:58:46PM +0200, Alice Ryhl wrote:
> On 9/25/23 23:55, Boqun Feng wrote:
> > On Mon, Sep 25, 2023 at 09:03:52PM +0000, Benno Lossin wrote:
> > > On 25.09.23 20:51, Boqun Feng wrote:
> > > > On Mon, Sep 25, 2023 at 05:00:45PM +0000, Benno Lossin wrote:
> > > > > On 25.09.23 18:16, Boqun Feng wrote:
> > > > > > On Mon, Sep 25, 2023 at 03:07:44PM +0000, Benno Lossin wrote:
> > > > > > > ```rust
> > > > > > > struct MutatingDrop {
> > > > > > >         value: i32,
> > > > > > > }
> > > > > > > 
> > > > > > > impl Drop for MutatingDrop {
> > > > > > >         fn drop(&mut self) {
> > > > > > >             self.value = 0;
> > > > > > >         }
> > > > > > > }
> > > > > > > 
> > > > > > > let arc = Arc::new(MutatingDrop { value: 42 });
> > > > > > > let wr = arc.as_with_ref(); // this creates a shared `&` reference to the MutatingDrop
> > > > > > > let arc2: Arc<MutatingDrop> = wr.into(); // increments the reference count to 2
> > > > > > 
> > > > > > More precisely, here we did a
> > > > > > 
> > > > > > 	&WithRef<_> -> NonNull<WithRef<_>>
> > > > > > 
> > > > > > conversion, and later on, we may use the `NonNull<WithRef<_>>` in
> > > > > > `drop` to get a `Box<WithRef<_>>`.
> > > > > 
> > > > > Indeed.
> > > > > 
> > > > 
> > > > Can we workaround this issue by (ab)using the `UnsafeCell` inside
> > > > `WithRef<T>`?
> > > > 
> > > > impl<T: ?Sized> From<&WithRef<T>> for Arc<T> {
> > > >       fn from(b: &WithRef<T>) -> Self {
> > > >           // SAFETY: The existence of the references proves that
> > > > 	// `b.refcount.get()` is a valid pointer to `WithRef<T>`.
> > > > 	let ptr = unsafe { NonNull::new_unchecked(b.refcount.get().cast::<WithRef<T>>()) };
> > > > 
> > > > 	// SAFETY: see the SAFETY above `let ptr = ..` line.
> > > >           ManuallyDrop::new(unsafe { Arc::from_inner(ptr) })
> > > >               .deref()
> > > >               .clone()
> > > >       }
> > > > }
> > > > 
> > > > This way, the raw pointer in the new Arc no longer derives from the
> > > > reference of `WithRef<T>`.
> > > 
> > > No, the code above only obtains a pointer that has provenance valid
> > > for a `bindings::refcount_t` (or type with the same layout, such as
> > > `Opaque<bindings::refcount_t>`). But not the whole `WithRef<T>`, so accessing
> > > it by reading/writing will still be UB.
> > > 
> > 
> > Hmm... but we do the similar thing in `Arc::from_raw()`, right?
> > 
> >      	pub unsafe fn from_raw(ptr: *const T) -> Self {
> > 	    ..
> > 	}
> > 
> > , what we have is a pointer to T, and we construct a pointer to
> > `ArcInner<T>/WithRef<T>`, in that function. Because the `sub` on pointer
> > gets away from provenance? If so, we can also do a sub(0) in the above
> > code.
> 
> Not sure what you mean. Operations on raw pointers leave provenance
> unchanged.

Let's look at the function from_raw(), the input is a pointer to T,
right? So you only have the provenance to T, but in that function, the
pointer is casted to a pointer to WithRef<T>/ArcInner<T>, that means you
have the provenance to the whole WithRef<T>/ArcInner<T>, right? My
question is: why isn't that a UB?

Regards,
Boqun

> 
> Alice
>