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

    > @@ -157,6 +158,12 @@ void rust_helper_init_work_with_key(struct work_struct *work, work_func_t func,
    > }
    > EXPORT_SYMBOL_GPL(rust_helper_init_work_with_key);
    >
    > +struct file *rust_helper_get_file(struct file *f)
    > +{
    > + return get_file(f);
    > +}
    > +EXPORT_SYMBOL_GPL(rust_helper_get_file);

    This is ridiculous. A function call instead of doing the
    atomic_long_inc() in Rust?

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