linux-kernel - Re: [PATCH v2 4/7] rust: file: add `FileDescriptorReservation`

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <ooDN7KSgDTAyd45wWcPa1VLmvo-fiqwDmffX1Yl8uiesYUcgnCZq5dcd6fGw5sVYMusZGpEI4L5avLCqNXXM7opR627oUp1NB-TIHOwJufg=@proton.me>
Date:   Fri, 08 Dec 2023 07:37:03 +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 v2 4/7] rust: file: add `FileDescriptorReservation`

On 12/6/23 12:59, Alice Ryhl wrote:
> +    /// Commits the reservation.
> +    ///
> +    /// The previously reserved file descriptor is bound to `file`. This method consumes the
> +    /// [`FileDescriptorReservation`], so it will not be usable after this call.
> +    pub fn fd_install(self, file: ARef<File>) {
> +        // SAFETY: `self.fd` was previously returned by `get_unused_fd_flags`, and `file.ptr` is
> +        // guaranteed to have an owned ref count by its type invariants.

There is no mention of the requirement that `current` has not changed
(you do mention it on the `_not_send` field, but I think it should also
be in the safety comment here).

> +        unsafe { bindings::fd_install(self.fd, file.0.get()) };
> +
> +        // `fd_install` consumes both the file descriptor and the file reference, so we cannot run
> +        // the destructors.
> +        core::mem::forget(self);
> +        core::mem::forget(file);
> +    }
> +}
> +
> +impl Drop for FileDescriptorReservation {
> +    fn drop(&mut self) {
> +        // SAFETY: `self.fd` was returned by a previous call to `get_unused_fd_flags`.

Ditto.

> +        unsafe { bindings::put_unused_fd(self.fd) };
> +    }
> +}
> +
>  /// Represents the `EBADF` error code.
>  ///
>  /// Used for methods that can only fail with `EBADF`.
> diff --git a/rust/kernel/types.rs b/rust/kernel/types.rs
> index fdb778e65d79..a4584d6b26c0 100644
> --- a/rust/kernel/types.rs
> +++ b/rust/kernel/types.rs
> @@ -387,3 +387,13 @@ pub enum Either<L, R> {
>      /// Constructs an instance of [`Either`] containing a value of type `R`.
>      Right(R),
>  }
> +
> +/// Zero-sized type to mark types not [`Send`].
> +///
> +/// Add this type as a field to your struct if your type should not be sent to a different task.
> +/// Since [`Send`] is an auto trait, adding a single field that is `!Send` will ensure that the
> +/// whole type is `!Send`.
> +///
> +/// If a type is `!Send` it is impossible to give control over an instance of the type to another
> +/// task. This is useful when a type stores task-local information for example file descriptors.
> +pub type NotThreadSafe = PhantomData<*mut ()>;

This should be in its own commit.

Then you can also change the usages of `PhantomData<*mut ()>` in
`Guard` and `TaskRef`.

It would be nice to use `NotThreadSafe` as the value instead of
`PhantomData`, since it is a bit confusing... 
I think we might be able to also have a constant with the same name
that is just `pub const NotThreadSafe: NotThreadSafe = PhantomData;`.

-- 
Cheers,
Benno