Date:   Sat, 24 Jun 2023 14:55:03 +0000
From:   Björn Roy Baron <bjorn3_gh@...tonmail.com>
To:     Benno Lossin <benno.lossin@...ton.me>
Cc:     Miguel Ojeda <ojeda@...nel.org>,
        Wedson Almeida Filho <wedsonaf@...il.com>,
        Alex Gaynor <alex.gaynor@...il.com>,
        Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>,
        Alice Ryhl <aliceryhl@...gle.com>,
        Andreas Hindborg <nmi@...aspace.dk>,
        rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org,
        patches@...ts.linux.dev, Asahi Lina <lina@...hilina.net>
Subject: Re: [PATCH 2/7] rust: add derive macro for `Zeroable`

On Saturday, June 24th, 2023 at 11:25, Benno Lossin <benno.lossin@...ton.me> wrote:

> Add a derive proc-macro for the `Zeroable` trait. The macro supports
> structs where every field implements the `Zeroable` trait. This way
> `unsafe` implementations can be avoided.
> 
> The macro is split into two parts:
> - a proc-macro to parse generics into impl and ty generics,
> - a declarative macro that expands to the impl block.
> 
> Suggested-by: Asahi Lina <lina@...hilina.net>
> Signed-off-by: Benno Lossin <benno.lossin@...ton.me>

Reviewed-by: Björn Roy Baron <bjorn3_gh@...tonmail.com>

> ---
>  rust/kernel/init/macros.rs | 28 ++++++++++++++++++++++++++++
>  rust/kernel/prelude.rs     |  2 +-
>  rust/macros/lib.rs         | 20 ++++++++++++++++++++
>  rust/macros/quote.rs       |  6 ++++++
>  rust/macros/zeroable.rs    | 25 +++++++++++++++++++++++++
>  5 files changed, 80 insertions(+), 1 deletion(-)
>  create mode 100644 rust/macros/zeroable.rs
> 
> diff --git a/rust/kernel/init/macros.rs b/rust/kernel/init/macros.rs
> index fbaebd34f218..e8165ff53a94 100644
> --- a/rust/kernel/init/macros.rs
> +++ b/rust/kernel/init/macros.rs
> @@ -1213,3 +1213,31 @@ macro_rules! __init_internal {
>          );
>      };
>  }
> +
> +#[doc(hidden)]
> +#[macro_export]
> +macro_rules! __derive_zeroable {
> +    (parse_input:
> +        @sig(
> +            $(#[$($struct_attr:tt)*])*
> +            $vis:vis struct $name:ident
> +            $(where $($whr:tt)*)?
> +        ),
> +        @impl_generics($($impl_generics:tt)*),
> +        @ty_generics($($ty_generics:tt)*),
> +        @body({
> +            $(
> +                $(#[$($field_attr:tt)*])*
> +                $field:ident : $field_ty:ty
> +            ),* $(,)?
> +        }),
> +    ) => {
> +        // SAFETY: every field type implements `Zeroable` and padding bytes may be zero.
> +        #[automatically_derived]
> +        unsafe impl<$($impl_generics)*> $crate::Zeroable for $name<$($ty_generics)*>
> +        where
> +            $($field_ty: $crate::Zeroable,)*
> +            $($($whr)*)?
> +        {}
> +    };
> +}
> diff --git a/rust/kernel/prelude.rs b/rust/kernel/prelude.rs
> index c28587d68ebc..ae21600970b3 100644
> --- a/rust/kernel/prelude.rs
> +++ b/rust/kernel/prelude.rs
> @@ -18,7 +18,7 @@
>  pub use alloc::{boxed::Box, vec::Vec};
> 
>  #[doc(no_inline)]
> -pub use macros::{module, pin_data, pinned_drop, vtable};
> +pub use macros::{module, pin_data, pinned_drop, vtable, Zeroable};
> 
>  pub use super::build_assert;
> 
> diff --git a/rust/macros/lib.rs b/rust/macros/lib.rs
> index 3fc74cb4ea19..9f056a5c780a 100644
> --- a/rust/macros/lib.rs
> +++ b/rust/macros/lib.rs
> @@ -10,6 +10,7 @@
>  mod pin_data;
>  mod pinned_drop;
>  mod vtable;
> +mod zeroable;
> 
>  use proc_macro::TokenStream;
> 
> @@ -246,3 +247,22 @@ pub fn pin_data(inner: TokenStream, item: TokenStream) -> TokenStream {
>  pub fn pinned_drop(args: TokenStream, input: TokenStream) -> TokenStream {
>      pinned_drop::pinned_drop(args, input)
>  }
> +
> +/// Derives the [`Zeroable`] trait for the given struct.
> +///
> +/// This can only be used for structs where every field implements the [`Zeroable`] trait.
> +///
> +/// # Examples
> +///
> +/// ```rust
> +/// #[derive(Zeroable)]
> +/// pub struct DriverData {
> +///     id: i64,
> +///     buf_ptr: *mut u8,
> +///     len: usize,
> +/// }
> +/// ```
> +#[proc_macro_derive(Zeroable)]
> +pub fn derive_zeroable(input: TokenStream) -> TokenStream {
> +    zeroable::derive(input)
> +}
> diff --git a/rust/macros/quote.rs b/rust/macros/quote.rs
> index dddbb4e6f4cb..b76c198a4ed5 100644
> --- a/rust/macros/quote.rs
> +++ b/rust/macros/quote.rs
> @@ -124,6 +124,12 @@ macro_rules! quote_spanned {
>          ));
>          quote_spanned!(@proc $v $span $($tt)*);
>      };
> +    (@proc $v:ident $span:ident ; $($tt:tt)*) => {
> +        $v.push(::proc_macro::TokenTree::Punct(
> +                ::proc_macro::Punct::new(';', ::proc_macro::Spacing::Alone)
> +        ));
> +        quote_spanned!(@proc $v $span $($tt)*);
> +    };
>      (@proc $v:ident $span:ident $id:ident $($tt:tt)*) => {
>          $v.push(::proc_macro::TokenTree::Ident(::proc_macro::Ident::new(stringify!($id), $span)));
>          quote_spanned!(@proc $v $span $($tt)*);
> diff --git a/rust/macros/zeroable.rs b/rust/macros/zeroable.rs
> new file mode 100644
> index 000000000000..cddb866c44ef
> --- /dev/null
> +++ b/rust/macros/zeroable.rs
> @@ -0,0 +1,25 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +use crate::helpers::{parse_generics, Generics};
> +use proc_macro::TokenStream;
> +
> +pub(crate) fn derive(input: TokenStream) -> TokenStream {
> +    let (
> +        Generics {
> +            impl_generics,
> +            ty_generics,
> +        },
> +        mut rest,
> +    ) = parse_generics(input);
> +    // This should be the body of the struct `{...}`.
> +    let last = rest.pop();
> +    quote! {
> +        ::kernel::__derive_zeroable!(
> +            parse_input:
> +                @sig(#(#rest)*),
> +                @impl_generics(#(#impl_generics)*),
> +                @ty_generics(#(#ty_generics)*),
> +                @body(#last),
> +        );
> +    }
> +}
> --
> 2.41.0

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