Re: [PATCH 1/7] rust: file: add Rust abstraction for `struct file`

[Benno Lossin <benno.lossin@proton.me>]

On 11/29/23 16:13, Matthew Wilcox wrote:
> On Wed, Nov 29, 2023 at 12:51:07PM +0000, Alice Ryhl wrote:
>> This introduces a struct for the EBADF error type, rather than just
>> using the Error type directly. This has two advantages:
>> * `File::from_fd` returns a `Result<ARef<File>, BadFdError>`, which the
>>   compiler will represent as a single pointer, with null being an error.
>>   This is possible because the compiler understands that `BadFdError`
>>   has only one possible value, and it also understands that the
>>   `ARef<File>` smart pointer is guaranteed non-null.
>> * Additionally, we promise to users of the method that the method can
>>   only fail with EBADF, which means that they can rely on this promise
>>   without having to inspect its implementation.
>> That said, there are also two disadvantages:
>> * Defining additional error types involves boilerplate.
>> * 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. (However, it works fine in methods
>>   that return `Error`.)
> 
> I haven't looked at how Rust-for-Linux handles errors yet, but it's
> disappointing to see that it doesn't do something like the PTR_ERR /
> ERR_PTR / IS_ERR C thing under the hood.

In this case we are actually doing that: `ARef<T>` is a non-null pointer
to a `T` and since `BadFdError` is a unit struct (i.e. there exists only
a single value it can take) `Result<ARef<T>, BadFdError>` has the same
size as a pointer. This is because the Rust compiler represents the
`Err` variant as null.

We also do have support for `ERR_PTR`, but that requires `unsafe`, since
we do not know which kind of pointer the C side returned (was it an
`ARef<T>`, `&mut T`, `&T` etc.?) and can therefore only support `*mut T`.

-- 
Cheers,
Benno