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Date:   Tue, 8 Aug 2023 23:59:04 -0300
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From:   Martin Rodriguez Reboredo <yakoyoku@gmail.com>
Subject: Re: [RFC PATCH v1 1/5] rust: file: add bindings for `struct file`
To:     Alice Ryhl <aliceryhl@google.com>, rust-for-linux@vger.kernel.org,
        linux-fsdevel@vger.kernel.org, Miguel Ojeda <ojeda@kernel.org>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Christian Brauner <brauner@kernel.org>
Cc:     Wedson Almeida Filho <wedsonaf@gmail.com>,
        Alex Gaynor <alex.gaynor@gmail.com>,
        Boqun Feng <boqun.feng@gmail.com>, Gary Guo <gary@garyguo.net>,
        =?UTF-8?Q?Bj=C3=B6rn_Roy_Baron?= <bjorn3_gh@protonmail.com>,
        Benno Lossin <benno.lossin@proton.me>,
        linux-kernel@vger.kernel.org, patches@lists.linux.dev,
        Wedson Almeida Filho <walmeida@microsoft.com>,
        Daniel Xu <dxu@dxuuu.xyz>
References: <20230720152820.3566078-1-aliceryhl@google.com>
 <20230720152820.3566078-2-aliceryhl@google.com>
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On 7/20/23 12:28, Alice Ryhl wrote:
> From: Wedson Almeida Filho <walmeida@microsoft.com>
> 
> Using these bindings it becomes possible to access files from drivers
> written in Rust. This patch only adds support for accessing the flags,
> and for managing the refcount of the file.
> 
> Signed-off-by: Wedson Almeida Filho <walmeida@microsoft.com>
> Co-Developed-by: Daniel Xu <dxu@dxuuu.xyz>
> Signed-off-by: Daniel Xu <dxu@dxuuu.xyz>
> Co-Developed-by: Alice Ryhl <aliceryhl@google.com>
> Signed-off-by: Alice Ryhl <aliceryhl@google.com>
> ---
> In this patch, I am defining an error type called `BadFdError`. I'd like
> your thoughts on doing it this way vs just using the normal `Error`
> type.
> 
> Pros:
>   * The type system makes it clear that the function can only fail with
>     EBADF, and that no other errors are possible.
>   * Since the compiler knows that `ARef<Self>` cannot be null and that
>     `BadFdError` has only one possible value, the return type of
>     `File::from_fd` is represented as a pointer with null being an error.
> 
> Cons:
>   * Defining additional error types involves boilerplate.
>   * The return type becomes a tagged union, making it larger than a
>     pointer.

These two are kinda passable, as a `impl_null_ptr_err` macro can be opted
to implement error types from nulls. Also if we consider that
`File::from_fd` isn't going to be called a gorillion times a second or a
recursive call is done, then I'd say that the tagged union won't bring
any other problems, except that a compiler optim is skipped.

>   * The question mark operator will only utilize the `From` trait once,
>     which prevents you from using the question mark operator on
>     `BadFdError` in methods that return some third error type that the
>     kernel `Error` is convertible into.

Although, I want this to be mentioned in the doc of `BadFdError` as this
cannot be overlooked when said usage of `?` is done.

> 
>   rust/bindings/bindings_helper.h |   2 +
>   rust/helpers.c                  |   7 ++
>   rust/kernel/file.rs             | 176 ++++++++++++++++++++++++++++++++
>   rust/kernel/lib.rs              |   1 +
>   4 files changed, 186 insertions(+)
>   create mode 100644 rust/kernel/file.rs
> 
> [...]
> diff --git a/rust/bindings/bindings_helper.h b/rust/bindings/bindings_helper.h
> [...]
> diff --git a/rust/helpers.c b/rust/helpers.c
> [...]

This one is making my head spin, shouldn't they go first?