Re: [PATCH v5 6/9] rust: file: add `FileDescriptorReservation`

From: Trevor Gross
Date: Sat Feb 10 2024 - 02:41:50 EST

On Fri, Feb 9, 2024 at 5:21 AM Alice Ryhl <aliceryhl@xxxxxxxxxx> wrote:
> From: Wedson Almeida Filho <wedsonaf@xxxxxxxxx>
> Allow for the creation of a file descriptor in two steps: first, we
> reserve a slot for it, then we commit or drop the reservation. The first
> step may fail (e.g., the current process ran out of available slots),
> but commit and drop never fail (and are mutually exclusive).
> This is needed by Rust Binder when fds are sent from one process to
> another. It has to be a two-step process to properly handle the case
> where multiple fds are sent: The operation must fail or succeed
> atomically, which we achieve by first reserving the fds we need, and
> only installing the files once we have reserved enough fds to send the
> files.
> Fd reservations assume that the value of `current` does not change
> between the call to get_unused_fd_flags and the call to fd_install (or
> put_unused_fd). By not implementing the Send trait, this abstraction
> ensures that the `FileDescriptorReservation` cannot be moved into a
> different process.
> Signed-off-by: Wedson Almeida Filho <wedsonaf@xxxxxxxxx>
> Co-developed-by: Alice Ryhl <aliceryhl@xxxxxxxxxx>
> Reviewed-by: Benno Lossin <benno.lossin@xxxxxxxxx>
> Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@xxxxxxxxx>
> Signed-off-by: Alice Ryhl <aliceryhl@xxxxxxxxxx>

Reviewed-by: Trevor Gross <tmgross@xxxxxxxxx>