linux-kernel - Re: [PATCH 10/13] rust: introduce `Task::current`

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Message-ID: <ZCeubhgETf84thv9@wedsonaf-dev>
Date:   Sat, 1 Apr 2023 01:09:18 -0300
From:   Wedson Almeida Filho <wedsonaf@...il.com>
To:     Gary Guo <gary@...yguo.net>
Cc:     rust-for-linux@...r.kernel.org, Miguel Ojeda <ojeda@...nel.org>,
        Alex Gaynor <alex.gaynor@...il.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Björn Roy Baron <bjorn3_gh@...tonmail.com>,
        linux-kernel@...r.kernel.org,
        Wedson Almeida Filho <walmeida@...rosoft.com>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH 10/13] rust: introduce `Task::current`

Gary, thanks for reviewing!

On Fri, Mar 31, 2023 at 03:47:01AM +0100, Gary Guo wrote:
> 
> I don't think this API is sound, as you can do `&*Task::current()` and
> get a `&'static Task`, which is very problematic.

One thing that isn't clear to me is: how do you get a 'static lifetime in the
example above?

Altough `TaskRef` does have an arbitrary lifetime param, that's not the lifetime
that the returned `Task` reference gets. For illustration, I've explicitly added
a lifetime 'a in the impl below:

impl Deref for TaskRef<'_> {
    type Target = Task;
    fn deref(&'a self) -> &'a Self::Target {
        self.task
    }
}

Which means that the borrow of the `TaskRef` you use to call `deref` must
outlive the returned `Task`.

So how do you get a `TaskRef` with a static lifetime to begin with? Or is there
another trick to get the `&'static Task` that I can't see?

Thanks,
-Wedson