Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Sat, 1 Apr 2023 08:01:00 +0100
From:   Gary Guo <gary@garyguo.net>
To:     Wedson Almeida Filho <wedsonaf@gmail.com>
Cc:     rust-for-linux@vger.kernel.org, Miguel Ojeda <ojeda@kernel.org>,
        Alex Gaynor <alex.gaynor@gmail.com>,
        Boqun Feng <boqun.feng@gmail.com>,
        =?UTF-8?B?QmrDtnJu?= Roy Baron <bjorn3_gh@protonmail.com>,
        linux-kernel@vger.kernel.org,
        Wedson Almeida Filho <walmeida@microsoft.com>,
        Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH 10/13] rust: introduce `Task::current`
Message-ID: <20230401080100.5fc898ef.gary@garyguo.net>
In-Reply-To: <ZCeubhgETf84thv9@wedsonaf-dev>
References: <20230330043954.562237-1-wedsonaf@gmail.com>
        <20230330043954.562237-10-wedsonaf@gmail.com>
        <20230331034701.0657d5f2.gary@garyguo.net>
        <ZCeubhgETf84thv9@wedsonaf-dev>
Content-Type: text/plain; charset=US-ASCII
Content-Transfer-Encoding: 7bit
MIME-Version: 1.0
X-MS-Exchange-AntiSpam-MessageData-ChunkCount: 1
X-MS-Exchange-AntiSpam-MessageData-0: =?us-ascii?Q?5IKffKeoW7WhJ/sZSrlcJOpgFSMaMEeokdOVI/irdlYd7DJX2D6In015yMWl?=
 =?us-ascii?Q?NimiknhaZPFTyxBDUYHU00MZvhBV0lEAbkT7ukvudeRsFU0H3X4CUfJoeJ/C?=
 =?us-ascii?Q?r/Tf3Tn1PsJghDPEhm07uRxFH/YtR7XmfiB8sgaM8uHHwziL73vYV7p3hnOK?=
 =?us-ascii?Q?xZZiYYjP+hHXl8THX4tPSx2V3SRyoESMH7y5SJHRHgqgNeySZcoMrEq7kWHP?=
 =?us-ascii?Q?3/2BZ4E6XaiurOFjM5CtRW6DHEUybmnYHQnlEyZsK0nvhp/dDAvIezYPHAtu?=
 =?us-ascii?Q?iOTzRm2Xb2MYjnud4JXilAt08n1/bZR3PTnzyMcm8/iSesyOuAPgqyAfNldR?=
 =?us-ascii?Q?R/1deEeuDxCCoNKpI9yooqc/XHnGaIZetYteIQ58yaHPvFOmfViQr32k7zqE?=
 =?us-ascii?Q?CUjs4uZBHyd4cag4aOJJHsCkZkX3gEg34JEIhwB8sjF/EBhzt13LJ7tFmxxn?=
 =?us-ascii?Q?IBAyvdkZi5moETrQzoalddiQhDlK6PMkE2fQJwo8USBEnLbhDucR9p1FlbN7?=
 =?us-ascii?Q?HZoZ4shR4V+A3dZeXKsSJYgttPjOc0AkWPXcYlwjqRmFd7D8KnEwGSy/AElq?=
 =?us-ascii?Q?EOrFSoJI7ZKZTLrdS9mURhf4CxmCZkTXIr81yDkwelXv+JC5ep0xYrY6cdxx?=
 =?us-ascii?Q?xMPw5AOS+8E33u4ge6rDBS4YFUQGNVNd5l2nCy6NJS96xAQG0CKR29fme/VU?=
 =?us-ascii?Q?Uite5b2ZWqdD8D3w4bWfGm3QdTrY3AFb86mu2zZCbbswib1ziPbC8vW1YNul?=
 =?us-ascii?Q?kVHAzGm9Wja9GcVU0xr+CFyZp6otCQJwus6YRie6G/QbFQTKPcVxVN89LIak?=
 =?us-ascii?Q?4p8e5ev1U9EXCNdIoTKbgTeFfQugAp0cQVdoSrkAIzB48xkqsWAMjM+vWhLc?=
 =?us-ascii?Q?MGdAvvTo/yShFvr6thgCLE/i+s28kV8Y/Y6ZcvAhKR3ifPofuBY6bqo+T3wI?=
 =?us-ascii?Q?HIWCmXQaJ6pvowj9G2ijcm/msemIKsebt01XexuTqHg7Tkl5xgDeUJP4Kyuz?=
 =?us-ascii?Q?KdSwXRjKzfNhs3QibWWwEyROQP6agCVOrpFP0hRj++xmXZ3pGq92squA7SyM?=
 =?us-ascii?Q?1hv9+Z9dcAdhCKrZzNecTQsHNxylQW2oWB8OXCCtNWIW323qGVPm7H+vGt5a?=
 =?us-ascii?Q?HqUTy3sNJkRX2MiupUzB8pJ8LXNG4dI9AEr4X2dDT/Kov2CS21oqWtMf6kJK?=
 =?us-ascii?Q?dl21czPCkYbyyOg1+a2aNXnFDcmruBbPivDmXhpOYkbpD9F6fZSzvBkBhD0+?=
 =?us-ascii?Q?L8YMKKf1vtLg+43LbmhfeDNpAHTjDD70Bgm6nwgHucQg2AxWqXYSijggMnk0?=
 =?us-ascii?Q?BNVeoNKheQCmmWUJzGPiWh0nfHg6OL9t3Nu9iK6TXGMOXF+ku0icnTpBYpYs?=
 =?us-ascii?Q?UVcnRDkHig7dnmWhv01YSKguli/breM1sVxkz2iY3YTMsDkSsvdl0ETst1NI?=
 =?us-ascii?Q?2z1Fp3DN9/crHefN/c8RNti2+c/xWR9Hz/n3h1Y4ujiaq8pZNFHZIRws8hkh?=
 =?us-ascii?Q?LKxGYk7cB1xwIhiuJw9kYYspUaw6aj930UdR4T7JlgR4pyTSZGclzzFsXPEz?=
 =?us-ascii?Q?Cd6zkjL4zWis+kJpj7tTI7DOqktEkAbciMQoXcdL?=
X-OriginatorOrg: garyguo.net
X-MS-Exchange-CrossTenant-Network-Message-Id: 68d57f9a-7e0c-4289-1500-08db327ee9f4
X-MS-Exchange-CrossTenant-AuthSource: LO2P265MB5183.GBRP265.PROD.OUTLOOK.COM
X-MS-Exchange-CrossTenant-AuthAs: Internal
X-MS-Exchange-CrossTenant-OriginalArrivalTime: 01 Apr 2023 07:01:27.6759
 (UTC)
X-MS-Exchange-CrossTenant-FromEntityHeader: Hosted
X-MS-Exchange-CrossTenant-Id: bbc898ad-b10f-4e10-8552-d9377b823d45
X-MS-Exchange-CrossTenant-MailboxType: HOSTED
X-MS-Exchange-CrossTenant-UserPrincipalName: 4wfjFOyPoOvR7/VgFdnf5BR+TvuhQqfIy1jcjZeO11jQaFHeeXxV/N3g/t11pYfC5RBTfZGBhgNczRy7Rl+tdA==
X-MS-Exchange-Transport-CrossTenantHeadersStamped: CWXP265MB2358
X-Spam-Status: No, score=-0.2 required=5.0 tests=DKIM_SIGNED,DKIM_VALID,
        DKIM_VALID_AU,DKIM_VALID_EF,RCVD_IN_DNSWL_NONE,RCVD_IN_MSPIKE_H2,
        SPF_HELO_PASS,SPF_PASS autolearn=unavailable autolearn_force=no
        version=3.4.6
X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on
        lindbergh.monkeyblade.net
Precedence: bulk
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Sat, 1 Apr 2023 01:09:18 -0300
Wedson Almeida Filho <wedsonaf@gmail.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