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Date:   Sat, 24 Jun 2023 15:11:35 +0000
To:     Benno Lossin <benno.lossin@proton.me>
From:   =?utf-8?Q?Bj=C3=B6rn_Roy_Baron?= <bjorn3_gh@protonmail.com>
Cc:     Miguel Ojeda <ojeda@kernel.org>,
        Wedson Almeida Filho <wedsonaf@gmail.com>,
        Alex Gaynor <alex.gaynor@gmail.com>,
        Boqun Feng <boqun.feng@gmail.com>, Gary Guo <gary@garyguo.net>,
        Alice Ryhl <aliceryhl@google.com>,
        Andreas Hindborg <nmi@metaspace.dk>,
        rust-for-linux@vger.kernel.org, linux-kernel@vger.kernel.org,
        patches@lists.linux.dev, Asahi Lina <lina@asahilina.net>
Subject: Re: [PATCH 5/7] rust: init: add `..Zeroable::zeroed()` syntax for zeroing all missing fields
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On Saturday, June 24th, 2023 at 11:25, Benno Lossin <benno.lossin@proton.me=
> wrote:

> Add the struct update syntax to the init macros, but only for
> `..Zeroable::zeroed()`. Adding this at the end of the struct initializer
> allows one to omit fields from the initializer, these fields will be
> initialized with 0x00 set to every byte. Only types that implement the
> `Zeroable` trait can utilize this.
>=20
> Suggested-by: Asahi Lina <lina@asahilina.net>
> Signed-off-by: Benno Lossin <benno.lossin@proton.me>
> ---
>  rust/kernel/init.rs        |  16 +++++-
>  rust/kernel/init/macros.rs | 114 ++++++++++++++++++++++++++++++++++++-
>  2 files changed, 128 insertions(+), 2 deletions(-)
>=20
> diff --git a/rust/kernel/init.rs b/rust/kernel/init.rs
> index ecf6a4bd0ce4..44bc3e77419a 100644
> --- a/rust/kernel/init.rs
> +++ b/rust/kernel/init.rs
> @@ -508,14 +508,18 @@ macro_rules! stack_try_pin_init {
>  /// - Fields that you want to initialize in-place have to use `<-` inste=
ad of `:`.
>  /// - In front of the initializer you can write `&this in` to have acces=
s to a [`NonNull<Self>`]
>  ///   pointer named `this` inside of the initializer.
> +/// - Using struct update syntax one can place `..Zeroable::zeroed()` at=
 the very end of the
> +///   struct, this initializes every field with 0 and then runs all init=
ializers specified in the
> +///   body. This can only be done if [`Zeroable`] is implemented for the=
 struct.
>  ///
>  /// For instance:
>  ///
>  /// ```rust
>  /// # use kernel::pin_init;
> -/// # use macros::pin_data;
> +/// # use macros::{pin_data, Zeroable};
>  /// # use core::{ptr::addr_of_mut, marker::PhantomPinned};
>  /// #[pin_data]
> +/// #[derive(Zeroable)]
>  /// struct Buf {
>  ///     // `ptr` points into `buf`.
>  ///     ptr: *mut u8,
> @@ -528,6 +532,10 @@ macro_rules! stack_try_pin_init {
>  ///     ptr: unsafe { addr_of_mut!((*this.as_ptr()).buf).cast() },
>  ///     pin: PhantomPinned,
>  /// });
> +/// pin_init!(Buf {
> +///     buf: [1; 64],
> +///     ..Zeroable::zeroed(),
> +/// });
>  /// ```
>  ///
>  /// [`try_pin_init!`]: kernel::try_pin_init
> @@ -547,6 +555,7 @@ macro_rules! pin_init {
>              @data(PinData, use_data),
>              @has_data(HasPinData, __pin_data),
>              @construct_closure(pin_init_from_closure),
> +            @munch_fields($($fields)*),
>          )
>      };
>  }
> @@ -603,6 +612,7 @@ macro_rules! try_pin_init {
>              @data(PinData, use_data),
>              @has_data(HasPinData, __pin_data),
>              @construct_closure(pin_init_from_closure),
> +            @munch_fields($($fields)*),
>          )
>      };
>      ($(&$this:ident in)? $t:ident $(::<$($generics:ty),* $(,)?>)? {
> @@ -616,6 +626,7 @@ macro_rules! try_pin_init {
>              @data(PinData, use_data),
>              @has_data(HasPinData, __pin_data),
>              @construct_closure(pin_init_from_closure),
> +            @munch_fields($($fields)*),
>          )
>      };
>  }
> @@ -650,6 +661,7 @@ macro_rules! init {
>              @data(InitData, /*no use_data*/),
>              @has_data(HasInitData, __init_data),
>              @construct_closure(init_from_closure),
> +            @munch_fields($($fields)*),
>          )
>      }
>  }
> @@ -700,6 +712,7 @@ macro_rules! try_init {
>              @data(InitData, /*no use_data*/),
>              @has_data(HasInitData, __init_data),
>              @construct_closure(init_from_closure),
> +            @munch_fields($($fields)*),
>          )
>      };
>      ($(&$this:ident in)? $t:ident $(::<$($generics:ty),* $(,)?>)? {
> @@ -713,6 +726,7 @@ macro_rules! try_init {
>              @data(InitData, /*no use_data*/),
>              @has_data(HasInitData, __init_data),
>              @construct_closure(init_from_closure),
> +            @munch_fields($($fields)*),
>          )
>      };
>  }
> diff --git a/rust/kernel/init/macros.rs b/rust/kernel/init/macros.rs
> index 1e0c4aca055a..5dcb2e513f26 100644
> --- a/rust/kernel/init/macros.rs
> +++ b/rust/kernel/init/macros.rs
> @@ -989,6 +989,7 @@ impl<$($impl_generics)*> $pin_data<$($ty_generics)*>
>  ///
>  /// This macro has multiple internal call configurations, these are alwa=
ys the very first ident:
>  /// - nothing: this is the base case and called by the `{try_}{pin_}init=
!` macros.
> +/// - `with_update_parsed`: when the `..Zeroable::zeroed()` syntax has b=
een handled.
>  /// - `init_slot`: recursively creates the code that initializes all fie=
lds in `slot`.
>  /// - `make_initializer`: recursively create the struct initializer that=
 guarantees that every
>  ///   field has been initialized exactly once.
> @@ -1007,6 +1008,82 @@ macro_rules! __init_internal {
>          @has_data($has_data:ident, $get_data:ident),
>          // `pin_init_from_closure` or `init_from_closure`.
>          @construct_closure($construct_closure:ident),
> +        @munch_fields(),
> +    ) =3D> {
> +        $crate::__init_internal!(with_update_parsed:
> +            @this($($this)?),
> +            @typ($t $(::<$($generics),*>)? ),
> +            @fields($($fields)*),
> +            @error($err),
> +            @data($data, $($use_data)?),
> +            @has_data($has_data, $get_data),
> +            @construct_closure($construct_closure),
> +            @zeroed(), // nothing means default behavior.
> +        )
> +    };
> +    (
> +        @this($($this:ident)?),
> +        @typ($t:ident $(::<$($generics:ty),*>)?),
> +        @fields($($fields:tt)*),
> +        @error($err:ty),
> +        // Either `PinData` or `InitData`, `$use_data` should only be pr=
esent in the `PinData`
> +        // case.
> +        @data($data:ident, $($use_data:ident)?),
> +        // `HasPinData` or `HasInitData`.
> +        @has_data($has_data:ident, $get_data:ident),
> +        // `pin_init_from_closure` or `init_from_closure`.
> +        @construct_closure($construct_closure:ident),
> +        @munch_fields(..Zeroable::zeroed()),
> +    ) =3D> {
> +        $crate::__init_internal!(with_update_parsed:
> +            @this($($this)?),
> +            @typ($t $(::<$($generics),*>)? ),
> +            @fields($($fields)*),
> +            @error($err),
> +            @data($data, $($use_data)?),
> +            @has_data($has_data, $get_data),
> +            @construct_closure($construct_closure),
> +            @zeroed(()), // `()` means zero all fields not mentioned.
> +        )
> +    };
> +    (
> +        @this($($this:ident)?),
> +        @typ($t:ident $(::<$($generics:ty),*>)?),
> +        @fields($($fields:tt)*),
> +        @error($err:ty),
> +        // Either `PinData` or `InitData`, `$use_data` should only be pr=
esent in the `PinData`
> +        // case.
> +        @data($data:ident, $($use_data:ident)?),
> +        // `HasPinData` or `HasInitData`.
> +        @has_data($has_data:ident, $get_data:ident),
> +        // `pin_init_from_closure` or `init_from_closure`.
> +        @construct_closure($construct_closure:ident),
> +        @munch_fields($ignore:tt $($rest:tt)*),
> +    ) =3D> {
> +        $crate::__init_internal!(
> +            @this($($this)?),
> +            @typ($t $(::<$($generics),*>)? ),
> +            @fields($($fields)*),
> +            @error($err),
> +            @data($data, $($use_data)?),
> +            @has_data($has_data, $get_data),
> +            @construct_closure($construct_closure),
> +            @munch_fields($($rest)*),
> +        )
> +    };
> +    (with_update_parsed:
> +        @this($($this:ident)?),
> +        @typ($t:ident $(::<$($generics:ty),*>)?),
> +        @fields($($fields:tt)*),
> +        @error($err:ty),
> +        // Either `PinData` or `InitData`, `$use_data` should only be pr=
esent in the `PinData`
> +        // case.
> +        @data($data:ident, $($use_data:ident)?),
> +        // `HasPinData` or `HasInitData`.
> +        @has_data($has_data:ident, $get_data:ident),
> +        // `pin_init_from_closure` or `init_from_closure`.
> +        @construct_closure($construct_closure:ident),
> +        @zeroed($($init_zeroed:expr)?),
>      ) =3D> {{
>          // We do not want to allow arbitrary returns, so we declare this=
 type as the `Ok` return
>          // type and shadow it later when we insert the arbitrary user co=
de. That way there will be
> @@ -1024,6 +1101,17 @@ macro_rules! __init_internal {
>                  {
>                      // Shadow the structure so it cannot be used to retu=
rn early.
>                      struct __InitOk;
> +                    // If `$init_zeroed` is present we should zero the s=
lot now and not emit an
> +                    // error when fields are missing (since they will be=
 zeroed). We also have to
> +                    // check that the type actually implements `Zeroable=
`.
> +                    $(
> +                        fn is_zeroable<T: Zeroable>(ptr: *mut T) {}

Maybe call this assert_zeroable?

> +                        // Ensure that the struct is indeed `Zeroable`.
> +                        is_zeroable(slot);
> +                        // SAFETY:  The type implements `Zeroable` by th=
e check above.
> +                        unsafe { ::core::ptr::write_bytes(slot, 0, 1) };
> +                        $init_zeroed // this will be `()` if set.

How does this work? Shouldn't there be a ; after $init_zeroed to consume th=
e () value?

> +                    )?
>                      // Create the `this` so it can be referenced by the =
user inside of the
>                      // expressions creating the individual fields.
>                      $(let $this =3D unsafe { ::core::ptr::NonNull::new_u=
nchecked(slot) };)?
> @@ -1064,7 +1152,7 @@ macro_rules! __init_internal {
>          @data($data:ident),
>          @slot($slot:ident),
>          @guards($($guards:ident,)*),
> -        @munch_fields($(,)?),
> +        @munch_fields($(..Zeroable::zeroed())? $(,)?),
>      ) =3D> {
>          // Endpoint of munching, no fields are left. If execution reache=
s this point, all fields
>          // have been initialized. Therefore we can now dismiss the guard=
s by forgetting them.
> @@ -1157,6 +1245,30 @@ macro_rules! __init_internal {
>              @munch_fields($($rest)*),
>          );
>      };
> +    (make_initializer:
> +        @slot($slot:ident),
> +        @type_name($t:ident),
> +        @munch_fields(..Zeroable::zeroed() $(,)?),
> +        @acc($($acc:tt)*),
> +    ) =3D> {
> +        // Endpoint, nothing more to munch, create the initializer. Sinc=
e the users specified
> +        // `..Zeroable::zeroed()`, the slot will already have been zeroe=
d and all field that have
> +        // not been overwritten are thus zero and initialized. We still =
check that all fields are
> +        // actually accessible by using the struct update syntax ourselv=
es.
> +        // Since we are in the `if false` branch, this will never get ex=
ecuted. We abuse `slot` to
> +        // get the correct type inference here:
> +        unsafe {
> +            let mut zeroed =3D ::core::mem::zeroed();
> +            // We have to use type inference her to make zeroed have the=
 correct type. This does

*here

> +            // not get executed, so it has no effect.
> +            ::core::ptr::write($slot, zeroed);
> +            zeroed =3D ::core::mem::zeroed();
> +            ::core::ptr::write($slot, $t {
> +                $($acc)*
> +                ..zeroed
> +            });
> +        }
> +    };
>      (make_initializer:
>          @slot($slot:ident),
>          @type_name($t:ident),
> --
> 2.41.0

Cheers,
Bj=C3=B6rn