linux-kernel - Re: [PATCH v3 7/9] rust: file: add `Kuid` wrapper

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Date: Wed, 24 Jan 2024 09:55:21 +0000
From: Benno Lossin <benno.lossin@...ton.me>
To: Alice Ryhl <aliceryhl@...gle.com>, 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>
Cc: 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 v3 7/9] rust: file: add `Kuid` wrapper

On 18.01.24 15:36, Alice Ryhl 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.
> 
> Link: https://lore.kernel.org/all/202312080947.674CD2DC7@keescook/ [1]
> Signed-off-by: Alice Ryhl <aliceryhl@...gle.com>
> ---
>   rust/bindings/bindings_helper.h |  1 +
>   rust/helpers.c                  | 45 ++++++++++++++++++++
>   rust/kernel/cred.rs             |  5 ++-
>   rust/kernel/task.rs             | 74 ++++++++++++++++++++++++++++++++-
>   4 files changed, 122 insertions(+), 3 deletions(-)

Reviewed-by: Benno Lossin <benno.lossin@...ton.me>

-- 
Cheers,
Benno