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Date:   Fri, 08 Dec 2023 16:13:26 +0000
To:     Alice Ryhl <aliceryhl@google.com>
From:   Benno Lossin <benno.lossin@proton.me>
Cc:     Miguel Ojeda <ojeda@kernel.org>,
        Alex Gaynor <alex.gaynor@gmail.com>,
        Wedson Almeida Filho <wedsonaf@gmail.com>,
        Boqun Feng <boqun.feng@gmail.com>, Gary Guo <gary@garyguo.net>,
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        Alexander Viro <viro@zeniv.linux.org.uk>,
        Christian Brauner <brauner@kernel.org>,
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        Matthew Wilcox <willy@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Daniel Xu <dxu@dxuuu.xyz>, linux-kernel@vger.kernel.org,
        rust-for-linux@vger.kernel.org, linux-fsdevel@vger.kernel.org
Subject: Re: [PATCH v2 2/7] rust: cred: add Rust abstraction for `struct cred`
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On 12/6/23 12:59, Alice Ryhl wrote:
> +/// Wraps the kernel's `struct cred`.
> +///
> +/// # Invariants
> +///
> +/// Instances of this type are always ref-counted, that is, a call to `g=
et_cred` ensures that the
> +/// allocation remains valid at least until the matching call to `put_cr=
ed`.
> +#[repr(transparent)]
> +pub struct Credential(pub(crate) Opaque<bindings::cred>);

Why is the field `pub(crate)`?

[...]

> +// SAFETY: The type invariants guarantee that `Credential` is always ref=
-counted.
> +unsafe impl AlwaysRefCounted for Credential {
> +    fn inc_ref(&self) {
> +        // SAFETY: The existence of a shared reference means that the re=
fcount is nonzero.
> +        unsafe { bindings::get_cred(self.0.get()) };
> +    }
> +
> +    unsafe fn dec_ref(obj: core::ptr::NonNull<Self>) {
> +        // SAFETY: The safety requirements guarantee that the refcount i=
s nonzero.

Can you also justify the `cast()`?

--=20
Cheers,
Benno

> +        unsafe { bindings::put_cred(obj.cast().as_ptr()) };
> +    }
> +}