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:   Thu, 19 Jan 2023 11:02:08 -0800
From:   Lucas De Marchi <lucas.demarchi@intel.com>
To:     Masahiro Yamada <masahiroy@kernel.org>
CC:     Michal =?utf-8?B?U3VjaMOhbmVr?= <msuchanek@suse.de>,
        Gary Guo <gary@garyguo.net>, Kees Cook <keescook@chromium.org>,
        <linux-kbuild@vger.kernel.org>,
        Wedson Almeida Filho <wedsonaf@google.com>,
        Joel Stanley <joel@jms.id.au>,
        Alex Gaynor <alex.gaynor@gmail.com>,
        "Miguel Ojeda" <ojeda@kernel.org>,
        Wedson Almeida Filho <wedsonaf@gmail.com>,
        Nicolas Schier <nicolas@fjasle.eu>,
        <rust-for-linux@vger.kernel.org>,
        "Guo Zhengkui" <guozhengkui@vivo.com>,
        Boqun Feng <boqun.feng@gmail.com>,
        "Nicholas Piggin" <npiggin@gmail.com>,
        =?utf-8?B?QmrDtnJu?= Roy Baron <bjorn3_gh@protonmail.com>,
        Nathan Chancellor <nathan@kernel.org>,
        "Nick Desaulniers" <ndesaulniers@google.com>,
        <linux-kernel@vger.kernel.org>,
        Julia Lawall <Julia.Lawall@inria.fr>,
        Luis Chamberlain <mcgrof@kernel.org>,
        <linuxppc-dev@lists.ozlabs.org>, <linux-modules@vger.kernel.org>
Subject: Re: [PATCH] modpost: support arbitrary symbol length in modversion
Message-ID: <20230119190208.2towhelvz7s6dn3u@ldmartin-desk2.lan>
References: <20230111161155.1349375-1-gary@garyguo.net>
 <20230112214059.o4vq474c47edjup6@ldmartin-desk2>
 <20230113181841.4d378a24.gary@garyguo.net>
 <20230117175144.GI16547@kitsune.suse.cz>
 <20230117192059.z5v5lfc2bzxk4ad2@ldmartin-desk2.lan>
 <CAK7LNATsuszFR7JB5ZkqVS1W=hWr9=E7bTf+MvgJ+NXT3aZNwg@mail.gmail.com>
Content-Type: text/plain; charset="iso-8859-1"; format=flowed
Content-Disposition: inline
Content-Transfer-Encoding: 8bit
In-Reply-To: <CAK7LNATsuszFR7JB5ZkqVS1W=hWr9=E7bTf+MvgJ+NXT3aZNwg@mail.gmail.com>
MIME-Version: 1.0
X-MS-Exchange-AntiSpam-MessageData-ChunkCount: 1
X-MS-Exchange-AntiSpam-MessageData-0: =?iso-8859-1?Q?5DcbA2zD+Ki7TrJ0QzW4uwxi/46SSYaU388igBOlWJChy6A7Ug8xs/AA1d?=
 =?iso-8859-1?Q?zYUIFpuNf+xe94Zc4Ol8YSZG8DOT4HuIGgWvw8wZ7r+qZ0oqyELtXV3l5b?=
 =?iso-8859-1?Q?PZEH04+t8tjYsvUJV7tisuepDT79crz0JxmTqU8liZjAV7FcYOqxuM6mpR?=
 =?iso-8859-1?Q?3kEX/BRNgVkvIbU5b11F+v8tzoqDtSs6WuD78Enj0OPZ/c56WZ5SdhqSN5?=
 =?iso-8859-1?Q?ZYl9KHTUr+665FjKXIpSbyCVYBtJF1RSf3llyCoqgC+NIIn2jT/j2b/gnO?=
 =?iso-8859-1?Q?oPCUrA61iWrbzXEQSLC3i2ov8ZHsWg45pGurSyP6AXTD3euhkcgKHJxKH7?=
 =?iso-8859-1?Q?x36uHxmKPfdQ/bTle3FJrfr7otG55o+RP4cNLzej9QF8hs1pI1AhO2Q32z?=
 =?iso-8859-1?Q?SaaNE3fQqUDFsaMMKLCLgwg0VQj36fS+n37HKOcUE0lbtmye3pYztoiNr4?=
 =?iso-8859-1?Q?/g4eSiMCWdKUAaEq/N4E3HYEdAvWbNWwiZFVHfiCER/idw6S2VojfCBmf8?=
 =?iso-8859-1?Q?pLSffVH9NMWG/jKc7NKqjFrUAObQdtwKWbDux9pIiytSzPw7jWcccTG7Rj?=
 =?iso-8859-1?Q?1Z1625r6MycWHI0QKN741K2BbnzuulUOIxG8iU941SelvEBtfU/PMNw167?=
 =?iso-8859-1?Q?BLw4TqIgmoduDJgVCqcn0ux1MARb0PTyXRv3zek2EOahjVcgiJTMMQ6fs5?=
 =?iso-8859-1?Q?TI4jFoTnb0q3rPu9tSmUsB0IIm2FK1lIIaYDQVz1mBBmoCqB8GCxbhGnT3?=
 =?iso-8859-1?Q?oXsxxLEW0iXxW1Zkxyk+RSd6k+y074JDB4ZNFl/FFAM8MjHyektpjnblp8?=
 =?iso-8859-1?Q?+hYanVoUnJMlXK/6ucyUIVB6xL3PFww31nL6DnnL1arpo11q9sfiqLSG6L?=
 =?iso-8859-1?Q?DQRZys2Skl4JrFRpJ6LVsKMPs24bnRoQ2tkwkdexAiejyppkVGzCqsdGDV?=
 =?iso-8859-1?Q?i8AWiTwhYl+97tkXcuheTgvR9OTtNx0JQQu7xp7zINrmJJHjgeL7iYMjqJ?=
 =?iso-8859-1?Q?XX5AX4G2wuhIiemCLAZId4/nhNnCrvqpkA4dhepcPVBH3oxaiI7rUQL6YY?=
 =?iso-8859-1?Q?kwK4CZb0Cclw+H0NMUPhTn6R0aUVMHYpLlKiooyULjMY6FNyiOCGLOQs6V?=
 =?iso-8859-1?Q?gfz1bRplmYLeftbfPdyRfedlsQ0paGpba5S4acSoDXnNnxcqNW4MI2obGV?=
 =?iso-8859-1?Q?KRqUqtzCmwXyite/weK5nsdfaeA0Wf68LQMSUBrrxmGmDKVFleR9mTrbCD?=
 =?iso-8859-1?Q?sBoMZDDnRgqb9XMxQUCXsbEJ0TF7IZdm92wucOVrsGHPKKxEzePvJBKKYD?=
 =?iso-8859-1?Q?6C6d7jABJB7hwmIeV0rhMbqCgncvPC5e5Xs5INeFLQXPuKIhLJEEJ7iQKa?=
 =?iso-8859-1?Q?aRblmhnUCo3Yk3XagD8KHGtfzoJgit0TG6zbQGzhvmvDlfokM581j1AdkM?=
 =?iso-8859-1?Q?3MbF3BYtHDl6R/4U1XYVPUikctjqf6M/nS+IWtitkIDVKnhwh4xJl/E83d?=
 =?iso-8859-1?Q?uX0/X9vlA0EUcypdBnbbDHi/jW8Z+tGwdkKAUDmlKFYipPDw/9YZBsL/6O?=
 =?iso-8859-1?Q?jBHW6A+if6anHr4COQcDq6vGZD4sMvYHE1ReQH6sWnLhoJ8Ja94F8bdCcj?=
 =?iso-8859-1?Q?FNnj/vy4qogR7AHJo0g2lHYJWiApXXuGul8jYXnEulGLEKjO/SyU4qiQ?=
 =?iso-8859-1?Q?=3D=3D?=
X-MS-Exchange-CrossTenant-Network-Message-Id: 550501be-1ab2-4e69-5619-08dafa4fad7c
X-MS-Exchange-CrossTenant-AuthSource: CY5PR11MB6139.namprd11.prod.outlook.com
X-MS-Exchange-CrossTenant-AuthAs: Internal
X-MS-Exchange-CrossTenant-OriginalArrivalTime: 19 Jan 2023 19:02:15.3677
 (UTC)
X-MS-Exchange-CrossTenant-FromEntityHeader: Hosted
X-MS-Exchange-CrossTenant-Id: 46c98d88-e344-4ed4-8496-4ed7712e255d
X-MS-Exchange-CrossTenant-MailboxType: HOSTED
X-MS-Exchange-CrossTenant-UserPrincipalName: mgyhFSRTrzNcJJB7KZTSlPVoS88vIaBUbmW08wtCo3yLvR9Zp/CH0naIoFOpEq6ZZ0yCG4QJZ+XGS9ZRRyDO/7xSB0i9AaeToFFI1PDnis4=
X-MS-Exchange-Transport-CrossTenantHeadersStamped: PH8PR11MB7046
X-OriginatorOrg: intel.com
X-Spam-Status: No, score=-4.4 required=5.0 tests=BAYES_00,DKIMWL_WL_HIGH,
        DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF,RCVD_IN_DNSWL_MED,
        SPF_HELO_PASS,SPF_NONE 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 Wed, Jan 18, 2023 at 04:01:29PM +0900, Masahiro Yamada wrote:
>On Wed, Jan 18, 2023 at 4:23 AM Lucas De Marchi
><lucas.demarchi@intel.com> wrote:
>>
>> On Tue, Jan 17, 2023 at 06:51:44PM +0100, Michal Such?nek wrote:
>> >Hello,
>> >
>> >On Fri, Jan 13, 2023 at 06:18:41PM +0000, Gary Guo wrote:
>> >> On Thu, 12 Jan 2023 14:40:59 -0700
>> >> Lucas De Marchi <lucas.demarchi@intel.com> wrote:
>> >>
>> >> > On Wed, Jan 11, 2023 at 04:11:51PM +0000, Gary Guo wrote:
>> >> > >
>> >> > > struct modversion_info {
>> >> > >- unsigned long crc;
>> >> > >- char name[MODULE_NAME_LEN];
>> >> > >+ /* Offset of the next modversion entry in relation to this one. */
>> >> > >+ u32 next;
>> >> > >+ u32 crc;
>> >> > >+ char name[0];
>> >> >
>> >> > although not really exported as uapi, this will break userspace as this is
>> >> > used in the  elf file generated for the modules. I think
>> >> > this change must be made in a backward compatible way and kmod updated
>> >> > to deal with the variable name length:
>> >> >
>> >> > kmod $ git grep "\[64"
>> >> > libkmod/libkmod-elf.c:  char name[64 - sizeof(uint32_t)];
>> >> > libkmod/libkmod-elf.c:  char name[64 - sizeof(uint64_t)];
>> >> >
>> >> > in kmod we have both 32 and 64 because a 64-bit kmod can read both 32
>> >> > and 64 bit module, and vice versa.
>> >> >
>> >>
>> >> Hi Lucas,
>> >>
>> >> Thanks for the information.
>> >>
>> >> The change can't be "truly" backward compatible, in a sense that
>> >> regardless of the new format we choose, kmod would not be able to decode
>> >> symbols longer than "64 - sizeof(long)" bytes. So the list it retrieves
>> >> is going to be incomplete, isn't it?
>> >>
>> >> What kind of backward compatibility should be expected? It could be:
>> >> * short symbols can still be found by old versions of kmod, but not
>> >>   long symbols;
>> >
>> >That sounds good. Not everyone is using rust, and with this option
>> >people who do will need to upgrade tooling, and people who don't care
>> >don't need to do anything.
>>
>> that could be it indeed. My main worry here is:
>>
>> "After the support is added in kmod, kmod needs to be able to output the
>> correct information regardless if the module is from before/after the
>> change in the kernel and also without relying on kernel version."
>> Just changing the struct modversion_info doesn't make that possible.
>>
>> Maybe adding the long symbols in another section? Or ble
>> just increase to 512 and add the size to a
>> "__versions_hdr" section. If we then output a max size per module,
>> this would offset a little bit the additional size gained for the
>> modules using rust. And the additional 0's should compress well
>> so I'm not sure the additional size is that much relevant here.
>
>
>
>
>
>I also thought of new section(s) for long symbols.
>
>
>
>One idea is to have separate sections for CRCs and symbol names.
>
>
>
>
>section __version_crc:
>   0x12345678
>   0x23456789
>   0x34567890
>
>
>section __version_sym:
>  "very_very_very_very_long_symbol"
>  "another_very_very_very_very_very_long_symbol"
>  "yet_another_very_very_very_very_very_long_symbol"
>
>
>
>
>You can iterate in each section with this:
>
>   crc += sizeof(u32);
>   name += strlen(name) + 1;
>
>
>Benefits:
>  - No next pointer
>  - No padding
>    - *.mod.c is kept human readable.

I like this option. It would be better than the current one.

Lucas De Marchi

>
>
>
>
>
>BTW, the following is impossible
>because the pointer reference to .rodata
>is not available at this point?
>
>struct modversion_info {
>     u32 crc;
>     const char *name:
>};
>
>
>
>-- 
>Best Regards
>Masahiro Yamada