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Date:   Wed, 2 Aug 2023 18:59:02 +0100
From:   Gary Guo <gary@garyguo.net>
To:     Benno Lossin <benno.lossin@proton.me>
Cc:     Miguel Ojeda <ojeda@kernel.org>,
        Wedson Almeida Filho <wedsonaf@gmail.com>,
        Alex Gaynor <alex.gaynor@gmail.com>,
        Boqun Feng <boqun.feng@gmail.com>,
        =?UTF-8?B?QmrDtnJu?= Roy Baron <bjorn3_gh@protonmail.com>,
        Alice Ryhl <aliceryhl@google.com>,
        Andreas Hindborg <nmi@metaspace.dk>,
        rust-for-linux@vger.kernel.org, linux-kernel@vger.kernel.org,
        Martin Rodriguez Reboredo <yakoyoku@gmail.com>
Subject: Re: [PATCH v3 06/13] rust: init: make initializer values
 inaccessible after initializing
Message-ID: <20230802185902.3753dba0.gary@garyguo.net>
In-Reply-To: <20230729090838.225225-7-benno.lossin@proton.me>
References: <20230729090838.225225-1-benno.lossin@proton.me>
        <20230729090838.225225-7-benno.lossin@proton.me>
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On Sat, 29 Jul 2023 09:09:53 +0000
Benno Lossin <benno.lossin@proton.me> wrote:

> Previously the init macros would create a local variable with the name
> and hygiene of the field that is being initialized to store the value of
> the field. This would override any user defined variables. For example:
> ```
> struct Foo {
>     a: usize,
>     b: usize,
> }
> let a = 10;
> let foo = init!(Foo{
>     a: a + 1, // This creates a local variable named `a`.
>     b: a, // This refers to that variable!
> });
> let foo = Box::init!(foo)?;
> assert_eq!(foo.a, 11);
> assert_eq!(foo.b, 11);
> ```
> 
> This patch changes this behavior, so the above code would panic at the
> last assertion, since `b` would have value 10.
> 
> Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>
> Reviewed-by: Alice Ryhl <aliceryhl@google.com>
> Signed-off-by: Benno Lossin <benno.lossin@proton.me>

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

> ---
> v2 -> v3:
> - added Reviewed-by's from Martin and Alice.
> 
>  rust/kernel/init/macros.rs | 20 +++++++++++---------
>  1 file changed, 11 insertions(+), 9 deletions(-)
> 
> diff --git a/rust/kernel/init/macros.rs b/rust/kernel/init/macros.rs
> index 2bad086cda0a..cfeacc4b3f7d 100644
> --- a/rust/kernel/init/macros.rs
> +++ b/rust/kernel/init/macros.rs
> @@ -1075,13 +1075,13 @@ macro_rules! __init_internal {
>          // In-place initialization syntax.
>          @munch_fields($field:ident <- $val:expr, $($rest:tt)*),
>      ) => {
> -        let $field = $val;
> +        let init = $val;
>          // Call the initializer.
>          //
>          // SAFETY: `slot` is valid, because we are inside of an initializer closure, we
>          // return when an error/panic occurs.
>          // We also use the `data` to require the correct trait (`Init` or `PinInit`) for `$field`.
> -        unsafe { $data.$field(::core::ptr::addr_of_mut!((*$slot).$field), $field)? };
> +        unsafe { $data.$field(::core::ptr::addr_of_mut!((*$slot).$field), init)? };
>          // Create the drop guard:
>          //
>          // We rely on macro hygiene to make it impossible for users to access this local variable.
> @@ -1107,12 +1107,12 @@ macro_rules! __init_internal {
>          // In-place initialization syntax.
>          @munch_fields($field:ident <- $val:expr, $($rest:tt)*),
>      ) => {
> -        let $field = $val;
> +        let init = $val;
>          // Call the initializer.
>          //
>          // SAFETY: `slot` is valid, because we are inside of an initializer closure, we
>          // return when an error/panic occurs.
> -        unsafe { $crate::init::Init::__init($field, ::core::ptr::addr_of_mut!((*$slot).$field))? };
> +        unsafe { $crate::init::Init::__init(init, ::core::ptr::addr_of_mut!((*$slot).$field))? };
>          // Create the drop guard:
>          //
>          // We rely on macro hygiene to make it impossible for users to access this local variable.
> @@ -1138,11 +1138,13 @@ macro_rules! __init_internal {
>          // Init by-value.
>          @munch_fields($field:ident $(: $val:expr)?, $($rest:tt)*),
>      ) => {
> -        $(let $field = $val;)?
> -        // Initialize the field.
> -        //
> -        // SAFETY: The memory at `slot` is uninitialized.
> -        unsafe { ::core::ptr::write(::core::ptr::addr_of_mut!((*$slot).$field), $field) };
> +        {
> +            $(let $field = $val;)?
> +            // Initialize the field.
> +            //
> +            // SAFETY: The memory at `slot` is uninitialized.
> +            unsafe { ::core::ptr::write(::core::ptr::addr_of_mut!((*$slot).$field), $field) };
> +        }
>          // Create the drop guard:
>          //
>          // We rely on macro hygiene to make it impossible for users to access this local variable.