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

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Date: Wed, 20 Mar 2024 16:33:01 +0100
From: Christian Brauner <brauner@...nel.org>
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>, Benno Lossin <benno.lossin@...ton.me>, 
	Andreas Hindborg <a.hindborg@...sung.com>, Peter Zijlstra <peterz@...radead.org>, 
	Alexander Viro <viro@...iv.linux.org.uk>, 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, 
	Martin Rodriguez Reboredo <yakoyoku@...il.com>
Subject: Re: [PATCH v5 6/9] rust: file: add `FileDescriptorReservation`

On Fri, Feb 09, 2024 at 11:18:19AM +0000, Alice Ryhl wrote:
> From: Wedson Almeida Filho <wedsonaf@...il.com>
> 
> 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@...il.com>
> Co-developed-by: Alice Ryhl <aliceryhl@...gle.com>
> Reviewed-by: Benno Lossin <benno.lossin@...ton.me>
> Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@...il.com>
> Signed-off-by: Alice Ryhl <aliceryhl@...gle.com>
> ---

This looks ok to me now.