linux-kernel - Re: [PATCH v5 7/9] rust: file: add `Kuid` wrapper

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Message-ID: <CALNs47tTTbWL3T9rk_ismPT0Bwi8Kcm5aT9k8jfPsh=1wKvrPA@mail.gmail.com>
Date: Sat, 10 Feb 2024 01:43:14 -0600
From: Trevor Gross <tmgross@...ch.edu>
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>, 
	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, 
	Martin Rodriguez Reboredo <yakoyoku@...il.com>
Subject: Re: [PATCH v5 7/9] rust: file: add `Kuid` wrapper

On Fri, Feb 9, 2024 at 5:22 AM Alice Ryhl <aliceryhl@...gle.com> wrote:
>
> Adds a wrapper around `kuid_t` called `Kuid`. This allows us to define
> various operations on kuids such as equality and current_euid. It also
> lets us provide conversions from kuid into userspace values.
>
> Rust Binder needs these operations because it needs to compare kuids for
> equality, and it needs to tell userspace about the pid and uid of
> incoming transactions.
>
> To read kuids from a `struct task_struct`, you must currently use
> various #defines that perform the appropriate field access under an RCU
> read lock. Currently, we do not have a Rust wrapper for rcu_read_lock,
> which means that for this patch, there are two ways forward:
>
>  1. Inline the methods into Rust code, and use __rcu_read_lock directly
>     rather than the rcu_read_lock wrapper. This gives up lockdep for
>     these usages of RCU.
>
>  2. Wrap the various #defines in helpers and call the helpers from Rust.
>
> This patch uses the second option. One possible disadvantage of the
> second option is the possible introduction of speculation gadgets, but
> as discussed in [1], the risk appears to be acceptable.
>
> Of course, once a wrapper for rcu_read_lock is available, it is
> preferable to use that over either of the two above approaches.

Is this worth a FIXME?

> Link: https://lore.kernel.org/all/202312080947.674CD2DC7@keescook/ [1]
> Reviewed-by: Benno Lossin <benno.lossin@...ton.me>
> Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@...il.com>
> Signed-off-by: Alice Ryhl <aliceryhl@...gle.com>

Reviewed-by: Trevor Gross <tmgross@...ch.edu>