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Date:   Mon, 3 Apr 2023 19:56:27 +0200
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From:   Alice Ryhl <alice@ryhl.io>
Subject: Re: [PATCH v5 10/15] rust: init: add `stack_pin_init!` macro
To:     Benno Lossin <y86-dev@protonmail.com>
Cc:     rust-for-linux@vger.kernel.org, linux-kernel@vger.kernel.org,
        patches@lists.linux.dev, Alice Ryhl <aliceryhl@google.com>,
        Andreas Hindborg <a.hindborg@samsung.com>,
        Miguel Ojeda <ojeda@kernel.org>,
        Alex Gaynor <alex.gaynor@gmail.com>,
        Wedson Almeida Filho <wedsonaf@gmail.com>,
        Boqun Feng <boqun.feng@gmail.com>, Gary Guo <gary@garyguo.net>,
        =?UTF-8?Q?Bj=c3=b6rn_Roy_Baron?= <bjorn3_gh@protonmail.com>
References: <20230403154422.168633-1-y86-dev@protonmail.com>
 <20230403160511.174894-3-y86-dev@protonmail.com>
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On 4/3/23 18:05, Benno Lossin wrote:
> The `stack_pin_init!` macro allows pin-initializing a value on the
> stack. It accepts a `impl PinInit<T, E>` to initialize a `T`. It allows
> propagating any errors via `?` or handling it normally via `match`.
> 
> Signed-off-by: Benno Lossin <y86-dev@protonmail.com>
> Cc: Alice Ryhl <aliceryhl@google.com>
> Cc: Andreas Hindborg <a.hindborg@samsung.com>
> Cc: Gary Guo <gary@garyguo.net>
> ---

If you fix the issue below, then you may add
Reviewed-by: Alice Ryhl <aliceryhl@google.com>

> +    /// Initializes the contents and returns the result.
> +    #[inline]
> +    pub fn init<E>(self: Pin<&mut Self>, init: impl PinInit<T, E>) -> Result<Pin<&mut T>, E> {
> +        // SAFETY: We never move out of `this`.
> +        let this = unsafe { Pin::into_inner_unchecked(self) };
> +        // The value is currently initialized, so it needs to be dropped before we can reuse
> +        // the memory (this is a safety guarantee of `Pin`).
> +        if this.1 {
> +            // SAFETY: `this.1` is true and we set it to false after this.
> +            unsafe { this.0.assume_init_drop() };
> +            this.1 = false;
> +        }

This would double-free the value if `assume_init_drop` panics. I know 
that we configure panics to abort the kernel, but someone could copy 
this into another codebase and then they would have this issue.

You can fix it by setting `this.1` to false *before* calling 
`assume_init_drop`.