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:   Wed, 7 Jun 2023 19:42:10 +0100
From:   Gary Guo <gary@garyguo.net>
To:     Boqun Feng <boqun.feng@gmail.com>
Cc:     Qingsong Chen <changxian.cqs@antgroup.com>,
        linux-kernel@vger.kernel.org,
        =?UTF-8?B?55Sw5rSq5Lqu?= <tate.thl@antgroup.com>,
        Miguel Ojeda <ojeda@kernel.org>,
        Alex Gaynor <alex.gaynor@gmail.com>,
        Wedson Almeida Filho <wedsonaf@gmail.com>,
        =?UTF-8?B?QmrDtnJu?= Roy Baron <bjorn3_gh@protonmail.com>,
        Benno Lossin <benno.lossin@proton.me>,
        Martin Rodriguez Reboredo <yakoyoku@gmail.com>,
        Alice Ryhl <aliceryhl@google.com>,
        Asahi Lina <lina@asahilina.net>, rust-for-linux@vger.kernel.org
Subject: Re: [PATCH v2 1/3] rust: kernel: add ScatterList abstraction
Message-ID: <20230607194210.380d195f.gary@garyguo.net>
In-Reply-To: <ZH3+jARBbn2iG1dW@Boquns-Mac-mini.local>
References: <20230602101819.2134194-1-changxian.cqs@antgroup.com>
        <20230602101819.2134194-2-changxian.cqs@antgroup.com>
        <ZH3+jARBbn2iG1dW@Boquns-Mac-mini.local>
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?OBsAPFnDC7/XKZ/IofwvvOH8hItvK21MlM9ZHWJ7B8VapcvAqPmiltmapDhe?=
 =?us-ascii?Q?wHLE4pXgmaK1/Depj5J3EKkU8g34+njEmeebAxTAgKkxhU641WbmHbVXpbDx?=
 =?us-ascii?Q?yehu5eB/uAngTFX1zdA96NIAVS/Xoa5v3au4uiZi7WNyT5sUXCnMEACve7tS?=
 =?us-ascii?Q?rx7wgrwG2YHE1rGP0wSuWizzB7NJqQfgLNsbJUIkUr+6zH6D0lzLOJ4/+sot?=
 =?us-ascii?Q?/MbLHMriKar1GPEXsHLdQ3rju+yWQGQopvtqfPpXie1vfuYqZXedgNagjToh?=
 =?us-ascii?Q?3RdpdER0Bp02grZulBmdWWsSHz6qSTz9vqqDbYOmXjzNyPQaUKyUheqMjA6w?=
 =?us-ascii?Q?lVdOHcYr3VDqkGR9O4uAKFo3b8/82Pmb4GPUZwlQbFX9c7ASuMmxHIqU08SA?=
 =?us-ascii?Q?hS6gOyJJRYApbH7oqC5QQJxsxaOabCr7c+/6nNXClIQIjt29xUdb28VK7kPg?=
 =?us-ascii?Q?UVYeN1eI5yxzGk5WzrOnPqpMlnAvoWu7yDRySUEpbvIgttMg/sUKaNkUr5dL?=
 =?us-ascii?Q?lMR9cNs7+gaMtUI0ubvpmGk/0OOMxD8CLr+Yut0h2tB9bwJTF8xOy7vL/dCU?=
 =?us-ascii?Q?uWuBWpJQyD/9pQu+zimBU0CY/U8j8NYnGvnyHjIAcaAGsrAlijhCVtZzYmtD?=
 =?us-ascii?Q?Wp7+AAvMniAmyKY0ynW7adIKvMmXOuPn+xUWsf7ZKV0m1gS4JE2B/dEx6vkn?=
 =?us-ascii?Q?bDNKjN+Oh7S6THG88ctuA7iS3sK18wUmrf5QfcISxzQdYl93eI7N4CtcIMhQ?=
 =?us-ascii?Q?1P9MNScgQtBXouar6T2ltl4KZLvv6MYXyRkOBPSVH9AEOBPgEXiZj8Up42Fo?=
 =?us-ascii?Q?U4idyqqaYYnAW8WTwPhpDdw5vGNVO5hG8koGxxyiwuyXGN3A2OHAq9pgN2u7?=
 =?us-ascii?Q?u7/ekmF8gpcRYh0T6XiKg9NsLT+20NeMyVxjEfLj/I4AG55NKuUmpQ8ovj+H?=
 =?us-ascii?Q?waxtorP9h5ORpfXUatqsItrljgMz6slxn/Ww4/q1VYiL7PkUK5rvBD7dG4Xj?=
 =?us-ascii?Q?/J20l8J4EDWB7g/xl4EfgQeTVEMBv+FpSrVL/MuwtcBZ8pVQmYb5GfyQIkyz?=
 =?us-ascii?Q?pu64qBnAptGK0FdoGG/twDg3t8/0jqancf8e5yzxcQ+0S+lr5ys3RQIcwQLy?=
 =?us-ascii?Q?oXX/vMnVZiIq3SIr+sK3SWMEHI051CRAlmJXAHI/UFYhxxy6F6l6OF7NcXgv?=
 =?us-ascii?Q?5C57hqizEf6FRoY4csI0dKKGnZZL1BMrLzvQ1ri6MntM7CN2COBwuh4TBKLF?=
 =?us-ascii?Q?V5iBL4925MFB84DwjnL6daMOcmL2LQCXPvzfvghgS4lIPlS6uWg9Wtxyv9DK?=
 =?us-ascii?Q?3DydVGmVeOtLvyqDzRN8u50LdtkUdtRw6oybRdbrYU1nhxEr5TIigX6vi0Fk?=
 =?us-ascii?Q?uErSE8hw8Xtc/PxS7zUjTC1/lB31THe11QIS+2yYzf87T8+B1UmpoxzyHdzT?=
 =?us-ascii?Q?OnDeEYGIF3KXw/Z7KKE4gcQJ8oy2O7suimqHs9+JyjD2F+Trr/6HEbrB4xvR?=
 =?us-ascii?Q?M8lLuF/9FrQE5ZHAZ8p0qWfGeO3kM3zcKNXxdRSLeRXMqC1S+43oQikMF0kE?=
 =?us-ascii?Q?u19TZxPrJuzRbITtI9IfXtcsBLWyxHKcNR2BxctA?=
X-OriginatorOrg: garyguo.net
X-MS-Exchange-CrossTenant-Network-Message-Id: b2abeb39-cbe9-4194-7b9d-08db6786e83b
X-MS-Exchange-CrossTenant-AuthSource: LO2P265MB5183.GBRP265.PROD.OUTLOOK.COM
X-MS-Exchange-CrossTenant-AuthAs: Internal
X-MS-Exchange-CrossTenant-OriginalArrivalTime: 07 Jun 2023 18:42:12.4581
 (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: /FH2AMLJ4zFbCvTM6WiPOGY84NchLumrpT5Ma4UfhkI+2STKPBPdQjyIdDJCw1HGrHAwgu6YRAGeE4rtZUFnLw==
X-MS-Exchange-Transport-CrossTenantHeadersStamped: CWLP265MB7005
X-Spam-Status: No, score=-2.1 required=5.0 tests=BAYES_00,DKIM_SIGNED,
        DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF,SPF_HELO_PASS,SPF_PASS,
        T_SCC_BODY_TEXT_LINE,URIBL_BLOCKED autolearn=ham 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 Mon, 5 Jun 2023 08:26:04 -0700
Boqun Feng <boqun.feng@gmail.com> wrote:

> On Fri, Jun 02, 2023 at 06:18:17PM +0800, Qingsong Chen wrote:
> [...]
> > +impl<'a> ScatterList<'a> {
> > +    /// Construct a new initializer.
> > +    pub fn new(buf: &'a Pin<&mut [u8]>) -> impl PinInit<ScatterList<'a>> {
> > +        // SAFETY: `slot` is valid while the closure is called, the memory
> > +        // buffer is pinned and valid.
> > +        unsafe {
> > +            init::pin_init_from_closure(move |slot: *mut Self| {
> > +                (*slot).set_buf(buf);
> > +                (*slot).mark_end();  
> 
> Benno can provide more information, but you cannot dereference or create
> a reference to `*slot`, since `slot` points to an uninitialized object
> (see `try_pin_init` implementations), and referencing uninitialized
> objects is UB (or may cause UB).

This is fine for `Self`, because the only non-ZST field in there is
`Opaque`, which can be uninitialised.

> 
> Note that you could do the following for `set_buf`:
> 
> 	// `addr_of!`[1] is special since it won't create references
> 	// (even temporary onces).
> 	let opaque = addr_of!((*slot).opaque); // <- *const Opaque<bindings::scatterlist>
> 
> 	let ptr = Opaque::raw_get(opaque); // <- *mut bindings::scatterlist
> 
> 	// Maybe this can be wrapped as a Rust function with a
> 	// parameter: *mut bindings::scatterlist.
> 	unsafe {
> 		bindings::sg_set_buf(ptr, buf.as_ptr(), buf.len());
> 	}
> 
> [1]: https://doc.rust-lang.org/core/ptr/macro.addr_of.html
> 
> Regards,
> Boqun
> 
> > +                Ok(())
> > +            })
> > +        }
> > +    }
> > +  
> [...]