linux-kernel - Re: [PATCH 1/2] rust: arc: add explicit `drop()` around `Box::from

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Date:   Wed, 23 Aug 2023 18:35:19 +0100
From:   Gary Guo <gary@...yguo.net>
To:     Miguel Ojeda <ojeda@...nel.org>
Cc:     Wedson Almeida Filho <wedsonaf@...il.com>,
        Alex Gaynor <alex.gaynor@...il.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Björn Roy Baron <bjorn3_gh@...tonmail.com>,
        Benno Lossin <benno.lossin@...ton.me>,
        Andreas Hindborg <a.hindborg@...sung.com>,
        Alice Ryhl <aliceryhl@...gle.com>,
        rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org,
        patches@...ts.linux.dev
Subject: Re: [PATCH 1/2] rust: arc: add explicit `drop()` around
 `Box::from_raw()`

On Wed, 23 Aug 2023 18:02:42 +0200
Miguel Ojeda <ojeda@...nel.org> wrote:

> `Box::from_raw()` is `#[must_use]`, which means the result cannot
> go unused.
> 
> In Rust 1.71.0, this was not detected because the block expression
> swallows the diagnostic [1]:
> 
>     unsafe { Box::from_raw(self.ptr.as_ptr()) };
> 
> It would have been detected, however, if the line had been instead:
> 
>     unsafe { Box::from_raw(self.ptr.as_ptr()); }
> 
> i.e. the semicolon being inside the `unsafe` block, rather than
> outside.
> 
> In Rust 1.72.0, the compiler started warning about this [2], so
> without this patch we will get:
> 
>         error: unused return value of `alloc::boxed::Box::<T>::from_raw` that must be used
>         --> rust/kernel/sync/arc.rs:302:22  
>         |
>     302 |             unsafe { Box::from_raw(self.ptr.as_ptr()) };
>         |                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>         |
>         = note: call `drop(Box::from_raw(ptr))` if you intend to drop the `Box`
>         = note: `-D unused-must-use` implied by `-D warnings`
>     help: use `let _ = ...` to ignore the resulting value
>         |
>     302 |             unsafe { let _ = Box::from_raw(self.ptr.as_ptr()); };
>         |                      +++++++                                 +
> 
> Thus add an add an explicit `drop()` as the `#[must_use]`'s
> annotation suggests (instead of the more general help line).
> 
> Link: https://github.com/rust-lang/rust/issues/104253 [1]
> Link: https://github.com/rust-lang/rust/pull/112529 [2]
> Signed-off-by: Miguel Ojeda <ojeda@...nel.org>

Reviewed-by: Gary Guo <gary@...yguo.net>

> ---
>  rust/kernel/sync/arc.rs | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/rust/kernel/sync/arc.rs b/rust/kernel/sync/arc.rs
> index 172f563976a9..4c14d540a581 100644
> --- a/rust/kernel/sync/arc.rs
> +++ b/rust/kernel/sync/arc.rs
> @@ -299,7 +299,7 @@ fn drop(&mut self) {
>              // The count reached zero, we must free the memory.
>              //
>              // SAFETY: The pointer was initialised from the result of `Box::leak`.
> -            unsafe { Box::from_raw(self.ptr.as_ptr()) };
> +            unsafe { drop(Box::from_raw(self.ptr.as_ptr())) };
>          }
>      }
>  }