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

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Message-ID: <877cpl7xhp.fsf@samsung.com>
Date:   Thu, 24 Aug 2023 06:41:55 +0000
From:   Andreas Hindborg <a.hindborg@...sung.com>
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>, Gary Guo <gary@...yguo.net>,
        Björn Roy Baron <bjorn3_gh@...tonmail.com>,
        Benno Lossin <benno.lossin@...ton.me>,
        Alice Ryhl <aliceryhl@...gle.com>,
        "rust-for-linux@...r.kernel.org" <rust-for-linux@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "patches@...ts.linux.dev" <patches@...ts.linux.dev>
Subject: Re: [PATCH 1/2] rust: arc: add explicit `drop()` around
 `Box::from_raw()`


Miguel Ojeda <ojeda@...nel.org> writes:

> `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://protect2.fireeye.com/v1/url?k=9e9028e1-ff1b3dd7-9e91a3ae-74fe485cbff1-f2bfda5f7702c68e&q=1&e=66bd90a6-86db-48e7-8538-c118c3f45baa&u=https%3A%2F%2Fgithub.com%2Frust-lang%2Frust%2Fissues%2F104253 [1]
> Link: https://protect2.fireeye.com/v1/url?k=89300613-e8bb1325-89318d5c-74fe485cbff1-fcf76e46b008b25a&q=1&e=66bd90a6-86db-48e7-8538-c118c3f45baa&u=https%3A%2F%2Fgithub.com%2Frust-lang%2Frust%2Fpull%2F112529 [2]
> Signed-off-by: Miguel Ojeda <ojeda@...nel.org>
> ---

Reviewed-by: Andreas Hindborg <a.hindborg@...sung.com>

>  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())) };
>          }
>      }
>  }