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:   Fri, 31 Mar 2023 01:20:06 +0100
From:   Gary Guo <gary@garyguo.net>
To:     Alice Ryhl <alice@ryhl.io>
Cc:     y86-dev@protonmail.com, Miguel Ojeda <ojeda@kernel.org>,
        Alex Gaynor <alex.gaynor@gmail.com>,
        Wedson Almeida Filho <wedsonaf@gmail.com>,
        Boqun Feng <boqun.feng@gmail.com>,
        =?UTF-8?B?QmrDtnJu?= Roy Baron <bjorn3_gh@protonmail.com>,
        rust-for-linux@vger.kernel.org, linux-kernel@vger.kernel.org,
        patches@lists.linux.dev
Subject: Re: [PATCH v3 09/13] rust: init: add `Zeroable` trait and
 `init::zeroed` function
Message-ID: <20230331012006.251b5729.gary@garyguo.net>
In-Reply-To: <19db6213-297b-a33d-7331-3c1a5b72fad9@ryhl.io>
References: <20230329223239.138757-10-y86-dev@protonmail.com>
        <19db6213-297b-a33d-7331-3c1a5b72fad9@ryhl.io>
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?g3zOGwd39VI0uE1hDVMDVIdYMlH+Tgt7oJN2vAvjXL4oMunnsSxqNvm0uh3J?=
 =?us-ascii?Q?MHS/8MjLlbmndgyQ7R7myr0Dm5q/JlRh1MMo+QeuBJ8pD4txqdrTBfkza5RM?=
 =?us-ascii?Q?ZLjU96f1VlKK330TRvBhb+2rvoz7F3zUgyFv8q+Rtm59UbRdk4nwmrZIK8sR?=
 =?us-ascii?Q?FFhctHQ8x4bj0FXI+0vgFE7wbacJ6uTO9zHOLlGNU0GiVngY2QNM3gaKC57R?=
 =?us-ascii?Q?Q+EjXhEXLENl7ncoIMbqslCMrCQCWVllhApF5l2xW3i2xp/9+fKixSkoBRt/?=
 =?us-ascii?Q?UzXyZLWUF3X3/Nr5vaGVy4c63xMq1+1ylz3+aSscxaxT27xF33+qxTEL/F9u?=
 =?us-ascii?Q?EOq4ZiRlPLWLVWwrlzkO2bxuODAXr5QMjUnYrBg5iY/XWWmwhpXF9u8qOcpU?=
 =?us-ascii?Q?pHZ8vgwyMikybEBbSKHTK/XsmmEGZux53NJ3dg5lydHl/mymUHYWY/6NT7jn?=
 =?us-ascii?Q?ZndiOFJRpH+2BESsur1uXp2MovxmoRxsouq8Au3VjdA8zJ3k+LCTDilpKEOA?=
 =?us-ascii?Q?7WTHQWrrMDhjBQCKsrI+MvMxK2X9eZHPx2ICLeFVTWbwVMglUVqTKI7pbcWk?=
 =?us-ascii?Q?7c8e44+OZZSthstfHZABPD8P0DFsmgTxkenhzQRztFV5cFOGfOzH4+Xrpv2V?=
 =?us-ascii?Q?qOxkISyRatRDJsQHNEZphN8ZPOoqy/CsKplARLAsoRnUC8JAvFZAjYHgpBq8?=
 =?us-ascii?Q?kLhMmE0apErwjPnK8X+B7QbSna4gwF1ZlzkD+58yRESigyKLYarWhtnL0JIN?=
 =?us-ascii?Q?x/LE0oWd6RuSXBUsXU0wdrTCLO5wxPznGqK+7XjJzpetJ2RkBR7Ektq2bMM7?=
 =?us-ascii?Q?Sl1Z74z+glbgMlva4NpHCZL6wTU84qs1HG2LoY100Fm4gJvjpGjtwYQhJ2P/?=
 =?us-ascii?Q?2iTxEWkUkVBntanWGZj8pqYr9Lz1Rg6iumfaENstlOKm/+M4xJGZWe9XnCYy?=
 =?us-ascii?Q?hG4eKj9GZ7mK6zLuk+GfiE1c7BbnxmyL39XwOVJ4C+nRRr5L1sy6B8ORjw2x?=
 =?us-ascii?Q?G5qwOjjYTtlCb37Sa8zcDD2agrlilbT94DSyswtru3bV45WjaWUO7p1csik2?=
 =?us-ascii?Q?lEh6OGxpr3eXawPt1dzMudbyZR9QpHNOzjBE09CF/NtDjCsGpmqM+xo5vRcc?=
 =?us-ascii?Q?7DaFNDfVcictPBT1Mae1K1JQA4kyd4H25EVEPz05CmCMs1+/LFNMxQgxlzrn?=
 =?us-ascii?Q?quoLP8h+XSXR/MQMYKAaWcFSOlyE+E3tOhNyF099+MPLnU5CAsEhhcE/kDJO?=
 =?us-ascii?Q?s/T4fmjVbLQRNFn4uuHHIHiRizX/mG1auPaXcm2c0PyGNuXuvytzd8lmlDbz?=
 =?us-ascii?Q?mHyf24aE34av+r+CuGDdirdcel67AKG9Cf0f9TerEvDKUEmfTk2n7FmdCDAM?=
 =?us-ascii?Q?G5QyVJ62Te6FbGjAMgdYEvHkVQHlLMzOCCWqe0KB07Yi9G/z9jyJxmih41cF?=
 =?us-ascii?Q?iQEQaJAwm/HrWp/cUMrisyc6Vglz9AaSSelpWPZraJHAKFeKtjsD9PQFiqmV?=
 =?us-ascii?Q?1xu9Cupo117kkZcs+NMPyCdbuJUHHOegTZYrFnrWWBwckHzz0KFVw/pSf5pl?=
 =?us-ascii?Q?PCzJt6km7ZmOS9nQL7oLrbYvp+kroixufXH8FWrr?=
X-OriginatorOrg: garyguo.net
X-MS-Exchange-CrossTenant-Network-Message-Id: 3bec74f5-5e68-48e4-8f0e-08db317dbbcd
X-MS-Exchange-CrossTenant-AuthSource: CWLP265MB5186.GBRP265.PROD.OUTLOOK.COM
X-MS-Exchange-CrossTenant-AuthAs: Internal
X-MS-Exchange-CrossTenant-OriginalArrivalTime: 31 Mar 2023 00:20:29.5908
 (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: aHsjOZ8LRFShUJlHo0FJ3dfjXpSa/bJs7n2eWtRqrJTXOxYScSPYEbsdItnIUDilGiDpRKZYqupLkD7KGkC4Gw==
X-MS-Exchange-Transport-CrossTenantHeadersStamped: LO0P265MB5525
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 Fri, 31 Mar 2023 00:40:34 +0200
Alice Ryhl <alice@ryhl.io> wrote:

> On 3/30/23 00:33, y86-dev@protonmail.com wrote:
> > From: Benno Lossin <y86-dev@protonmail.com>
> > 
> > Add the `Zeroable` trait which marks types that can be initialized by
> > writing `0x00` to every byte of the type. Also add the `init::zeroed`
> > function that creates an initializer for a `Zeroable` type that writes
> > `0x00` to every byte.
> > 
> > Signed-off-by: Benno Lossin <y86-dev@protonmail.com>
> > ---
> > +// SAFETY: `null` pointer is valid.
> > +unsafe impl<T: ?Sized> Zeroable for *mut T {}
> > +unsafe impl<T: ?Sized> Zeroable for *const T {}  
> 
> Actually, I just realized that this is not ok for unsized types. When T 
> is unsized, the raw pointer is a fat pointer with a vtable, and the 
> vtable part is not necessarily zeroable.
> 
> However, it would be ok to do it for `*const [T]` since the fat part of 
> the pointer is just the length in this case, and a length of zero is fine.
> 
> See more here:
> https://github.com/Lokathor/bytemuck/blob/8391afa876ba2e99dffb0c991cc7fa775287d106/src/zeroable.rs#L56-L65

Good catch. vtable completely slipped my mind when I am reviewing this
code.

Vtable is not *not necessary zeroable*, but actually never zeroable.
Although currently not yet formally specified, the compiler has always
assumed vtable part of fat pointers to be non-null, well aligned and
dereferenceable.

Best,
Gary