Re: [PATCH v4 03/13] rust: add derive macro for `Zeroable`

[Gary Guo <gary@garyguo.net>]

On Mon, 14 Aug 2023 08:46:41 +0000
Benno Lossin <benno.lossin@proton.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@asahilina.net>
> Signed-off-by: Benno Lossin <benno.lossin@proton.me>

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

> ---
> v3 -> v4:
> - add support for `+` in `quote!`.
> 
> v2 -> v3:
> - change derive behavior, instead of adding `Zeroable` bounds for every
>   field, add them only for generic type parameters,
> - still check that every field implements `Zeroable`,
> - removed Reviewed-by's due to changes.
> 
> v1 -> v2:
> - fix Zeroable path,
> - add Reviewed-by from Gary and Björn.
> 
>  rust/kernel/init/macros.rs | 35 ++++++++++++++++++
>  rust/kernel/prelude.rs     |  2 +-
>  rust/macros/lib.rs         | 20 +++++++++++
>  rust/macros/quote.rs       | 12 +++++++
>  rust/macros/zeroable.rs    | 72 ++++++++++++++++++++++++++++++++++++++
>  5 files changed, 140 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 9182fdf99e7e..78091756dec0 100644
> --- a/rust/kernel/init/macros.rs
> +++ b/rust/kernel/init/macros.rs
> @@ -1215,3 +1215,38 @@ 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::init::Zeroable for $name<$($ty_generics)*>
> +        where
> +            $($($whr)*)?
> +        {}
> +        const _: () = {
> +            fn assert_zeroable<T: ?::core::marker::Sized + $crate::init::Zeroable>() {}
> +            fn ensure_zeroable<$($impl_generics)*>()
> +                where $($($whr)*)?
> +            {
> +                $(assert_zeroable::<$field_ty>();)*
> +            }
> +        };
> +    };
> +}
> 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 b4bc44c27bd4..fd7a815e68a8 100644
> --- a/rust/macros/lib.rs
> +++ b/rust/macros/lib.rs
> @@ -11,6 +11,7 @@
>  mod pin_data;
>  mod pinned_drop;
>  mod vtable;
> +mod zeroable;
>  
>  use proc_macro::TokenStream;
>  
> @@ -343,3 +344,22 @@ pub fn paste(input: TokenStream) -> TokenStream {
>      paste::expand(&mut tokens);
>      tokens.into_iter().collect()
>  }
> +
> +/// 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..33a199e4f176 100644
> --- a/rust/macros/quote.rs
> +++ b/rust/macros/quote.rs
> @@ -124,6 +124,18 @@ 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 + $($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..0d605c46ab3b
> --- /dev/null
> +++ b/rust/macros/zeroable.rs
> @@ -0,0 +1,72 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +use crate::helpers::{parse_generics, Generics};
> +use proc_macro::{TokenStream, TokenTree};
> +
> +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();
> +    // Now we insert `Zeroable` as a bound for every generic parameter in `impl_generics`.
> +    let mut new_impl_generics = Vec::with_capacity(impl_generics.len());
> +    // Are we inside of a generic where we want to add `Zeroable`?
> +    let mut in_generic = !impl_generics.is_empty();
> +    // Have we already inserted `Zeroable`?
> +    let mut inserted = false;
> +    // Level of `<>` nestings.
> +    let mut nested = 0;
> +    for tt in impl_generics {
> +        match &tt {
> +            // If we find a `,`, then we have finished a generic/constant/lifetime parameter.
> +            TokenTree::Punct(p) if nested == 0 && p.as_char() == ',' => {
> +                if in_generic && !inserted {
> +                    new_impl_generics.extend(quote! { : ::kernel::init::Zeroable });
> +                }
> +                in_generic = true;
> +                inserted = false;
> +                new_impl_generics.push(tt);
> +            }
> +            // If we find `'`, then we are entering a lifetime.
> +            TokenTree::Punct(p) if nested == 0 && p.as_char() == '\'' => {
> +                in_generic = false;
> +                new_impl_generics.push(tt);
> +            }
> +            TokenTree::Punct(p) if nested == 0 && p.as_char() == ':' => {
> +                new_impl_generics.push(tt);
> +                if in_generic {
> +                    new_impl_generics.extend(quote! { ::kernel::init::Zeroable + });
> +                    inserted = true;
> +                }
> +            }
> +            TokenTree::Punct(p) if p.as_char() == '<' => {
> +                nested += 1;
> +                new_impl_generics.push(tt);
> +            }
> +            TokenTree::Punct(p) if p.as_char() == '>' => {
> +                assert!(nested > 0);
> +                nested -= 1;
> +                new_impl_generics.push(tt);
> +            }
> +            _ => new_impl_generics.push(tt),
> +        }
> +    }
> +    assert_eq!(nested, 0);
> +    if in_generic && !inserted {
> +        new_impl_generics.extend(quote! { : ::kernel::init::Zeroable });
> +    }
> +    quote! {
> +        ::kernel::__derive_zeroable!(
> +            parse_input:
> +                @sig(#(#rest)*),
> +                @impl_generics(#(#new_impl_generics)*),
> +                @ty_generics(#(#ty_generics)*),
> +                @body(#last),
> +        );
> +    }
> +}