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Date:   Thu, 30 Nov 2023 16:17:03 +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>,
        =?utf-8?Q?Bj=C3=B6rn_Roy_Baron?= <bjorn3_gh@protonmail.com>,
        Andreas Hindborg <a.hindborg@samsung.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Christian Brauner <brauner@kernel.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        =?utf-8?Q?Arve_Hj=C3=B8nnev=C3=A5g?= <arve@android.com>,
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        Martijn Coenen <maco@android.com>,
        Joel Fernandes <joel@joelfernandes.org>,
        Carlos Llamas <cmllamas@google.com>,
        Suren Baghdasaryan <surenb@google.com>,
        Dan Williams <dan.j.williams@intel.com>,
        Kees Cook <keescook@chromium.org>,
        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 2/7] rust: cred: add Rust abstraction for `struct cred`
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On 11/29/23 13:51, Alice Ryhl wrote:
> +    /// Returns the credentials of the task that originally opened the f=
ile.
> +    pub fn cred(&self) -> &Credential {
> +        // This `read_volatile` is intended to correspond to a READ_ONCE=
 call.
> +        //
> +        // SAFETY: The file is valid because the shared reference guaran=
tees a nonzero refcount.
> +        //
> +        // TODO: Replace with `read_once` when available on the Rust sid=
e.
> +        let ptr =3D unsafe { core::ptr::addr_of!((*self.0.get()).f_cred)=
.read_volatile() };
> +
> +        // SAFETY: The signature of this function ensures that the calle=
r will only access the
> +        // returned credential while the file is still valid, and the cr=
edential must stay valid
> +        // while the file is valid.

About the last part of this safety comment, is this a guarantee from the
C side? If yes, then I would phrase it that way:

    ... while the file is still valid, and the C side ensures that the
    credentials stay valid while the file is valid.

--=20
Cheers,
Benno

> +        unsafe { Credential::from_ptr(ptr) }
> +    }
> +