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

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Date:   Sat, 1 Apr 2023 08:01:00 +0100
From:   Gary Guo <gary@...yguo.net>
To:     Wedson Almeida Filho <wedsonaf@...il.com>
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`

On Sat, 1 Apr 2023 01:09:18 -0300
Wedson Almeida Filho <wedsonaf@...il.com> wrote:

> 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

Hi Wedson,

My apologies for the confusion. `&*Task::current()` is not
sufficient. I typed too quick without double checking.

However it is still true that `TaskRef<'static>` is unsound, and it can
be retrieved from `current()`. The missing step is `&'static
TaskRef<'static>`.

So you can write `&*Box::leak(Box::try_new(Task::current()).unwrap())`
and get `&'static Task`.

Best,
Gary