linux-kernel - Re: [PATCH 2/7] rust: cred: add Rust abstraction for `struct cred`

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Date:   Thu, 30 Nov 2023 16:17:03 +0000
From:   Benno Lossin <benno.lossin@...ton.me>
To:     Alice Ryhl <aliceryhl@...gle.com>
Cc:     Miguel Ojeda <ojeda@...nel.org>,
        Alex Gaynor <alex.gaynor@...il.com>,
        Wedson Almeida Filho <wedsonaf@...il.com>,
        Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>,
        Björn Roy Baron <bjorn3_gh@...tonmail.com>,
        Andreas Hindborg <a.hindborg@...sung.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Christian Brauner <brauner@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Arve Hjønnevåg <arve@...roid.com>,
        Todd Kjos <tkjos@...roid.com>,
        Martijn Coenen <maco@...roid.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Carlos Llamas <cmllamas@...gle.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Kees Cook <keescook@...omium.org>,
        Matthew Wilcox <willy@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Daniel Xu <dxu@...uu.xyz>, linux-kernel@...r.kernel.org,
        rust-for-linux@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 2/7] rust: cred: add Rust abstraction for `struct cred`

On 11/29/23 13:51, Alice Ryhl wrote:
> +    /// Returns the credentials of the task that originally opened the file.
> +    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 guarantees a nonzero refcount.
> +        //
> +        // TODO: Replace with `read_once` when available on the Rust side.
> +        let ptr = unsafe { core::ptr::addr_of!((*self.0.get()).f_cred).read_volatile() };
> +
> +        // SAFETY: The signature of this function ensures that the caller will only access the
> +        // returned credential while the file is still valid, and the credential 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.

-- 
Cheers,
Benno

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