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Date:   Wed, 5 Apr 2023 21:18:40 +0100
From:   Gary Guo <gary@garyguo.net>
To:     Benno Lossin <y86-dev@protonmail.com>
Cc:     Miguel Ojeda <ojeda@kernel.org>,
        Alex Gaynor <alex.gaynor@gmail.com>,
        Wedson Almeida Filho <wedsonaf@gmail.com>,
        Boqun Feng <boqun.feng@gmail.com>,
        =?UTF-8?B?QmrDtnJu?= Roy Baron <bjorn3_gh@protonmail.com>,
        Alice Ryhl <alice@ryhl.io>,
        Andreas Hindborg <nmi@metaspace.dk>,
        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>
Subject: Re: [PATCH v6 13/15] rust: types: add common init-helper functions
 for `Opaque`
Message-ID: <20230405211840.022ee6e2.gary@garyguo.net>
In-Reply-To: <20230405193445.745024-14-y86-dev@protonmail.com>
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On Wed, 05 Apr 2023 19:36:46 +0000
Benno Lossin <y86-dev@protonmail.com> wrote:

> Add helper functions to more easily initialize `Opaque<T>` via FFI and
> rust raw initializer functions.
> These functions take a function pointer to the FFI/raw initialization
> function and take between 0-4 other arguments. It then returns an
> initializer that uses the FFI/raw initialization function along with the
> given arguments to initialize an `Opaque<T>`.
> 
> Signed-off-by: Benno Lossin <y86-dev@protonmail.com>
> Reviewed-by: Alice Ryhl <aliceryhl@google.com>
> Reviewed-by: Andreas Hindborg <a.hindborg@samsung.com>
> Cc: Gary Guo <gary@garyguo.net>
> ---
>  rust/kernel/init.rs  |  9 +++++++++
>  rust/kernel/types.rs | 47 ++++++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 56 insertions(+)
> 
> diff --git a/rust/kernel/init.rs b/rust/kernel/init.rs
> index ffd539e2f5ef..a501fb823ae9 100644
> --- a/rust/kernel/init.rs
> +++ b/rust/kernel/init.rs
> @@ -177,6 +177,14 @@
>  //! }
>  //! ```
>  //!
> +//! For the special case where initializing a field is a single FFI-function call that cannot fail,
> +//! there exist helper functions [`Opaque::ffi_init`]. These functions initialize a single
> +//! [`Opaque`] field by just delegating to the FFI-function. You can use these in combination with
> +//! [`pin_init!`].
> +//!
> +//! For more information on how to use [`pin_init_from_closure()`], take a look at the uses inside
> +//! the `kernel` crate. The [`sync`] module is a good starting point.
> +//!
>  //! [`sync`]: kernel::sync
>  //! [pinning]: https://doc.rust-lang.org/std/pin/index.html
>  //! [structurally pinned fields]:
> @@ -187,6 +195,7 @@
>  //! [`impl PinInit<T, E>`]: PinInit
>  //! [`impl Init<T, E>`]: Init
>  //! [`Opaque`]: kernel::types::Opaque
> +//! [`Opaque::ffi_init`]: kernel::types::Opaque::ffi_init
>  //! [`pin_data`]: ::macros::pin_data
> 
>  use crate::{
> diff --git a/rust/kernel/types.rs b/rust/kernel/types.rs
> index ff2b2fac951d..dbfae9bb97ce 100644
> --- a/rust/kernel/types.rs
> +++ b/rust/kernel/types.rs
> @@ -2,6 +2,7 @@
> 
>  //! Kernel types.
> 
> +use crate::init::{self, PinInit};
>  use alloc::boxed::Box;
>  use core::{
>      cell::UnsafeCell,
> @@ -248,6 +249,52 @@ impl<T> Opaque<T> {
>      }
>  }
> 
> +macro_rules! opaque_init_funcs {
> +    ($($abi:literal $name:ident($($arg_name:ident: $arg_typ:ident),*);)*) => {
> +        impl<T> Opaque<T> {
> +            $(
> +                /// Create an initializer using the given initializer function.
> +                ///
> +                /// # Safety
> +                ///
> +                /// The given function **must** under all circumstances initialize the memory
> +                /// location to a valid `T`. If it fails to do so it results in UB.
> +                ///
> +                /// If any parameters are given, those need to be valid for the function. Valid
> +                /// means that calling the function with those parameters complies with the above
> +                /// requirement **and** every other requirement on the function itself.
> +                pub unsafe fn $name<$($arg_typ),*>(
> +                    init_func: unsafe extern $abi fn(*mut T $(, $arg_typ)*),
> +                    $($arg_name: $arg_typ,)*
> +                ) -> impl PinInit<Self> {
> +                    // SAFETY: The safety contract of this function ensures that `init_func` fully
> +                    // initializes `slot`.
> +                    unsafe {
> +                        init::pin_init_from_closure(move |slot| {
> +                            init_func(Self::raw_get(slot) $(, $arg_name)*);
> +                            Ok(())
> +                        })
> +                    }
> +                }
> +            )*
> +        }
> +    }
> +}

I personally don't like the fact that this is using function pointers,
as that generally causes a difficulty for static analysis tools.

How useful would this helper be?

unsafe {
    Opaque::ffi_init2(
        bindings::__init_waitqueue_head,
        name.as_char_ptr(),
        key.as_ptr(),
    )
}

doesn't save much from

unsafe {
    init::pin_init_from_closure(move |slot| {
        bindings::__init_waitqueue_head(
            Opaque::raw_get(slot),
            name.as_char_ptr(),
            key.as_ptr(),
       )
    )
}

> +
> +opaque_init_funcs! {
> +    "C" ffi_init();
> +    "C" ffi_init1(arg1: A1);
> +    "C" ffi_init2(arg1: A1, arg2: A2);
> +    "C" ffi_init3(arg1: A1, arg2: A2, arg3: A3);
> +    "C" ffi_init4(arg1: A1, arg2: A2, arg3: A3, arg4: A4);
> +
> +    "Rust" manual_init();
> +    "Rust" manual_init1(arg1: A1);
> +    "Rust" manual_init2(arg1: A1, arg2: A2);
> +    "Rust" manual_init3(arg1: A1, arg2: A2, arg3: A3);
> +    "Rust" manual_init4(arg1: A1, arg2: A2, arg3: A3, arg4: A4);
> +}
> +
>  /// A sum type that always holds either a value of type `L` or `R`.
>  pub enum Either<L, R> {
>      /// Constructs an instance of [`Either`] containing a value of type `L`.
> --
> 2.39.2
> 
>