linux-kernel - Re: [PATCH 1/2] rust: sync: arc: implement Arc<dyn Any + Send + Sync>::downcast()

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Date:   Fri, 24 Feb 2023 11:34:29 -0300
From:   Martin Rodriguez Reboredo <yakoyoku@...il.com>
To:     lina@...hilina.net
Cc:     alex.gaynor@...il.com, asahi@...ts.linux.dev,
        bjorn3_gh@...tonmail.com, boqun.feng@...il.com, gary@...yguo.net,
        linux-kernel@...r.kernel.org, ojeda@...nel.org,
        rust-for-linux@...r.kernel.org, wedsonaf@...il.com
Subject: Re: [PATCH 1/2] rust: sync: arc: implement Arc<dyn Any + Send + Sync>::downcast()

On Fri, Feb 24, 2023 at 05:09:47PM +0900, Asahi Lina wrote:
> This mirrors the standard library's alloc::sync::Arc::downcast().
> 
> Based on the Rust standard library implementation, ver 1.62.0,
> licensed under "Apache-2.0 OR MIT", from:
> 
>     https://github.com/rust-lang/rust/tree/1.62.0/library/alloc/src
> 
> For copyright details, please see:
> 
>     https://github.com/rust-lang/rust/blob/1.62.0/COPYRIGHT
> 
> Signed-off-by: Asahi Lina <lina@...hilina.net>
> ---
>  rust/kernel/sync/arc.rs | 22 ++++++++++++++++++++++
>  1 file changed, 22 insertions(+)
> 
> diff --git a/rust/kernel/sync/arc.rs b/rust/kernel/sync/arc.rs
> index a94e303217c6..752bd7c4699e 100644
> --- a/rust/kernel/sync/arc.rs
> +++ b/rust/kernel/sync/arc.rs
> @@ -22,6 +22,7 @@ use crate::{
>  };
>  use alloc::boxed::Box;
>  use core::{
> +    any::Any,
>      fmt,
>      marker::{PhantomData, Unsize},
>      mem::{ManuallyDrop, MaybeUninit},
> @@ -220,6 +221,27 @@ impl<T: 'static> ForeignOwnable for Arc<T> {
>      }
>  }
>  
> +impl Arc<dyn Any + Send + Sync> {
> +    /// Attempt to downcast the `Arc<dyn Any + Send + Sync>` to a concrete type.
> +    // Based on the Rust standard library implementation, ver 1.62.0, which is
> +    // Apache-2.0 OR MIT.
> +    pub fn downcast<T>(self) -> core::result::Result<Arc<T>, Self>
> +    where
> +        T: Any + Send + Sync,
> +    {
> +        if (*self).is::<T>() {
> +            // SAFETY: We have just checked that the type is correct, so we can cast the pointer.
> +            unsafe {
> +                let ptr = self.ptr.cast::<ArcInner<T>>();
> +                core::mem::forget(self);

I see, we cast the inner pointer of the `Arc` to the target type which
is wrapped in an `ArcInner` that is then used for another `Arc`. This is
important as this new value now owns the pointer and, by consequence,
it is safe to forget the passed `Arc` as it won't leak the pointer.
Thus, this method is safe.

> +                Ok(Arc::from_inner(ptr))
> +            }
> +        } else {
> +            Err(self)
> +        }
> +    }
> +}
> +
>  impl<T: ?Sized> Deref for Arc<T> {
>      type Target = T;

Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@...il.com>